Old Tunes On New Fiddles:

The Enya 4-Stroke Diesels

by Adrian Duncan

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In this article we break tradition yet again by examining a contemporary model engine series, albeit from a manufacturer having a proud tradition of quality going back over 63 years at the time of writing (2012), a record matched by few current model engine producers. Our usual focus on the history of model engines from the increasingly-distant past reflects the admitted fact that we find the majority of current model engine products to be distressingly stereotyped and hence lacking in interest from anything other than a user's standpoint. However, once in a while a contemporary product jumps out at us ...

The new series with which we are concerned here combines two of the classic model engine design concepts—four-stroke technology and compression ignition. As die-hard model "diesel" aficionados, we couldn't resist the temptation to see for ourselves how well these two design features worked out in combination. As always, we'll start with a little background to our tale.

Background

Contrary to a seemingly widespread present-day belief, the application of four-stroke technology to model engines is far from new. In fact, it goes back to the very earliest days of power modelling. It's impossible to be sure any more regarding who first made and sold practical four-stroke model aero engines, but a strong claim to that honour can surely be made on behalf of the British modeller David Stanger, who is known to have made and later sold a few multi-cylinder four-stroke engines intended for model use beginning as early as 1907. Indeed, Stanger made the first officially-recorded British flight by a power model aircraft using one of his four-stroke engines.

From that time onwards, considerations of power-to-weight ratio steered the further development of model engines primarily towards the two-stroke principle. The odd commercial four-stroke design appeared from time to time, the names of Grayson, Morton (later Burgess), Feeney, Jensen, Gannet coming immediately to mind. A number of home constructors also produced very successful four-stroke models during this period, including some which achieved considerable competition success in the field of model hydroplane racing.

All of these were of course spark ignition models. Following the arrival of the commercial glow-plug in 1948, the issue of whether or not a four-stroke model engine could be made to run on glow-plug ignition was debated from time to time, the question being finally settled very decisively by British model engineering icon Professor DH Chaddock in the mid 1960's when he designed and built a very efficient 5cc four-stroke engine for an attempt on the world model aircraft flight duration record. This engine was found to run perfectly well on glow-plug ignition. An initial description of this groundbreaking unit was published in an article by Chaddock entitled "Why Not 4-Stroke?" which appeared in the January 1966 issue of Aeromodeller. Constructional details were subsequently published in the April 21st, 1967 issue of Model Engineer.

In the early 1970's, Glen Hargrave began work on five and seven cylinder four-stroke, glow plug radials which would eventually become the Technopower-II series. A prototype was displayed at the Nuremberg Hobby trade fair of 1974. According to Peter Chinn, writing in Model Four-Stroke Engines, this was the year that OS commenced design work on the OS FS-60 with the objectives of developing an engine that would (a) set new standards of quietness of operation for model internal combustion engines, and (b) run on glowplug ignition and on ordinary ("two-stroke") glow fuel. This very impressive unit was freleased in 1976. It was a fine performer which was hailed at the time as a major technological breakthrough.

The initial success of the original OS four-stroke model soon led to the entry into this market of several other manufacturers, most notably Saito, who entered the market in 1979 with their light and compact open-rocker FA-30 5cc model. The Enya Metal Products Co of Tokyo, always arch-rivals of OS, were a little slower to become involved, releasing their first four-stroke design in 1980.

OS broke further new ground in 1990 when they released the world's first commercial supercharged model engine, the 20cc FS-120S-SP four-stroke model. This featured a Roots-type supercharger, allowing the unit to set a new four-stroke performance standard which directly challenged the two-strokes. It was successful to the point that Giichi Naruke won the 1995 FAI F3A World Championship for R/C aerobatics using one of these engines—a major breakthrough for the four-stroke concept.

All three of the above-mentioned Japanese companies remain active today, continuing their long-standing involvement with the development and manufacture of four-stroke model engines along with constructors from other countries. Consequently, four-stroke engines have continued their advance to the point where they are now a mainstream model engine type, finding application in all branches of power modelling other than all-out speed and combat events. Their advantages include significantly reduced noise levels, a more "realistic" sound, enhanced mid-range torque and improved fuel consumption. They are unquestionably heavier, more complex, less powerful (minus the supercharger!) and more expensive than their two-stroke counterparts, but this is more than offset by their other advantages for many modelling applications.

All of the commercial four-stroke engines developed from 1976 onwards were designed for glow-plug operation, with a few also being offered in spark ignition configuration. It appears that for the better part of 25 years it never occurred to anyone that a four-stroke model engine might be operated on compression ignition. As a die-hard diesel user myself, I often used to indulge in idle fantasies about the possibility of making such a concept work, but I never got around to testing the concept in any way. [...as did others. See MEN for April 2010, amd April 2006, plus the efforts in the Engine Gallery of Phil Cox, and Malcolm Beak. Ed.]

It was not until the new millennium had got underway that the designers at Enya began to give serious consideration to the possibility of making a successful commercial four-stroke diesel. Although a few Laser four-strokes had been made in diesel form to custom order, the viability of the development of such a model for wider commercial sale was still unproven. Accordingly, Enya's efforts were naturally kept fairly quiet during the prototype stage. However, the designers were eventually satisfied that they had a viable product on their hands, resulting in the 2006 release of the world's first commercial model four-stroke compression-ignition engine, the Enya 41-4CD diesel.

Design Considerations

It's been several decades now since there was any remaining mystery about the practical business of designing a functional four-stroke model engine. Professor Chaddock's mid-1960's effort was a masterpiece of logical and economical engineering design which in my opinion has never been bettered and which unmistakably pointed the way towards the future. OS have been following in Chaddock's footsteps since 1976, with Saito and Enya (among others) joining in soon thereafter. By the early years of the new millennium, all three Japanese companies had acquired a wealth of practical experience in the application of four-stroke technology to model aero engines.

