Engine braking on diesels

[fangio]

And you don't switch it on, it's a spring-loaded button on the floor, pressed with a foot when needed.

[GC8]

A Jacobs brake, you mean?

[redtwin]

fangio said:

And you don't switch it on, it's a spring-loaded button on the floor, pressed with a foot when needed.

That spring loaded button on the floor is still a switch though, but understand the point you are trying to make, most exhaust brakes (or I should say, the ones I have driven) use a spring loaded floor switch.

I have driven with Jacobs brakes and with an exhaust brake. The Jacobs brake is significantly more effective when on the maximum setting than the exhaust brake, considerably louder too. The one I drove had 3 settings which would activate the valves on 2, 4 or all 6 cylinders.

Drivers of the Jacobs equipped truck were always warned to switch off the Jacobs when driving on wet roads or without laden trailer attached as it would lock up the rear wheels with ease. I could certainly understand a truck operating (not being competitive, mind) on a racetrack using Jacobs only as technically the service brakes would only be needed to come to a complete stop.

[StottyZr]

My 1.5d had rediculous amounts of engine braking, my new diesel has virtually none whatsoever.

Weird eh.

[fangio]

GC8 said:

A Jacobs brake, you mean?

Sorry, exhaust brakes!

And none I ever drove had a dashboard switch.

[GC8]

Conversely I havent ever driven a vehicle with a worthwhile exhaust brake which didnt have a dash or stalk mounted switch...

[Nick1point9]

Oh for god's sake this comes up every couple months.

Drawing air against a vacuum is one element of engine braking, which you have in a petrol but not a diesel.

Some of the others are as follows, which both engines have:

pumping losses: energy put into compressing the air in the cylinders heats the air up and some of this energy is lost. This means that the downward stroke as the compressed gas (but not burning fuel) expands is delivers less torque than the cylinders in compression, so there is net negative torque. This has a bigger effect in diesels because of higher compression and more air in the cylinder.

engine friction: pretty straight forward. net negative torque.

parasitic losses: oil pumps, alternators, water pumps, air con, brake servo vac pumps all require drive from the engine, so there will be net negative torque.



So yes, of course a diesel engine will engine brake off throttle.

And because of the pumping losses associated with compressing nigh on full cylinders, engine braking is normally greater in a diesel than a petrol.

[P I Staker]

Thank you.

[supersingle]

Nick1point9 said:

Oh for god's sake this comes up every couple months.

Drawing air against a vacuum is one element of engine braking, which you have in a petrol but not a diesel.

Some of the others are as follows, which both engines have:

pumping losses: energy put into compressing the air in the cylinders heats the air up and some of this energy is lost. This means that the downward stroke as the compressed gas (but not burning fuel) expands is delivers less torque than the cylinders in compression, so there is net negative torque. This has a bigger effect in diesels because of higher compression and more air in the cylinder.

engine friction: pretty straight forward. net negative torque.

parasitic losses: oil pumps, alternators, water pumps, air con, brake servo vac pumps all require drive from the engine, so there will be net negative torque.



So yes, of course a diesel engine will engine brake off throttle.

And because of the pumping losses associated with compressing nigh on full cylinders, engine braking is normally greater in a diesel than a petrol.

It all depends.

A petrol engine running throttle bodies will have massive engine braking due to the small volume of air between throttle plates and cylinders. The engine draws big vacuum which leads to big pumping losses and engine breaking (and big fuel consumption).

An engine with a large plenum chamber will have much lower engine braking and better mpg.

Edited by supersingle on Monday 19th March 18:36

[thinfourth2]

Nick1point9 said:

Oh for god's sake this comes up every couple months.

Drawing air against a vacuum is one element of engine braking, which you have in a petrol but not a diesel.

Some of the others are as follows, which both engines have:

pumping losses: energy put into compressing the air in the cylinders heats the air up and some of this energy is lost. This means that the downward stroke as the compressed gas (but not burning fuel) expands is delivers less torque than the cylinders in compression, so there is net negative torque. This has a bigger effect in diesels because of higher compression and more air in the cylinder.

engine friction: pretty straight forward. net negative torque.

parasitic losses: oil pumps, alternators, water pumps, air con, brake servo vac pumps all require drive from the engine, so there will be net negative torque.



So yes, of course a diesel engine will engine brake off throttle.

And because of the pumping losses associated with compressing nigh on full cylinders, engine braking is normally greater in a diesel than a petrol.

Ah but the rules state that petrol must be better then diesel and diesel is better then electric

So the worst engine braking you can get is an EV in full regen mode, then a big sod off diesel and by far the best is a little petrol engine

[Gooly]

Also depends on how heavy/light the flywheel is, regardless of engine type.

[Nick1point9]

supersingle said:

Nick1point9 said:

Oh for god's sake this comes up every couple months.

Drawing air against a vacuum is one element of engine braking, which you have in a petrol but not a diesel.

