>Sstones replies: >>But... Once the turbocharged intake strok is over... You get a >>compression stroke at a much higher pressure, there in lies the problem. This is correct > >I reply: >Ah-hah! That makes sense. With more gas (& pressure) in the cylinder at the >beginning of the compression stroke, there will be higher ultimate pressure >at the end of the stroke. This must be why turbo engines generally run a >lower compression ratio - to allow for the additional compression effect of >the turbo. afraid not. It's not the little extra compression you get from the turbo booster, it's the fact that while being compressed, gases heat up. The lower compression ratio is used to prevent knocking (premature ignition). as the turbo stuffs more air into the cylinder (so that you get more Oxygen to burn the extra fuel with) the fuel-air mix heats up due to compression, even before the piston starts to compress the mixture. The same fuel-air mixture will have different 'ignition pressures' at different temperatures -- the higher the temp, the lower the pressure it will ignite under. This (and low-octane gasoline) makes for very good conditions for knocking. One way to deal with is to change the compression ratio. Another is to put intercoolers. SAAB had the best solution early on, I do not know if they still use the same system: They had a knock-sensor (a microphone) attached to the engine, which, when sensed the engine knocking, would reduce the turbo boost ratio.
Cetin
Cetin Seren Direct: 412-635-3479 Software Development Engineer Main: 412-772-6600 Outbound Technology Group fax: 412-635-3350 FORE Systems, Inc. email: cseren@fore.com Research Park, 5800 Corporate Drive URL: http://www.fore.com Pittsburgh, PA 15237-5829
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