Accordingly, the challenge facing the Enya designers was not that of designing a successful four-stroke model engine (they already knew how to do that!) but rather that of making such an engine operate on compression ignition without any assistance from a glow-plug.

The question that one might logically ask at this point is—WHY set out to design a four-stroke diesel at all? After all, model four-strokes have proved themselves to be more than capable performers using glow-plug ignition. Why mess with a good thing?

Here we have to speculate a little. It's a well-known fact that two-stroke model diesel engines can generally speaking be designed to produce more low-end torque than their glow-plug cousins. Presumably there was some expectation (given the relatively low speeds of a typical four-stroke model engine) that this principle might apply to four-strokes as well. The issue of fuel consumption might also have come into play—a low-speed two-stroke diesel is famously economical on fuel, so a four-stroke would presumably be even better. Finally, there would have been the undeniable attraction of establishing a technological "first", always a feather in the cap of any model engine manufacturer.

Regardless of the reasons for pursuing this concept, the first decision must clearly have been the type of fuel on which the engine would run. This should have been a pretty easy one—the goal would surely have been to design an engine that would run on the same type of model diesel fuel used in the two-stroke diesel engines both from Enya and from other makers. An engine aimed at something of a niche market which needed its own special fuel would stand little chance in the marketplace. So we would expect to see a four-stroke diesel engine being designed to run on conventional ether/kerosene/oil/improver diesel fuel.

This would of course set a bench-mark figure for the required compression ratio. Using normal diesel fuel with ignition improver, a compression ratio in the vicinity of 18:1 or more is generally required as a minimum for dependable ignition, even at relatively low speeds. This is a far higher ratio than those typically incorporated into four-stroke glow-plug motors. Nonetheless, a compression ratio in this range would have to be incorporated into the engine if success was to be achieved.

Another issue which would immediately arise at this point would be that of valve-to-piston clearances. The required higher compression ratio would dictate a smaller combustion chamber with consequently less room for the valves to operate. The valve lift and timing provided by the camshaft might well be affected by this consideration, as indeed might the diameter of the valve heads and the design of the piston crown. The engine's tolerance of valve float might well be significantly reduced as well, possibly leading to a lower recommended maximum speed or the use of more powerful valve springs.

The higher compression ratio coupled with the greater torque and "denotation ignition" syndrome inherent with model diesels would also impose considerably increased internal stress on the engine's working components. We might therefore expect to see an engine designed for diesel operation being far more heavily constructed than its glow-plug equivalent as far as its stressed components go.

A further criterion which would be affected by the higher-than-normal compression ratio would be that of piston seal. It would be essential to provide a piston/cylinder configuration capable of maintaining a seal against the far higher pressures developed in a diesel cylinder during the working cycle, particularly if good starting was to be achieved.

Finally, there's the issue of ignition timing. In a conventional diesel, this can readily be adjusted during operation by judicious use of the compression screw. However, the presence of the valve gear in the head of an overhead valve four-stroke engine would absolutely preclude the use of a conventional contra piston arrangement. Could this be an over-riding objection to the concept of a four-stroke diesel?

At this point, it's worth reminding ourselves (as Enya evidently did) that ignition timing on glow-plug is very far from precise. In fact, glow-plug ignition timing is dictated by a given combination of compression ratio, plug heat range, needle setting, engine speed under load and fuel, meaning that there's probably only a very narrow range of engine speeds at which the ignition timing on glow-plug is spot-on for a given combination. Put another way, the fact that glow-plug engines work as well as they do over a considerable speed range in some cases seems to suggest that the issue of ignition timing is somewhat less critical than might at first appear, at least in a general-purpose context.

A further factor is that an engine of this type might be expected to operate at far lower speeds and hence within a substantially narrower speed range than a two-stroke model. Keeping all of these factors in mind, it's a short step to the conclusion that provided the initial compression ratio and fuel are well matched to the engine's design speed, the ability to vary the compression ratio (and hence the ignition timing) during operation might be less important than with a higher-performance unit that would logically be expected to operate over a relatively wider speed range.

A quite rational conclusion resulting from this line of inquiry might well be that a four-stroke diesel engine should logically be able to operate on fixed compression. After all, the old Drone, Micron and Owat fixed-compression two-stroke diesels ran (and still run) very well indeed using this approach if properly managed. This is largely due to the fact that they operate efficiently within a very narrow speed range, to which they can easily be confined through the use of appropriate airscrews. The same approach should surely work with a four-stroke diesel.

That said, it would clearly be desirable to provide some means of making compression adjustments to suit different fuels and operating conditions as well as to accommodate engine wear. The most obvious approach would be to make some convenient provision for re-shimming the cylinder head as required. With a four-stroke engine, such a procedure is complicated by the fact that any such adjustment will inevitably affect the valve gear clearances and may also give rise to problems with valve contact with the piston. So great care is required when undertaking such an adjustment, particularly if compression is being raised by the removal of a shim—the valve train will have to be re-regulated for sure and the valve-to-piston clearances checked. Even so, there's no reason whatsoever to believe that such an adjustment would be impracticable. It would merely require a little time and care to accomplish, that's all.

Not only that, but the same effect could likely be achieved through the far less intrusive approach of adjusting the fuel mixture. It's a well-known fact arising from long experience with fixed-compression two-stroke diesels like the Drone that changes in the oil content of the fuel will have some impact upon the effective operating compression ratio. The presence of oil in the combustion chamber reduces the available gas volume within the chamber. As a result, an increase in the oil content of the fuel will have the effect of slightly increasing the effective operating compression ratio, since more oil will displace more of the combustion chamber volume at and near top dead centre.

