Unusual engine question?

[lusty v-tec]

I understand that on modern engines:

If when going down a hill for example you leave the car in gear and dont touch the throttle then the engine uses no fuel at all.

Why then does the exhaust still release gases if the engine is simply running by the use of the gears? Id have thought the exhaust would be silent.

Cheers.

 

[Turner]

The engine is still running thou thats why you hear the exhaust but engine just not injecting petrol?

 

[kully]

if your engine is running you are using fuel , no matter what other circumstances are used

 

[FeelX]

No fuel is used, but air is still pumped through the engine.

 

[White16valver]

But the sound coming out of the exhaust is the sound of the petrol 'exploding' in the cylinders (isn't it?) - so how does it still make noise?

This is a question I have pondered myself... lol

 

[Harry182]

The engine is still running thou thats why you hear the exhaust but engine just not injecting petrol?

Thats correct, If you dip the cluth the engine will tick over using fuel.

 

[mattjk]

Not convinced - would be cool for an engineer to clear this one up

 

[Kung Fu Chef]

Surely it will be using the same amount of petrol as if the engine was at idle?

 

[kully]

Surely it will be using the same amount of petrol as if the engine was at idle?

yep ....

 

[DrR]

Drive your car till it runs out of petrol, then we'll know the answer.

Or switch the ignition off when driving along?

 

[Andy_RT]

If u r goin down hill u r using no fuel. U still hear sound from the exhaust cos the engine is still being turned over from the gearbox, which causes the pistons to go up n down as normal and still compresses the air in the combustion chamber, but not actually exploding. Which in turn does not generate any power. The compressed air needs to escape which is out of the exhaust.

 

[Dan@SJM]

no.

The engine on over run is injecting no fuel. otherwise it would still be wanting to accelerate a bit. it only starts fuelling again once the engine is at tick over speed.

The noise you hear is the air coming out the cylinder, there will still be air going through as the engine is turning.


EDIT : AS andy said

 

[Andy_RT]

correct lol

 

[Andy_RT]

If u wana save petrol go down a massive hill lmao

 

[White16valver]

Except you have to get up that massive hill at some point, therefore petrol saved = nil... (unless you pushed it up lol)

 

[FeelX]

No fuel is used, but air is still pumped through the engine.

I will explain myself clearer.

I am an mechanical engineer, but I did not design any engine or something.

Not using fuel

The engine will not be using any fuel, when the engine is spinned around by it's own kinetic energy. So when you are braking on the engine....coming from high revs. to low revs. the engine used energy that the car has from movement. (thus increasing braking)

(dipping the clutch isn't working)

Ok I have understood that the new engines let say late 1990's are able to shut down injection totally when the above is happening.

This is for economic reasons.

The sound of the exhaust

Because the engine isn't using fuel it doesn't mean the engine isn't using air or moving air. The engine exctually works like a pump. Inhaling air...and pressing it out. (this motion is using the kinetic energy that the car has from movement) The air is blown into the exhaust that is why you will still hear it. It won't be so loud as on full throttle ofcourse, but that's obvious.

On tuned cars the total shut down of injection could be made undone, so then you car is still using fuel, this makes the exhaust pop. (unburnt fuel in exhaust)

Hope this is clear. Ofcourse there will be any fault is my story, but I believe the biggest part is true. I'm only an engineer.

PS. sorry for my bad English as I am Dutch

 

[Bash]

so if I was coasting down a very big hill in 2nd and reset the trip computer will the mpg display read off the scale and if I was to do it in test mode would the fuel flow rate read zero?

 

[Andy_RT]

prob read 99.9 or sumet m8, wont read 0 cos u r still movin. If its on zero thn u would be parked cos u r doin 0mpg, if u get me

 

[FeelX]

If u wana save petrol go down a massive hill lmao

You would probably extra heat your engine, because the petrol also has an cooling function, inside the combustion chamber

Extra heat = probably kill

 

[Bash]

I reccon the same for mpg rate, anyone wanna give it a go my clio's broken.
sorry andy I ment the fuel flow rate in test mode when u hold ur trip computer stick in and start the engine.

 

[White16valver]

If u wana save petrol go down a massive hill lmao

You would probably extra heat your engine, because the petrol also has an cooling function, inside the combustion chamber

Extra heat = probably kill

Isn't that what the radiator and fan's for?? :S

 

[DrR]

If u wana save petrol go down a massive hill lmao

You would probably extra heat your engine, because the petrol also has an cooling function, inside the combustion chamber

Extra heat = probably kill

Engine won't be making any heat (apart from friction) so won't be an issue.

 

[FeelX]

Mostly yes, but when working at the Renault (not as an mechanic) one of the few good renault mechanics told me this. Not that the extra heat will kill the engine, but when reving to 6000rpm and then only braking on the engine will generate more heat.

