Operating Fixed Compression Diesels

But here the advantages end. In other respects, ether is a bad diesel fuel! It has a decided tendency to detonate (i.e., to ignite explosively rather than burn progressively), which imposes greatly increased stresses on already-stressed engine components. Furthermore, either has a considerably lower calorific value (the amount of energy released by burning a given amount) than other fuels such as kerosene and gasoline. In other words, you get substantially less energy from burning a given amount of ether than you do from burning the same amount of other fuels. For this reason, modern model diesel fuels tend to use only as much ether as is required for good starting, and to make up the rest of the combustible content of the fuel with a high-energy smooth-burning constituent such as kerosene.

However, we can turn some of the characteristics of ether to our advantage when looking at fixed-compression operation. One factor in model diesel operation that often gives rise to a need for a reduction in compression during running is that of over-heating. Since we can't respond in this manner to any overheating issues with our fixed-compression diesel, we need to do everything that we can to prevent an overheating situation from arising in the first place. This means that the use of hot-burning fuels is definitely out—do not attempt to run a fixed-compression diesel on conventional modern diesel fuel!! If you do, damage will likely result.

To keep temperatures under control, fixed-compression diesels are generally run on a fuel consisting of straight ether and mineral oil, with no power-enhancing additives such as kerosene (which burns far hotter than ether) or ignition improvers such as amyl nitrate. Ether has a high latent heat of evaporation, and the fuel mixture with an ether-based fuel thus enters the working parts of the engine at a very low temperature and does much to promote cooling of those parts. And the relatively low calorific value of ether reduces the tendency for the engine to run hot, although it also reduces the power potential of the fuel. But the imperative is to prevent overheating, and fixed-compression diesels run very cool on an all-ether fuel, so there is little chance of the overheating problem occurring.

Apart from its low calorific value, the main limitation of ether as a diesel fuel is its previously-mentioned tendency to detonate rather than burn smoothly under self-ignition conditions. For reasons which are unclear, the use of mineral oil in the fuel does much to counteract this tendency—in this respect, the more oil, the better! The point is that the oil content of the fuel does more than merely lubricate the engine—it also reduces the tendency of the fuel to detonate. For some reason, castor oil does not seem to have this same "dampening" effect, so fixed compression diesels are definitely best run on mineral-based fuels. I've had excellent results using SAE 20-50 multi-grade mineral-based motor oil. I've also had satisfactory results using straight SAE 30. You can buy mineral-based oils at most automotive supply stores. I've never tried the modern synthetic oils, and have no idea how they would work.

In his most interesting write-up on the Drone which appeared in MECA Bulletin no. 71, in January 1981 (over a quarter century ago—how time flies!!), long-time Drone Diesel user Gus Munich recommended the use of medicinal mineral oil. I can only say that I've tried this, with very indifferent results. I've had far better luck with conventional mineral-base motor oil. Perhaps the 1940's medicinal mineral oil recommended by Gus was a different substance from that sold in more recent years?!? The manufacturers of the Drone engine recommended "Heavy or Extra Heavy Mineral Oil (also known as Liquid Petrolatum)" or straight SAE 20 oil, with the addition of a little extra SAE 30 mineral oil during break-in. There are some clear inconsistencies in these recommendations, and I can only repeat that the oils that I have noted above work very well in my experience. I believe that the slightly thicker oil provides better wear protection at the high bearing stresses under which model diesel engines operate.

As far as ether goes, the ready availability of this material is becoming increasingly compromised, largely because of its use in the making of illicit drugs such as crystal meth and consequent crack-downs on its availability. The most accessible source at present is the automotive "starting fluid" available from many automotive supply stores. predominantly ether, and fuel made up using this material is perfectly satisfactory. The better grades of this material have a high ether content in the 60% plus range, with the bulk of the balance being heptane in most cases.

Heptane is actually a member of the ether family in a chemical sense, although it has a somewhat higher self-ignition temperature. Its combustion characteristics are however far closer to those of ether than those of kerosene in that it tends to burn more "explosively" than kerosene. Accordingly, the presence of a proportion of heptane in the mix does not appear to greatly affect the performance of an either-based diesel fuel as long as the ether predominates.

