|Type||Compression Ignition||Capacity||0.1 to 2.5cc|
|Production run||unknown||Country of Origin||England|
|Photo by||Ron C||Year of manufacture||circa 1990|
As mentioned in the story about the Allen Mercury Series, following some years making Mercos and glow plugs, Dennis Allen returned to diesel engines of his own design in the 1990's with a range of sport engines under the name Allen Engineering (AE). These used no castings; the crankcases being milled from bar stock and finished with a black anodizing treatment. The range, as shown here, comprised capacities 0.1cc, 0.2cc, 0.5cc, 1.0cc, 1.5cc, and 2.5cc (the 0.1cc engine is not shown, being the only one missing from my collection). Like the AM series before it, the engines are identified as "AE" with a suffix formed by multiplying the capacity in cc's by 10. Thus the AM15 is a 1.5cc engine, not a 0.15cuin. But, just to confuse us, the engines below 1cc are not AE1, AE2, and AE5 as would be expected, but "AE.1", "AE.2", and "AE.5", so go figure. The engines were distributed in the UK and abroad through IRVINE ENGINES.
At first, the engines look to be simple, and they are, but close examination through disassembly shows the culmination of Den Allen's years of design and manufacturing experience. The subject for the "exploded" shots here is the 0.5cc model. The main crankcase a simple cruciform section produced by a shaper, bored to accept the front journal housing and cylinder liner. There is no "backplate" as such; it is integral with the case. If you look at the rear views above, what appears to be a backplate is simply a counter-bore to remove some excess material, and an arty turning on the end to clean-up and give the illusion of a traditional, screwed-in backplate. Now look at the massive crankweb. The reason for this is is the crank-pin, which is pressed in. No attempt has been made on any engine of the range to counter balance the crank. Experience suggests this to be a waste of time in sport class engines of this size.
The front housing is turned to a conical form and counter bored to provide seats for the 4 8BA (?) mounting screws, resulting in a "strengthening web" like appearance. The venturi fits into a hole bored into the cone, very much like David Owen's Mate design, although the AE's secure it with two small set-screws, allowing it to be angled, or replaced (presumably) by an R/C carburetor. The needle and spray bar of the AM.5, AM10, and AM15 is a conventional design with a split thimble and single jet hole, while the two smaller engines and the AM25 use a two piece spraybar. The turned and knurled prop-drive washer is pressed onto knurled splines on the front of the crankshaft, rather discouraging disassembly to see if there is any shaft bushing (which I rather doubt). The piston is turned from steel rather than the more usual cast iron, fitted with a captive gudgeon (wrist) pin and a turned aluminum conrod. Notice how the milling of the "big" end of the rod shown here has been carried just a bit too far and has nibbled away on the rod shank. Not that I'd presume to instruct anyone as experienced as Dennis Allen, but for another approach, see the method deduced from Gordon Burford's Taipan series. The piston crown is a truncated cone, presumably to assist scavenging without the need to produce a concave cone in the contra-piston.
On all but the 2.5cc model, the cylinder attaches with three long screws. These are cheese-headed and finished to a pleasant "blue" apparently be heating. The liner is from steel and has a flange into which the exhaust slits are cut on 120 degree radials. The inter-exhaust "posts" are large, allowing the transfer ports to be milled at an angle to assist gas flow and to reduce the delay between exhaust and transfer opening, thus increasing the transfer duration and reducing the amount the piston must descend below the bottom of the exhaust ports. As seen here, the transfer passages are formed by shallow milling inside the lower aluminum finned section that sits clamped on two thin brown paper gaskets between crankcase and cylinder flange. The location of the cylinder "posts" relative to the cylinder bolts provides sufficient visual clues to correct assembly.
The engine pulled down for this investigation was a New In Box (NIB) example, so I was surprised to find the black glue-like accretion of congealed castor oil much in evidence. This indicates that the engine has been factory run. So big tick for this extra QA expense, though a few drops of after-run oil would have been nice.
