The reason diesels are efficent at idle? Solved!

[theholycow]

Originally Posted by milesgallon.com

The high pressure makes the temperature in the cylinder so high that the fuel will ignite by itself as soon as it's sprayed into the cylinder.

Sounds like new direct injection gasoline engines may get approximately the same benefit.

[GasSavers_bobski]

Originally Posted by theholycow

Sounds like new direct injection gasoline engines may get approximately the same benefit.

Sort of. You get the similar benefit of being able to ignore the air charge that isn't needed, but direct injection doesn't rely on autoignition - it still uses a spark plug. In that respect, I think it has more in common with stratified charge engine designs than diesel.

The problem diesels have is with getting all the air/fuel to burn, and quickly enough to make efficient use of the released energy (which incidentally is one of the reasons diesels typically have low redlines). In that respect, gasoline direct injection has an advantage. GDI can spray a small amount of fuel into the pocket around the spark plug, ignoring the surrounding unneeded air to get diesel-like operation. Or, to get typical gasoline engine peak power output, it can inject fuel during the intake and compression strokes, evenly pre-mixing the air and fuel without worrying about the mixture auto-igniting before it is mechanically advantageous (aka pre-ignition) as would happen if you tried the same thing in a diesel engine. It could also operate at points in between those two extremes (mimicking stratified charge engine operation) by providing a leaned-out premix and tossing in a quick burst of additional fuel around the plug to light it off.

[milesgallon.com]

Are there any direct injection gasoline engines used in real mass production cars yet?

Simon

[theholycow]

Yes, plenty of them. GM has been putting them in the Cadillac CTS for a few years, and the same engine is in the V6 Camaro. I think the Ecotec I4 is too.

Volkswagen's 2.0 FSI-series engines are DI.

I think most manufacturers have at least one DI gas engine.

[JanGeo]

Landspeed - one thing you didn't get on the engine braking with the ignition off is when you opened up the throttle you got that the intake valve closes at the bottom of the stroke and compresses the gas all the way to TDC then the plug would fire and you should get all the compression effort back on the down stroke BUT you forgot that the exhaust valve opens BEFORE the piston goes very far down the power stroke so it ends up dumping the compressed air out the eshaust before the compressed air can put that compression energy back into the piston stroke. That is why giving it more throttle didn't reduce engine braking effect - it can in engines with later exhaust valve openings like my Old 65 Rambler American Flat Head 6 however.

[GasSavers_bobski]

Originally Posted by JanGeo

BUT you forgot that the exhaust valve opens BEFORE the piston goes very far down the power stroke so it ends up dumping the compressed air out the eshaust before the compressed air can put that compression energy back into the piston stroke. That is why giving it more throttle didn't reduce engine braking effect - it can in engines with later exhaust valve openings like my Old 65 Rambler American Flat Head 6 however.

I think you've got that backwards. When the exhaust valve opens, pressure in the combustion chamber drops, so the force pressing against the piston disappears; The energy stored as compressed air is wasted. The sooner in the power stroke the valve opens, the less energy is recovered. Going to one extreme, if the exhaust valve opened at the beginning of the power stroke so that no energy was recovered, the degree of engine braking would be entirely dependent on how much work it takes to push the piston to the top of the cylinder during the compression stroke. The higher the initial air pressure in the cylinder (dependent on manifold pressure and thus throttle position), the greater the effort it will take to compress it.

[JanGeo]

NO I got it right just like you said bobski - the exhaust opens too soon letting the pressure out so it can't continue pushing the piston down to recover the energy.

[GasSavers_bobski]

Originally Posted by JanGeo

NO I got it right just like you said bobski - the exhaust opens too soon letting the pressure out so it can't continue pushing the piston down to recover the energy.

Right... I'm saying your conclusion about later exhaust valve openings increasing the throttle-modulated engine braking effect is backwards. I guess I could have cropped the quote a little better.

[DarbyWalters]

Problem is Diesels do not really have throttle plates...they jump pump air unrestricted. You acceleration is solely thru addition of diesel fuel. That is why Diesels idle so efficiently. Air to Fuel Ratios can be VERY LEAN.

[rh13]

Actually, if it makes any difference, opening the throttle plate should increase the braking effect. When coasting, the engine is functioning as a pump. The pumping losses are the energy used to pump air through the engine. This happens under power, too, but under power the energy produced by the expansion of gas in the combustion chamber overpowers the pumping losses. In a pump, the energy drawn by the pump is positively related to the amount of fluid pumped. Hence, opening the throttle plate will increase the resistance. To really reduce the pumping losses, some engines close off the valves of some of the cylinders. With no air going through those cylinders, there are no pumping losses at those cylinders.