Diesel Engine Braking - Reasons for an increase?
Discussion
All,
I'll try to keep it brief....I understand from reading around on here that peoples opinions on diesel vs petrol engine braking vary but can anyone explain why a turbo diesel may suffer an increased amount over that what there should be?
I ask this question as I think my current car ( an Alfa 1.9 jtd) is suffering from excessive engine braking of some sort. The car seems to slow down excessively following release of the throttle possibly having a negative effect on fuel economy as the car cannot coast in gear as easily.
I have been running a 2010 Megane 1.9dci for quite a while and this seems to have alot less engine braking. The 320d and 118d before this also seemed to have alot less.
My thoughts at the moment are that it may vary between different turbo diesels or it could be exhaust blockages or issues with variable vanes in the turbo ( though I expect the car would have issues with high/low down power etc if this was the case).
Any ideas?
I'll try to keep it brief....I understand from reading around on here that peoples opinions on diesel vs petrol engine braking vary but can anyone explain why a turbo diesel may suffer an increased amount over that what there should be?
I ask this question as I think my current car ( an Alfa 1.9 jtd) is suffering from excessive engine braking of some sort. The car seems to slow down excessively following release of the throttle possibly having a negative effect on fuel economy as the car cannot coast in gear as easily.
I have been running a 2010 Megane 1.9dci for quite a while and this seems to have alot less engine braking. The 320d and 118d before this also seemed to have alot less.
My thoughts at the moment are that it may vary between different turbo diesels or it could be exhaust blockages or issues with variable vanes in the turbo ( though I expect the car would have issues with high/low down power etc if this was the case).
Any ideas?
Could be a flywheel issue, dual mass flywheel stuck in one position would make a difference if it has changed since you drove it before. Alfa probably setup to be a bit more sporty so might have a lighter flywheel and lower gearing to start with and show more engine braking as a result.
The following may or may not help you... though I wrote it to try and help either way :-).
What is "engine braking" in a petrol engined car?
When you lift the throttle pedal, in a petrol car the throttle disc closes, basically blocking off the inlet tract, meaning the motion of the piston moving down the cylinder (on the suck part of the 4stroke)now is trying to fight to suck a full cylinder of air through the tiny closed throttle disc or through the little idle speed control valve 'throttle' opening. Imagine holding a syringe, putting your finger over the end and then trying to pull the plunger up. Much more resistance than if the end of the syringe was not blocked. This is why you get a good bit of braking on a petrol engine when you close the throttle and at the higher the rpm, the more times this is happening per second, meaning the more braking you get
What is "engine braking" in a diesel?
When you release the throttle pedal, all that happens is that the level of fuel flowing into the engine reduces. The ability of air to fill the cylinders remains roughly the same as the inlet tract doesn't become blocked as there is no throttle. It is wide open at all times. So the piston moving down the cylinder can still suck as much air as it wants. Thus you get less engine braking. This also means it will still have lots of air to compress on the "squeeze" part of the 4stroke cycle, so I guess there is still some resistance there but it seems to be much less. Plus diesels have longer gear ratios so the engine tends to be doing less rpm than a petrol equivalent, so the resistance is happening less times per second, thus less braking.
SO: as for what makes you see an increase. I guess the question is an increase compared to what? Compared to the same car on another morning? Compared to a petrol car? Compared to another car that your mate has that is the same and so on?
Interesting topic.
Diesel bus and some trucks I guess, have a pedal that blocks the exhaust (you use this when off the throttle and wanting to do some light braking, e.g. on downhills to stop it running away from you), so that the engine is fighting more compression and this creates more resistance to the 4stroke cycle, which creates engine braking.
I "guess" some diesel cars could have an automatic equivalent nowadays to this exhaust blockage, but didn't believe I had ever heard of it. Dual mass flywheel chat is interesting, but I didn't think they worked like that.
Cheers,
Steven RW
What is "engine braking" in a petrol engined car?
When you lift the throttle pedal, in a petrol car the throttle disc closes, basically blocking off the inlet tract, meaning the motion of the piston moving down the cylinder (on the suck part of the 4stroke)now is trying to fight to suck a full cylinder of air through the tiny closed throttle disc or through the little idle speed control valve 'throttle' opening. Imagine holding a syringe, putting your finger over the end and then trying to pull the plunger up. Much more resistance than if the end of the syringe was not blocked. This is why you get a good bit of braking on a petrol engine when you close the throttle and at the higher the rpm, the more times this is happening per second, meaning the more braking you get
What is "engine braking" in a diesel?
