Engine Breaking?

The_Dark_Side · 22 Jul 2006 at 00:39

paradigm said:
No, it uses momentum of the car to keep the engine spinning, hence I can get infinite miles to the gallon when rolling in-gear.

so let's say you come off the gas as 5kRPM and engine brake until you're revs have dropped to 3kRPM then you hit the gas and away you go....are you saying that the engine immediately shuts off fuel supply as soon as you come off the throttle then re-introduces supply the second you hit the throttle?

The_Dark_Side · 22 Jul 2006 at 00:40

saitrix said:
Pure momentum of the wheels turning the crankshaft over.

if this is true then you're effectively saying the ignition system shuts down on a trailing throttle right?

MrMatteh · 22 Jul 2006 at 00:41

The_Dark_Side said:
if this is true then you're effectively saying the ignition system shuts down on a trailing throttle right?

well, on a diesel its done by compression

The_Dark_Side · 22 Jul 2006 at 00:43

paradigm said:
No, because without the throttle being depressed, there is no openings for ANYTHING to get into the combustion chamber, not just fuel.

the throttle openings are not air tight though.
even in the fully closed position air can get in and as such will be mixed with some fuel, although not enough even to maintain a constant speed.

saitrix · 22 Jul 2006 at 00:43

The_Dark_Side said:
if this is true then you're effectively saying the ignition system shuts down on a trailing throttle right?

Well yes it just about shuts off, i think they still have it feeding a little bit but hardly anything.

Scuzi · 22 Jul 2006 at 00:44

The_Dark_Side said:
so let's say you come off the gas as 5kRPM and engine brake until you're revs have dropped to 3kRPM then you hit the gas and away you go....are you saying that the engine immediately shuts off fuel supply as soon as you come off the throttle then re-introduces supply the second you hit the throttle?

That's how I believe it works. I'm no mechanic so I may be wrong but I've read from numerous sources that this is the case.

The_Dark_Side · 22 Jul 2006 at 00:44

MrMatteh said:
well, on a diesel its done by compression

true, but as the op mentioned petrol, coupled with the fact diesels don't have an ignition system i thought that was obvious

Vanilla · 22 Jul 2006 at 00:45

Doesn't engine breaking put more strain on the engine? Perhaps leading to faults sooner?

The_Dark_Side · 22 Jul 2006 at 00:47

Scuzi said:
That's how I believe it works. I'm no mechanic so I may be wrong but I've read from numerous sources that this is the case.

so all of a sudden at 3kRPM your engine breathes itself back into life and roars off.
now isn't this exactly the same reason you shouldn't bump start a car?
because if an engine fires up at a higher RPM than tickover all the unburnt hydrocarbons damage the cat?

until told otherwise by a more knowledgable source, i firmly believe the only time you're engine is using zero fuel is when you turn the key to the "off" position

agw_01 · 22 Jul 2006 at 00:47

Remember that cars have breather pipes and what not.

Surely these must lead somewhere for the car to take in air.

Also, with the Rover MEMs ECU, if you come off the throttle at say 4000rpm and keep it in gear, it'll cut all fuel until under 2000rpm, where it'll start to use a little bit of fuel in order to keep the engine ticking over.

I'd be suprised if it wasn't the same for many other cars.

The_Dark_Side · 22 Jul 2006 at 00:53

paradigm said:
No, because without the throttle being depressed, there is no openings for ANYTHING to get into the combustion chamber, not just fuel.

again, i may be wrong here but to the best of my knowledge all the throttle controls is the air intake.the fuel system works out how much fuel to add and this is something you have no control over.by closing the throttle all you're doing is closing the air intake alone, and as i said earlier even in the closed position it certainly doesn't form an air tight seal by any stretch of the imagination therefore some air will get in.
and if some air gets in it will be mixed with some fuel.

The_Dark_Side · 22 Jul 2006 at 00:58

lmfy2k said:
Jeremy Clarkson said on one of the top gear episodes that when you are approaching some traffic lights and you let the engine brake i.e leave it in what gear and coast to the traffic light line, (remembering to press the clutch before engine stalls!)

