esmarkey

Hey, wasn't there a forum on mileage/hyper-miling?

Anyway, my question is:
If you lift your foot off the throttle while in gear is there ANY fuel going through the injectors?

The reason I ask is that I have a habit of putting my cars in neutral and coasting when I am coming up to a stop/red light and nobody behind me. I figure I am saving on fuel and brakes, but got to thinking that it takes fuel to make the engine idle, but not when it is in gear and moving.

Are there differences in engines (2.0, 2.2, 2.4), or auto vs. manual?

Being an Engineer, I think about stupid stuff like this

GDZHHR

The engine require fuel to run, period. It will run at a much lower rpm, lower rpm=less fuel used, without pressure on the throttle, but it still requires fuel or else the engine would stall.

masterchief1112

theres alwas fuel going through the injectors i believe. i do the same with neutral and i know i save on gas while doing it.

esmarkey

That's not true, haven't you ever ran out of gas? If you were doing 75mph on the expressway and ran out of fuel while in gear (manual trans). The engine would continue to turn until you came to a complete stop.

I would think that in this fuel economy minded world that the guys who program the ECUs would have zero fuel programed while coasting, but not sure? There may be a reason fuel is needed?
Yes, I know that below a certain RPM you would want fuel going to the engine so that it does not stall when the clutch is pushed in.....

Come on now, anyone know for sure?

GDZHHR

I have run out of gas, the engine stalled and only the momentum kept the car going until it stopped.

The transmission may crank the motor but it will not be running or producing any power. Try jump starting a car with a manual. Sure it will turn the motor, but without fuel, it will NOT start and continue to run.

It's quite simple, no fuel, no running engine.

Think about an even simpler engine, a lawn mower. No matter how many time you crank it, it will not run/start without fuel.

pitbull76

Mike, I don't think esmarkey is directly arguing with your point. No gas definitely means the motor produces no power, but the deal is some cars (and I think the HHR is one of them) will cut fuel to the motor after you are coasting for a certain amount of time+ above a particular speed. The engine continues turning, but uses no gas and produces no power. As soon as the vehicle speed gets too slow to keep the motor turning, or the gas pedal is pressed, the injectors will open back up and gas will flow again.

JoeR

Well, so far nobody has given the correct answer.

Most of the newer engines today have what is called DFCO (Deceleration Fuel CutOff). When certain operating parameters are met, fuel is cut to the injectors. It helps with both mileage and emissions.

When the engine approaches idle, fuel resumes to prevent stalling.

I see it all the time with my wideband O2 sensor and during tuning data logs.

Whether manual or automatic, putting it into neutral will circumvent DCFO. So, for mileage, you are better off just leaving it in gear.

Hope that helps.

EDIT... Well pitbull answered right while I was typing.

GDZHHR

Thanks for the real poop Joe!

JoeR

Real poop... anytime!!

esmarkey

In the below situation I do not see why fuel would need to be flowing into the injectors.

I am going about 65mph on a rural highway. About 1/2 mile ahead of me a traffic light turns yellow. I lift my foot off of the throttle and start to decelerate. Why would fuel be required as I am decelerating (Like the lawn mower running out of gas and the blade slowing). I know at some point under 1,000 RPM, I would need to push in the clutch and fuel would be required for the engine to continue running at that point, but why during deceleration from high speeds?

2nd situation, same hwy, same speed, same light 1/2 mile up the road turning yellow, only this time I push in the clutch and put the car in neutral. Now, I know there has to be fuel to keep the engine idling.

I don't know either way, I just wanted to understand myself, so don't feel that I am arguing