It's important to realize that not all starting fluids are created equal! Some in fact (like the appropriately-named Australian product "Start, Ya Bastard!") have relatively low either contents in the 25% range and are unsuitable for our purpose. The best by far appears to be John Deere starting fluid, which is 80% ether by weight. A portion of the balance is an upper cylinder lubricant, which doesn't seem to upset things to any meaningful extent. I've also had reasonable results with Gunk Liquid Fire, which has a guaranteed ether content of at least 60% with much of the balance being heptane and only a very small upper cylinder lubricant fraction which can be ignored when mixing.

Starting fluid is sold in an aerosol spray can, and my own method of extraction is very simple. I begin by cooling the can in the freezer for a while and then holding the can upside down and pressing the spray button to open the valve. The pressure bleeds off while the liquid contents remain in place inside the can. This process needs to be repeated several times with a pause between attempts, since part of the content consists of dissolved gasses which take time to come out of the ether as the pressure is relieved.

Once the pressure has been completely relieved and repeated presses produce no more hissing, I then take a sharp-ended camping can-opener and cut a small hole in the base of the de-pressurized aerosol can. This releases any remaining pressure in the system. I then cut a second slightly larger hole through which the contents can be drained into whatever container is to be used to store the ingredient. Needless to say, I do this outdoors well away from any ignition source! I don't claim that this is necessarily a safe technique—I can only say that it has worked well for me so far, and I'm still here to prove it! Others may or may not wish to try this technique themselves—if so, it's entirely at their own discretion and at their own risk. End of liability disclaimer .

OK, so we have our basic ingredients. How much oil should we use in our fuel mix?? Well, that depends, as we'll see when we get onto the actual operational issues.

Running Characteristics

So to accommodate the inevitable changes in fuel draw which will take place under real operating conditions, we need to set the needle somewhere in the middle of the smooth-running range. Fortunately, the wide explosive limits of ether mean that this is by no means critical, and a reliable setting is easy to find. The trick is to make a couple of runs to establish the point at which missing begins to occur (bordering on too lean) and the point at which the engine cuts (too rich). The most dependable running setting is somewhere right in the middle.

Starting

If you follow the above advice, you'll find that the operation of these engines is if anything less challenging than it is with a more conventional later-model diesel!

Optimizing performance

This all seems very complicated, no doubt, and there's little question that the provision of some means of varying the compression is a godsend in that it eliminates the need for all this testing and tuning. But if one perseveres and establishes the optimum combination of geometric compression ratio, fuel mix and flight prop, the results can be impressive. My personal experiences with actually running and flying fixed compression diesels (notably the Drone) are that once you get things dialled in they are every bit as easy to use as their more modern counterparts and perform extremely dependably.

Maintenance

My own preference is not to store such fuels for any length of time, but rather to make up fresh fuel each time I plan to do any fixed-compression running. That way, I can be sure of my oil content. A single can of starting fluid will make up enough fuel for a day's flying, and I generally use up the entire batch at one session, thus eliminating the need to store any such fuel at all. If it is necessary to store some of this fuel, that's OK as long as it is kept in a well-sealed airtight container in a cool, dark place well away from any ignition source. It will safely keep for quite a while under such conditions.

Notes On Specific Engines

Here are a few comments on some fixed-compression engines with which I have actual operating experience. More details regarding the engines themselves may be found in The Finder.

Drone

In my part of the world, the most commonly-encountered fixed compression diesels are the American-made 5cc Drone units. These were made in two models—the plain bearing first model and the single ball-race second model. They are extremely well-made and very sturdy—a credit to their designer and manufacturer, in fact. About 10,000 examples of the first model were made, and perhaps 5,000 examples of the second model. A substantial number of these survive today.

The two models of the Drone are quite distinct—in fact, the only interchangeable parts are the prop driver and prop washer! The first model has a plain bearing crankshaft and was supplied with a fixed compression ratio of 18:1. This is on the low side for model diesel operation—high-speed diesels frequently require compression ratios of up to 20:1 for optimum performance. But the Drones are definitely not high-speed engines, so the compression ratio adopted is well suited to their normal operating speeds.