When you release the throttle pedal, all that happens is that the level of fuel flowing into the engine reduces. The ability of air to fill the cylinders remains roughly the same as the inlet tract doesn't become blocked as there is no throttle. It is wide open at all times. So the piston moving down the cylinder can still suck as much air as it wants. Thus you get less engine braking. This also means it will still have lots of air to compress on the "squeeze" part of the 4stroke cycle, so I guess there is still some resistance there but it seems to be much less. Plus diesels have longer gear ratios so the engine tends to be doing less rpm than a petrol equivalent, so the resistance is happening less times per second, thus less braking.
SO: as for what makes you see an increase. I guess the question is an increase compared to what? Compared to the same car on another morning? Compared to a petrol car? Compared to another car that your mate has that is the same and so on?
Interesting topic.
Diesel bus and some trucks I guess, have a pedal that blocks the exhaust (you use this when off the throttle and wanting to do some light braking, e.g. on downhills to stop it running away from you), so that the engine is fighting more compression and this creates more resistance to the 4stroke cycle, which creates engine braking.
I "guess" some diesel cars could have an automatic equivalent nowadays to this exhaust blockage, but didn't believe I had ever heard of it. Dual mass flywheel chat is interesting, but I didn't think they worked like that.
Cheers,
Steven RW
Steven_RW said:
The following may or may not help you... though I wrote it to try and help either way :-).
What is "engine braking" in a petrol engined car?
When you lift the throttle pedal, in a petrol car the throttle disc closes, basically blocking off the inlet tract, meaning the motion of the piston moving down the cylinder (on the suck part of the 4stroke)now is trying to fight to suck a full cylinder of air through the tiny closed throttle disc or through the little idle speed control valve 'throttle' opening. Imagine holding a syringe, putting your finger over the end and then trying to pull the plunger up. Much more resistance than if the end of the syringe was not blocked. This is why you get a good bit of braking on a petrol engine when you close the throttle and at the higher the rpm, the more times this is happening per second, meaning the more braking you get
What is "engine braking" in a diesel?
When you release the throttle pedal, all that happens is that the level of fuel flowing into the engine reduces. The ability of air to fill the cylinders remains roughly the same as the inlet tract doesn't become blocked as there is no throttle. It is wide open at all times. So the piston moving down the cylinder can still suck as much air as it wants. Thus you get less engine braking. This also means it will still have lots of air to compress on the "squeeze" part of the 4stroke cycle, so I guess there is still some resistance there but it seems to be much less. Plus diesels have longer gear ratios so the engine tends to be doing less rpm than a petrol equivalent, so the resistance is happening less times per second, thus less braking.
SO: as for what makes you see an increase. I guess the question is an increase compared to what? Compared to the same car on another morning? Compared to a petrol car? Compared to another car that your mate has that is the same and so on?
Interesting topic.
Diesel bus and some trucks I guess, have a pedal that blocks the exhaust (you use this when off the throttle and wanting to do some light braking, e.g. on downhills to stop it running away from you), so that the engine is fighting more compression and this creates more resistance to the 4stroke cycle, which creates engine braking.
I "guess" some diesel cars could have an automatic equivalent nowadays to this exhaust blockage, but didn't believe I had ever heard of it. Dual mass flywheel chat is interesting, but I didn't think they worked like that.
Cheers,
Steven RW
Understood! Thanks for the informative post.What is "engine braking" in a petrol engined car?
When you lift the throttle pedal, in a petrol car the throttle disc closes, basically blocking off the inlet tract, meaning the motion of the piston moving down the cylinder (on the suck part of the 4stroke)now is trying to fight to suck a full cylinder of air through the tiny closed throttle disc or through the little idle speed control valve 'throttle' opening. Imagine holding a syringe, putting your finger over the end and then trying to pull the plunger up. Much more resistance than if the end of the syringe was not blocked. This is why you get a good bit of braking on a petrol engine when you close the throttle and at the higher the rpm, the more times this is happening per second, meaning the more braking you get
What is "engine braking" in a diesel?
When you release the throttle pedal, all that happens is that the level of fuel flowing into the engine reduces. The ability of air to fill the cylinders remains roughly the same as the inlet tract doesn't become blocked as there is no throttle. It is wide open at all times. So the piston moving down the cylinder can still suck as much air as it wants. Thus you get less engine braking. This also means it will still have lots of air to compress on the "squeeze" part of the 4stroke cycle, so I guess there is still some resistance there but it seems to be much less. Plus diesels have longer gear ratios so the engine tends to be doing less rpm than a petrol equivalent, so the resistance is happening less times per second, thus less braking.
