Is it possible to run an engine without fuel?

nist7

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Many people are saying that it's possible. Including Jeremy Clarkson, who I believe said the same thing during an episode of Top Gear. But the way they say it is quite ... quirky. So I shall present the argument as best as I can:

When a car is approaching a red light or stop sign or is otherwise in need to decelerate, it is better to stay in gear than to shift into neutral. Because if you simply stay in gear and let off the gas pedal, then the engine is consuming no fuel while if you put it in neutral then the engine must use some fuel to prevent stalling.


I don't believe that is a correct statement. The engine management system is not smart enough to cut off fuel flow because it somehow "knows" that the driver wants to stop. And also if NO fuel is being used, then how is the engine running at all? Are they saying that for a brief period of time all those 4, 8, 12 pistons can still move up and down with enough ferocity to keep the crank going with no combustion????
 
No, it's not possible to actually run an engine with no fuel.

This is simple physics. The car had to start rolling somehow, right? It used the engine to convert chemical potential energy into thermal energy which is then converted into kinetic energy. That kinetic energy makes the car roll until friction turns the kinetic energy into heat. If you use the car's momentum (kinetic energy) to turn the engine over when it's in gear then the car will decelerate faster because there is more friction at work and the car will stop sooner, but you are still using the energy initially put into the equation by the fuel (minus whatever amount for engine inefficiency).

What about if you dragged a car to the top of a hill and rolled it down in gear? Well, same thing. You had to convert chemical energy (ATP, the stuff that fuels your cells) in your muscles to kinetic energy to move the car, resulting in potential energy due to gravity. Where do you get energy? From food, and what fuels life on earth? The sun. There is your fuel source. You will always be inputing energy into the equation.

Now can you *reduce* the amount of fuel consumed by the engine by keeping the car in high gear as you roll up to a stop? Compared to idling? Probably, just don't stall it or you will use more fuel trying to re-start the engine than if you had left it the hell alone.
 
Confusing post, but I'll see what I can do:

First off, what you're talking about is down shifting/engine braking, in which you use the engine to brake (duh :p). In this instance, the rolling momentum of the car is trying to turn over the engine rather than the engine itself, so it causes the car to slow down. If anything, you're using more fuel this way, because the engine is forced to rev higher than if you were to throw the car into neutral and let the engine idle. Also, you'll eventually have to pull the car out of gear, otherwise you will stall the car :p


True, the engine management system might not know that the driver wants to stop, but it does know how much throttle he's inputting, and how much the engine is revving. Management systems typically go off of throttle input and revs, which are rapidly decreasing when you are stopping.


As for it being better, it's really a matter of opinion to the driver. I use downshifting occasionally, but when I do, it's only through 3rd gear. However, for the most part, I tend to just put the Volvo in neutral and hit the brakes.

Again, the engine is still using fuel. Also, it is possible to turn over an engine without running it, however, if you do it fast enough, the engine will start itself. This is how "popping the clutch" works.

Hope that helps, if not, I'm sure someone will answer...


Edit: I actually started typing this before Blind_Io made his post, so don't think I'm trying to contradict him :p
 
You guys are making this really complicated.

Ok, simple answer. When the car is in gear and the clutch is engaged, the entire drivetrain from the crankshaft to the wheels is one piece. So the engine keeps turning cause the wheels are still turning.

And modern engine management is smart enough to detect when the driver wants to slow down. It's really easy actually, the computer looks at two variables, RPMs and throttle position. If the throttle pedal is closed and the RPMs are greater than idle then the computer goes "oh, he's coasting" and turns off the fuel injectors.
 
Isn't it possible that its no fuel going in and the drivetrain spining at speed is in turn spinning the crank w/o combustion? I dunno I have no expertise in such matters - it just seemed logical to me.
 
Modern fuel-injected engines use fuel only when your foot is on the accelerator or when the car is idling. Period.

I don't think I need to explain it any further, as Blind_Io has already done an excellent job of elaborating on the technical details.

Isn't it possible that its no fuel going in and the drivetrain spining at speed is in turn spinning the crank w/o combustion?
Yes, that's exactly what happens.

