Pistons Rings
Bourke Engine
The Bourke engine was designed by Russell Bourke in the late 1930s, who endeavored to improve upon the Otto cycle engine. Despite finishing his redesign and building several working engines; bad luck (onset of World War II), bad health and a know-it-all attitude compounded to prevent his engine from ever coming to market despite its claimed advantages. Well into the 2000’s there are several small groups extolling the virtues of the design. The Bourke engine has two opposed cylinders with the pistons in a Scotch yoke mechanism. Because the motion of the pistons is a perfect sine wave with regards to time vs displacement the fuel burns in a smaller volume, and so burns hotter. The Bourke engine also has a looser coupling with the output shaft, preventing excess vibration. The intake valves are replaced by ports, saving on parts.
It is thought that the design features that increase its efficiency, namely the detonation mode of combustion, may cause emission problems. The higher combustion temperatures combined with the increased cycle time around top dead center may lead to increased nitrogen oxide emissions. There have not been any verified nitrous oxide tests on running engines to verify the emission problem.
Design features
* Scotch yoke instead of connecting rods to translate motion to rotary motion
o Fewer (only 3) moving parts
o Smoother operation
o Longer percentage of cycle spent at top dead center and bottom-dead-center for more complete combustion and exhaust scavenging
* Two power strokes for every rotation from the opposed pistons instead of one every other rotation (4-stroke) resulting in nearly twice the power at a given engine speed
* Expanding gasses cause adiabatic cooling reaction as opposed to a drawn out combustion and heating reaction.
* Lean fuel/air mixture combined with the adiabatic cooling reaction resulting in zero unburnt hydrocarbons in the exhaust
* Sealed underside of the piston to isolate the fuel/air mixture from the crankcase
o Eliminate the need to mix oil with the fuel as with standard two-stroke engines
o Prevents the piston ring blow by from polluting the crankcase oil extending the life of the oil
Simplified explanation:
The Bourke engine should be considered a detonation or “explosion” engine because of the extremely fast burn time of the fuel air mixture.
The rising piston compresses the mixture heating it, because of the cool piston head and cylinder walls the mixture is not heated enough to start a burn. The piston “latches” at top dead center because of the action of the scotch yoke. Spark plug fires causing an explosive burn. All of fuel is burned completely because of explosive burn time. Sharp rise in pressure causes scotch yoke to “unlatch” piston. Piston moves down cylinder, expanding gases following piston cause cooling of piston and cylinder walls. Since all of fuel was burned during “latching” of piston no fuel is being burned at this time, no heat is being added, all expansion results in cooling. When piston reaches the bottom of the cylinder it again “latches” until “unlatched” by the explosion in the opposite cylinder. This gives time for scavenging of explosive gases and injection of fuel air mixture.
The mixture must be lean enough so that compression heating of the mixture does not cause it to ignite prior to the “latching”. If this occurs engine will not run, or will run rough. Mixture of the fuel must be in the “explosive” range for the engine to run correctly. Any fuel that is used that is mixed into its “explosive” range will work on this engine thus giving it multi-fuel capabilities.
The maximum pressure on the piston occurs right after the release of the piston from top dead center. It is not spread over the whole length of the piston travel as in a conventional engine. For this reason this is an extremely high torque engine.
The complete burn of the fuel while the piston is “latched” gives this engine its high efficiency and low emissions.
The cooling caused by the expanding gases, behind the piston head and the non burning of fuel while piston is traveling, causes the low temperature exhaust gases. This also prevents dieseling during the next compression cycle, because of the cooling of the piston head.
Air enters under the piston head, where it is compressed (turbocharging). It then leaves from under the piston and is mixed with fuel, prior to being injected above the piston. This injection occurs at the same time, but at the other side of the cylinder port, that the exhaust gasses leave. Because of the shape of the piston the injected mixture hitting the sloped piston head cause a swirling action that leads to complete mixing of fuel/air. This mixing contributes to the complete explosive burn. The sloped piston head effectively separate the incoming mixture from the scavaging of the exhaust gases.
Piston is connected to the Scottish yoke through a “triple slipper bearing”. This bearing absorbs and smooths out the force from the explosive burn preventing deformation/breaking of engine parts. The bearing also absorbs any lateral forces, preventing vibrations. This bearing is the “key” to the engine. Without this bearing the explosive forces would “tear the engine apart”.
Compression ratio needs to be adjusted for type of fuel burnt. If compression ratio is to high speed control becomes erratic do to dieseling. Adjusting the compression ratio allows burning of both low and high octane fuels.
Piston shape, with higher volume around edges, contributes to complete explosive burn.
About the Author
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