The Bourke Engine Could Revolutionize Private Aviation
by Yukon Jack
Bourke is to engines as Tesla is to alternators, this was no accident, Robert Bourke taught engine maintenance at the Unites States Air Service School at Kelly Field, Texas in 1918. He fully understood the theoretical limitations of the Otto 4 cycle and sought a simpler design and for years worked on the problem until 1932 when he built a working prototype. Bourke set out to overcome the complex design of gasoline engines, he succeeded. The Army Air Force was impressed and awarded him a contract to build an aviation prototype. According to legend he did and connected a wooden prop to the crank, when he gunned the engine the torque was so great the prop sheered, the blades could not keep up with the engine RPM acceleration.
No real pilot ever complains about too much thrust, the rapid acceleration of a Bourke engine can be overcome with a gear box or throttle acceleration limiter.
The Bourke design is considered by some to be a ‘one-stroke’ engine. Every half turn of the engine, or 180 degrees of revolution, the engine has a power stroke, this is compared to a 2-stroke engine that fires every 360 degrees and 4-stroke every 2 revolutions, or 720 degrees. Bourke’s engine is simple compared to high performance aircraft radial engines:
The Bourke engine lacks the many parts of regular 4 stroke piston engines, there’s no valves or rocker arms, camshaft, springs or precision ground valve seats. Even the flywheel has been eliminated because the engine is balanced at all crank angles. In fact there are only two moving internal parts, the piston – rod – Scottish yoke and the crank – bearing assemblies.
Disassembling the engine is very easy and requires only one wrench on easy to reach external flange nuts. There is no head or head gasket, the piston sleeve is a jug just like a 2-cycle engine sealed with an O-ring, the piston rods are solid and immovable and supported in two places making a very strong assembly. With an impact wrench or ratchet the entire overhaul disassembly in accomplished less than 5 minutes. No special torx driver or extension needed to remove the jugs like many modern 2 cycle engines, the large macro size bolts are a breeze to remove.
The Bourke engine is so simple you’ll say “It can’t be that simple”, but it is. The crank is enclosed and is isolated from piston ring blowby thus this engine doesn’t require an oil filter! The piston rods go through the crank housing, keeping the pistons isolated from the crank oil. The engine is rugged, the piston cylindrical rod is supported at two points, the rod is a straight throw, there is no piston slap or cylinder wear.
The four stroke piston engine is a thing of wonder, so many moving parts and precision, it is amazing they work as well as they do, they produce lots of torque but they are not thermally efficient. Even with all of the modern improvements of controlled combustion and Exhaust Gas Recirculation (EGR) system, a modern gasoline engine is unable to use 25-35% of the available heat in a gallon of gasoline. Modern car engines also require high octane fuel, but not the Bourke engine, it runs smoothly on low grade fuels, even 20 Octane, hydrogen or highly diluted alcohols! The Bourke engine has cool exhaust, what this implies is very impressive thermal efficiency. The reason for this is the incredibly long ‘dwell’ angle as the piston pauses at TDC longer because of the Scottish Yoke design.
Because of this inherent design and very high 15:1 to 24:1 compression ratio, the fuel is burned before the piston reacts, thus Bourke’s engine is similar to a gun firing a projectile, as the hot burnt gas expands the temperature cools, to the point that during the exhaust stroke the gas is cool enough to touch at the exhaust port. Regular engines are still combusting during the power stroke but Bourke’s engine burns most of the fuel before the piston starts the power stroke.
Lower compression gasoline engines (8:1) are not good at complete combustion. In fact at car races you can see flame shooting out the exhaust pipes – the fuel is still burning even as it is exits the cylinder and is vented to the atmosphere. I’ve worked on pickups that had pollution control sparkplugs that fired on the exhaust stroke, or at classic car shows where the rat rods shoot a flame out the tail pipe. Diesel engines have compressions of 20:1, thus they don’t require sparkplugs and radiators require covers during cold weather. Bourke’s engine has thermal results like a diesel engine burning low grade fuels.
Low Grade Fuels
Burning hydrocarbons burn better when all the molecules are squished together and Bourke’s engine allows the combustion to reach completion because of the long dwell at TDC. With higher compression you can also burn crappier lower cost fuels or cut fuels. You may be able to cut (dilute) gasoline with water if you add alcohol which dissolves water and mixes well will gasoline. Or you may be able to get combustion on alcohols heavily diluted with water.
You can check this out for yourself, go to the store and get some straight grain alcohol (200 proof ethyl alcohol) or 90% isopropyl rubbing alcohol then cut it with water in different test amounts, alcohol will burn with water. Wine or beer won’t burn because the alcohol is only 5-12% unless you heat it, the flash point of ordinary wine containing 12.5% ethanol is 126 °F. Chefs like to impress diners with burning wine flambe, table wine poured into a hot skillet will burn. With sufficient compression and heat even wine will burn.
The engine can run at 20,000 RPM. After 2,000 hours the engine shows no cylinder wear. Two stroke beats four cycle engines for weight and power, but the Bourke beats them all. Bourke’s 30 cubic inch delivers 76 horsepower at 10,000 RPM and it only weighs 38 lbs.
The reported thermodynamic efficiency greater than 55%, which is an exceedingly high figure for a small gasoline engine and in the range of the best diesel engine.