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Jerry/MT
09-09-2003 15:25:43
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Re: Question about what happens when the throttle in reply to Ron, 09-09-2003 13:02:15
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Ron, I'm not sure I understand your question either, but I'll risk answering what I think you are asking. If it's a normally aspirated spark ignition engine (carbretor of fuel injection) the basic contol parameter for power is airflow since the engine will only run over a narrow range of fuel to air ratios. The compression ratio is fixed and is limited to mainly by the fuel to be used. (Typical range of modern compresion ratios, Vol @ TDC/Vol @BDC, is in the range of 8-9) When we open the throttle plate wide open at any given rpm, at constant ambient pressure and temperature, the air pressure (not vacuum)in the manifold ahead of the intake valves is a maximum and the airflow is a maximum for that rpm. Therefore the power is maximum for that rpm. Conversely, at idle rpm, the throttle plate is nearly closed, the pressure is at a minuimum and, (for constant ambient pressure and temperature)and the power is at a minimum.
In a diesel engine, there is no throttle plate and the mode of control is fuel to air ratio. The compression ratio is fixed and in my 4610 I believe it is about 16 or 17:1. The pressure in the intake manifold ahead of the intake valves is almost constant across the entire range of rpm (It actually is a minumum at max power due to pressure losses in the intake passages but we are talking relatively small losses not like that imposed by the throttle plate in a spark ignition engine.)To achieve maximum power at any given rpm with constant ambient conditions, we add the maximum amount of fuel that is limited usually by temperature (we don't want to fry the top of the piston) cooling requirements and/or stregth and load considertions). At idle, the rpm is low, the airflow is low and the fuel flow is low, giving us a low fuel to air ratio. At max power, the rpm is high, the airflow is high and the fuel flow is high and the fuel/ air ratio is high.
Hope this helps.
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