RPM drops when coasting

[gmonag]

Originally Posted by dambat

A 20:1 compression results in a terminal temp of 1050C for air at the end of the compression stroke. Assuming it is cooled to 500C, this results in some 80% of the work being lost to coolant, and a 2.5L diesel engine providing some 20kW via engine braking at 2000 rpm.

But it is not cooled to 500C. It is an adiabatic process, although not perfect. The losses are not significant to the compression/expansion process.

Diesels do not produce as much engine braking as petrol engines.

[owenfackrell]

Originally Posted by gmonag

But it is not cooled to 500C. It is an adiabatic process, although not perfect. The losses are not significant to the compression/expansion process.

Diesels do not produce as much engine braking as petrol engines.

Completely agree. Most of the engine braking comes form the petrol trying to pull a vacuum. A petrol engine the throttle controls the amount of air entering the engine and the injection matches this with the amount of fuel. In a diesel you actual control the amount of fuel being out in and there is no real restrictions in the air intake. This is also the reason that diesels have to have a vacuum pump for the brake servo and petrols rely on the vacuum at the inlet manifold on a closed throttle.
In really old diesel, to stop the. You quite literally pulled a lever that closed of the air intake and if you released it too soon the engine would catch again and keep going. They had mechanical fuel pumps.

[Tannaton]

Hmmm.... contrary to what I thought....

https://en.m.wikipedia.org/wiki/Engine_braking

[Bashy]

That's where I got the info from....

Diesel engines Edit
Diesel engines in personal cars provide little engine braking as they are not equipped with a throttle body and thus cannot draw a vacuum in the intake manifold.

In heavy vehicles the engine is often made to provide extra braking power to take some strain off the vehicle's regular brake system, and to help avoid it getting too hot. In its simplest form this consists of a butterfly valve that stops the exhaust flow. This is referred to as an exhaust brake and mostly found on older trucks. It has a limited effect, and more advanced systems as described below are near universal on newer heavy vehicles.

[moncman]

Lots of interesting chat about engine braking but I'm sure you would like to hear if the problem/artefact was resolved.

Wheel bearing has been replaced and had nothing to do with the ABS fault. The ABS sensor ring (toothed ring on the CV joint assembly) on the other side was the culprit. All repaired now and codes cleared. The car is driving as normal.

So, in summary, it appears that the TC lock-up function is indeed disabled when the ABS detects a fault and the anti-skid disabled and DSTC off messages are displayed.

Thanks for all your posts.