Even so, it would surely be advantageous to provide some means of making minor adjustments to the compression ratio that did not involve disturbing the head. We might therefore expect to see some provision of this sort built into the design of a well thought-out model four-stroke diesel.

Apart from the above points, the rest of the design should logically follow established model four-stroke engine practise, with which Enya already had vast experience. Let's have a look at the Enya 41-4CD to see how the designers went about dealing with the issues just discussed.

The Enya 41-4CD Described

The Enya 41-4CD was a 0.41 cuin. (6.64cc) overhead valve four-stroke engine which was in effect a diesel conversion of Enya's previously-established 41-4C glow-plug model. Bore and stroke were 22.3mm and 17.0mm respectively, making the engine considerably over-square. The diesel model weighed in at 370 gm (13.05 oz) without muffler.

In most respects, the 41-4CD was very similar to its glow-plug counterpart. It was a more or less conventional overhead valve four-stroke model engine which followed the well-established design pattern for engines of this type, with no obvious design anomalies. There were however a few differences which represented the designer's approach to the resolution of the issues discussed in the previous section.

As anticipated in the above discussion, the specified fuel was a basically conventional model diesel brew. The recommended mixture was 36% kerosene, 36% ether, 25% castor oil and 2 to 3% amyl nitrate or equivalent, a blend which should suit the majority of model diesel engines regardless of type. As usual with contemporary four-stroke model engines, no extra provisions were made for lubrication, the bleeding of oil past the piston to the engine's bottom end being rightly considered sufficient for lubrication purposes. The crankcase breather nipple had a pin-hole orifice which ensured the maintenance of a slightly positive pressure within the crankcase, thus forcing accumulated oil into the various bearings as well as the camshaft assembly. Any excess oil will drain off through the breather.

No compression ratio figure was given for this engine, and I don't have an example to measure. All that can be said on this subject was that a number of posters on various modelling forums reported that by feel the compression ratio was very noticeably higher than that of the glow-plug version. Several individuals claimed to have made volumetric measurements which suggested a ratio of around 20:1, but this can't be confirmed. This however is just what we would expect—indeed, a high compression ratio would be a prerequisite for successful operation on the specified fuel in the absence of glow-plug assistance. The higher ratio was apparently achieved very simply by removing metal from the lower face of a standard 41-4C glow-plug cylinder head.

The design of the engine strongly reflected the need for all stressed components to be suitably beefed-up to deal with the greater loads imposed by diesel operation. The eyes at both ends of the drop-forged double-bushed con-rod were increased in size, with both the gudgeon pin and the crankpin increased in diameter by comparison with their equivalents from the glow-plug model. In addition, the front housing of the diesel model was machined from bar stock rather than being cast as in the glow-plug version.

The issue of piston sealing was addressed very directly through the use of a cast-iron piston running in a steel liner—the traditional diesel set-up, in fact. Enya have long been noted for the excellence and longevity of the piston/cylinder fits in their lapped-piston products, and this one was reportedly no exception. The piston included two rebates machined into its crown, presumably to address the issue of valve-to-piston contact using such a high compression ratio. The downside of this set-up was a very high piston weight which reportedly gave rise to a significant level of operational vibration.

Major compression adjustments could of course be made by re-shimming the entire head using the three shims supplied with each engine. Indeed, it was clear that the manufacturers expected their customers to make their own decision regarding the number of shims to be used (the fewer shims, the higher the compression ratio and therefore the higher the optimum speed) since the valve cover was not installed as supplied and the head screws were not tightened down. This made it abundantly clear that this engine was aimed at the more knowledgeable engine types in the collector or "motor boy" categories.

Naturally, any change in the shimming would affect the valve clearances, requiring complete re-adjustment of the valve gear. Following such an adjustment, it would be highly advisable to turn the engine over very slowly and carefully with the compression released (see following paragraph) to ensure that there were no problems with valve/piston contact.

However, the desirability of having some easy means of making minor adjustments to the compression ratio without disturbing the head was not overlooked. To allow for fine-tuning of the compression ratio following shimming, the tapped hole at the front of the head for the glow-plug was retained, with what appeared at first sight to be a standard glow-plug being installed in this hole. However, the apparent glow-plug was actually a dummy—it was a solid threaded plug. The depth to which it extended into the plug hole when tightened could be adjusted by varying the number of sealing washers, thus changing the combustion chamber volume and hence the compression ratio. Very neat ...

This provision had another and perhaps even more important function. It is actually quite easy with heavy-handed choking to get too much fuel into the head of one of these engines, creating a virtual hydraulic lock. Electric starter users, take warning ...Unlike a two-stroke engine, draining the cylinder through the exhaust port is not a convenient option with a four-stroke. The answer in this case is to remove the threaded plug and flip the engine over a few times, ejecting excess fuel out of the threaded hole. Problem solved ...

One feels that it should be possible to improve on this system by making a modified head insert with an internal plunger that could be externally adjusted for depth. This would allow minor compression adjustments to be made while the engine was running—a significant operational advantage. It's also worth recalling the previously-noted fact that changes in the oil content of the fuel will have some impact upon the effective operating compression ratio.

In all other respects, the Enya 41-4CD was a completely conventional model four-stroke engine. Given the familiarity and general standardization of such designs these days, there is no need for a complete description here.

Claimed output of the 41-4CD was a respectable 0.8 BHP. Enya have generally been very honest with their power claims, many of which have been exceeded on independent test. There seems no compelling reason to doubt the claimed figure in this case. Maximum recommended speed was 9,000 rpm.