Also, it is possible to turn over an engine without running it, however, if you do it fast enough, the engine will start itself. This is how "popping the clutch" works.
That sort of made sense up until the sentence I put in bold. What do you mean by that? Are you talking about starting an engine that has a broken starter?
 
He is talking about a "roll start" for a car with a dead battery. In a manual you can get the car rolling, put it in gear and pop the clutch. This uses the momentum of the car to crank the engine and therefore turn the alternator to generate enough power for a spark. Simply pushing a car and popping the clutch won't start a car, you still have to have the ignition on for there to be spark. I don't think it's possible to roll start a diesel because you have no power for the glow plugs.
 
No, it's not possible to actually run an engine with no fuel.

This is simple physics. The car had to start rolling somehow, right? It used the engine to convert chemical potential energy into thermal energy which is then converted into kinetic energy. That kinetic energy makes the car roll until friction turns the kinetic energy into heat. If you use the car's momentum (kinetic energy) to turn the engine over when it's in gear then the car will decelerate faster because there is more friction at work and the car will stop sooner, but you are still using the energy initially put into the equation by the fuel (minus whatever amount for engine inefficiency).
This is nicely explained, but somewhat confusing. Of course, you need fuel to get the car moving in the first place (put things like pulling it aside). BUT ... when you approach a red light, don't press the accelerator and stay in gear, the engine uses no fuel at all at this very moment. It is still turning because it is still connected to the wheels (like Raven already pointed out), so the rolling wheels will turn the engine. Obviously, this does save fuel, because you are moving without using any.

As a conclusion: yes, an engine can run without fuel in certain situations.

YF19pilot said:
First off, what you're talking about is down shifting/engine braking, in which you use the engine to brake. In this instance, the rolling momentum of the car is trying to turn over the engine rather than the engine itself, so it causes the car to slow down. If anything, you're using more fuel this way, because the engine is forced to rev higher than if you were to throw the car into neutral and let the engine idle. Also, you'll eventually have to pull the car out of gear, otherwise you will stall the car.
This is totally wrong and confuses even more. If you get off the accelerator and stay in gear, the engine will use no fuel independently from the current revs it is doing. The revs only determine how fast the car will slow down in this situation.

Regards
the Interceptor

EDIT: I must say I am really surprised that for a petrolhead forum, this technique seems to be so uncommon. This has been around in cars for many years, I even learned to do it in driving school.
 
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I don't think it's possible to roll start a diesel because you have no power for the glow plugs.

Well, you can because i've done it more than once. For the very same reason you pointed out: "This uses the momentum of the car to crank the engine and therefore turn the alternator to generate enough power ", not for a spark, but for the glow plugs.
 
You guys are making this really complicated.

Ok, simple answer. When the car is in gear and the clutch is engaged, the entire drivetrain from the crankshaft to the wheels is one piece. So the engine keeps turning cause the wheels are still turning.

And modern engine management is smart enough to detect when the driver wants to slow down. It's really easy actually, the computer looks at two variables, RPMs and throttle position. If the throttle pedal is closed and the RPMs are greater than idle then the computer goes "oh, he's coasting" and turns off the fuel injectors.

aight, been wondering about that for a while, if it turns them off completely, or keeps injecting idleing-amounts of fuel

(maybe a reason why you don't have (many) 2 strokers with injections? if it cuts off completely, there is no lubrication no more...- although it must be able to program a CPU to keep injecting a small amounth)
 
Are you guys getting confused my the de-acceleration fuel cut on most EFI cars?

Normally, say on a Nissan ECCS style computer (Skyline, Silvia etc) with a gearbox neutral switch (which detects if the gearbox is in gear or not) if you jump off the throttle in gear over 3000rpm the ECCS computer will cut the fuel to the motor, hence saveing on fuel as you de-accelerate.

If you did this out of gear the motor will quite literally stall (the new systems, say post 94 didn't stall but the motor wasn't happy about it), hence why you have the neutral position switch to prevent this from happening -- The ECCS computer changes modes, fuel cut de-acceleration is not avaliable. On these cars there is also a switch on the clutch, which does the same job as the neutral position switch -- Infact they are wired in paralell.