Read DrRollo's post, so probably not a problem, I am not an expert

 

[DrR]

I'm no mechanic, just using basic physics/science Fuel makes the heat so if no fuel is being used shouldn't get hotter.

 

[Vish]

Is this why Clarkson said (in the 800mile Audi trip) that when coming to a stop/junction, leave the car in gear instead of dipping clutch so it wont use any fuel? Never understood that until now.

So effectively, the engine will only use fuel when you press the accelerator or on tick over to stop it stalling? Driving in gear, with no accelerator, uses no petrol?

 

[mattjk]

Probably wont use fuel above 2000 revs or something. as you can pull the car at idle speed in 1st gear

 

[Peperami]

Surely it will be using the same amount of petrol as if the engine was at idle?

yep ....

 

[FeelX]

@DrRollo,

I guess you are right, but the when fuel/air is inhaled, the cilinder and piston are cooled.

And compressing only air also generates heat, I don't know in what sort of factor but still. (As you probably know, diesel engines work on the heat generated by compression)

 

[DrR]

Probably wont use fuel above 2000 revs or something. as you can pull the car at idle speed in 1st gear

???

 

[Dan@SJM]

right, engines on over run do not use fuel.

look in all of the aftermarket fuel maps, and in omex etc mapping software you set the throttle position for fuel cut off for over run...

as for the air, when ebeings are on over run, they are still sucking in air. otherwise how would it not? just stop the inlet valves from opening? ffs. think about it people!

 

[FeelX]

Actually there are engines that can do so.

Rail road locomotive running on diesel mostly have this function. On full load they use all cilinders and on normal load they eg. use only 8 cilinders of 12 in total.

I believe there are cars that also have this function. probably american because of there big displacement.

Google --> multi displacement engine

 

[Dan@SJM]

i understand that, i was just saying about the clio engines that were being discussed.

 

[edde]

Is this why Clarkson said (in the 800mile Audi trip) that when coming to a stop/junction, leave the car in gear instead of dipping clutch so it wont use any fuel? Never understood that until now.

So effectively, the engine will only use fuel when you press the accelerator or on tick over to stop it stalling? Driving in gear, with no accelerator, uses no petrol?

Yep in a diesel the engine creates no restriction when you life off so as long as the cars moving and the rpm is over idle then no fuel being used. On a petrol its a restriction so it can be better to let the car idle depends though.

Probably wont use fuel above 2000 revs or something. as you can pull the car at idle speed in 1st gear

When the rpm gets to or below the idle the engine i jects fuel and uses its idle control to keep it at idle above idle though and there no fuel used.

I believe there are cars that also have this function. probably american because of there big displacement.

Some big Merc's etc.

 

[White16valver]

I think the gangster cadillac thing has this. (My memory fails me today.)

 

[izard-industries]

When the throttle is let off, surely its just using the same fuel as on tick over, thats what gives you the barbling sound when you throttle off surely?

 

[edde]

When the throttle is let off, surely its just using the same fuel as on tick over, thats what gives you the barbling sound when you throttle off surely?

Nope.

If rpm is above idle and thottle is closed no fuel is being injected.

Only when throttle is applied or rpm comes to idle does the engine start injecting fuel after all engin e has to keep idle going.

 

[JAY172]

some engines can shut off fueling fullstop on over run and will start injecting as rpm drops to about 1500rpm.

some engines adopt like a stepper injection stratadgy i dont know how to explain it. they do inject a small amount of fuel still but not on every revolution

 

[GordonD]

This thread is based on a misconception. The sound you hear from the exhaust is only partly the sound of the petrol-air mix exploding. When that occurs the valves are closed. The pulses that the intake system and exhaust system are "tuned" for are the pressure pulses created when the valves open and close. And that's some of the "sound" you hear. When you go down a hill the pistons are still going up and down and compressing whatever amount of air is allowed past the throttle butterfly, and the valves are all opening and closing, and those sound pulses are still being generated.

Its the same when you're starting an engine. If its turning over but not firing you get some sound from the intake and exhaust system, but as soon as it catches you can hear it has by the fact is gotten louder and its tone has changed.

I suspect, though I've never thought about this before, is that the sound of the explosions when the engine is firing actually comes directly from the engine itself.

 

[GordonD]

If u wana save petrol go down a massive hill lmao

You would probably extra heat your engine, because the petrol also has an cooling function, inside the combustion chamber

Extra heat = probably kill

Racing karts (air-cooled, 2-stroke) seize when you take your foot off the throttle at the end of the straight. But that's because the oil is mixed in with the fuel, so you've got an engine that's temperature has gone up under full throttle down the straight, and when you close the throttle its got no lubrication. A water-cooled four-stroke engine is still going to heat generated from friction, and heat generated from compressing the air in the cylinder, but its still got lubricant and the water is still flowing through the cooling system, so its going to run at a lower temperature than if there was fuel going in and exploding and generating more heat.

 

[lusty v-tec]

cheers for the responses guys......I didn't expect a big debate to generate. lol