# how does engine braking work if the manifold vacuum is equally applied to all cylinders?

102 views

Jake brakes are trivial to understand: you’re compressing a spring and “magically” losing that energy instead of allowing it to be returned. But I don’t get how gasoline engine brakes work: from what I can tell the manifold holds a vacuum which resists the piston downstroke, but the vacuum returns the same energy in the piston’s upstroke (minus friction which is negligible). Furthermore, once all cylinders have undergone one full cycle, they all hold a vacuum so if one cylinder is being retarded, it’s opposite is being actuated meaning the force balance is pretty close to neutral. So where’s the energy loss here?

In: Engineering

> So where’s the energy loss here?

The gasoline engine is still exhausting at the top of the stroke. It blows out high pressure exhaust and then needs to work again to draw in more air, being slowed by the resulting vacuum.

You say the energy is being “returned” in the form of the upstroke compressing the air drawn in by the manifold vacuum, but it isn’t returned to the crankshaft because the pistons aren’t pushed down by that compression for the next stroke.

This is what I gathered from the wikipedia article on the subject:
When you suck, you suck on a vacuum, which requires energy (because the other side of the piston is at atmospheric pressure).
When you squeeze, you squeeze that vacuum, getting your energy back.
When you bang, there’s no bang, and you’re required to suck on the vacuum again, which requires energy.
When you’re about to blow, air from the exhaust manifold is allowed to enter the cylinder, killing the vacuum and ensuring that you won’t get that energy back. Then you blow this air out again, and the cycle starts anew.

the vacuum is being pulled in the expansion stroke and in the injection stroke, because no fuel has been injected there is no explosion.

in the upwards cycle after this the exhaust valve will be open so the vacuum will not pull the pistons upwards.

even in cars without direct injection this works because the car can simply stop igniting the fuel, which is also why engine braking happens so quickly and why some sports cars have popping sounds coming from the exhaust as the left over fuel gets into the exhaust and ignites there.

The energy loss is occurring at the throttle body/throttle plate. Fundamentally, an automobile engine is a pump that moves air from the intake manifold to the exhaust manifold. When you engine brake you close the throttle and as a result more work is required to pull air through the closed throttle.

Air in the cylinder behaves like a perfect spring only while the valves are closed. When the valves are open, the energy is not conserved.

Strokes:

1 – lose energy to suck air in the cylinder

2 – lose energy by compressing air

3 – expand air and recover energy from step 2

4 – lose energy by pushing air outside the cylinder.

Steps 1 and 4 are where the engine brake is effectly slowing down the vehicle.

**With the throttle closed:**

* Intake stroke costs energy (sucking against vacuum, forcing air past closed throttle)
* Compression stroke costs energy (compressing air)
* Power stroke returns most of that compression energy (compressed air pushes piston)
* Exhaust stroke costs energy (pushing air out of cylinder and through exhaust)
* On top of all that you’ve got engine friction that’s just straight up wasted energy too.

So the net balance of all that is that you’re spending a lot of energy moving air around and overcoming mechanical friction, and only reclaiming a little of it.

**In cars that do cylinder deactivation, you keep all the valves closed, so it’s just**

* Exhaust stroke compresses air (Costs energy)
* Intake stroke uncompresses air (reclaims most of that energy)
* Compression stroke compresses air (costs energy)
* Power stroke uncompresses air. (reclaims most of that energy)

So then yeah, you’ve just made a spring, you’re still losing energy to friction etc, but not to inhaling or exhaling air into the cylinder.

I would assume you’re also losing a tiny bit of that compression pressure past the piston rings too.

**In diesels doing compression braking, its:**

* Intake stroke sucks air in (costs energy)
* Compression stroke compresses air (costs energy)
* Exhaust valve opens at the end of compression, stays open during entire power stroke (compressed air noisily blows out of cylinder, wasting it’s energy)
* Exhaust valve remains open during exhaust stroke (costs energy)

A big reason this system exists, is because diesels don’t have a throttle, so if you didn’t do this, you wouldn’t get much engine braking on deceleration, as there isn’t the energy loss to pulling air through a closed throttle.

Even with the throttle closed, airflow occurs and the engine pumps air, consuming energy.

Also, the engine has its own internal friction of the piston rings, valve springs on the cam, bearings, oil, crankcase windage, etc.

Jake brakes are mechanically blocking a port to create vacuum, engine braking in a normal car is simply the engine compressing air on one of every four strokes each piston but not doing anything with it.

That’s why jake brakes exist, because while engine braking will slow down a heavy truck, it’s not enough drag to prevent acceleration on it’s own, even in a diesel which has incredibly high compression.