Engine braking on diesels

[mat777]

The following is a snippet from the 80s Racing Trucks thread:

wst said:

mat777 said:

wst said:

mat777 said:

wst said:

Wait, how... how the hell does he drive 15 laps with no brakes without going around at a snail's pace? That sounds amazing.

engine braking?

Didn't think diesels had that much engine braking, unless the racing trucks are all fitted with jake brakes!

wait, what?????

Diesel engines have a massive amount of engine braking, many times more than petrol. Think of the compression ratio

No throttle. No vacuum. Nothing to inhibit the engine from turning. They have pretty much no engine braking...

Seriously, WTF? According to this, a diesel engined vehicles has no engine braking if you lift off the throttle. Hmmmmm.... every single bit of off roading I'ev ever seen, not to mention my own experiences learning to drive in a diesel, are wrong then? As is the physics of compressing a charge in the cylinder with the valves closed, even if there is very little fuel in it?

[chris1roll]

He's right.


Having started driving in cars with rubbish brakes, I learnt to use engine braking a lot, and still do it.
On getting my first diesel car I noticed the comparative lack of engine braking compared to my petrol cars.


EDIT to reassure myself I'm not wrong: http://en.wikipedia.org/wiki/Engine_braking

[sparkybean]

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

[mrmr96]

sparkybean said:

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

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

[Magictrousers]

mrmr96 said:

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

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

[thinfourth2]

sparkybean said:

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

No it isn't

It gets bloody hot and that heat goes into the water jacket

Also you are assuming the compression stroke is the same length as the power stroke

[mat777]

So surely that means that every time I or everyone else goes "feet off" in a Defender off road, it is defying what it should do by crawling down the hill with increased revs from idle, instead of shooting off the don the hill at idle?
No to mention every off road driving manual I've ever read being wrong in stating that "the engine braking effect in a diesel 4x4 will be greater than that in a petrol, in some cases requiring a higher gear to be selected for hill descents"?

Or am I missing something?

[thinfourth2]

mat777 said:

Or am I missing something?

yes diesels are worse at everything unless they come from BMW

[Patrick Bateman]

I was sure my old 2 litre diesel compact and the vectra that preceded it had greater engine braking than my 330ci.

[wst]

Oh of course there is a bit of engine braking from inefficiencies in engineering (or we'd have defied some basic physical laws) but on the whole most of the pressure produced by the compression stroke is relieved when the cylinder reaches TDC by pushing the piston down again. I drive a van far too much and that thing needs me to brake on downhills where in my petrol vehicle I would just drop a cog and let the engine hold the speed with the vacuum.

Varies a bit possibly depending on how the engine handles 'off-right-pedal' as well. Some vehicles might have magical gubbins that do extra things to artificially induce a 'throttle-closed'-like drag on the engine or so on.

Tbh on that thread I was wondering how engine braking of any sort would be good enough to race against people with fully functioning brakes but that's a matter for that thread ain't it?...

[tercelgold]

Motorcycles have huge engine braking

[-Pete-]

Last time this came up I argued that my diesel definitely had engine braking but a few years later I realize it doesn't really, compared to a petrol engine. It stops accelerating if you lift of, but it's mainly aerodynamics that are slowing it down.

[chris1roll]

mat777 said:

So surely that means that every time I or everyone else goes "feet off" in a Defender off road, it is defying what it should do by crawling down the hill with increased revs from idle, instead of shooting off the don the hill at idle?
No to mention every off road driving manual I've ever read being wrong in stating that "the engine braking effect in a diesel 4x4 will be greater than that in a petrol, in some cases requiring a higher gear to be selected for hill descents"?

Or am I missing something?

Gear ratios?

[P I Staker]

mat777 said:

So surely that means that every time I or everyone else goes "feet off" in a Defender off road, it is defying what it should do by crawling down the hill with increased revs from idle, instead of shooting off the don the hill at idle?
No to mention every off road driving manual I've ever read being wrong in stating that "the engine braking effect in a diesel 4x4 will be greater than that in a petrol, in some cases requiring a higher gear to be selected for hill descents"?

Or am I missing something?

I'm with you on this as well.

My Suzuki (1.6 Petrol) has very little engine braking and compared to my Dads Defender TD5 it has next to none.

Its not uncommon to swap diesel engines in to Suzukis, I know of someone who did this and raised the final drive to lower the revs at cruising speed and the engine braking is still much improved over standard.

[Eighteeteewhy]

WTF are Jake brakes?

[The Wookie]

Quite a lot of diesels actually have throttles, although they don't need them as part of the combustion cycle...

[TheEnd]

Eighteeteewhy said:

WTF are Jake brakes?

http://www.youtube.com/watch?v=kl3NLWrj_sA

I keep expecting to see KnightRider or Airwolf appear in that.

[tercelgold]

Eighteeteewhy said:

WTF are Jake brakes?

Basically it's about the children, jake brakes are banned in most states from the effects they cause.

[Urban Sports]

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.

[P I Staker]

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?