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When will a diesel be available for my aircraft? |
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News, facts, and comments on the coming revolution for piston-engine aircraft.
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News of March 08, 2010Answer received from Wilksch Airmotive on our comparizon of 2-stroke versus 4-strokeDear Andre - Some comments on your response to a subscriber who was asking “Could you please expound on the differences of the harmonics produced between a 2 stroke and a 4 stroke diesel.” Firstly, I think that you are right in assuming that the subscriber did not actually wish to know about harmonics – these being the higher order frequencies (of vibration, noise etc) that are produced when any system does not resonate or operate in a true simple harmonic (sinusoidal) fashion. However, if it were indeed information about differences in harmonics that is being requested then I think it will take more than this discussion to answer. You are of course absolutely correct in stating that, in theory, the 2 stroke generates less vibration because of the more frequent cylinder firing (for a given configuration of engine). The point is that with more firing strokes in a given time (given that the prop rpm is fixed, and the engine is direct drive) then each one will involve less energy input to the crankshaft system, to achieve the same power output. Of course geared engines will change that situation by the gear ratio used, but bring unwanted complexity and cost. In the case of diesel cycle engines the compression ratio needs to be high to ensure ignition of the fuel by the heat of compression alone (neglecting the use of glow plugs as starting aids). So diesel cycle engines universally have to have high compression ratios, of pretty much the same value, as ignition is dependent upon the fuel properties. This is of course not true for SI engines. Post ignition the pressure rise above compression pressure is a function of the combustion system and the fuel injection equipment. These other variables bring influences that can modify the simplistic answer significantly. An indirect injection (IDI) engine (such as the WAM series of engines) has a slower fuel burn and a delayed increase in cylinder pressure compared to a direct injection (DI) engine because much of the combustion takes place in a separate chamber (the DI engine’s combustion takes place above the piston). This leads to a small loss of efficiency, but significantly less harsh, smoother combustion and hence lower vibration energy. Variably timed and pulsed injection such as is only made practically possible by common rail FIE systems can reduce peak cylinder pressure for DI engines to similar levels to that of an IDI engine, but with slightly less loss in efficiency. For a constant speed application such as an aircraft engine this is the only real benefit of a complex and expensive common rail FIE system. Interestingly (and rarely noted) 2 stroke diesel engines universally have lower exhaust temperatures than their 4 stroke counterparts because the 2 stroke needs to purge the cylinders with fresh air at each BDC to ensure good combustion. This is not required (or possible) for a 4 stroke, which as a result has an exhaust temperature usually 50 – 100 deg C (90 to 180 deg F) higher at full load. Incidentally one of the side benefits of diesel combustion is that the exhaust temperature falls rapidly at part load, thus greatly increasing the cruise life of components (cf an SI engine where the temperature is more a function of A:F ratio than load, so at leaned cruise the head components may be at their highest stress). We at WAM believe that the principles of operation and construction that we have used give the best possible situation for an aircraft engine. The 2-stroke IDI combustion gives low vibrational energy, combined with the valve-in-head exhaust and ported liners for inlet giving longevity to these areas, and a “bottom end” with pressure fed bearings to ensure uncompromised bearing life. The balance masses employed in the WAM engines reduce mechanically induced vibration to insignificant levels, certainly as good as those found in the majority of flat, V or radial engines, which nearly all have practical compromises in their construction that add mechanical imbalances. Propeller testing has demonstrated the WAM engine’s forgiving characteristics and our customers have frequently commented on the smoothness of operation. Best regards, keep up the great website! Phil Franklin, Chief Engineer, Wilksch Airmotive Ltd posted at 9:20 AM |
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