SO: as for what makes you see an increase. I guess the question is an increase compared to what? Compared to the same car on another morning? Compared to a petrol car? Compared to another car that your mate has that is the same and so on?
Interesting topic.
Diesel bus and some trucks I guess, have a pedal that blocks the exhaust (you use this when off the throttle and wanting to do some light braking, e.g. on downhills to stop it running away from you), so that the engine is fighting more compression and this creates more resistance to the 4stroke cycle, which creates engine braking.
I "guess" some diesel cars could have an automatic equivalent nowadays to this exhaust blockage, but didn't believe I had ever heard of it. Dual mass flywheel chat is interesting, but I didn't think they worked like that.
Cheers,
Steven RW
One thing to consider is that many diesels do actually have a throttle, I think it's mainly for EGR and emissions although in some circumstances it could possibly be because they need vacuum to power something (e.g. brake servo).
Generally though, unless you've got a warning light on, it sounds rough, feels gutless or has poor fuel consumption compared to what is expected for that model then I wouldn't worry about it. It's probably just variation between vehicles.
Generally though, unless you've got a warning light on, it sounds rough, feels gutless or has poor fuel consumption compared to what is expected for that model then I wouldn't worry about it. It's probably just variation between vehicles.
The Wookie said:
One thing to consider is that many diesels do actually have a throttle
Ah interesting. I was wondering if they had advanced to such a setup. Just monitoring fuel flow seemed a bit crude. :-)As for inlet tract stuff. If the airfilter was blocking or collapsing in the inlet tract, OFF throttle, then I can only imagine it would be seriously collapsing on throttle when the turbo is sucking like blazes.
RW
steven RW said:
g may or may not help you... though I wrote it to try and help either way :-).
What is "engine braking" in a petrol engined car?
When you lift the throttle pedal, in a petrol car the throttle disc closes, basically blocking off the inlet tract, meaning the motion of the piston moving down the cylinder (on the suck part of the 4stroke)now is trying to fight to suck a full cylinder of air through the tiny closed throttle disc or through the little idle speed control valve 'throttle' opening. Imagine holding a syringe, putting your finger over the end and then trying to pull the plunger up. Much more resistance than if the end of the syringe was not blocked. This is why you get a good bit of braking on a petrol engine when you close the throttle and at the higher the rpm, the more times this is happening per second, meaning the more braking you get
What is "engine braking" in a diesel?
When you release the throttle pedal, all that happens is that the level of fuel flowing into the engine reduces. The ability of air to fill the cylinders remains roughly the same as the inlet tract doesn't become blocked as there is no throttle. It is wide open at all times. So the piston moving down the cylinder can still suck as much air as it wants. Thus you get less engine braking. This also means it will still have lots of air to compress on the "squeeze" part of the 4stroke cycle, so I guess there is still some resistance there but it seems to be much less. Plus diesels have longer gear ratios so the engine tends to be doing less rpm than a petrol equivalent, so the resistance is happening less times per second, thus less braking.
SO: as for what makes you see an increase. I guess the question is an increase compared to what? Compared to the same car on another morning? Compared to a petrol car? Compared to another car that your mate has that is the same and so on?
Interesting topic.
Diesel bus and some trucks I guess, have a pedal that blocks the exhaust (you use this when off the throttle and wanting to do some light braking, e.g. on downhills to stop it running away from you), so that the engine is fighting more compression and this creates more resistance to the 4stroke cycle, which creates engine braking.
I "guess" some diesel cars could have an automatic equivalent nowadays to this exhaust blockage, but didn't believe I had ever heard of it. Dual mass flywheel chat is interesting, but I didn't think they worked like that.
Cheers,
Steven RW
On a like for like basis, a diesel should have significantly more engine braking than a petrol, as the compression ration is in the order of twice as high, therefore requiring much more work from the crank to compress the intake air.What is "engine braking" in a petrol engined car?
When you lift the throttle pedal, in a petrol car the throttle disc closes, basically blocking off the inlet tract, meaning the motion of the piston moving down the cylinder (on the suck part of the 4stroke)now is trying to fight to suck a full cylinder of air through the tiny closed throttle disc or through the little idle speed control valve 'throttle' opening. Imagine holding a syringe, putting your finger over the end and then trying to pull the plunger up. Much more resistance than if the end of the syringe was not blocked. This is why you get a good bit of braking on a petrol engine when you close the throttle and at the higher the rpm, the more times this is happening per second, meaning the more braking you get
What is "engine braking" in a diesel?
