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Old 13th January 2009, 17:34   #136
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But how would you do engine breaking if the engine is off?
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Why would some one have engine not running & try to stop the car by using engine braking under normal circumstances?
Err, I'm not quite clear where the doubt is.

As for why engine braking might be used with the engine off, nobody ought to be doing it in the 'normal' course. Normally engine braking is used with the engine running.
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Old 13th January 2009, 18:22   #137
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Engine braking is dependent on compression. Regardless of whether the engine is on or off, compression WILL occur.
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Yes, I agree, engine breaking will still be there with the engine turned off, because compression will still be there.
Hi guys, a n00b question here... Is it frictional losses or compression that helps during engine braking ? While you're expending energy in trying to compress air during the compression stroke, won't it be returned once the piston is past TDC since the compressed air would just act as a spring ? Here, I'm assuming that the ignition's off... With the wheels driving the engine, whatever energy that's used for compression would be returned back at once wouldn't it ?
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Old 13th January 2009, 19:08   #138
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Normally engine braking is used with the engine running.
Thanks, then we are on the same page
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Old 13th January 2009, 19:25   #139
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Thanks, then we are on the same page
Good. Now what happens if you're going downhill and the engine dies on you, AND the brakes 'seem' to have failed, AND the steering has become unbelievably heavy?!!
This is where engine braking becomes your best friend!
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Old 13th January 2009, 19:32   #140
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Hi guys, a n00b question here... Is it frictional losses or compression that helps during engine braking ? While you're expending energy in trying to compress air during the compression stroke, won't it be returned once the piston is past TDC since the compressed air would just act as a spring ? Here, I'm assuming that the ignition's off... With the wheels driving the engine, whatever energy that's used for compression would be returned back at once wouldn't it ?
Ah no , firctional losses do not stop the engine. otherwise your engine would burnout when running. (more than 1k rpms)

OT

however the friction does increase marginally when the engine is off and it would be bad. I do not know how the oil is sent to the valves, valvesprings, Rocker arms and camshaft and camchain. if its by some pump then the oil wouldnt be circulated resulting in the parts running dry. someone crosscheck this explanation
/OT


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Good. Now what happens if you're going downhill and the engine dies on you, AND the brakes 'seem' to have failed, AND the steering has become unbelievably heavy?!!
This is where engine braking becomes your best friend!
^exactly why to stick to it !
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Old 13th January 2009, 19:39   #141
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Now what happens if you're going downhill and the engine dies on you, AND the brakes 'seem' to have failed, AND the steering has become unbelievably heavy?!!
This is where engine braking becomes your best friend!
Well am I in gear or in neutral?
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Old 13th January 2009, 19:47   #142
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however the friction does increase marginally when the engine is off and it would be bad. I do not know how the oil is sent to the valves, valvesprings, Rocker arms and camshaft and camchain. if its by some pump then the oil wouldnt be circulated resulting in the parts running dry. someone crosscheck this explanation
The oil pump is mechanically driven by the engine. As long as the engine turns, or is externally driven, full lubrication is maintained!
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Old 13th January 2009, 19:48   #143
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^but how would being in gear be helpful if you don't have engine braking ?
(if no engine braking was there in IGNITION OFF mode then the car would seem as if the clutch is released all the time)
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Old 13th January 2009, 20:14   #144
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I do not know how the oil is sent to the valves, valvesprings, Rocker arms and camshaft and camchain. if its by some pump then the oil wouldnt be circulated resulting in the parts running dry. someone crosscheck this explanation
Hi Xeno, the oil pump's usually found at one end of the crankshaft & is driven directly by it. So I guess, as long as the crank's rotating, you'd have oil being pumped around the engine. The pressure would depend on what RPM the engine's being turned by the wheels though.

My question is whether engine braking is due to compression at all since the compressed air would just act as an 'air spring'.

Even with the ignition on, during engine braking, modern ECUs are said to cut off fuel (heard it on Top-Gear once !), so I'm guessing the same principle (air-spring) would apply...
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Old 13th January 2009, 20:48   #145
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The oil pump is mechanically driven by the engine. As long as the engine turns, or is externally driven, full lubrication is maintained!
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Hi Xeno, the oil pump's usually found at one end of the crankshaft & is driven directly by it. So I guess, as long as the crank's rotating, you'd have oil being pumped around the engine. The pressure would depend on what RPM the engine's being turned by the wheels though.

My question is whether engine braking is due to compression at all since the compressed air would just act as an 'air spring'.
thank you for the clearance. I thought it would be some electrical pump.

