Engine braking on diesels

[thinfourth2]

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

[mrmr96]

P I Staker said:

I don't mean to say you're wrong but why do my experiences and those of others not back this up?

Gear ratios.

[PlugUgly]

I feel like I should be quite well positioned to answer this, the Pug 206 HDI van I drive for work feels like it has has a LOT more engine braking than my petrol Fiesta. Both are 1.4s, both are broadly the same size, weight and shape, the 206 has longer gearing than the Fiesta. So anecdotally diesels (or at least turbo diesels) should have greater engine breaking.

[Urban Sports]

Just google it. Although that isn't where I got my information from.

[Urban Sports]

mrmr96 said:

sparkybean said:

Any energy spent compressing the air in the cylinder is returned after tdc.

Unless it's released... using a jake brake.

Who wants to send their kids to school thinking they are going to be machine gunned?

[P I Staker]

mrmr96 said:

Gear ratios.

The final drive in the Suzuki is 5.12, the Landy is 3.54, Suzuki 1st gear is 3.78 Landy 1st gear is 3.692, Suzuki high box is 1.00, Landy high box is 1.410 ...

So the higher the ratio the more engine braking? This means the Land Rover should worse engine braking then the Suzuki when you take in to account the diesel engine?

Or have I got the stty end of the stick.

[Urban Sports]

PlugUgly said:

I feel like I should be quite well positioned to answer this, the Pug 206 HDI van I drive for work feels like it has has a LOT more engine braking than my petrol Fiesta. Both are 1.4s, both are broadly the same size, weight and shape, the 206 has longer gearing than the Fiesta. So anecdotally diesels (or at least turbo diesels) should have greater engine breaking.

Yes, from the information you have given you are indeed very well informed

[cpas]

So all the engine braking force on a petrol car is 'held back' by the tine butterfly valve in the carburettor. In all the petrol engined cars I've ever owned, this could simply be opened with one finger so I can't imagine the suction involved would not also open it!

[P I Staker]

cpas said:

So all the engine braking force on a petrol car is 'held back' by the tine butterfly valve in the carburettor. In all the petrol engined cars I've ever owned, this could simply be opened with one finger so I can't imagine the suction involved would not also open it!

The butterfly valve is usually in the throttle body these days, you try putting two fingers either side of the valve and apply equal pressure.

[mrmr96]

cpas said:

So all the engine braking force on a petrol car is 'held back' by the tine butterfly valve in the carburettor. In all the petrol engined cars I've ever owned, this could simply be opened with one finger so I can't imagine the suction involved would not also open it!

When you opened it with your finger did you push on both sides of the pivot with equal pressure at the same time?

[R300will]

P I Staker 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.

I don't mean to say you're wrong but why do my experiences and those of others not back this up?

learn't to drive in my instructors 1.5 diesel micra new model. It had much more engine braking than my swift 1.3 petrol. its not because of the 0.2l difference there was considerably more braking from the diesel so i would also think along the lines that the compression ratio of the diesel gives it more engine braking?

[mat777]

Ive been thinking somewhat and I now realise that this "vacuum" talk is misleading.

Correct me if I am wrong, but a diesel engine draws in the same amount of air in every induction stroke, the only thing controlling engine speed is how much fuel is injected each time. Ergo, the engine braking is the piston compressing all or mainly air, which doesnt combust so provides no power on the power stroke, at the massively high compression ratio.

[Urban Sports]

Let's not let fact get in the way of speculation though fellas eh!

[P I Staker]

Urban Sports said:

Let's not let fact get in the way of speculation though fellas eh!

Try explaining properly. Apparently its down to gearing, I have an example and would like it explained as currently I do not fully understand what you're saying and I would like to.

[wst]

Got to say the issue seems less clear to me now than it did when I made that initial comment to start the thread off. All the diesel I've driven has been annoyingly un-engine-braked, while the petrol stuff has been pretty well engine braked. Anyone here an engine surgeon with real knowledge?

[GC8]

Magictrousers said:

mrmr96 said:

Unless it's released... using a jake brake.

Only real explanation, especially from relative speed and with a relative bulk to slow.

Not really. Jacobs brakes are rare, most 'Jake brakes' are actually exhaust brakes. A good exhaust brake slow you down enough on a race track, but they can lift you out of your seat in an unladen lorry, if you forget that its turned on.

[GC8]

Eighteeteewhy said:

WTF are Jake brakes?

Google "Jacobs brake". 'Jake brake' is to auxiliary brakes what 'Hoover' is to vacuum cleaners, and most aux brakes referred to as 'jake brakes' arent Jacobs brakes, theyre exhaust brakes, which work by constricting the exhaust, providing substantial engine braking.

[Gooly]

AFAIK a diesel engine in theory will have hardly any engine braking, however a turbo-diesel will have some engine braking caused by the turbo creating back pressure in the exhaust, leading to the same sort of effect as a closed throttle. There aren't many non-turbo-diesels about nowerdays hence why the majority of people are associating engine braking with their experience with diesels.

I've driven a diesel corsa and a petrol fiesta (Among other cars, but these are the closest to compare) and the Fiesta has much more significant engine braking. Granted the Corsa is heavier with slightly different gear ratios, but the effect still remained, IMO at least.

[Urban Sports]

GC8 said:

Eighteeteewhy said:

WTF are Jake brakes?

Google "Jacobs brake". 'Jake brake' is to auxiliary brakes what 'Hoover' is to vacuum cleaners, and most aux brakes referred to as 'jake brakes' arent Jacobs brakes, theyre exhaust brakes, which work by constricting the exhaust, providing substantial engine braking.

And sound like this

http://www.youtube.com/watch?v=WOKf9lqD1-8

[GC8]

Ive only watched the first two, but they appear to be Jacobs brakes. Exhaust brakes alter the exhaust tone, but they dont really make it any louder.

At the risk of repeating myself: virtually every auxiliary referred to as a 'jake brake' isnt actually a Jacobs brake, its an exhaust brake.