The Enya 41-4CD On Test

Following the release of the Enya 41-4CD, a very complete analysis of the engine by Steve Dorling appeared on the website which hosts RCME magazine in the UK. For those interested in obtaining more details of this groundbreaking model engine, this article may still be accessed online.

After describing the unit in great detail, Steve subjected his example to a fairly extensive bench test. He found the engine to be almost unbelievably easy to start—a single choked turn, a few leisurely flicks and away she went, every time. No need for an electric starter, nor would the use of such a device be at all advisable with this small a combustion chamber and the potential noted earlier for a hydraulic lock to develop.

Steve admitted that he was frankly surprised at how easy the engine was to start, describing the process as something which he had expected to find rather challenging but which actually turned out to be a total non-event in terms of difficulty. He put this unexpected ease of starting down in part to the truly excellent piston fit and also to the fact that a four-stroke engine is inherently more efficient than a two-stroke in completely filling the combustion chamber with fresh mixture, there being no attenuation through the open exhaust.

The engine was found to run beautifully on all props tried, the really remarkable thing being the slowness of the idle which could be held. Steve had no trouble holding an idle speed of 1200 rpm using a 13.5 x 8 Bolly prop. However, the engine would only get this prop up to 5,500 rpm, a speed at which it seemed to be over-compressed given the fact that detonation was evident when the throttle was opened wide on a leaned-out needle.

Further tests using Bolly 11.5 x 5 and 12.5 x 6 propellers gave more encouraging results. The engine would actually get past 9,000 rpm on the 11.5 x 5 prop, suggesting that it was under-propped on this airscrew. It also exhibited signs of under-compression on this prop. The manufacturers suggested that speeds be kept to around 9,000 rpm for general-purpose use, and the engine as supplied appeared to be well set-up to operate efficiently at around that figure.

In summary, Steve was very impressed indeed with this engine, praising both its handling and running characteristics and the quality of its construction. As an old hand with Enya engines, the latter comment comes as little surprise to me personally—from the very beginning way back in 1949, Enya engines have always been right at the top of the quality scale. It appears that their long tradition of quality was well upheld in the 41-4CD!

A further test of the 41-4CD was published in the Australian modelling magazine "Airborne", the tester on that occasion being Brian Winch. Brian obtained very parallel results to those reported by Steve Dorling, finding the engine to be a very easy starter which ran extremely well on a matching airscrew. The engine turned an APC 12x7 prop at 7,800 rpm and a 14x6 at 6,900 rpm, the latter probably being close to an ideal ground speed for an effective flight prop, allowing for airborne pickup.

Overall, a very worthy effort by Enya to break new ground in the commercial application of the compression ignition principle to model four-stroke technology. Omedeto gozaimasu, minna-san!

The 2nd Generation Enya 4-Stroke Diesel—The Enya 36-4CD

Despite the general excellence of the 41-4CD, all was not perfect. The engine was definitely aimed at the more knowledgeable engine types rather than at the average model flier, a view seemingly confirmed by the previously-noted fact that it was supplied with the rocker cover not yet installed and the head screws slack, presumably on the assumption that the owner might want to do some tinkering with the head shimming before running the engine. In addition, as we noted earlier, the operation of what amounted to a fixed-compression diesel required a deeper understanding of model diesel engine operation than the average modeller might reasonably be expected to possess. In particular, the ability to resist the temptation to use an electric starter was a prerequisite for any owner who wished the engine to survive for any length of time.

As a result of these factors, the engine played to mixed reviews as far as the modelling public was concerned. While a number of purchasers expressed themselves as well satisfied (as confirmed by the above reviews as well as a number of YouTube videos of the engine in operation), others seemed to have a great deal of trouble extracting an acceptable performance from the unit. The more these unfortunates tinkered, the worse things seemed to get. Funny how often it works out that way ...leave it alone and just run it is a more effective approach than many people seem willing to admit!

All of this was widely discussed on the various international modelling forums. One noted model diesel authority went so far as to publicly dismiss the Enya four-stroke diesel outright on the grounds that fixed-compression diesels were inherently impracticable, regardless of type. This statement was of course made without the commentator having actually tried an example ...

Be that as it may, the inevitable result was that (rightly or wrongly) people became rather cautious regarding the viability of the Enya design approach. Sales of the 41-4CD were naturally affected by this, to the extent that the engine was eventually withdrawn from production. Examples remained available on eBay and elsewhere, but it looked for a while as if the commercial four-stroke diesel experiment was a thing of the past.

All this changed on May 8th, 2012 when Enya added a new second-generation version of their four-stroke diesel to their catalogue. This was the 36-4CD, an engine which showed clear evidence of its 41-4CD origins while introducing a few new technological features. I had missed out on the 41-4CD, having other calls upon my finances at the time when it was current. I decided that I wasn't going to make the same mistake twice and set about ordering one of the new models for inspection and test. I must have been among the earliest purchasers of this model.

Enya engines must surely be among the best-kept secrets in the modelling world. The Enya company is one of the oldest model engine manufacturing concerns still operating today, having been founded in 1949 as a small family business. The company has remained in Enya family ownership throughout, being operated today by Ken Enya, who is (I believe) a nephew of Saburo Enya. Their quality has always been right at the very top, as have their designs. However, for some reason they don't appear to be at all well promoted, a situation which has become steadily more obvious as the years have gone by. It's been quite a while since I've seen an Enya engine in any local model shop.

However, the engines remain readily available worldwide, from the excellent Enya online store.

Here one can browse their currently-available products and make purchases of engines, spare parts and accessories, using Paypal or any other preferred method of payment. It's pretty much like making a purchase on eBay—very straightforward. Their shipping is lightning-fast and the goods are extremely well packed.

