Oven cleaning it is then.

 

[QUOTE]Originally posted by Moggy
[B]

so overall then, acceleration is a combination of torque, engine size, weight, and gearing.

 

ohhh sh'''@ttttt----thought I had this sussed willread this and pay more attention in class when I have not had a drink

seriously though this stuff is all interesting-keep em coming

 

I like the idea of something like IMA in a sportscar, the electric motor gives max torque at zero-ish revs and the rev-happy engine gives you the high-rev racecar feel.
Can any of the gurus out there explain BMEP (brake mean effective pressure, I think?). Something to do with engine efficiency?

 

BMEP - now you're in trouble!!!!!!!

Sometime last century, my honours project was developing closed loop control of an engine to optimise BMEP.

You're quite right about it being something to do with engine efficiency - really it's the best measure of how good the engine is as an engine - if the BMEP figure was the same for all engines then there really would be no substitute for cubes.

NOW THEN CLASS FAITE L'ATTENTION ...

For 4 stroke engines the work cycle goes suck, squeeze, bang, blow (no tittering at the back there).

Suck - the pressure in the cylinder is less than atmospheric and so Effective Pressure on the piston is negative

Squeeze - the pressure in the cylinder is greater than atmospheric and it is being produced by the piston so Effective Pressure on the piston is still negative

BANG - the pressure in the cylinder is (much) greater than atmospheric and so Effective Pressure on the piston is positive

Blow - the pressure in the cylinder is greater than atmospheric and it is being produced by the piston so Effective Pressure on the piston is negative again

Now if we take the average pressure on the piston over the whole cycle we get a figure for the Mean Effective Pressure. The Brake bit is just because we do the measurement when the engine is being held at specific revs against a brake.

Pressure x Piston Area = Force on the piston.

Force on the piston x crank throw (Stroke) = Torque. (this is why 'long stroke' engines are good for torque)

Torque x Engine Speed = Power.

... and that's all there is to it!!

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apart from some arcane stuff about cylinder filling, throttle sizing, port shaping, combustion swirl, dynamic loads on the piston, crank, connecting rods, valve bounce, cam profiles ...