engine braking effect

[silver343124]

As I understand engine braking effect in an internal combustion pistons engine it is due to following things:
-friction between pistons and cylinder walls
-throttle losses because of the back pressure effect
-losses durring compresion stroke when air is compressed and thus warmed

in diesel engine there is no throttling losses, but the braking effect is even biger because of higher compression ratio. The proof is gasoline engine, when key is off - there is no difference between full and none throttle.

But how is with so called air-spring effect. Is this because some gases gets below pistons, during power and compression stroke and thus affect as air spring, lowering the engine braking.

The question is also if the engine has some kind of release of the gases below pistons.

[curtis73]

I haven't really heard it referred to as the air-spring effect, but since air is compressible, its a good analogy.

There is a small difference in gasoline engines when the key is off between full throttle and none, but its almost unmeasurable. The amount of small vacuum being pulled by the closed throttles is very insignificant compared to a 3000-lb car tugging on the pistons. The big thing is that you still have a compression stroke without the benefit of a power stroke to offset the pump's work.

Some gasses get below the pistons because rings don't seal perfectly, but in a properly running engine, very little gets past the rings. There is no specific air bleed to let air get below the pistons, but since its not a perfect seal, a tiny bit does.

[UncleBob]

can't speak for diesel engines, but gas engines, naturally asperated, the negative hp (engine braking) is roughly equal to 10-15% peak, of the peak hp at WOT

This is mostly to do with the cc of the engine,the CR of the engine, and the cam shaft. IE, the more efficient air pump or larger air pump you have, the more efficient engine braking you have.

I would assume a diesel (minus jake brakes, or some version of it) would follow simular rules

[534BC]

The reason most of you realise a minimum engine braking difference between WOT and closed throttle (engine off or on) is because the throttle plates are open so far. If the idle was slowed down and not allowed to open during decel then the braking would be very noticeable.

New cars may hold the plates open further than the idle stop screw and defeats most all engine braking, older cars sometime require more spring to close the plates at higher rpm and vacuum. It depends quite a bit on butterfly offset, but if you were to go down a hill with the plates closed you'd feel braking that is better than any exhaust butterfly brake and rivals many jake type brakes.

A gage in the intake between valves and throttle plates will tell you much during engine decel. If it's not over 20"-25" then almost no braking is realised.

[silver343124]

Originally Posted by curtis73

I haven't really heard it referred to as the air-spring effect, but since air is compressible, its a good analogy.

from http://en.wikipedia.org/wiki/Engine_braking 3rd chapter, last paragraph

"The exhaust brake is used in large diesel vehicles because the rate of conversion of mechanical energy into waste thermal energy is low compared to the mechanical returns to kinetic energy from the air-spring effect in the engine."

[KiwiBacon]

It's probably better to replace "throttling losses" with "pumping losses". Where pumping losses include throttling losses too.
Your average diesel, not having a throttle, has lower pumping losses.

The so called "air spring effect" is still a big waster of energy (as you want it to be in an engine braking situation), bigger in engines with higher compression ratios (try hand cranking a diesel compared to a similar size petrol).

I have heard people claim that petrols offer a better engine brake because of the closed throttle. But in my experience driving diesels that's nowhere near true. Exhaust brakes are fitted to diesel vehicles not because the engine has less engine braking, but because diesel engines are fitted to heavier vehicles which need alternative braking (engine and additional retarders) to avoid overloading and overheating the wheel brakes.

[534BC]

My experience has been opposite, running same trucks, weights, hills, speeds, engines, trans, gears, makeing hundreds of trips.

Notice the reason that most users will not notice an increase in "engine braking" with a throttle plate. It's pretty easy to tell when going down a long constant hill with the key turned off while pushing throttle open and closed. The braking can be felt easily as long as the throttle will actually close (ie slower idle and stiff spring makes a huge difference) as just a 1/4-1/2 turn on an idle screw will negate most of the braking realised.

Here's some things that don't seem to make a difference, fuel, ignition,ect.

Compression, rpm, size,load makes quite a difference along with the amount of throttle plate opening. Cutting thru all the variables seems to indiate that if one can achieve 30" of vacuum or more the compressin braking will be realised. Any component that makes this figure go down will negate the braking effect. Same is true on the exhaust side when talking about and exhaust butterfly brake. (anything that makes the pressure go down negates the braking effect)