Using this very convenient facility, I soon found myself the proud owner of an early example of the latest Enya four-stroke diesel—the 36-4CD. It wasn't cheap—the engine cost me the equivalent of some US$460.00 including shipping. Let's have a good look at this engine to see what sort of value I got for my money and also to determine what changes have been made as a result of experience with the now-defunct 41-4CD.

The Enya 36-4CD Described

As was the case with the Enya 41-4CD, the new diesel model is a compression ignition version of a less expensive companion glow-plug model, the 36-4C. Immediately upon removing the engine from the box, one is impressed with the general air of quality with which this engine is endowed. Typical of Enya products in my experience, but gratifying nonetheless.

The engine is supplied with an almost overwhelming selection of additional components in small plastic bags. Chief among these are a three-part muffler/exhaust pipe assembly as well as a very comprehensive tool kit of quite reasonable quality which includes all tools required to maintain the engine. There are two spanners, three Allen keys and a small screwdriver. The package also includes two extra head shims for compression adjustment, one shim being fitted as supplied.

Interestingly enough, the manufacturers appear more than usually concerned with the integrity of the airscrew attachment set-up. A lock-nut is provided for use with the standard steel nut and prop washer which are fitted to the engine as supplied. There is also a steel sleeve nut with matching aluminium alloy prop washer which can be used as an alternative means of prop mounting.

As with the previous model, the rocker cover is not installed as supplied—a logical move in recognition of the fact that adjustment of the valve clearances will likely be required at several points during break-in. Somewhat less logically, the pushrod tubes are also not installed—a bit odd, because installing them following break-in (as recommended in the instructions) requires the removal of the head and hence the disturbance of the head seal as well as a re-adjustment of the valve clearances following a few shake-down runs after re-assembly. There actually seems to be no good reason for the omission of these tubes as supplied.

Superficially, the new model appears to be very little changed from its predecessor. However, internally there have been quite a few alterations. The most obvious change that confronts us with the new model is the reduction in displacement from 6.64cc down to 5.89cc. This has been accomplished through a reduction in the bore from 22.3mm down to 21.0mm, the stroke remaining unchanged at 17.0mm. This still leaves the engine very much over-square, like its predecessor.

One might question why this change was made. Presumably one driving force behind it was a desire to reduce the weight and perhaps the bulk of the engine, a factor which manifests itself in other areas, as we shall see. Another might have been the fact that the torque produced by these four-stroke diesels is such that it was seen as unnecessary to maintain the larger displacement and still have an acceptable performer.

The next feature that we encounter is the fact that the piston/cylinder set-up is completely different. Instead of the conventional lapped iron-and-steel set-up used on the earlier model, we now find a plain un-ringed aluminium piston running in a chrome-plated drop-in aluminium cylinder liner. A standard AAC set-up, in fact. The effects of this are two-fold—first, to reduce overall engine weight and second, to significantly reduce reciprocating mass. The reduced bore also plays a role in this latter respect. The 41-4CD with its large cast iron piston was known as a bit of a vibration producer, and this change is clearly a very direct and welcome response to that issue.

It's worth noting in passing that the initial entry for this engine on the Enya web site was incorrect in that it stated that the engine has a ringed aluminium piston. I was surprised to read this, since the use of a ringed piston is highly unusual in a model diesel. Diesels need a really excellent compression seal for dependable starting, particularly given the far higher pressures developed on the compression stroke during starting. These factors place a premium upon ring fits and finishes which not many manufacturers would care to take up. Indeed, very few have done so in the past, preferring to stick with the tried-and-true lapped iron-and-steel, ABC or AAC set-ups.

It should also be recalled at this point that Enya started using chrome-plated steel bores as long ago as 1960 in a few of their diesel and glow-plug two-stroke models. Their first-ever four-stroke model of 1980 used a chromed aluminium bore—an early application of this technology. The new model thus represents a return to the original specification. There was an unsubstantiated rumour going the rounds a while back that Enya had experienced some trouble with their chromed aluminium bores. This was sparked by the fact that their AAC models were seemingly withdrawn for a time, allegedly while this issue was being sorted out. If such problems did in fact arise, it would appear that they have now been ironed out, hence the return to the earlier technology with the new diesel model.

The 36-4CD weighs in at 352 gm (12.4 ounces) without the muffler. While this represents only an 18 gm weight reduction compared with the earlier 41-4CD model, it must be remembered that a substantial portion of this reduction is in the form of reciprocating weight—a very worthwhile improvement. The muffler assembly adds a further 42 gm, giving the engine an all-up weight ready to run with muffler of 394 gm (13.9 ounces).

By comparison, a PAW 35 TBR of essentially identical displacement weighs 279 gm with muffler but without throttle. The Enya is thus considerably heavier than its two-stroke counterpart, as we would expect. Despite this, it is in fact surprisingly little more bulky than the PAW.

Apart from the points just discussed, the engine appears little changed from its predecessor. The same fixed compression arrangement is retained, there being no sign of any form of adjustable plunger insert of the type which I postulated earlier. The blank plug with its variable number of sealing washers remains the sole means of varying the compression ratio without disturbing the head for re-shimming. The adjustment of the oil content in the fuel also remains a non-intrusive option.

The construction of the engine generally follows standard current practise for a model OHV four-stroke engine. Hopefully the attached exploded view of the engine will help to clarify the main features. The very beefy shaft is carried on two ball bearings which are installed in a fully machined front housing, exactly as in the earlier 41-4CD model. The two valves are actuated by a pair of camshafts enclosed in a rear housing. The cams are driven at half engine speed by a spur gear cut into the journal of the timing gear shaft which in turn is driven by an extension of the crankpin. The timing gear shaft is carried in a single ball bearing.

