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

Question

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

Answer 1

**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.