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The flame-front moves at roughly 33.5 m/second (110 feet/second) during normal combustion. It is only when the remaining unburned mixture is heated and pressurized by the advancing flame front for a certain length of time that the detonation occurs. It is caused by an instantaneous ignition of the remaining fuel/air mixture in the form of an explosion. The cylinder pressure rises dramatically beyond its design limits and if allowed to persist detonation will damage or destroy engine parts.
A non conventional engine that makes use of detonation to improve efficiency and decrease pollutants is the 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 design and building several working engines, bad luck (onset of World War II) and bad health compounded to prevent his engine from ever coming to market. This engine was in pre-production by Hudson Motor Car Company (now know as American Motors). For almost a decade there have been several small groups extolling the virtues of the design and engines in testing. Yet not one has produced comprehensive test results conducted by an independent party to back up their performance claims. The main virtues of the design are its small size, light weight, and low number of parts.Design featuresThe following design features have been identifiedScotch yoke instead of connecting rods to translate linear motion to rotary motionScotch Yoke does not create sideforces on piston, reducing friction and vibrationFewer (only 3) moving partsSmoother operation due to elimination of crank and slider mechanismLonger percentage of cycle spent at top dead center and bottom-dead-center for more complete combustion and exhaust scavengingOne power stroke per piston, for every rotation from the opposed pistons instead of one every other rotation resulting in nearly twice the power at a given engine speed, compared with a 4-stroke engine.Eliminate the need to mix oil with the fuel as with standard two-stroke enginesPrevents the piston ring blow by from polluting the crankcase oil extending the life of the oilLow exhaust temperature (below that of boiling water) so metal exhaust components are not required, plastic ones can be usedExtremely fast burn time of the fuel air mixture so the engine can be considered to be a detonation or "explosion" engine.15:1 compression ratio for high efficiencyMultifuel capability - although different fuels will need different compression ratios.Fuel is vaporised by the high temperature in the transfer port, and the mechanical action in the combustion chamber.Lean burn for increased efficiency and reduced emissionsThe piston is connected to the Scotch yoke through a slipper bearing (a type of hydrodynamic tilting-pad fluid bearing).Piston shape, with higher volume around edges, contributes to complete explosive burnEngineering Critique of the Bourke EngineThe Bourke Engine has some interesting features, but the extravagant claims for its performance are unlikely to be borne out by real tests. Many of the claims are contradictory.1) Seal friction from the seal between the air compressor chamber and the crankcase, against the conrod, will reduce the efficiency.2) Pumping losses, the air charge is compressed and expanded twice but energy is only extracted for power in one of the expansions.3) Engine weight is likely to be high as it will have to be very strongly built to cope with the high peak pressures seen as a result of the rapid high temperature combustion, and the scotch yoke/triple slipper bearing are heavier than a conventional crankshaft.4) Each piston pair is highly imbalanced. This will limit the speed range and hence the power of the engine, and increase its weight due to the strong construction necessary to react the high forces in the components. (videos of running engines do not appear to show imbalance)5) High speed two-stroke engines tend to be inefficient compared with four-strokes because some of the intake charge escapes unburnt with the exhaust.6) When the charge is transferred from the compressor chamber to the combustion chamber it will cool down, reducing the efficiency of the engine.7) Use of excess air will reduce the torque available for a given engine size. Forcing the exhaust out through small ports will incur a further efficiency loss.9) Operating an internal combustion engine in detonation reduces efficiency due to heat lost from the combustion gases being scrubbed against the combustion chamber walls by the shock waves.10) Emissions - although some tests have shown low emissions in some circumstances, these were not necessarily at full power. As the scavenge ratio (ie engine torque) is increased more HC and CO will be emitted. See HCCI engine