The very small combustion chamber has a wedge configuration with squish areas fore and aft to promote swirl. The two valves are of generous size for an engine of this displacement. They are both angled to the rear in keeping with the rear location of the pushrods. The very tight clearances necessitated by the engine's diesel-mandated compression ratio of around 19 to 1 (by rough volumetric measurement) require the provision of a pair of rebates in the piston crown to avoid piston-to-valve contact.

The engine is supplied with a single head shim installed between the top of the liner and the sealing face of the head. Two extra shims are provided, meaning that the owner can go one shim higher than the compression ratio as supplied or two shims lower. The measured thickness of each shim is 0.11mm, so that the addition of a single shim combines with the 17mm bore to yield an increase in the combustion chamber volume of 0.025cc. With a single shim fitted, the measured combustion chamber volume of the Enya at top dead centre is in the vicinity of 0.325cc, which combines with the 5.89cc displacement to yield the cited as-supplied compression ratio of just over 19:1. The addition of a single shim increases the combustion chamber volume to 0.350cc, resulting in a compression ratio of 17.8 to 1 - a very significant reduction. A similar calculation shows that the removal of the single shim which is fitted as supplied raises the compression ratio to around 20.6 to 1. Clearly, changes in the shimming should be approached with some caution!

The compression plug which replaces the glow-plug in this design is similar to a standard glow plug but with the space normally occupied by the filament stopped up with a brass insert. It is installed as supplied with a single washer. Extra washers can be added to reduce compression ratio. It might be imagined that the effect of this arrangement would be rather insignificant. However, this is very far from being the case.

The thread base diameter of a 1/4-32 plug is approximately 5.5mm. A typical plug washer has a thickness of around 0.9 mm. Based upon the above figures, adding a single plug washer to the compression plug will increase head volume by around 0.021cc. A similar calculation to that applied above to the head shims demonstrates that the effect on compression ratio of the addition of a single extra washer is to reduce the compression ratio from the 19.12 to 1 figure as supplied down to 18.02 to 1. This is actually a quite significant change. We have to conclude that when it comes to compression reduction to suit larger props, the compression plug arrangement is actually quite effective - in fact, at least as effective as re-shimming the head.

A more dramatic compression reduction could doubtless be achieved by using a burned-out glow-plug with the filament completely removed and the space left unfilled. However, the above analysis makes it clear that such measures are unnecessary.

The carburettor is one of Enya's standard four-stroke units, designated their TN 6.5 model. The intake tube is joined to the head through a high-temperature soft plastic insert which provides a perfect seal for good suction. The material is not standard silicone since it appears unaffected by diesel fuel. The carburettor features both high and low-speed mixture adjustments as well as the usual idle speed adjustment.

The muffler assembly is another standard set-up designated as the M534C unit. The short member of the trio of components is a threaded socket which mounts directly into the alloy head and in turn accepts the threaded end of the main exhaust pipe. It would seem advisable to leave this in place at all times when the engine is in service, thus avoiding the need to wear the alloy threads when removing and re-installing or simply tightening the muffler. Both this component and the longer exhaust pipe itself are suitably bent to allow a wide range of muffler discharge locations to be established.

The suggested operating speed range for this engine is 1,800—9,000 rpm, pretty much the same as for the 41-4CD but substantially less than the 2,500—13,000 rpm range suggested for the companion 36-4C glow-plug model. The fact that an identical power output is claimed for both the diesel and glow versions of the 36-4C despite the speed range difference implies that the diesel generates substantially greater torque at the lower end of the rev scale.

Enya claim an output of 0.6 BHP for this model, significantly down from the 0.8 BHP claimed for the 41-4CD. As noted earlier, Enya have not in the past been noted for exaggerated performance claims, but the difference in this case is actually a little surprising given the relatively small reduction in displacement of the new model. Therefore, this claim certainly warrants testing. Let's get to that right away ...

The Enya 36-4CD On Test

When testing a new engine possessing an unfamiliar combination of technologies, it's always advisable to begin by following the manufacturer's recommendations. After all, it's in his best interests to ensure that his customers succeed, rendering him highly unlikely to lead them astray! This approach was further encouraged in this instance by the heady aroma of diesel fuel which wafted from the newly-opened box, indicating that the engine as received had undergone a successful test run at the factory. Presumably it passed this test, otherwise it seems unlikely that Ken Enya would have sent it to me!

So ... let's look at the manufacturer's recommendations for starting the beast. First, the instruction manual recommends that initial runs be conducted with the engine set up as supplied with a single head shim installed. In this configuration, the unit is claimed to be set for efficient running in the vicinity of its suggested maximum speed of 9,000 rpm or thereabouts. The idle mixture is evidently set at the factory during the test run, resulting in a recommendation that this too not be tampered with. The clear message is—leave well alone and just run the thing! So that's exactly what I did ...

Recommended fuel is 35% ether, 40% kerosene, 25% castor oil and 1-2% ignition improver. To be fair to the manufacturers, I brewed up a fresh batch of fuel to this precise specification.

The suggested airscrews for initial runs are 13x5 or 13x6 items. Particular emphasis is placed upon the need for the airscrew to have plenty of flywheel effect—understandable given the fact that the engine has to carry over two revolutions between fairly hefty compression strokes. The manufacturers suggest the use of well-balanced glass fibre props for this reason. If a wood prop is used, they recommend the use of a heavy spinner to add flywheel effect. Not having a glass fibre prop of the suggested size, I used a 12x8 APC airscrew, a relatively massive unit which I felt would turn at around the same speed as a 13x6.