At the bottom end of the scale was a 0.1cc model (0.006 cuin). Unlike the others, this one is radial mounted using the flange which covers the rear of the integral fuel tank. The rather poor picture is a photograph of a reproduction appearing in the Aeromodeller report of the 0.1 and 0.2cc versions [AM674].
Next up is the 0.2cc model (0.0122 cuin). This engine differs slightly from the 0.5,1.0 and 1.5cc versions in being fitted with a rear mounted fuel tank, and a spring starter. This latter is almost essential for very small diesels (even though the 0.15cc "Bambi" is capable of first flick starts with the correct technique). Two other difference are apparent in the shape of the compression screw (a El, rather than a Tee), and the needle valve, which uses a short length of PVC fuel tube as the friction device. The spray bar comprises a threaded length to hold the needle, and a separate jet and fuel nipple--both screwed into the venturi. The PVC tube trick, more normally seen on amateur constructed engines, is very effective at this task and as a side-effect, provides a good seal against air-leaks--which can spell doom to obtaining reliable running at the miniscule mixture range required by these tiny engines. There is naturally a down-side, which we'll discuss later.
The 0.5, 1.0 and 1.5 are all very similar, both in construction and in appearance. The photos here show the 1.5cc engine, although you'd never be able to tell from the photo as the AE range carry no external markings. If you squint very closely at the "backplate" region of the engine in the vicinity of the rear head bolt, you may be able to make out the worm track raised by the tap as a result of insufficient material having been left on the rear of this engine. None of the others exhibit this "feature", so it is probably peculiar to this 1.5 alone (I'd hope!)
The AE25 is different from the others in two major respects. First and most obviously, it has a single exhaust port and is fitted with a muffler/gunk collector that the Aeromodeller test, published in August 1993, reports as being very effective [AM691]. Like the babies of the range, it is fitted with a rather spindly EL-type comp screw and a two-piece spray bar with PVC tubing needle valve friction/air seal device. The Aeromodeller test reported that this results in a degree of spring-back after releasing the needle. This sounds very inconvenient to me, although the tester asserts the effect can be compensated for.
The head is secured with four long screws. In characteristic fashion, these, like all external steel parts, are heat-blued. The piston, made again from EN1A leaded steel (according to the Aeromodeller and Model Engine World articles by Ron Prentice), is flat topped and the crankshaft sports a pressed-in pin with a massive, un-balanced web. Not so obvious are the twin ball races supporting the crankshaft, and the Schnuerle porting cut into the liner and cylinder jacket. The liner for this engine is cylindrical with a flanged top. In the photo, the Schnuerle "boost" transfer passage is towards the camera; the exhaust is on the far side. Notice the cut-away spigot that locates the aluminum head into the case, providing the transfer openings. The whole assembly is clamped tight by the plain head with a brown paper gasket sealing jacket to case. The head bolt pattern is not equidistant. Those arranged to fit between the boost and transfer passages are closer together than those arrayed either side of the exhaust. Bit of a shame really, as if they were symmetrical, the engine could be assembled with the exhaust wherever you wanted it.
The four smaller engines shared a common box and instruction sheet. The capacity being identified by a gold, stick-on address label to front and ends. The AE25 needed a larger box. The common instruction sheet gave the recommended prop sizes for the mid three engines. An extra type written fragment provides some additional handling and recommendations for the 0.2cc gnat. For no readily apparent reason, the instructions for one of the two 0.5cc engines in my collection are A4 sized and have a copy of a copy effect around a large point ".5cc" qualifier. Don't ask me why.
|[AM672]||Prentice, Ron: Dennis Allen: Motor Maestro, Aeromodeller Magazine #672 January 1992, Argus Publications, England, p20.|
|[AM674]||Robarts, John: Engines Old and New, Aeromodeller Magazine #674 March 1992, Argus Publications, England, p19.|
|[AM691]||Roberts, Dick: Engines Old and New, Aeromodeller Magazine August 1993, Argus Publications, England, p20.|
Prentice, Ron: Dennis Allen: Motor Maestro, Model Engine World Magazine #51 July 1998, John Goodall editor/publisher, England, p12.
(partial reprint of Aeromodeller article of the same name, with update noting that Dennis was now retired and production had ceased)