Some of the others are as follows, which both engines have:

pumping losses: energy put into compressing the air in the cylinders heats the air up and some of this energy is lost. This means that the downward stroke as the compressed gas (but not burning fuel) expands is delivers less torque than the cylinders in compression, so there is net negative torque. This has a bigger effect in diesels because of higher compression and more air in the cylinder.

engine friction: pretty straight forward. net negative torque.

parasitic losses: oil pumps, alternators, water pumps, air con, brake servo vac pumps all require drive from the engine, so there will be net negative torque.



So yes, of course a diesel engine will engine brake off throttle.

And because of the pumping losses associated with compressing nigh on full cylinders, engine braking is normally greater in a diesel than a petrol.

It all depends.

A petrol engine running throttle bodies will have massive engine braking due to the small volume of air between throttle plates and cylinders. The engine draws big vacuum which leads to big pumping losses and engine breaking (and big fuel consumption).

An engine with a large plenum chamber will have much lower engine braking and better mpg.

Edited by supersingle on Monday 19th March 18:36

Well that's not strictly true: the stronger the vacuum the engine is pulling against the less air in the cylinder and therefore lower pumping losses. Obviously this relationship won't be linear, so the two effects don't cancel each other out, but as one increases, the other has to decrease.

If anything, pumping against a vacuum probably doesn't have much of an effect: pumping against a vacuum will make the induction stroke torque negative but the compression stroke will be torque positive, as the low pressure in the cylinder wants to decrease the volume of the cylinder, helping draw up the piston.

And since there would be less heat generated (and therefore lost to the water jacket) when dealing with negative gauge pressures I'd imagine the actual process may in fact be more efficient in terms of net torque.

[Eighteeteewhy]

Urban Sports said:

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

It's like anti-lag for trucks

I guess they're not legal over here? I've never heard them.

[Nick1point9]

Gooly said:

Also depends on how heavy/light the flywheel is, regardless of engine type.

That's a good point to raise. In principle yes but its flywheel inertia (mass x effective radius) not mass that's the important factor (just being picky now). But that is only during accelerating of the engine, i.e. as you drop a gear. In terms of the absolute value of negative torque (engine braking) the engine makes at any one operating point the flywheel plays no part. Flywheels only have a reaction torque when they are being accelerated/decelerated, and so driving down hill at constant speed a flywheel plays no part in engine braking.

In the case originally given though of slowing the car down with engine breaking, yes, a bit flywheel will help whilst engaging a lower gear and accelerating the engine, but once the gear is engaged it will in fact have a detrimental effect, resisting the engine slowing down.

Edited by Nick1point9 on Monday 19th March 22:26

[redtwin]

Eighteeteewhy said:

It's like anti-lag for trucks

I guess they're not legal over here? I've never heard them.

I don't know whether they are legal or not, but they are a 3rd party option and most if not all manufacturers have their own exhaust brake systems that work well enough. I suppose not many people are willing to pay to have it installed on an already expensive truck especially when they are trying to earn an income from that truck. You certainly wouldn't find large company owned fleets paying for it.

Even in the US, it would only be the independent owner/driver trucks or heavy haulage dump (tipper) trucks that needed the extra braking help that you would hear with it. Only one of the 7 trucks I drove had a Jacobs brake and that truck was bought used with it already installed by the previous owner.

[AJB]

Nick1point9 said:

If anything, pumping against a vacuum probably doesn't have much of an effect: pumping against a vacuum will make the induction stroke torque negative but the compression stroke will be torque positive, as the low pressure in the cylinder wants to decrease the volume of the cylinder, helping draw up the piston.

No, I don't think so. The low pressure might make slightly less pumping loss on the compression vs power strokes, but the big difference is that the induction stroke is against a vacuum, whilst the exhaust stroke is against probably about atmospheric. The pumping loss isn't induction vs compression.

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.

[AJB]

thinfourth2 said:

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

Yes, but it cools down again as it expands on the "power" stroke, taking almost all of the heat back out and leaving it not using much energy at all overall.

Also, what engines have different length compression and power strokes? Not doubting you on that one, but I can't picture how it could be without some sort of very complicated mechanical arrangement...

[thinfourth2]

AJB said:

Yes, but it cools down again as it expands on the "power" stroke, taking almost all of the heat back out and leaving it not using much energy at all overall.

Also, what engines have different length compression and power strokes? Not doubting you on that one, but I can't picture how it could be without some sort of very complicated mechanical arrangement...

Cam timing

Not hard, not complicated

[AJB]

thinfourth2 said:

AJB said:

Also, what engines have different length compression and power strokes? Not doubting you on that one, but I can't picture how it could be without some sort of very complicated mechanical arrangement...

Cam timing

Not hard, not complicated

Oh, I see what you mean. I thought you were saying different length in mm rather than in seconds! Yes - if, for example, exhaust opens early then energy will be wasted and you won't get back all that you put in.

[Mr-B]

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.