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To a certain extent; diesel more than petrol for obvious reasons.
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It's very gearbox, gear, and selector position specific. Sometimes a one way clutch which allows overrun is blocked when the selector is moved to numbered gear positions instead of D.
for obvious reasons.
I'm not sure they're too obvious.
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I'm not sure they're too obvious.
Higher compression.
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>>Higher compression.
But, the pressure of the compressed gas also presses down on the piston on what would have been the power stroke.
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But the pressure of the compressed gas also presses down on the piston on what would have been the power stroke.
Okay, let's try again. Which is easier to bump-start, petrol or diesel ? I respectfully rest my case.
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I respectfully rest my case.
No you don't!, because you haven't explained it. It really isn't as obvious as you might think.
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No you don't! because you haven't explained it. It really isn't as obvious as you might think.
Well it works for me, and another poster has noted it as you'll have seen. I had a BMW 525TD auto which had a modicum of engine braking, and also occasionally drove a 520 auto which had virtually none. Given the reasonable assumption that both cars had the same transmission, I can only put it down to the higher rolling resistance offered by the higher compression of the diesel engine.
Perhaps you would care to enlighten us with a more scientific explanation? ;-)
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>>Well it works for me
I'm not saying that some diesels don't offer more overrun resistance - it's just that the reason behind it isn't as straightforward as many might think.
If engines had negligible losses, all of the energy you put into compression, you would get back as expansion, and so compression wouldn't matter at all, and you would not get any engine braking at all from this source in an averaged sense (while push starting, you would still need to put the energy in to get the engine turning, and to provide th first compression of each cylinder once).
However, the real situation is that the compressed air gets hot, and this heat is transferred to the metalwork of the engine. This transfer of heat lowers the pressure of the air in the cylinder, and so, during expansion, you do not get back the same energy that you put in. This difference in energy input and energy returned is where the overrun braking comes from - the kinetic energy of the car becomes heat energy in the coolant. So, it's the losses associated with the air being heated, rather than the compression itself.
To complicate matters, petrol engines are throttled to a greater or lesser extent depending upon how the engine management deals with overrun (some systems open the throttle or idle air control valve on overrun). So, a petrol engine has to do work pumping air against a closed throttle, which is a source of loss which the diesel doesn't have (as mentioned by manatee below). Also, as a petrol has lower compression, the temperature difference between the compressed charge and the block is lower, and so less heat is transferred between gas and metalwork.
Differences in valve timing between engines will also have an effect.
Of course, in common between engine types are mechanical, aerodynamic, and hydrodynamic friction in the powertrain, pumping losses in the oil pumps, coolant pumps, and electrical loads from the alternator.
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Thanks for the explanation; no offence but I'll stick to "suck squeeze bang pop" if you don't mind, too much science makes me brain hurt! ;-)
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>>no offence but I'll stick to "suck squeeze bang pop" if you don't mind
No offence taken!
The easy way to think of it is that for braking to be effective, you've got to complete the chain all the way from the kinetic energy of the vehicle to heat energy being lost from the vehicle.
When you begin to look at it that way, it's processes which produce a real loss, rather than those which temporarily store energy which matter.
Edited by Number_Cruncher on 23/11/2008 at 12:49
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"Well it works for me, and another poster has noted it as you'll have seen. I had a BMW 525TD auto which had a modicum of engine braking, and also occasionally drove a 520 auto "
The auto box in the 525TD has a one way clutch because the engine has too much engine braking. So when you lift off, it coasts.
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I see this with our cars. Our diesel S80 with an AW 5 speed box seems to benefit from some engine braking at 30mph downhill in 3rd, but our petrol Zafira with an AW 4 speed box has no engine braking at all in these conditions and just runs quicker and quicker down a hill unless you hold it back with the brakes.
Is that down to the difference between diesel and petrol or just the different boxes? NC please enlighten me! :)
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I don't know the detail differences between AW4 and AW5 gearboxes, but if you have seperate gear ratios on your selector, like the 3, 2, and 1 of
P R N D 3 2 1,
for example, you will obtain engine braking if the selector lever is in, say, 3 while you wouldn't obtain engine braking from 3rd gear in D. The car's manual should make this clear for each specific case.
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Thanks NC, I'll try that. The manual does talk about this for towing and mountain driving, so I hadn't thought about it for everyday use...
Still a right pain to have to manually switch the 4 speeder into 3rd to get that effect when the 5 speeder does it automatically.
I still don't understand though, is it as simple as one is set-up to engage engine braking automatically and one doesn't or is it the petrol/diesel difference?
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>>or is it the petrol/diesel difference?
I don't know in your specific case, but, you can tell by watching the rev counter when on overrun.
If the engine goes back to tickover, you have an overrun clutch, and you'll never get engine braking in that ratio/selector position combination with any engine.
On the other hand, if the engine speed drops a bit, but is held above tickover, and the engine speeds up with the car, you're getting some engine braking.
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CVT's do if they're programmed to do it. On the Nissan Qashqai you can feel the effect going downhill if you lift slightly off the accelerator whereas a conventional auto will simply roll on.
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CVT's do if they're programmed to do it. On the Nissan Qashqai you can feel the effect going downhill if you lift slightly off the accelerator >>
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On the Audi I had on holiday. RTFM says something like - if you use the brakes going down hill the box will not change up thus giving better engine braking.
Seemed to work quite well. I thought it was quite clever.
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