How to drive a diesel

[gazza12345]

Originally Posted by Jim314

Cutting off fuel does not cause engine braking. It is the air pumping resistance on the motion of the pistons that causes engine braking. Diesels, due to no restriction of intake air, don't have any pumping resistance. The lack of pumping resistance is one of the reasons diesels are more efficient than petrol engines.

The Land Rover Defender must have an automatic control system which detects a downhill attitude and automatically applies the friction brakes. Alternatively, the Defender may have an exhaust restriction or special hill descent throttle which engages and provides engine braking.

I feel you have contradicted your self.
(Cutting off fuel does not cause engine braking: yes it does.)
(It is the air pumping resistance on the motion of the pistons that causes engine braking: Thats right.)
(Diesels, due to no restriction of intake air, don't have any pumping resistance: Yes they do.)
(The lack of pumping resistance is one of the reasons diesels are more efficient than petrol engines: Thats not true.)
A diesel engine has a higher compression ratio & no air intake restriction. A petrol engine as a lower compression ratio & air intake restriction. Let me explain. Due to no restriction of intake air in a diesel engine it is still sucking & filling the cylinders up with air when coasting. So it as to compress this air without fuel. But a petrol engine as air intake restriction. so there's not much air to compress when coasting.
Diesel as a bigger bang per drop of fuel then petrol.
Diesels run at high turbo boost then petrols.
Diesel pumps run at around 1700bar/25000psi. This is to atomize fuel better so it gives a better burn without wasting fuel.

[Jim314]

Don't take my word for it. Read up on this and you will see that I did not figure this out myself. Don't take my comments personally--I am 68 years old and have learned a thing or two, and I admit that I can be wrong and occasionally am wrong.

The newer diesels are turbocharged with variable vane turbochargers. This can provide exhaust restriction which provides engine braking.

Comparing engine braking in naturally aspirated petrol and diesel engines:

Naturally aspirated petrol engines have strong engine braking due to the vacuum in the intake manifold resisting the intake stroke. Diesels have no resistance on the intake stroke so no engine braking. There is energy consumed by movement against the resistance on the compression stroke, but this energy is mostly recovered in the expansion stroke (i.e. the power stroke when fuel is injected). In the exhaust stroke there is no energy generated or consumed (unless the variable vane turbocharger is resisting).

Read up on it:

http://en.wikipedia.org/wiki/Engine_braking


http://en.wikipedia.org/wiki/Diesel_engine

[wimorrison]

Engine braking on any engine is performed by more energy being expended to compress the combustion chamber gases than is provided from the ignition stroke. This applies whether it is a compression ignition or spark ignition engine and is not a binary situation. At very light throttle very little fuel is injected into either engine type, whilst the volume of air taken is remains the same - the result is that more power is used compress this weak mixture than you get when it ignites. The method of aspiration, normal, turbo or compressor has minimal effect on the engine braking performance.

Exhaust braking is often used on heavy goods vehicles where the exhaust gases are restricted thus placing preventing the engine from fully purging the cylinder on the exhaust cycle, thus reducing the air intake on the induction stroke, and consequently reducing the power from the engine - this gives a mechanical brake to the engine by it needing to compress exhaust gases and no power from the ignition stroke - note this is not applied to spark ignition engines.