Topsy
0.375cc Diesel

 

Name Topsy Designer G Hugh
Bore 0.312" (8.41mm) Stroke 0.300" (7.62mm)
Type Compression Ignition Capacity 0.02293 cuin (0.3753 cc)
Production run Plans built:   unknowable! Country of Origin England
Photos by Les Stone,
Bert Streigler
Year of manufacture Plans Published in 1969

 

Background

The Topsy 0.375cc compression ignition (aka "diesel") engine was designed by Mr G Hugh and presented as a four part series in the Aero Modeller commencing with the February issue of 1969 [1]. Following the pattern established previously with the Sugden Special, Ron Moulton, the editor at the time, commissioned a special engine review of the Topsy from Peter Chinn. This appeared in the August issue [2] providing a third party—but hardly independent—critique for the fourth and last home-constructed engine design to be published in the magazine. Plans for the engine were added to the plan service as a two-sheet set, E992. The first sheet reprinted the machining set-ups illustrated in the magazine series; the second gave fully dimensioned parts in four times full size.

New life was breathed into the design in earlt 1980's by the addition to the ARGUS plans line of a nice little 12 page booklet that reprinted a re-edited version of the construction series and the Chinn review to accompany plans sales. About this time, Dunham Engineering (UK) produced a die-cast crankcase and an unknown number of "kits" comprising plan, booklet, and casting were sold on both sides of the Atlantic. Dunham also produced cases turned from barstock which were sold through their distributor network. As might be expected, the Motor Boys have assorted associations with the engine. Streigler's RC Supply (Houston, TX) sold the kits; Owen Engines sold the turned versions; Les Stone made one (from a Streigler RC supplied kit); and I have original and reprint plans squirrel'd away for a rainy day! The photos at the head of this page show Les' engine made from the casting.

The designer—as mentioned, one G Hugh, first name unknown—decided to build his own miniature diesel after his interest in Aeromodelling was rekindled in the late 1960's following a 20 year break. From the text, it seems that this was Mr Hugh's first attempt at engine design and engine construction, thus qualifying for a position in the Model Engine News review of self-constructed engines for the first-time builder. His goals were stated in the text of Part 1:

"It should be as small and light and compact as possible, allied to a design that would lend itself easily to being manufactured entirely on a center lathe out of materials easily obtained and without resorting to costly castings."

Construction began without detailed drawings—just a hand-drawn sketch without any dimensions. English speaking people of Mr Hugh's era would all have been familiar with the children's book, "Uncle Tom's Cabin". Such would not, I suspect, be the case today; in fact I rather suspect that all copies have been removed from libraries and school curriculums as being unutterably un-politically correct! Never the less, people of that age would instantly recognize that anything called "Topsy" must have "...just grow'd..."; which is to say, evolved without much planning. An apt description of the engine's genesis, if not its making. The actual quote emerges from a conversation between the daughter of a slave owner, Eva, and a young, black, slave girl, Topsy:

"Have you ever heard anything about God, Topsy?" The child looked bewildered, but grinned as usual. "Do you know who made you?" "Nobody, as I knows on," said the child, with a short laugh. The idea appeared to amuse her considerably; for her eyes twinkled, and she added, "I spect I grow'd. Don't think nobody never made me." [Chapter XX]

Now, assuming you are still reading and have not placed Model Engine News on your personal, private, permanent black-list, it should be apparent that the name is not all that perfect. The design may have evolved, but we sure knows' who made it!

Construction

Ok, enough of my literary pretensions. The engine is front-rotary induction, plain bearing, featuring backplate mounting and an integral tank. The cylinder is steel with integral, tapered, cooling fins and Oliver Porting. The piston and contra-piston are cast-iron. The connecting rod is "Dural" and features an intricate, tapered H-section. The crankshaft, which runs in a cast-iron bush, was machined from a high-tensile bolt. It has a screw-in 4BA stud (close to 6-32) for the pointed spinner-nut—a feature I totally endorse for small engines.

The prop-driver is keyed to the crankshaft in an unusual way that is worth remembering. The driver is drilled through #27 to clear the 4BA stud, then counter-bored 3/16" diameter for a depth of 0.075" to fit over the end of the shaft where it protrudes from the front bushing. A 1/16" diameter peg is pressed into a hole drilled in the driver to protrude a mere 0.015" into the counter-bore. This keys to a small flat filed on the shaft. I would not use it on engines much larger than this, but it seems quite practical for the Topsy.

So is this an engine suitable for the "Beginner"? I'd have to give a resounding no-way, Hoe-Xai! First there is the size. The bore is 0.312" (8.41mm) and the stroke is 0.300" (7.62mm), giving a capacity of 0.3753cc , or 0.02293 cuin (about the same as a Cox 020 PeeWee). And as if watch-making was not enough to bump it out of the beginners' category, the builder must cut so many fine 32 TPI threads as to almost qualify as an apprentice machinist exercise in thread-cutting. The backplate/fuel tank screws into the crankcase and the head screws into the top of the cylinder. Then there's the rather labour-intensive transfer system. The end of the cylinder is threaded, with an area above the thread for the angled transfer ports drilled between the slit exhaust ports—the latter requiring the manufacture of a special tool. The cylinder screws into an aluminium carrier that is threaded on its outside diameter as well to screw into the crankcase. This "carrier" is screwed and glued to the cylinder, then the intersection is drilled axially for six 1/16" transfer passages. Count them. Including the backplate, that's EIGHT 32 TPI threads to be cut.

