Engine Braking?

[JohnL]

yeh string is right,allthough obviously u wont get it all back on the power stroke,u'll get enough to negate the compression effect.

I can see that on the down stroke the compressed charge will 'give back' at least some of the energy 'stored' in the compressed charge, but minus the energy converted by the compression into heat. It seems to me that this must ultimately be what happens to the kinetic energy of the moving mass, i.e. it's converted to heat through the vector of gas compression, thus creating the engine braking effect.

for a normal car engine,u'd prob have to go back one stroke & most of the effect would be intake trying to pull open a vacuum....

Which would have a limitation in that even with 100% efficiency the work is actualy done not by creating a vacuum, but by the underside of the piston 'pushing' against atmospheric pressure, which has a nominal value of only 14.7psi (give or take altitude variation and crankcase fluctuations, which won't be much).

So, even with an unrealistically high efficiency the work done creating at best a partial vacuum is not going to be the same as compressing in cylinder gas to pressures much greater than atmospheric...

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It also just occurred to me that cutting the fuel injection on the overun should add to engine longevity since the bore walls aren't being rinsed with unburnt fuel (washing off the bore oil film), and less oil degradation due to less unburnt fuel finding it's way into the oil (i.e. less oil dilution).