I've found that this version of the Drone actually operates best on a 30% oil mixture. The plain bearing benefits from the application of the extra oil, since wear in this bearing is devoutly to be avoided given the effect which any such wear would have on the geometric compression ratio. This is in fact a reported problem with this model of the Drone, and there's no easy fix—the bearing has to be re-bushed or a new shaft made. However, the bearing is very substantial and it will take tens of hours of running to wear the bearing to the point where a problem develops, as long as detonation is avoided and the oil content is of sufficient quantity and quality.

The engine seems to run very smoothly in the 5500 - 7500 rpm range, but starts to struggle with under-compression above that range. One tester reportedly rated it at 8,400 rpm on a 9 x 4 wooden prop and 4.400 rpm on a 14 x 8. The 9 x 4 is definitely operating past the engine's peak, which is probably in the region of 7000 rpm or so, and the 14 x 8 is definitely lugging the engine in my view. I obtain around 6,800 rpm on the bench on a 10 x 8 wood prop, and would recommend that or an 11 x 6 as a very suitable bench test prop for this engine. For control-line flying (for which these engines were primarily intended), one should aim for a ground speed of around 5,000 - 5500 rpm or so. The makers recommended 10 to 12 inch diameter props for control-line stunt, with pitches ranging from 8 inches to 10 inches. The purpose-built Drone stunt prop for this engine was an 11 x 10, which would probably work very well if one could be found. Failing that, a slightly cut-down 12 x 8 should work fine. Or you may be able to cut down a larger prop of higher pitch. The trick is not to under-prop the engine and to use a high pitch for good airspeed at the relatively low revs which the engine will deliver. Such large props will shift a lot of air—you'll be amazed at the static thrust developed! I don't recommend running a Drone below 5,000 rpm under any circumstances—at lower speeds the condition of premature ignition sets in, with potential harm to the engine.

The second model Drone has substantially stronger mounting lugs than the Mk. I. Even more significantly, it features a single ball-race located just in front of the crankweb in the usual place. This is a great step forward from the first model Drone, mainly because it essentially eliminates the issue of reduction in the geometric compression ratio due to main bearing wear. It also allows the engine to rev a bit more freely. For this reason, if you plan to do some serious fixed-compression running and/or flying, this is the motor with which to do it! In his previously-mentioned 1981 write-up, Gus Munich (whose all-time favourite engine this was) reported that he had a second model Drone which had over 500 running hours on it with no replacements! My own "flyer" second model Drone has around 25 hours in my hands, and who knows how many hours before that?!? It shows virtually no signs of wear.

The second model Drone also has a slightly higher geometric compression ratio of 18.5:1, which allows it to keep running evenly at slightly higher speeds than the earlier version or alternatively allows the use of a fuel with a slightly lesser oil content—I generally use 25% oil in the fuel for my Mk. II Drones, as recommended by the manufacturers. The same prop sizes as for the first model generally work, but it may be found advantageous to trim the blades slightly to allow the second model motor to reach higher revs in the air. It probably peaks at around 7,500 rpm as supplied.

Gus Munich recommended setting the engine a little lean for take-off and allowing centrifugal force to richen the mixture in flight to smooth out the running. Personally, I've found that the use of a well-located Uniflow or chicken-hopper tank eliminates any need for such measures since such tanks maintain a constant fuel head throughout the flight except insofar as manoeuvres may affect mixture. The trick is to locate the tank at or near the centre-line of the spraybar so that centrifugal force actually plays a minimal role in infuencing fuel head relative to the spraybar. If anything, the tank should be a little inboard of the spraybar centre, since (as Gus rightly pointed out) it's easier to adjust for a slight richening in flight than it is for a slight leaning-out. In a sidewinder mounting as used on a profile stunter (the recommended mounting position for the Drone), placing the tank in the usual position on the outside of the fuselage achieves this pretty well. The tank should also be tucked in as closely as possible behind the motor to minimize the effect of manoeuvres on the mixture.

The motor should run smoothly throughout the flight using this approach, although it will start to run lean just before the tank runs out, thus giving you plenty of warning that the flight is about to end. If it crackles momentarily in manoeuvres, this is a symptom of momentary leaning-out. Nothing to worry about—it won't stop and will pick up smoothly as soon as the momentary anomaly is passed! A slightly richer take-off setting may cure this as long as the setting stays within the rich-running limits of the engine. However, the opposite effect can also occur—the engine may suddenly cut out altogether if it is run too near the upper limit of fuel mixture richness and a temporary manoeuvre-induced fuel surge makes it go even richer. If this occurs, you'll have to adjust the fuel system or take off at a slightly leaner mixture. But the engine actually has pretty good suction, and troubles of this nature can generally be overcome by test-flying and appropriate adjustment. Once the system is fine-tuned, flying becomes smooth and trouble-free.