The engine went nicely into the test stand and was soon ready for a run. I wasn't expecting particularly high noise levels with the muffler fitted, so I elected to save travel time by doing this test at home—a thing that I would not consider trying with my 5cc classic two-stroke racing engines!

Suggested needle setting is 2-1/2 turns. It's also recommended that the engine be started at half throttle and be allowed to warm up for 30 seconds or so before opening out. I followed both recommendations. Having set the carburettor, I choked to fill the fuel line and get the carburettor wet and then gave it the recommended five additional turns (two induction cycles) without the choke to suck some fuel mist into the head. The compression ratio certainly feels considerable when you turn this engine over—it could never be mistaken for a glow-plug model! Compression seal is outstanding.

Now it was time to go for the start! This is a bit different with one of these engines, because you only get compression every two turns instead of each time round—tricky for an old two-stroke diesel type like me! Still, I got it up against compression and gave it a good flip, followed by another. It fired on the second flip and started on flip no. 5—as simple as that!

I subsequently learned that the amount of fuel in the combustion chamber during starting is rather critical—despite the downdraft intake which tends to drain excess fuel, it is possible to get too much fuel into the head and come close to a hydraulic lock. For this reason, I would view the use of an electric starter as a death sentence for one of these engines—you have to have the feedback which only a finger can give to be sure that you have the right amount of fuel in there. It doesn't take much ...! The manual specifically recommends against the use of an electric starter, and this advice should be followed.

If the engine does become flooded, as it did once for me, the cure is to remove the compression adjusting plug and flip it over fast a few times. The excess fuel comes shooting out of the hole and clears the head. Re-install the compression plug, and it should start up right away.

Overall, I would not rate the Enya as a first-flick starter—my initial start described above proved to be pretty much the norm. It usually takes a few flips to get the engine to fire. However, it generally only takes once—the first firing stroke almost invariably results in a start. The engine's preference for being started at half throttle remained noticeable throughout. Provided one respected this characteristic, starting was completely trouble-free.

Having achieved a first start with the carburettor set at half throttle, my initial too-early attempt to open the throttle resulted in the engine actually slowing down, indicating that it does need to warm up before being put to work. After about 30 seconds or so, I was able to open right out, with excellent results. The needle proved to be a touch rich for best performance, so I closed it down a bit to smooth out the running, keeping it a touch on the rich side as recommended for the break-in.

The optimum needle setting is very easy to find—too lean, and the engine begins to detonate and slow down, with puffs of light-coloured exhaust smoke. Too rich, and the thing slows down and begins to splutter, with gray exhaust smoke. In neither case does it stop, however. The range within which running is smooth is quite wide, so setting the engine for flight should be very easy. When correctly set, the exhaust is remarkably smoke-free—just the odd puff as a dollop of excess oil comes through.

The manufacturers recommend a break-in period of 30 to 60 minutes—an unusually wide range. I split the difference by putting on 45 minutes in 5-minute runs, allowing complete cooling between runs on each occasion. At that point, the engine felt truly superb—I could see little point in running it further prior to an initial test. So I proceeded to try the engine leaned out on the three suitable APC props which I had on hand. The following results were obtained:

Although I probably didn't reach the engine's peaking speed, these figures nonetheless imply a somewhat lower peak output than that claimed by the manufacturers. However, they also reflect excellent torque production, as witness the figure for the 13x7. Moreover, the numbers are very much in line with those reported by Brian Winch for the larger 41-4CD. That model reached 7,800 rpm on an APC 12x7 prop (a somewhat faster prop that the 12x8 that I used) and 6,900 rpm on an APC 14x6 (probably not that different from a 13x7 in terms of power absorption). Moreover, my engine as tested only had some 45 minutes of running time. It's highly likely that performance would improve further over the first few running hours. Regardless, these figures suggest that performance of the new 36 is not that far if at all below that of the earlier 41. This underscores the apparent inconsistency between the manufacturer's performance claims for the two models.

Whatever the output, running qualities are first-rate—the engine runs very smoothly without missing a beat and with no trace of detonation or sag provided the mixture is not allowed to go lean. The compression ratio as supplied is clearly very well tailored to the engine's design operating range. Moreover, the test props were all moving a lot of air! Given that the engine is claimed to peak out in the general vicinity of 9,000 rpm, I would suggest that the 12x8 would likely be an ideal flight airscrew. A well freed-up engine will probably turn this at close enough to the recommended 7,500 rpm on the ground and would almost certainly approach the 9,000 rpm limit in the air. Such a prop turning at that speed should provide quite adequate airspeed for most purposes.

I also tested the throttle response, a rather strange experience for a dyed-in-the-wool control line flier like me! Once the engine was well warmed up, it proved dead easy to hold a steady and dependable idle of around 1800 rpm on all of the props tested. This is exactly as claimed by the makers, who state that the engine's recommended idling range is between 1800 and 2200 rpm. The engine seemed willing to hold the lower speed for as long as was wanted, picking up almost instantaneously when the throttle was snapped open. Spot testing showed that the engine would in fact hold a substantially slower idle down in the 1400 rpm range, but throttle response became a little less immediate at such speeds. Even so, the engine never quit on me despite my best efforts to make it do so!

Noise levels are indeed quite reasonable with the muffler fitted—in fact, I got the impression that the prop was contributing almost as much to the noise level as the exhaust. Actually, I would like to hear a little more "engine noise" myself, but I'm probably in a minority there! A further good point was the relatively low level of vibration, which did not appear to me to be at all excessive. It would seem that the changes to the bore and the piston metallurgy have had a very positive effect in this regard.

