Engine braking on diesels

[Nick1point9]

Mr-B said:

AJB said:

FWIW, the only diesel I've owned didn't have as much engine braking as the petrols I've owned. Having said that, it was a big heavy engine (and that's where the flywheel weight comes in too), so there was a LOT of engine braking as you brought the clutch up in a lower gear and the engine had to spin up. But that's not really engine braking... Once the engine was up to speed, there wasn't much at all.

Just been out in mine and this ^ just about sums it up. Drop a gear and you get an initial jolt of deceleration but after that not much, I guess less than a petrol engine. This is with 3 litre turbo diesel.

Too many variables to say one car does this one car does that. I can make subjective unsubstantiated comments too that my wife's 1.6 petrol has virtually no engine braking whereas my 1.9 tdi has "lots", significantly more than the 3.2l petrol it replaced.





I didn't intend to get into an argument over whether petrols or diesels engine brake more of less, simply that diesels do engine brake, for the reasons I stated earlier.

[Otispunkmeyer]

thinfourth2 said:

Urban Sports said:

The throttle on a petrol car closes the air and fuel intake via a butterfly therefore throttles the engine of air and fuel which gives higher vacuum and maximum engine braking, the accelerator pedal on a diesel shuts off the fuel to the engine leaving the air intake open. On a closed accelerator pedal the engine has no fuel but still has air, thus the engine braking is not as great as a petrol engine.

So with the throttle closed the engine will have almost a complete vacuum in the bores.

So what is causing the engine braking as surely the vacuum would pull the piston back up

I'm not saying that you are wrong as owning a V8 petrol land rover 90 and a diesel land rover 90 I find far more engine braking with the diesel

But i must be wrong as petrols are better at everything

Well piston goes down, valves open. Butterfly valve shut, vacuum pulled. Valves shut, piston near BDC. So now there is a vacuum trapped in the cylinder. Compressing a vacuum requires no effort, expanding it again likewise, exhaust valves open and pressure equalises and pushes what exhaust enters back out and cycle repeats. So I can't see anywhere for the vacuum to pull the piston back up.

I wonder if I can get some one in thee test cells here to do me a pumping loop graph taken off both engine types under no throttle. Also the pressure traces.

[thinfourth2]

Otispunkmeyer said:

Well piston goes down, valves open. Butterfly valve shut, vacuum pulled. Valves shut, piston near BDC. So now there is a vacuum trapped in the cylinder.

Correct up to this point

Otispunkmeyer said:

Compressing a vacuum requires no effort, expanding it again likewise, exhaust valves open and pressure equalises and pushes what exhaust enters back out and cycle repeats. So I can't see anywhere for the vacuum to pull the piston back up.

You don't compress a vacuum the vacuum will pull the piston up as your sump will be at atmospheric pressure

[thinfourth2]

Well i carried out a little experiment with my car which is a petrol and has this old fashioned thing called a throttle cable.


Heading down a hill and switch off the engine so getting max engine braking.

Open the throttle and bugger all difference to the amount of braking compared to a closed throttle

[NRS91]

i got bored of reading all of this after half a page as you were mainly all confusing each other. so if im covering sumthing already mentioned i appologise.

petrols have engine braking through the intake vacuum when the butterfly is closed.
diesels have engine braking due to the high compression (18-20:1 compared to 8-14:1 in petrols) and also due to the leverage effect of the longer throw crank.

quite alot of UK trucks have jake brakes (cummins engines mainly IIRC) but also exhaust brakes feature greatly too just depends on the engine tbh. (cummins on jake with a load sounds just amazing on twin straight pipes)

when i let off the throttle in my landy (6l cummins) its almost as much braking force as the brakes supply (cant decide if its how strong the engine is or how crap landy brakes are :L)