The Drones can best be summed up as powerful and dependable engines which are one or two-flick starters and will run forever if well looked after. If you follow the above procedures, they can be run with complete confidence!

Micron 5 cc

This French engine seems by "feel" to have a very similar compression ratio to that of the Drone, although it has been reported as having a higher compression ratio of 20:1. All I can say is that its performance suggests otherwise—it definitely likes the lower speeds, appearing to peak at around 6000 rpm or so. It also does well on a 25% oil fuel mixture, which was in fact the mix recommended by the manufacturer. These engines were designed for free flight and as such were used with larger diameter lower-pitch props than the Drone. A 14 inch diameter prop was specified, with pitches around the 6 inch range. I've never flown one of these, but can report that it starts very easily and runs flawlessly. I get around 6000 rpm on a 13x 6 prop, which probably duplicates airborne performance on a 14 x 6. The thrust generated is amazing—despite the low revs, the large airscrew means that the thing moves a lot of air!!

Owat 5 cc

This British engine is pretty much a clone of the Micron, and the same comments generally apply. The makers recommended a 14 x 9 prop, which they claimed would turn at around 5000 rpm. They also recommended a very low oil content of only 10%, which would have reduced the effective operating compression ratio somewhat to match the low recommended speeds. I find that in practise the Owat runs fine on the bench on a 25% mixture, with a performance very similar to that of the Micron. I've never flown one, though.

Mite .099

This little American-made motor is the one that stands out from the rest in terms of its operating characteristics. The reason for this is its remarkably low compression ratio—only 13.5:1!! This is excessively low for any model diesel and in my view represents a design error on the part of the manufacturers. The makers appear to have recognized the implications of the low compression ratio employed, because the recommended fuel mix is 50% oil, 50% ether! And you really do have to use such a mix—with less oil, the effective operating compression ratio is simply too low for smooth running at any speed.

For starting, this engine is unusual in that you need to get a lot of fuel into the cylinder to reduce the unused space in the combustion chamber and generate enough heat for starting. Thanks to the very wide explosive limits of ether, you can get away with this. However, another problem rears its ugly head at this point—the method of mounting the prop driver on the shaft is highly unsatisfactory, with an absurdly small tapered shoulder against which the prop driver rests—more of a slip joint than an actual fixture! As a result, the prop tends to slip while starting the engine from cold. You have to really tighten it down, and even then it's problematic.

For this reason, the Mite is one fixed-compression diesel which is actually far easier to start when hot! There's no tendency to detonate—the low compression ratio and high oil content see to that! When hot, you just follow normal fixed-compression procedure as outlined above, and the engine re-starts very easily.

As far as props go, I've found that an 8 x 8 prop is the lightest load on which the engine will run smoothly on the bench. Try an 8 x 6 and you get past the speed range at which the timing is optimized. The Mite turns an 8 x 8 at around 5500 rpm on the bench. Naturally, it moves a fair bit of air while doing so, and would undoubtedly fly a small model very well. But it will not run smoothly above about 6500 rpm on the standard 50% fuel mix, and it seems absurd to consider adding even more oil to such a mix. So I'd recommend an 8 x 8 as the ideal test prop for this engine. Indeed, this prop size was recommended by several contemporary testers of this engine.

The castings on the Mite are all magnesium, so you have to follow the usual precautions to prevent corrosion while the engine is in storage. That said, the Mite is a neat little engine which runs well but would undoubtedly have benefited from being supplied with a higher compression ratio and a better method of mounting the prop driver.

I also have a Vivell fixed-compression diesel, but have never run it since I wish to preserve its New condition. It feels as if it has a rather low compression ratio as well, and I suspect that its running characteristics would not be unlike those for the Mite.

Well, there it is! There's no reason to fear setting up that old fixed-comper in the test stand and giving it a run! You'll be surprised at how instantly they start and how well they do in fact run once everything is sorted out. And the contest records compiled by several of them will no longer seem so improbable!