I did note that the oil coming out of the exhaust was a lot darker in colour than one usually finds with a new diesel. This cleared up a bit as the break-in progressed, but was still evident at the conclusion of the tests. Several owners of the earlier 41-4CD model commented upon the same symptom, so it may be a characteristic of four-stroke diesels rather than a fault with this particular design. It seems not unlikely that this effect is caused by a combination of the higher working pressures reached in a four-stroke cylinder and the tendency of the oil to hang around longer in a four-stroke cylinder during operation than it will in a two-stroke.

One notable feature of the engine was its incredibly low fuel consumption. Even running slightly rich, the thing took forever to get through a half fill of my usual test stand tank seen in the image. The manufacturers state that a tank having a capacity of 150-200 ml (5.0—6.75 ounces) is adequate for most purposes, and I see no reason to doubt this—the beast would run all day on such a tank.

At the conclusion of the tests with the engine having had around one hour of running time, I removed the head to install the pushrod tubes. The engine felt truly superb at this stage, with no trace of play developing anywhere and a nice smooth piston fit with outstanding compression. The unit clearly came through its break-in and initial test with flying colours.

A point to note when re-assembling the Enya 36-4CD is that the head screws must not be tightened too hard. The instruction manual makes a point of this, stating that over-tightening of the head screws can distort the cylinder liner. This is due to the fact that the expansions in the main casting for the head screw threads are quite shallow and hence poorly supported, meaning that serious tension being imposed upon them will tend to pinch the casting inwards at the top and put a squeeze on the aluminium liner inside.

This effect is very noticeable when re-assembling the engine—go just a little too tight and the top of the liner begins to pinch. It's best to snug the screws up and no more—the aluminium in which the screws are embedded will expand more than the steel screws when the engine is hot, so there's little chance of things coming undone while running. A check now and then between flights would be all that was necessary. Even with a relatively light torque on each screw when cold (using the short leg of the Allen wrench with light finger pressure), compression seal remains outstanding. Brian Winch noted this point in his earlier test of the 41-4CD. Just a pinch seems to be enough ...

A final point on reassembly—the rocker cover is not symmetrical, being higher at one end than at the other. It should be installed with the higher end to the rear of the engine, so that the name ENYA reads from the right-hand side of the engine. Unlike the earlier 41-4CD model, there is ample clearance between this cover and the valve gear, no shim or gasket being required to maintain clearance.

Conclusion

So how would I rate the Enya 36-4CD on the basis of this admittedly rather limited test-bench evaluation? Before committing myself on this point, I feel it necessary to remind readers that I am not, and never have been, an R/C modeller. This being the case, I'm in a somewhat poorly-informed position to make any comparison between this and any other R/C engine of a similar displacement. I can only comment on this particular engine as I found it.

With that clearly understood, I must confess that as a hard-core diesel man going back well over 50 years now, I bought this engine mainly out of plain old-fashioned curiosity regarding how well the compression-ignition principle would transfer to present-day model four-stroke technology. I have to say that I was not disappointed!

In terms of value, it's really hard to feel that one has not received fair value for money with this product. A glance at the parts list will show that close to 100 individual components go into the making of one of these engines, many of them tiny and all of them manufactured to Enya's usual extremely high standard. Overall, the quality of this engine is at a level which many model engineers would find it challenging to match. The assembly of this unit too is a far more time-consuming process than with the more familiar two-stroke model diesels to which I and others like me are more accustomed.

Frankly, far from being taken aback by the price charged for this fine product, I remain amazed and grateful that we can buy quality and design of this standard at such a price! In early post-war Britain, the cost of a simple no-frills two-stroke model diesel engine was generally in the order of a week's take-home wages for a skilled individual. Looked at in this light, the cost of the far more complex Enya actually appears quite reasonable in the context of today's economic environment!

In terms of performance, my lack of R/C experience leaves me less well-informed than usual regarding the degree to which this engine stacks up against the glow-plug opposition. I can only say that I was impressed by the engine in every way. Apart from being beautifully made, it started far more easily than I had initially expected, needled very well indeed and ran superbly within its design speed range. I'm no throttle expert, but the way in which the thing will sit there quietly ticking over at 1400 rpm or so like a full-sized motorcycle engine quite astonished me. I found no basis for supposing that the engine would not prove completely adequate for any task to which it might reasonably be assigned.

So we get right back to the question with which we started—what's the point of this engine? It's some 40% more costly to buy than its 36-4C glow-plug equivalent and even more costly compared to a two-stroke alternative. It's also a lot less powerful than a two-stroke model of similar or even lesser displacement, although it undoubtedly swings a much larger prop with authority. Try that 12x8 on your 2.5cc diesel of equal or greater output...

So who's going to bother? The answer is almost certainly limited to those four-stroke aficionados to whom diesel operation is a major attractant as well as those who enjoy tinkering with a new technological challenge or just flying something different. Speaking personally, I'd fly one just for that glorious aroma! Fuel economy may also be a selling point, albeit a minor one in my view.

The above account will have made it clear that this is no "bolt it in and fly it" model—the manufacturers clearly expect their customers to have a pretty good grasp of the principles and practise of engine management and maintenance. Taking all of the above factors into account, I suspect that the higher price of this engine is likely a reflection of the manufacturer's acceptance of the fact that the appeal of this model will likely be confined to something of a niche market. Hence they may have expectations of lower sales and hence smaller production figures than the glow-plug equivalent, reflecting the higher unit cost.

That may be how things work out, but I can say without reservation that if my experience is typical, those who do take the plunge and buy the Enya 36-4CD four-stroke diesel will get a quality product representing very good value for money along with a very positive diesel experience. So hats off to Ken Enya and his colleagues for a daring and in my view highly successful step into hitherto-uncharted territory! Domo arigato gozaimasu!