Gentlemen –
Thank you to all that have welcomed me to this site…much appreciated.
I was disappointed in the outcome of the Bourke engine evaluation, especially after all the time, money and dedication that I expended. There was good that came of this though. The Sport Aviation article put me in touch with a number of people that had experience with Russell Bourke personally, some of whom constructed and tested their own Bourke engines, and one person who actually dyno tested an original Bourke “30”. I found it interesting that his test results were almost identical to mine – that is, 9 bhp @ 4000 rpm. In conversations with those that knew Bourke personally (Melvin Vaux), I learned that Bourke was hot-tempered, hard to work with, and greatly exaggerated his claims.
The feature that initially attracted me to the Bourke engine was the combining of the scotch yoke crank mechanism with the two-stroke operational cycle. In studying the patent literature, it was readily apparent that Bourke was NOT the inventor of this configuration – patents extend back to the early 1900’s describing this embodiment almost exactly. One of particular interest is U.S. Patent 981995, entitled “Motor”, January 1911. The drawings describe a 6-cylinder 2-stroke radial layout using 3 scotch yokes with crankpin yoke rollers used – uncannily resembling what Bourke patented 25 years later in 1938. One wonders if this document was originally viewed by Bourke and provided him “inspiration”?
The real attractiveness in this mechanical combination is that, due to the pure linear motion of the piston rods, the cylinder scavenge pumps (normally the crankcase of a conventional 2-stroke) can be totally isolated from the crankcase mechanism. This allows a full, circulatory lubrication system to be employed, thereby supplying undiluted oil to all of the highly loaded bearings. The pure harmonic motion of the rod-yoke reciprocating assemblies offers the potential of excellent engine balance if properly configured and counterweighted. All of these advantages are gained without complicating the basic simplicity of the 2-stroke engine. Preliminary layouts showed a very compact engine could be achieved especially with oversquare bore/stroke ratios.
Encouraged by a good friend who offered financial support, I decided to design an engine that would correct all the design faults uncovered in my prior Bourke engine testing. Due to my love of aviation and being a member of EAA, an aircraft engine application was most appealing. The ARV (Air Recreational Vehicle) market, then becoming very popular, required engines of 18 – 25 bhp and this defined the preliminary goals of the project. Initial layouts and basic calculations showed that by using 2 recip assemblies oriented at 90 degrees from each other, a compact 4-cylinder X-radial engine was possible and that 100% primary balance was achievable with proper crankshaft counterweighting. As 25 bhp was the target power at 5000 rpm (direct drive turning a 36” diameter propeller) a realistic bmep of 80 psi was selected for good reliability. These parameters sized the displacement at 25.2 cu in, or 6.3 cu in/cylinder.
Over the next 18 months, I performed all of the engineering calculations, design work, and board drawings (no CAD back then) all as a spare-time endeavor (I have a very understanding wife). All of the features found detrimental in the Bourke engine were corrected and these were as follows:
• Scavenge pump compression ratio was reduced to 1.4:1, a typical value for high performance 2-stroke engines. This was achieved by incorporation of an integral volume chamber between the cylinder and crankcase that added nothing to the weight or physical size of the engine.
• Cylinders were designed with Schnurle porting (loop scavenging) that utilized 2 main and 2 boost transfers, allowing non-deflector (slightly domed) pistons to be used. Conventional piston-controlled intake and exhaust ports were utilized.
• In the Bourke design, piston rods were supported and guided by bushings in the crankcase. The reaction forces (torque applied to engine frame) of the rod yoke assembly was absorbed by relatively small surface area bearings, contributing to high unit loads and heat transfer into the crankcase. In my redesign, pistons are rigidly attached to the piston rods with piston skirts carrying this reaction force. Only Teflon lip seals with radial compliance were used at the crankcase/scavenge pump juncture, sealing around the ground and polished piston rods.
• Knowledge of 2-stroke gas dynamics and attention to gas flow allowed non-restrictive passages and proper port time-area relationships to be incorporated. A 4-into-1 exhaust system was designed to utilize inter-cylinder tuning, allowing properly timed positive pressure wave to arrive prior to exhaust closure, contributing to good trapping efficiency.
• A gerotor oil pump supplied oil from an oil tank for direct spray lubrication of the rod yoke bearings. I did use the yoke roller design as lubrication requirements are greatly simplified versus a pressure lubricated slider block. Oil return back to the tank was via gravity. Cylinder lubrication was via cylinder located, drilled oil ports communicating with oil mist in the crankcase, with one-way check valves being used. Vacuum created in the scavenge pump drew oil mist onto the bore surface.
Patterns were made, castings completed, and machining for the first prototype were done by local machine shops and friends. With very little development work (mostly sorting out the lubrication system), the engine made it’s target performance goals. After a little tweaking, best power recorded was 27.5 bhp @ 5300 rpm. For power measurement, I constructed a torque stand whereby the engine was rotationally supported and balanced. Torque measurement was read from a 1-foot moment arm directly to a spring scale. I had a selection of 3 different-pitched propellers such that I could run 3 load points – sufficient for initial evaluation.
I displayed and ran the engine for 2 consecutive EAA Fly-In events in Oshkosh WI. It received lots of interest, however it was never produced due to the legal liability aspects involved with such a product. I put about 30 hours of WOT testing on the first prototype with no failures or surprises. It was very smooth in operation, with only very slight vibration induced by the force couple due to the axial offset of the reciprocating assemblies. Best bsfc recorded was .606 lbm/bhp-hr at 4800 rpm (cruise setting) and .673 lbm/bhp-hr at 5000 rpm WOT. While these figures are by no means exemplary, they are typical of carbureted 2-stroke engines employing exhaust tuning. Final specifications were as follows:
Bore – 2.313”
Stroke – 1.500”
Total Displacement – 25.2 cu in
Compression Ratio – 7.0:1
Overall Diameter – 16.25”
Length – 21” (with Vertex Magneto)
Weight, Complete – 60 lbm
Fuel – Auto or Avgas, 100 octane low-lead avgas preferred
Best Ignition Timing – 28 deg btdc
Bmep – 78.6 psi
Rated Power – 25.0 bhp @ 5000 rpm
I wrote and presented a technical paper on this engine at the 1985 SAE convention in Milwaukee WI. For those interested, it is still available from SAE:
The paper describes the design rationale and specific engineering details that are too lengthy to discuss here.
I must advise those that are proponents of the Bourke engine –
my X-4 IS NOT A BOURKE ENGINE! It exhibits no magical combustion phenomena, no incredible fuel efficiency, no cool exhaust temperature, no reverse-toggle action, no cyclonic turbulation, etc. It is a 2-stroke, homogeneous-charged/scavenged engine that performs comparably to a conventional exhaust-tuned engine. Benefits gained by the configuration are in operational smoothness, small package size, and employment of a more-conventional 4-stroke lubrication system…that’s it!
The take-aways from this project were the personal satisfaction in designing my own engine that met all the performance targets without anyone micro-managing or “supervising”, no legal department to contend with, no sales-marketing-purchasing department, etc., that we as engineers face in the corporate environment. Most of all, it demonstrates that a scotch yoke 2-stroke engine is a perfectly workable concept. Scaled up and equipped with a direct-cylinder fuel injection system, it could be a very attractive aircraft powerplant for general aviation.
Pictures of the “short nose” version of my X-4 appear below. Engine shown with the Vertex mag and exhaust system removed.
