If it takes energy to compress the air in the cylinder - doesn't it also release most of that energy when that compressed air is expanded on the subsequent stroke?

If you release the compressed air without pushing the cylinder down you would lose that energy, but you would need a extra device to do so (by lifting a valve at the right time). This option does exist for large vehicles like trucks as a compression release engine brake [0], but this isn't something you'd have on a family car.

In a petrol engine you always want the same ratio of petrol to air in the mix that is taken into a cylinder. As you want to vary the amount of fuel, and therefore power developed, you have to be able to therefore limit the amount of air that is sucked in. Otherwise the engine would always run at full power.

There is a mechanical restrictor called a throttle plate that lives inside the throttle body that restricts how much air the cylinder can pull in (and therefore how much fuel is injected to get the same fuel/air mix). This is controlled by the throttle. When you are coasting, this plate is in its most closed position. This creates significant resistance on the intake stroke, and is where the majority of energy is lost during engine braking. This is also known as a pumping loss.

Diesels always intake the same amount of air, so they can compress it enough to autoignite the fuel. They vary the amount of fuel injected to the same volume of air. This means no throttle body or plate, so unless an extra exhaust restrictor has been added there is minimal engine braking on a diesel engine.

[0] https://en.wikipedia.org/wiki/Compression_release_engine_bra...

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potato3732842 2 days ago | parent | next [-]

What everyone is missing is that a petrol engine is sucking against a vacuum behind the throttle blade.

It's basically like an air compressor that just keeps running despite hitting max pressure and every pump just goes out the blow off valve.

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MindSpunk 2 days ago | parent | prev [-]

You're right. Petrol engines are air pumps so I should've realised the explanation wasn't correct.

Though the ECU would be doing the AFR management on modern EFI engines as the injectors aren't vacuum operated like Carburetors were. You should be able to cut fuel injection when coasting in a modern engine, can't run lean if there's no fuel at all. Not sure if carbs could do the same.

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agurk 2 days ago | parent [-]

On all the fuel injected engines I have owned there is a physical cable that controls the position of the throttle plate. There is an airflow mass sensor the other side of the plate to measure the amount of air and therefore how much fuel needs to be injected. So interestingly in these sort of engines you're really just controlling airflow to the engine rather than fuel/air mix like on one with a carburettor.

More modern engines have electronically controlled throttle plates, and this is definitely somewhere you could do something clever like you suggest - cutting fuel flow but also maximising airflow when there is zero throttle input.

I assume engine braking is generally considered a beneficial thing by manufacturers, but it could be fun to be able to customise the amount. Or do something like have the braking come on gently at first then harder. Maybe even try and have a linear or flat response curve vs. engine rpm.

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sokoloff 2 days ago | parent | next [-]

> cutting fuel flow but also maximising airflow when there is zero throttle input

You don't want to do this. Much of the engine braking effect is from pulling the intake air charge past the mostly closed throttle plate. On a car with a wide open throttle plate [even with no fuel], the engine is acting more like a spring than a damper. On the intake stroke, it will pull an intake air charge past the small restriction of the open intake valve(s), then compress it on the compression stroke, then release that compressed energy on the "power" stroke, then exhaust it past the small restriction of the open exhaust valves. Pushing air past the valves will cost energy, but it's not much.

This is why diesel trucks' engine braking works differently. (Diesels don't have a throttle plate.) They can open the exhaust valves to prevent the energy recovery in the "power" stroke to create a higher net braking force. Jake Brake: https://en.wikipedia.org/wiki/Compression_release_engine_bra...

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yetihehe 2 days ago | parent | prev | next [-]

> Or do something like have the braking come on gently at first then harder.

You can do this by letting go of gas pedal slowly. I have "current amount of fuel used" info in my car (liters/100km), it shows pretty clearly, that when going fast and slowly letting go of gas, amount of fuels slowly goes to 0. If I let go of gas fast, the engine is intelligent enough to not close throttle as fast as possible, still probably takes 1 second.

> More modern engines have electronically controlled throttle plates, and this is definitely somewhere you could do something clever like you suggest - cutting fuel flow but also maximising airflow when there is zero throttle input.

They cut fuel flow and close throttle plate almost completely but still allow some small amount of air, in order to actually do engine braking. If you need to coast, you can apply clutch in manual. Don't know that much about automatic, but from what I've driven, they use "lift gas" as a "engine braking" signal, so probably they can't really coast that good.

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MindSpunk 2 days ago | parent | prev | next [-]

I've got both at the moment on my two Hondas. Both manual, one with a throttle cable and one with throttle-by-wire. There's quite a bit of difference in how they handle off throttle. The cable will just slam the throttle shut if you just jump off the pedal (obviously) and it jerks pretty hard. The throttle-by-wire car hangs the throttle a little when letting off and doesn't just immediately start decelerating. Then it's much smoother once it does start slowing down. The ECU definitely doing something to smooth it out.

Funny because the cars build dates are only 2 years apart, 2005 and 2007, and they're both K20 engines but the engines handle so different.

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potato3732842 2 days ago | parent [-]

The ECU is doing that because rapid changes in state are bad for emissions. Letting you just slam the throttle closed could result in a tiny, but measurable at OEM scale, amount of extra fuel going half burnt out the tailpipe. Slamming it open can cause too lean combustion and oxide byproducts which.

The OEMs try real hard to prevent this because the amounts of emissions byproducts that aren't water or C02 they're allowed to produce are on the order of single digit grams per multiple miles (you can mentally file it as "about the baseline air quality in urban areas" though the rules are hugely more complex than that) so these edge cases matter.

	▲	tkj922 2 days ago \| parent [-]
		An idling I4 has about 10 injection cycles per second. And the ECU clocks injection time corrections at least at that rate, more likely at double that rate or more. So I think that the smoothing is mostly there for the owner's wellbeing, not emissions.

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vladvasiliu 2 days ago | parent | prev [-]

> it could be fun to be able to customise the amount

Some recentish motorbikes have an option to customise the amount of engine brake, I suppose cars could have something similar, too.