Does my car really use no fuel on the overrun?

Basically, I’d like to know: when cruising towards a set of red traffic lights, is it more fuel efficient to keep the car in gear with the throttle closed, or put it in to neutral? I have heard that modern engine management systems will shut off the fuel injectors in the former situation, so it will use less fuel than the latter because that requires fuel to keep the engine idling. Although this seems counter-intuitive because the former method results in higher engine revs (which usually gives higher fuel consumption), I can believe it. The problem is, my car isn’t really modern - it was built in 1993. It’s a 1.2 litre Vauxhall Nova, FWIW. I know the engine is fuel-injected and it does have an ECU. So, does this guarantee that it is clever enough to switch the injectors off, or had that technology not been developed at the time? It’s a toughie, but does anyone know?

I’m not 100% sure I understand the question. Do you have an automatic or manual transmission? With an automatic, I don’t see any point at all in placing the car in neutral at a stop light. With a manual transmission, you will normally have the clutch depressed, which is the same as neutral, unless you are on a hill and keep the clutch at the friction point. In that case, putting the transmission in neutral would also be pointless.

Shutting off the fuel injectors has a rather sudden effect on a fuel-injected engine: it stops. It should be reasonably obvious that stopping the fuel flow to an engine causes it to cease running.

Damn - I could have sworn I included the phrase “manual gearbox” in my OP, but it seems not :smack:. Yes, my car is a manual. My question does not relate to being stopped at the lights (there was a thread on that not long ago, IIRC), but the approach to them, when the car is moving but no impulse from the drivetrain is required to reach the stopline.

Cerowyn, that’s my intuition, too, but I believe that when the car is moving with the clutch engaged, the turning of the wheels keeps the engine running without requiring any fuel. Or not, which is my question, really. Is that any clearer?

If the engine is running, it’s using fuel.

No, that would be perpetual motion.

The engine requires fuel to run. If the wheels are turning while the engine is off, that’s called coasting.

Regardless of what you’re doing with transmission or clutch, if your foot is off the gas, and the engine is on, it’s going to be idling.

If you’re moving at some arbitrary speed and take your foot off the gas with the transmission and clutch engaged, you’ll experience “engine braking.” Gas is still being used - just not very much.

Not according to Wikipedia. From here (near the bottom of the page, under ‘Advantages’):

I’m sorry that I’m not making myself clear; the intention of this thread was not to reel in people with an obvious answer and then go: “Aha! You’re wrong! [cite]”. I’m glad that everyone so far shares my intuition. The intention of this thread is twofold. 1. Given that my only source so far is Wikipedia (and an uncited part of it, at that), is the above information correct? 2. If it is, then is it likely that the engine in my car operates in the same way? If the answer to both is yes, then it is slightly more economic for me to drive up to traffic lights with the clutch engaged, the engine in gear, and the throttle closed. If not, then I should put it in neutral and let the engine idle, since that will presumably use less fuel. Clear now?

No, because the friction of the system causes the car to slow to a stop, on a level surface at least.

Not if you have a gear engaged in your manual transmission. The engine speed will then be above idle, and, if Wiki is correct, not necessarily using any fuel.

That is indeed the effect I am talking about.

And this is where we disagree. I thought the same until I read something similar to my cite above. That’s why I want to know whether the cite is correct or not. So far three of you have indicated that it is not, but forgive me if I do not immediately accept your word without at least some justification.

I really hope I’m being unclear rather than completely stupid, here :).

You didn’t mention whether the hypothetical red light would be green by the time you got to it. If it will almost be green by the time you’d coast to it, you should probably immediately brake to the speed that will put you at the intersection when it turns green and put it in neutral, thereby allowing you to start thru the intersection at that speed, saving gas. Unless there are pesky annoyances called “other drivers” on the roads that do things like “pass people” who would be stopped at the light before you.

Go to about page seven of here Toyota manual (pdf) and you will find verification of the fuel shut off.

It does happen, and if you meet the criteria (high engine RPM + closed throttle) fuel is indeed shut off to the engine, and system power comes from shedding speed. Generally, this is only for a few seconds at best, since moving a decent distance in this method would involve aggresive down-shifting to keep the RPMs up, or winding the engine tight before-hand. Either method is probably more expensive in terms of maintenance (damaging to the system to continually near-redline) then cost-effective in terms of gasoline saved.

Also notice that Toyota’s system takes into account ambiant air temp, engine temps, AC demand, and many other not listed factors (I know transaxle temp is one). So even if you manage to find a sweet spot for DFCO it may not happen anyway.

(For clarification, all the systems I have seen from major automotive companies work in a similar fashion. I realize you drive a GM and not a Toyota.)