"i don't do facts, i do opinions"
quote, Jeremy Clarkson 2005.

something else that bothers me about that bogus A8 diesel run.Clarkson stated that he'd been told that engine braking was more economical than dipping the clutch and freewheeling as the engine needs to squirt in more fuel to keep it from stalling.
now unless i'm mistaken we've just described the job of the flywheel to a tee.
infact modern engines are becoming so well balanced than smaller and lighter flywheels are possible nowadays which makes me think Mr.Clarksons comment smells decidedly of rodent.

james.miller · 22 Jul 2006 at 01:10

i dont believe that no fuel get's in. at all. You still hear the combustion after all. if you were to switch the engine off rather than engine brake you wouldnt hear much else other than road noise and such but whats the difference between engine off and engine braking? becuase according to you guys they are both basically the same thing - no fuel getting in.

nope, doesnt add up.

Psycrow · 22 Jul 2006 at 02:21

james just hit the biggest nail on the head, even when engine braking you hear the VROOOM of the engine, this is combustion. If it was just momentum it would be almost silent, all the noise that comes from your exaust is from detonating petrol countless times a second. It has to still be burning fuel plain and simple.

TheOtherPhil · 27 Jul 2006 at 22:55

On modern engines, fuel is indeed cut on no throttle. It is then restored at a pre-determined rev setting. Mine is set at 980rpm. For those that don't believe me, I have made a short video of the commander unit on my Apexi Power FC Ecu. I have set it to monitor injector duty cycle and engine revs. When I rev the engine, the injector duty rises. When I release the throttle, a 0% injector duty cycle is shown - 0% = no fuel:

Clicky

The_Dark_Side · 27 Jul 2006 at 23:15

TheOtherPhil said:
On modern engines, fuel is indeed cut on no throttle. It is then restored at a pre-determined rev setting. Mine is set at 980rpm. For those that don't believe me, I have made a short video of the commander unit on my Apexi Power FC Ecu. I have set it to monitor injector duty cycle and engine revs. When I rev the engine, the injector duty rises. When I release the throttle, a 0% injector duty cycle is shown - 0% = no fuel:

Clicky

unless it's showing zero duty above idle.
as i said earlier if you're at 5k rpm then you release the throttle, coast down to 3000 rpm then floor it, if fuel had been completely shut down then re-applied you would have a massive surge of unburnt hydrocarbons leaving your exhaust system.
this is something that kills your catalytic convertor so surely the manufacturers wouldn't setup a system that damages the cat everytime you re-apply the gas?

TheOtherPhil · 27 Jul 2006 at 23:20

The_Dark_Side said:
unless it's showing zero duty above idle.

That's what I said in my post

Fuel is cut on zero throttle until you hit a pre-determined level set by your ECU. Mine is 980rpm at which point fuel is injected to maintain idle. The video clearly shows this so I can't see how you can argue otherwise

The_Dark_Side · 27 Jul 2006 at 23:22

TheOtherPhil said:
The video clearly shows this so I can't see how you can argue otherwise

because i havent watched it yet?

i'll get back to you,lol.

Clarkey · 27 Jul 2006 at 23:26

Hmm I think the motors forum nee to do some serious reading up on engines because it has become apparent that 90% know absolutely nothing.

Lets start with the basics, throttle controls the amount of throttling on the intake, hence why they call it a throttle.

On the over-run, almost all EFI engines will completely cut fuel, because it is not needed. Afaik carbs will cut fuel too, but not completely, might be wrong but it seems possible.

At idle, air is introduced through the idle air valve, because the throttle(s) are closed, and yes, that should make a pretty good seal.

Of course, it is possible for child-killing environment-destroying hooligans (like me

) to remove the overrun fuel cut, which = bwaaaaaaaaaaaaarp pop pop BANG, with a touch of flame for good measure

TheOtherPhil · 28 Jul 2006 at 00:12

Clarkey said:
Of course, it is possible for child-killing environment-destroying hooligans (like me ) to remove the overrun fuel cut, which = bwaaaaaaaaaaaaarp pop pop BANG, with a touch of flame for good measure

And me! One simple setting in my commander unit and I get even bigger flames than usual