Engine Braking?

**dsp26** · 22-07-2009 03:17 PM

so kinda the same affect as me turning my engine off completely while car is moving then putting it back into ign and simply putting gas on except it's fuel and not ign?

**aaronng** · 22-07-2009 03:17 PM

Originally Posted by dsp26

to simplify this for those who still don't understand... your throttlebody is still closed as if you were on idle coz your obviously not accelerating... hence acting as a 'brake' for intake air thus slowing the engine down.

If you are idling and the engine is not connected to the wheels (either neutral or clutch in), then the throttle is slightly open and air is being sucked in. That means fuel is being injected and used.

If the car is in gear and your foot is off the clutch and the vehicle is moving, the rotation of the wheels turn the gearbox which in turn keeps the engine turning. Not sure if the throttle body is totally closed (I am guess it would still be slightly open), but it would be smaller than if you were just idling.

**CRXer** · 22-07-2009 03:26 PM

Originally Posted by dsp26

so kinda the same affect as me turning my engine off completely while car is moving then putting it back into ign and simply putting gas on except it's fuel and not ign?

not quite following what u r saying,but if u put the igntion back on,then there is still no fuel if the engine is above 950rpm & the throttle is completely closed.
not sure whether spark turns off,but there would be no real need to turn it off in any case,theres still no air/fuel to fire.
dont forget,most of the noise u hear from a motor is not explosions but air pumping & rapid air expansion etc.

aarong,throttle plate is completely closed when foot off accelerator,idle passages past the throttle plate take over instaed

**aaronng** · 22-07-2009 05:45 PM

Originally Posted by CRXer

not quite following what u r saying,but if u put the igntion back on,then there is still no fuel if the engine is above 950rpm & the throttle is completely closed.
not sure whether spark turns off,but there would be no real need to turn it off in any case,theres still no air/fuel to fire.
dont forget,most of the noise u hear from a motor is not explosions but air pumping & rapid air expansion etc.

aarong,throttle plate is completely closed when foot off accelerator,idle passages past the throttle plate take over instaed

Ahh, ok, then that's different from my car, which controls the throttle on its own.

Does that idle passage close if you are driving at speed with zero throttle?

**jks24** · 22-07-2009 05:58 PM

good read this

**jords** · 22-07-2009 06:11 PM

http://en.wikipedia.org/wiki/Engine_braking

FYI Read this.

**Riviera** · 22-07-2009 06:13 PM

yea i was gunna post that but meh, and it is the same process for all engines
in the diesels though u can get a massive compression skid out of them lol

**JohnL** · 22-07-2009 06:38 PM

aaronng,
The throttle plate and IACV don't do anything out of the usual when the engine is on the overun. Overun fuel cut off isn't cotrolled using aiflow, though the IACV does remain momentarily open immediately after the throttle plate has been closed to (I think) assist in burning still unburnt fuel as it flows through the exhaust system (an emissions control measure).

My understanding of what happens is that the ECU detects that the throttle is at the closed position (using the TPS input) but also detects that engine rpm is above X, and using these inputs the ECU 'decides' that an overun condition must exist and as programmed cuts the injectors until either the throttle is opened again or the rpm reaches a set point somewhat above idle speed.

This feature lessens fuel consumptioon and emissions, and helps to prevent backfiring (which can damage the cat apart from anything else...).

The braking effect comes from internal engine friction (and to a much lesser degree from internal gearbox friction). Some of the braking effect comes from the compression of gas in the cylinders, but since the throttle plate is shut very little air should be able to flow into the cylinders from the intake side, so perhaps there is some reverse flowing gas coming in via the exhaust valve...?

**unity** · 22-07-2009 08:52 PM

Can anyone explain why engine braking kicks in sometimes and doesn't at other times? Anytime I approach a decline I take my foot off the accelerator to let the car cruise. But the car will only sometimes engine brake, and it gets pretty annoying when it only engine brakes on very small declines.

**CRXer** · 23-07-2009 01:01 AM

Originally Posted by aaronng

Ahh, ok, then that's different from my car, which controls the throttle on its own.

Does that idle passage close if you are driving at speed with zero throttle?

aaronng,wouldnt have a clue how your fancy dbw works,im too poor,my throttle still runs off a cable...
does the K24 dbw even have an idle valve? or does it do some fancy throttle plate work?

idle passages are always open,even if the idle valve was fully shut,the idle set screw is still bleeding,theres even a little bit of PCV at play. On EGR cars,theres a little bit of that too....

john,most of the effect is compression,theres always air available to compress,due to idle passages & overlap.

**string** · 23-07-2009 01:28 AM

I don't think compression has much effect on the engine braking. Any energy that you lose to heating the air (which is at near vacuum) during the compression stroke you'll get back on the power stroke.