Some people remove the fuel cut de-acceleration system since if you want to jump back on the throttle there is a moment (not much, half a second tops) of pause.

In english keep it in gear when your slowing down...well at least if your Nissan powered car (had to add that there!) has the ECCS computer system in it ;)
 
:shock2: oh my god... if you read most of the answers in this thread you?d think this is anything but a car-forum :blink:

Lots of people seem to have some of the concepts correct, but reach the wrong conclusions... in the end the Interceptor is the only one who?s got it right and kept it simple...

one thing thats worth adding: when the rpms fall below a certain value (it?s between 1800 and 2000 for most 4-cylinder cars) the ECU will start injecting fuel again, so the engine doesn?t stall, the closer you get to idle rpms the more fuel will be going in...

I?ve never heard of a "neutral switch", perhaps some cars have it, but the way most modern engines work is what I explained above... when you rev the engine in neutral or with the clutch pressed (say because you have a small penis and you?re standing at a traffic light with people watching you) and then let go of the gas, the ECU cuts the fuel, rpms go down again, until eventually it needs to get fuel again so they won?t go down below idle...

hope its not too confusing...
 
^No, it's actually quite simple, which is why i don't understand all the confusion.
 
You can see it in modern cars with a consumption display:

roll in neutral to red light: ~0.6-1l/100km consumption displayed (same as idling)
roll in gear: 0l/100km dispayed.

I do it all the time, it really saves fuel.

one thing thats worth adding: when the rpms fall below a certain value (it?s between 1800 and 2000 for most 4-cylinder cars) the ECU will start injecting fuel again, so the engine doesn?t stall, the closer you get to idle rpms the more fuel will be going in...

Actually mine doesn?t. It starts shaking (~800rpm) and then stalls ;). Even when rolling.
 
One should really think that car enthusiasts know a bit more about how an engine works. Read more wikipedia guys. Its not that complicated.
 
Without reading other posts, I see it this way.

When you're engine breaking and drop below, let's say 2000 rpm, the onboard computer recognizes it and stops supporting engine with fuel. It that moment you break by the mechanic resistance of transmission components. When you shift up, put the car in neutral or begin to accelerate, computer simply restart to pump the fuel and because the engine is rotating ignition's forced automaticly.

Of course you have to have an electronic fuel injection.
 
Actually mine doesn?t. It starts shaking (~800rpm) and then stalls ;). Even when rolling.

well it seems there is something wrong with your car... :blink:
anyway to save fuel you should not only avoid high rpms but should also aim not to drop too far below 2000... as weird as it may sound...
EDIT: that goes for petrol engines, I?m not sure what the deal is with diesel cars...

Without reading other posts, I see it this way.

When you're engine breaking and drop below, let's say 2000 rpm, the onboard computer recognizes it and stops supporting engine with fuel. It that moment you break by the mechanic resistance of transmission components. When you shift up, put the car in neutral or begin to accelerate, computer simply restart to pump the fuel and because the engine is rotating ignition's forced automaticly.

Of course you have to have an electronic fuel injection.

very confusing... could you rephrase that? but anyway I suspect you?ve got it wrong as well...
 
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If you guys read my post carefully, you would see that the explanation has to do with conservation of energy, not with fuel management.

You are all looking at the situation from a rolling car, I ask how did the car get rolling in the first place? The momentum is the result of expending fuel. Even if in that moment you are not burning any fuel you are using the energy of previously burned fuel to coast. Therefore you still had to use fuel to get the car going in the first place.

Forget about engine management for a moment and just think about it like a scientist. You have to put energy into the picture somewhere or the car would never overcome it's own inertia. That input of energy is what I am talking about.
 
anyway to save fuel you should not only avoid high rpms but should also aim not to drop too far below 2000... as weird as it may sound...
You can drop below 2000 if you are going at a steady speed or stopping, but when you are accelerating revs should be higher than that.
 
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