what happens during engine braking? I put it to PH

[DannyVTS]

As with all my motoring problems, i am putting a rather mechanical problem to you PH.

I want to know what happens, in a FWD car with a longitudinal engine what happens during engine braking

Ie is it just the wheels are turning too slowly for the gearbox? Where is the force acted?

Reason for asking is my car makes an odd noise when i let off the throttle, and the car wants to slow itself down. I have been told its the differential and will be ringing a mechanic tomorrow for a quote, but i want to know what exactly happens..

And if anyone knows how a differential works on a FWD car, i wouldn't mind an explanation, i imagine it looks something like this >|< where the two arrows are the CV joints. Are CV joints on each end of the gearbox? Argh too many questions

I hope someone can assist

Danny

[CraigyMc]

DannyVTS said:

As with all my motoring problems, i am putting a rather mechanical problem to you PH.

I want to know what happens, in a FWD car with a longitudinal engine what happens during engine braking

Ie is it just the wheels are turning too slowly for the gearbox? Where is the force acted?

Reason for asking is my car makes an odd noise when i let off the throttle, and the car wants to slow itself down. I have been told its the differential and will be ringing a mechanic tomorrow for a quote, but i want to know what exactly happens..

And if anyone knows how a differential works on a FWD car, i wouldn't mind an explanation, i imagine it looks something like this >|< where the two arrows are the CV joints. Are CV joints on each end of the gearbox? Argh too many questions

I hope someone can assist

Danny

Well, for a start, unless it's some sort of tardismobile, your VTS has a transverse engine. As for the whining noise, I'll leave that to someone who knows about bearings/transmissions etc to help you.

C

[jon-]

Engine braking is a result of piston compression without a spark. There's no mixture (if it's a FI car) but it's still pumping air off throttle.

[crofty1984]

Engine braking: Have you ever tried to bump start a car and noticed how it's very hard to keep pushing once the clutch is let out? (But keep pushing you must!) That's because you're acting against the force of the compression. When you're engine brakng the momentum of the car is being slowed down because it has to act against the force of the compression of whatever cylinder is on the compression stroke at that moment.

[Mave]

Normally when you're driving, the engine drives the wheels. During engine braking, the wheels drive the engine (unoess you put the clutch down). This means that the transmission is also loaded the opposite way to normal.

[DannyVTS]

CraigyMc said:

Well, for a start, unless it's some sort of tardismobile, your VTS has a transverse engine. As for the whining noise, I'll leave that to someone who knows about bearings/transmissions etc to help you.

C

Haha, that teaches me for trying to sound like i have a clue doesn't it!
FYI it was just a mistake, i knew i had a transverse engine

jon- said:

Engine braking is a result of piston compression without a spark. There's no mixture (if it's a FI car) but it's still pumping air off throttle.

crofty1984 said:

Engine braking: Have you ever tried to bump start a car and noticed how it's very hard to keep pushing once the clutch is let out? (But keep pushing you must!) That's because you're acting against the force of the compression. When you're engine brakng the momentum of the car is being slowed down because it has to act against the force of the compression of whatever cylinder is on the compression stroke at that moment.

Right, so, to concur unless i have misread this

The sound coming from my car is probable to be the result of 2 forces coming together under a bearning that is going/or has gone in my gearbox? Thus the audible grinding noise...

Correct?

(the two forces being, the momentum of my wheel/the speed of the driveshaft vs the air compressing in my cylinders?

[Mave]

Differential and CV joints are 2 different things.

CV joints (constand velocity joints) are just a snazzy rottaing hinge to allow the wheels to steer while still tranfering torque from the driveshaft. A bit like a flexy drinking straw. You have one on each driveshaft, ie one on each side of the car.

A differential allows the drive wheels to turn at different speeds to each other. Its needed because on a corner the outside wheels travel further than the inside ones. Imagine a wheel being driven around on its outside edge. That's your differential being driven by the gearbox. Instead of spokes, imagine axles with long thin gears rotating around them. Then on either side of the wheel, put a disc shaped gear, driven by the gears on the spokes. These then connect to your driveshafts. The average speed of the discs is equal to the speed of the wheel, but the discs can go around at different speeds to each other.

[littleredrooster]

jon- said:

Engine braking is a result of piston compression without a spark. There's no mixture (if it's a FI car) but it's still pumping air off throttle.

Only diesel engines pump air when the accelerator pedal is released. Petrol engines have a throttle, deliberately named because it throttles the air intake - i.e. closes it to all except the tiny amount passed by the pilot mixture/air channel.
Diesel engines run on open air intakes (most of the time anyway, there are some highly-technical exceptions...) and will therefore pump air at all times.

OP: the strange noise you hear when closing the throttle is often an exhaust head-shield rattling as the resonance of the exhaust changes. It is much cheaper than a new differential.

Edited by littleredrooster on Saturday 27th November 10:10

[Ozzie Osmond]

littleredrooster said:

Only diesel engines pump air when the accelerator pedal is released. Petrol engines have a throttle, deliberately named because it throttles the air intake - i.e. closes it to all except the tiny amount passed by the pilot mixture/air channel.
Diesel engines run on open air intakes (most of the time anyway, there are some highly-technical exceptions...) and will therefore pump air at all times.

Whichever internal combustion engine is being turned by the wheels during overrun/engine braking the VALVES will be CLOSED so there will be COMPRESSION and there will be engine braking as the PISTONS continue to PUMP air into the cylinders (when the inlet valve is open) and then COMPRESS it (when the valves are closed). Generally an engine with a higher compression ratio will give more engine braking.

[rottie102]

THE BEST explanation what a differential is:
http://www.youtube.com/watch?v=K4JhruinbWc

[DannyVTS]

rottie102 said:

THE BEST explanation what a differential is:
http://www.youtube.com/watch?v=K4JhruinbWc

thankyou that helped a lot

BTW I've checked under the car with the engine running and i'm certain it is not a heatshield etc

[LeeThr]

DannyVTS said:

Right, so, to concur unless i have misread this

The sound coming from my car is probable to be the result of 2 forces coming together under a bearning that is going/or has gone in my gearbox? Thus the audible grinding noise...

Correct?

(the two forces being, the momentum of my wheel/the speed of the driveshaft vs the air compressing in my cylinders?

The grinding noise could mean the signs of needing a new gearbox or yours being rebuilt in the none too distant future. Mine's making a grinding noise when i dont have the throttle on at all. Common fault with my 1.4 box not too sure with the 1.6 but doesnt mean it still cant go wrong. Its the differential bearings on the way out.