Quote:
Originally Posted by RustyLugNut
The quick answer is adequate.
With conversion into liter displacement input, the formula is simply:
BMEP = ( HP * 13000 ) / ( L * RPM ) , for a four cycle engine.
But the result is the same. 78.1 psi .
For those not familiar with BMEP, it is a quantity that represents an ideal, flat pressure head acting on the piston top across the angular range between TDC and BDC on the power stroke ( 180 degrees piston motion ). It's usefulness is in comparing ICE engines across a broad spectrum of design variables. It is usually introduced in undergrad engineering classes as a derivation exercise that is surprising in that the resulting pressure is dependent only on the variables of displacement, engine speed and power - three values that are easily found in literature for a particular engine, or measured in the college dynamometer lab.
We can use this equation to glean some insight into engine modifications.
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. . . just how relatively low the pressure is in this idealized example. This is because it is idealized ( mean = average of the pressures ) not an actual pressure trace. An actual pressure trace would have peaks that are much higher and plateaus that are much lower. It is also a NET idealized pressure, taking into account the losses of pressure before TDC and after BDC.
Also, it is representative of the power output we as ecommoders strive to reach in our efforts to maximize fuel consumption. Many of the Xprize cars like the Edison VLC, the Aptera and Jack McCornack's Max, need less power than this to cruise at road speeds. But it is certainly a bottom number many of us can strive for with some of our modified cars.