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Even with the ignition on, during engine braking, modern ECUs are said to cut off fuel (heard it on Top-Gear once !), so I'm guessing the same principle (air-spring) would apply...
But they'd cut off fuel to the optimum level and wouldnt stop supplying fuel would they (they = ECU)

case of the ignition being off (when the air fuel mixture AFM isnt ignited)

well the air spring query does seem to be very much interesting indeed. After all when the piston moves from BDC to TDC the AFM gets compressed. And when the piston moves from TDC to BDC the AFM releases the energy by decompression. so the energy isnt used to brake still.

hmmmmmmmm.

what about engine braking on automatic transmission cars and hybrid cars ? auto boxed cars shift up/down near specific conditions, they dont allow you to redline the engine in lower gears right ?

Cheers
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Old 13th January 2009, 20:55   #146
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Slightly OT but related question.

Which is better, Automatic with ABS or manual using downshifting (engine breaking)?

I drive a Auto with ABS and when it kicks in the wheels are still spinning trying to steady the car. Heck but I am still going. I miss manual cars but I dont know if its better to stop (downshifting) or would skid more. Both scenarios are for snow and wet weathers.
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Old 13th January 2009, 21:22   #147
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My question is whether engine braking is due to compression at all since the compressed air would just act as an 'air spring'.
Hey, if that is how things happened, you've got yourself the ever-elusive Perpetual Motion Machine! The wheels turn the engine which pushes the pistons to compress the air in the cylinders>the compressed air pushes back the pistons which would then turn the wheels!!!! So where's the need for fuel, except for the initial startup of this perpetual motion machine??

It is the compression that creates the 'resistance' that is offered, and this resistance is 'multiplied' by a ratio dependent on the gear the car happens to be in at that time! The lower the gear, the larger the multiplication factor.
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Old 13th January 2009, 21:39   #148
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Hey, if that is how things happened, you've got yourself the ever-elusive Perpetual Motion Machine! The wheels turn the engine which pushes the pistons to compress the air in the cylinders>the compressed air pushes back the pistons which would then turn the wheels!!!! So where's the need for fuel, except for the initial startup of this perpetual motion machine??
It is the compression that creates the 'resistance' that is offered, and this resistance is 'multiplied' by a ratio dependent on the gear the car happens to be in at that time! The lower the gear, the larger the multiplication factor.
Hi Anup, I'm just questioning the percentage of engine braking that is due to work spent in compressing the air within the cylinders. While there may be some losses due to friction, wouldn't a large portion of it be returned by the expanding compressed air ?

Of course, I'm assuming that the period for which the valves remain shut are the same in both directions of the piston...
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Old 13th January 2009, 21:45   #149
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Originally Posted by anupmathur View Post
Hey, if that is how things happened, you've got yourself the ever-elusive Perpetual Motion Machine! The wheels turn the engine which pushes the pistons to compress the air in the cylinders>the compressed air pushes back the pistons which would then turn the wheels!!!! So where's the need for fuel, except for the initial startup of this perpetual motion machine??

It is the compression that creates the 'resistance' that is offered, and this resistance is 'multiplied' by a ratio dependent on the gear the car happens to be in at that time! The lower the gear, the larger the multiplication factor.
aah that is something else ! he's asking HOW is engine braking effective and how does it arise when the compressed air/AFM is decompressed !.

I think the heat energy generated during compression is sapped by the cylinder wall, the piston etc etc and passed on to the engine shell which is cooled by the radiator ! so that's the only possible way to lose energy.


AFAIK frictional forces , I repeat cannot cause engine braking. otherwise ... frictional forces arising from the drivetrain, tyres, and other moving parts of the engine only slow the vehicle down when you let go of the accelerator pedal. In ideal conditions the car should keep cruising endlessly but it slows down due to these frictional forces.

Engine braking is really significant. basically i do not think that its easy to crank cars of today with a hand crank as they had in the previous century's begining.

normally the pistons move up and down due to the energy released by AFM, the inertial energy in the flywheel causes the next IC cycle's compression stroke to take place.

But when you downshift the car, it means the engine would have to work at the higher rpm's to maintain the same speed. now the movement of the car causes the engine's crankshaft to spin faster (which is reverse of the engine crank turning the driveshaft) because the engine is running at a lower speed because it was in a higher gear previously.

so its the car's mometum + gearing (that is the car is at a higher speed in the lower gear) which now cause the compression and not the energy from the AFM combustion (you arent increasing the throttle are you ? ) so the car loses its momentum which results in the increase of revv's of the engine WITHOUT consuming extra fuel (this i can support because an engine can still overrev if you let go of the gas pedal)

that is the only logical explanation i can give. if someone did get what i said and can put in better words please do so.
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Old 13th January 2009, 21:51   #150
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I follow the same practice ! although once something was on my head and i started off with the parking brakes (ashamed)
In my car (Optra Magnum) if park break is applied and you try to engage gear definitely your wrist will touch on park break. This is a great way of reminding you that park break is not dis-engaged. So never faced this issue
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