When you release the throttle pedal, all that happens is that the level of fuel flowing into the engine reduces. The ability of air to fill the cylinders remains roughly the same as the inlet tract doesn't become blocked as there is no throttle. It is wide open at all times. So the piston moving down the cylinder can still suck as much air as it wants. Thus you get less engine braking. This also means it will still have lots of air to compress on the "squeeze" part of the 4stroke cycle, so I guess there is still some resistance there but it seems to be much less. Plus diesels have longer gear ratios so the engine tends to be doing less rpm than a petrol equivalent, so the resistance is happening less times per second, thus less braking.
SO: as for what makes you see an increase. I guess the question is an increase compared to what? Compared to the same car on another morning? Compared to a petrol car? Compared to another car that your mate has that is the same and so on?
Interesting topic.
Diesel bus and some trucks I guess, have a pedal that blocks the exhaust (you use this when off the throttle and wanting to do some light braking, e.g. on downhills to stop it running away from you), so that the engine is fighting more compression and this creates more resistance to the 4stroke cycle, which creates engine braking.
I "guess" some diesel cars could have an automatic equivalent nowadays to this exhaust blockage, but didn't believe I had ever heard of it. Dual mass flywheel chat is interesting, but I didn't think they worked like that.
Cheers,
Steven RW
The throttle plate on a petrol engine doesn't make that much difference, as it should have a spring effect that holds it partly open on the overrun, so air can flow in and the suction effect doesn't rip the throttle plate down the intake ports.
The exhaust brake on trucks and buses doesn't 'block' the exhaust as it'd explode! What it actually does is holds the exhaust valve open during what would be the power stroke, so the compressed air doesn't push the piston back down the bore, and the excess pressure is vented off into the exhaust. That's why they make quite a noise.
hobbiniho said:
i think it is to do with the gear ratios rather than the engine itself, with the diesel having "longer" ratios than the petrol equivalent
That has the opposite effect, longer gear ratios reduce the engine braking effect.Edited by Super Slo Mo on Monday 2nd February 20:11
Super Slo Mo said:
On a like for like basis, a diesel should have significantly more engine braking than a petrol, as the compression ration is in the order of twice as high, therefore requiring much more work from the crank to compress the intake air.
The throttle plate on a petrol engine doesn't make that much difference, as it should have a spring effect that holds it partly open on the overrun, so air can flow in and the suction effect doesn't rip the throttle plate down the intake ports.
The exhaust brake on trucks and buses doesn't 'block' the exhaust as it'd explode! What it actually does is holds the exhaust valve open during what would be the power stroke, so the compressed air doesn't push the piston back down the bore, and the excess pressure is vented off into the exhaust. That's why they make quite a noise.
I'm crap at quoting so please excuse me here.The throttle plate on a petrol engine doesn't make that much difference, as it should have a spring effect that holds it partly open on the overrun, so air can flow in and the suction effect doesn't rip the throttle plate down the intake ports.
The exhaust brake on trucks and buses doesn't 'block' the exhaust as it'd explode! What it actually does is holds the exhaust valve open during what would be the power stroke, so the compressed air doesn't push the piston back down the bore, and the excess pressure is vented off into the exhaust. That's why they make quite a noise.
Re the throttle disc not mattering, that's interesting. So if I am at 6,000rpm on wide open throttle, and let go of the throttle, the disc doesn't really shut? It just sort of shuts and the spring effect opens it to feed as much air to the engine as it is trying to suck in at 6,000rpm? I don't quite buy that as on a non turbo application (N/A) I can't see why it would result in any reduction in power. Wouldn't the mega vacuum just open the disc fully resulting in the air going into the inlet manifold and the MAP/MAF sensor just giving it the required amount of fuel? Sounds like we are missing something from this explanation. Maybe its the fuel cut or the fact the disc only springs open a small amount?
I assume you have looked at a vacuum gauge on a car when you lift the throttle pedal at high rpm? It goes from zero psi or bar (not absolute) to negative .9bar, which is nearly a vacuum. That sounds like quite a big vacuum and so far my throttle disc hasn't been sucked down my inlet.
