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From what I am reading here, diesels dump their compression from their cylinders when decelerating faster than a gas? So they have lower compression in the cylinders when decelerating and the gas is higher?
Don
No, diesels don't "dump" compression at all. They use it. The diesel squeezes a full cylinder of air very tightly using crank energy up until top dead center. Then the crank rotates and starts the piston down. The compressed air in the cylinder expands similar to if it had fuel/air explosion - but at obviously much less power, and transfers the majority of the energy back into the crank. Yes, some energy was lost during compression to internal friction, heat conversion, blowby, etc, but the majority of the energy is still in the compressed air. Hence the air-spring. The expansion will continue until bottom dead center or until the valves open. Remember,
nearly all diesels do not have an air-throttled intake tract.
Gassers work exactly the same under Wide Open Throttle. Try coasting down a hill with the ignition off - and see whether there is more engine braking with the gas pedal down or up (providing you don't have throttle-by-wire). Open throttle will have less engine braking. It's compressing more air, but it is not fighting vacuum behind the throttle plate, and uses the air-spring effect. Close the throttle, and there's more engine braking.
The vacuum created by the closed throttle will offer a large amount of resistance to the engine (braking) and the partial charge in the cylinder will compress easily, and will not spring back with any force, further slowing the rotation of the crank.
Do diesels have engine braking? Yes. Do gassers have engine braking? Yes. All other things equal (weight, rpm, valve timing, etc.) the gasser will out engine-brake a diesel every time. Any time one uses engine braking, just watch the tach, and keep the revs reasonable.
Of course, if the TDI has clogged intake runners, it will have more engine braking than a clean TDI engine. That's another story, with the carbon deposits effectively air-throttling the engine.
Read up on the Jacobs Brake (Jake Brakes or compression brakes) if you do not believe me.
The compression-brake principle is to open the exhaust valve or supplementary valve to dump the air compressed in the cylinder just as it hits TDC. The compression has slowed the engine a little, and after the valve opens, the air-spring is gone, and now the piston travels down under the vacuum of closed valves, creating a new phase of engine braking.
The byproduct of the release of the compressed air suddenly is the expansion of the air released and what causes the extra noise under braking. That noise is quite substantial, and should indicate the amount of power lost purposefully. Also why there are signs on the outskirts of town telling truckers that use of engine braking or "Jake Brakes" use prohibited.
frank