The reason diesels are efficent at idle? Solved!

[landspeed]

I've been thinking about this for a while, and I've done experiements to see the effect of engine braking in my car, which show that full-throttle (with engine-off) seems to make no difference (although it should, as the 'pumping losses') are reduced.

My observations:

(1)

'Pumping losses' seem to make little difference, when I roll down a hill at 15mph, in 1st gear (very strong engine braking), with the engine off. Pushing my foot on the accelerator, or leaving the throttle plate fully closed, makes no difference in the degree of engine braking present. I think this is because:

If the throttle plate is closed (high vacuum), then the engine easily compresses the vacuum in the cylinder, but then has to work to 'expand' the vacuum in the cylinder again. This is an air spring, with the gas in the atmosphere being the 'spring'

If the throttle plate is open (full throttle), then the engine works hard to compress the gas in the cylinder, but then easily 'expands' the air in the cylinder again. This is an air spring' with the gas in the cylinder being the 'spring'.

Therefore, there is almost no difference between the throttle plate being open or closed.

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(2) If the mixture goes too lean (maybe above 17.0) in a petrol/gas engine, then the fuel economy drops (from a graph I was shown on this website in the past). Diesels do run lean at idle, but this isn't necessarily the reason that they are economical at idle (if it wasn't running lean, it would be revving to the redline in an instant - diesels even run lean at full throttle anyway).

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Therefore, I think that gas(petrol) engines are inefficient at idle, because, when they are running at a high vacuum, only a tiny amount of air actually gets into the cylinder!. Once the mixture has ignited, the final volume is not much bigger than the volume of the cylinder, so, a lot of the energy is wasted (because the ignited mixture will exert very little pressure on the piston, as the volume in the cylinder approaches the volume that the combusted mixture 'wants' to be at.

Diesels, on the other hand, run at full throttle. This means that any extra expansion caused by the combustion of tiny amounts of diesel gets turned into extra pressure, and as the cylinder is at full throttle anyway, a lot more of the extra pressure is 'useable'.

Therefore, when you run a gas/petrol engine at high vacuum (low power output) a lot of the energy is wasted, whereas, when you run a diesel engine at low power output, it still remains efficient!.

When you run a petrol engine at a more significant throttle level, there is more air in the cylinder, so it is running more like a diesel engine, and more of the available energy is useable.

Any comments on this idea?