As for an exhaust brake on an truck. I quote "An exhaust brake is typically mounted on the outlet side of the turbocharger and retards the engine's ability to push out or exhaust compression. A butterfly valve in the exhaust brake stays open until it's activated. Then it closes and restricts exhaust flow by keeping it in the cylinder. This causes the piston to force the compression into the exhaust brake, which absorbs the energy [source: Purcell]. A little confusing? Think of it this way: Take a deep breath and hold it in. Now force the air into your mouth and cheeks, but don't let it out. You've just created your own little exhaust brake.
Exhaust brakes don't produce the loud blatting sound for which engine brakes are known. They actually make no sound at all. Exhaust brakes are designed to be used all the time, not just when you need them [source: Purcell]. When used as recommended, they save money by reducing brake service costs and offer added security if you are traveling or pulling a rig in hill country."
Not looking for an argument, more just attempting to learn more about cars etc :-)
Thanks,
Steven
Edited by Steven_RW on Monday 2nd February 16:35
Maybe I'm thinking of engine brake then. They were always colloquially known as exhaust brake, although technically that's a misnomer as it releases pressure on the power stroke.
Yes, the throttle is only held open a bit. I was thinking about it some more, the retardation effect caused by the throttle plate will more or less be cancelled out by the lack of retardation caused by the compression as its compressing a near vacuum so doing very little. Diesels on the other hand don't have this issue.
Also, a throttle plate on a diesel has next to no effect on engine performance as the fuel burn doesn't begin until the moment fuel is injected. That's why 'throttle response' on a diesel isn't anywhere near as good as a petrol.
The thermodynamic cycle for a diesel is quite different to petrol.
Yes, the throttle is only held open a bit. I was thinking about it some more, the retardation effect caused by the throttle plate will more or less be cancelled out by the lack of retardation caused by the compression as its compressing a near vacuum so doing very little. Diesels on the other hand don't have this issue.
Also, a throttle plate on a diesel has next to no effect on engine performance as the fuel burn doesn't begin until the moment fuel is injected. That's why 'throttle response' on a diesel isn't anywhere near as good as a petrol.
The thermodynamic cycle for a diesel is quite different to petrol.
Super Slo Mo said:
Maybe I'm thinking of engine brake then. They were always colloquially known as exhaust brake, although technically that's a misnomer as it releases pressure on the power stroke.
Yes, the throttle is only held open a bit. I was thinking about it some more, the retardation effect caused by the throttle plate will more or less be cancelled out by the lack of retardation caused by the compression as its compressing a near vacuum so doing very little. Diesels on the other hand don't have this issue.
Also, a throttle plate on a diesel has next to no effect on engine performance as the fuel burn doesn't begin until the moment fuel is injected. That's why 'throttle response' on a diesel isn't anywhere near as good as a petrol.
The thermodynamic cycle for a diesel is quite different to petrol.
Surely releasing pressure on the power stroke will reduce the braking effect? Opening the exhaust valve will just allow any pressure built up to freely escape, providing less resistance, thus a freely spinning engine?Yes, the throttle is only held open a bit. I was thinking about it some more, the retardation effect caused by the throttle plate will more or less be cancelled out by the lack of retardation caused by the compression as its compressing a near vacuum so doing very little. Diesels on the other hand don't have this issue.
Also, a throttle plate on a diesel has next to no effect on engine performance as the fuel burn doesn't begin until the moment fuel is injected. That's why 'throttle response' on a diesel isn't anywhere near as good as a petrol.
The thermodynamic cycle for a diesel is quite different to petrol.
Causing a build up of pressure in the exhaust system would surely cause more resistance and make it harder for the engine to turn over, increasing the braking effect?
k with carbon. That's usually what causes diesels to have an excessive braking effect when lifting off, assuming there's nothing like brakes binding etc.Rickyy said:
Surely releasing pressure on the power stroke will reduce the braking effect? Opening the exhaust valve will just allow any pressure built up to freely escape, providing less resistance, thus a freely spinning engine?
Causing a build up of pressure in the exhaust system would surely cause more resistance and make it harder for the engine to turn over, increasing the braking effect?
No, what happens normally on a closed throttle is that the air in the cylinder that you've just compressed is now acting as a spring and pushes the piston back down, driving the crank and fighting against engine braking on the compression strokes on the other cylinders. Opening the exhaust valve during the power phase releases the pressure in the cylinder to the atmosphere, rather than pushing the piston down.Causing a build up of pressure in the exhaust system would surely cause more resistance and make it harder for the engine to turn over, increasing the braking effect?
Obviously you can't use this system in conjunction with a valve that blocks the exhaust flow completely, which I've never come across before by the way, but you learn new stuff every day!
So yes, the second part of your quote is correct.
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