Despite all this, the beginner designer and builder succeeded! He also documents the four design evolutions his cylinder/transfer porting went through on the way to the final, published design. I always like to see this as it indicates that merely running has not been good enough for our designer. He has tested, evaluated, and revised until his objectives were met.

Performance

Chinn began his review by congratulating Mr Hugh on the accuracy of his stated displacement "...as a .735 [sic] c.c.' engine!". My twisted sense of ironic humour is aroused by the text getting wrong the very thing it is praising having been got right ! In defence of Peter Chinn, I must point out that this error appears only in the booklet that accompanied the plans, so it is really the fault of the printers and proof readers of the booklet. The original text as it appears on page 434 of the August 1969 Aero Modeller is correct.

Creating performance curves for engines this small is highly problematical, so the review constrained itself to quoting RPM figures on selected prop sizes:

PropRPM
Top-Flite 7x3 wood7,600
5-1/2" home-made, wide blade wood  8,100
Top-Flite 6x4 nylon 8,400
Kielkraft 5x3 nylon10,600
Tordado 5-1/2x3 nylon11,200

Chinn pronounced these figures respectable and I have to agree. He also felt that the engine was better on the larger rather than the smaller props, making it best suited to small free-flight models, for which it was designed. Unsurprisingly, he found that like all small diesels, the engine required a definite technique in starting and handling. He described a tendency for the engine to unscrew from its backplate tank when starting, but observed that his mounting may have been at fault and suggesting a paper gasket and firm tightening of the tank/crankcase. Alternatively, a left-hand thread would fix that and add significantly to the intrinsic value of the thread-cutting exercise. Chinn's summary reflects my own thoughts, so I'll quote verbatim:

Advice for the 'Topsy' maker
In a diesel as small as this, the effects of any inaccuracy or sloppiness in its construction tend to be greatly magnified, resulting in difficult starting or erratic running. The best advice we can offer to those intending to make a
Topsy, therefore, is to maintain the greatest accuracy when machining, particularly where this affects the transfer port timing and gas tight joints, and to pay meticulous attention to the finish and fit of the individual parts, especially the piston and cylinder bore. [2]

I'd go a step further and suggest a steel piston running in a liner with minimal taper. This minimises friction, depending on the oil film for compression. It is not all that difficult to machine and hone, provided you've previously made a few cylinder/pistons with the more usual TDC "pinch" and so can judge from experience what is just right.

One reason fits are so critical on small engines is heat loss. The components can't be made paper-thin. They need mechanical strength and so that they don't distort unduly at operating temperature. But this assembly must be maintained at operating temperature, so as the displacement decreases, we have to raise the operating compression ratio to compensate for the loss of heat to the disproportionate mass of metal. So, to replace the lost heat, we have to compress the mixture more and to achieve the higher pressure we need closer and finer fits [4].

Conclusion

The Topsy is not suitable for a builder new to miniature IC engines and model engineering. It may suit an experienced model engineer—preferably a clock-maker—wishing to try IC and fancying a bit of a challenge. I met a just such a person while living in Portland. A professional watch-maker and jeweller, he chose Eric Whittle's Robin, a 1cc four-stroke as his first engine. Perseverance and meticulous work finally achieved success (see the cover of SIC #64, Aug/Sep '98). While Topsy may not be for the beginner, the design embodies a number of construction techniques that are worthy of study by the absolute beginner. The well illustrated construction series shows ways of tackling difficult tasks with minimal equipment. In fact they make it look so simple that I wonder how many began construction on the strength of the illustrations. I must also wonder how many were finished and actually ran.

To pre-empt a few emails, I've checked the X-List for the Topsy plan. The list dated August 2006 shows two different Topsy entries, neither of which has the correct plan number. I suggest contacting the X-List people before ordering. If the plan has gone the way of the Dodo, I'll consider CAD'ing it.

Footnote

A footnote to the first part of the construction series announced a Topsy Contest for '69, with "...generous prizes for winners in classes for Power, Workmanship, Individually constructed or School Projects" [3]. More details were promised in the next issue. The next issue, which printed a reduced size copy of the plan, just restated the previous announcement on the plan page. A search in subsequent issues for the next 12 months has failed to uncover any results. This suggests the readers found it a rather advanced project too.

References:

[1] Hugh, G: Topsy .375 c.c. Diesel (Part 1), Aero Modeller, Model and Allied Publications Ltd, England, Volume XXXIV, Number 397, February 1969, p89.
[2] Chinn, PGF: Latest Engine News, Aero Modeller, Model and Allied Publications Ltd, England, Volume XXXIV, Number 403, August 1969, p434.
[3] op. cit. p91.
[4] Walshaw, TD: Diesel Design, George Newness Ltd, London, 1949, p18.

 

Page created: 2006-10-21

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