::: sigh::: We have been over this before.
No. Wrong.
All modern electronic fuel injection systems that I am aware of (and that includes a very large percentage of the market here in the US) shut off fuel on deceleration. The reason for this is NOT fuel savings, it is for emissions. Cars nowadays have to meet some very serious emission requirements. Leaving fuel injectors on during deceleration gives no advantage.* They also have a very serious disadvantage in that if you leave the injectors on you then have to deal with unburnt hydrocarbons, and CO.

This is why Bosch K-Jetronic fuel injection went the way of the dodo bird. The K stood for Konstant, this system just pissed fuel into the engine if there was air coming in. Because it sprayed fuel all the time air was flowing into the engine, it was too dirty to meet newer emission requirements.

As far as the details go, typically the system will look at throttle position (duh!) engine temp, and possibly air temp. If the throttle is closed above say 2,000 RPM the ECU stops activating the injectors until a preset RPM is reached, and then they are turned back on. This cut in RPM might be about 1300 RPM warm, and somewhat higher cold, say 1800 RPM. If the driver steps on the gas, the injectors resume right away, regardless of engine speed. This off and on function of the fuel injectors is imperceptible to the driver, but can be demonstrated in a shop using shop tools. I have shown this to my students hundreds of times.

I am not sure of the first system that used fuel cut, but I know Volvo has used it since mid 1982.

Getting back to the OP, assuming your ECU used a fuel cut strategy the difference in fuel usage would be so small I doubt you could measure it. But leaving the car in gear and allowing it to coast would use less fuel on a modern engine. I am not familiar with your model car and engine, so I do not know if uses fuel cut. I suspect it does, but I am not sure.
*The only advantage of leaving the injectors on, is it will keep the engine temp up. If you descend a very long grade, it is possible to have the engine coolant cool off to the point that the heater stops blowing hot air. This complaint shows up during the winter at some car dealers in Denver after cars descend the Eisenhower grade.

Thanks, Rick and Grave. I have found out that apparently my engine is a GM Family II engine, just in case that information helps anyone pin down whether it uses DFCO (deceleration fuel cut-off) or not.

Indeed, I do this too. What I wanted to know was whether, in doing this, I should be in neutral or in gear. The answer is still unclear (because we don’t know the specifics for my engine), but the thread has been progressively more helpful :).

IIRC, it was mentioned on Top Gear on the BBC when the host Jeremy Clarkson was doing an economy run in a diesel Audi. He mentioned it was designed to use no fuel when coasting to a halt ahead of traffic lights.

The Wiki article you linked tells about the base engine but nothing about the fuel injection system used on your car.
If it were me, I would leave your car in gear, as shifting into drive particularly while moving does put a stress on your transmission. Admittedly it is a small stress, but a stress nonetheless. Transmissions repairs are expensive, gas is cheap by comparison. Especially in the amounts we are talking about here at most maybe a few CC of gas.

Remember, a diesel with the “clutch” engaged provides SERIOUS engine braking. I had a vintage Mercedes diesel sedan that would rock you forward in your seat if you lifted off the gas, and would do it with more force than it would use in the other direction if you floored the gas.

Rick, does this mean that my Scangauge tool, which uses air flow to estimate gas consumption, is in fact overestimating fuel usage since it still shows 0.5 Gallons per Hour when I’m going downhill at 3000 RPM, presumably with the fuel cut-off activated?

Probably. If this is really of interest, you can either buy a Noid light to fit your car, or rig up a 12V test light across the terminals of a fuel injector. Start the engine, watch the light flash. Rev the hell out of it, and snap the throttle shut. Does the light stop for a moment? If so your system uses a fuel cut on deceleration strategy.
Word of warning, do not run the engine for long with an injector disconnected, the ECU won’t like it. Also some modern systems may trip a check engine light if it detects too high or too low a load on an injector circuit. Noid lights have the same current consumption as an injector.

About the diesels, don’t forget that modern diesels are electronically controlled, and can do things that older diesels could only dream of.

I think as a rule of thumb the cost of the amount of gas you would save over a lifetime of coasting in neutral under these circumstances would not offset the possible one occasion when you urgently needed to accelerate in a hurry and couldn’t spare the time to put it back in gear. I was always taught not to coast with the clutch out or in neutral, simply in the interests of having the car under full control. Anyone think they’re eking as much as five miles out of each tankful by coasting to a stop?

Mercedes installed a throttle plate for a couple of model years to provided this. “normal” diesels have much less engine braking than a similar displacement spark engine.

Wow. Thanks for that tidbit. Really, it was quite helpful in city traffic, I almost never had to hit the brakes until I was at… 5 MPH or so.