Fuel/Braking Question....

[momo]

well its been quite awhile since I made a new thread and i had a question sooo...

here's the question: this is going to sound stupid but, remember I'm 14 so i dont have much real experience with cars.I was wondering if it takes less fuel to just let off the gas and slow down than to put the clutch in when you slow down/coast.

I thought it was better if you just used engine braking to slow you down because the ECU isn't needed to provide fuel to the cylinders because it can keep running by the inertia of the engine and the car....right?

So basically what I'm asking is if the ECU is still providing fuel flow to the cylinders when you take your foot all the way off the gas and its not at idle. Because I thought the idle was for to start adding fuel when the rpms got down to a certain point.

My dad and I got in a quite long(and a bit heated, too) discussion over this and I was just wondering. I think I'm right

[Die Frau]

Good question.

I don't know the answer to be quite honest, but I am sure that which ever way is more efficient is only marginally more efficient.

[silver saloon]

My guess is that the ECU is still providing fuel to the cylinders even when you let your foot off the throttle.

[V_hares]

or option three is taking it out of gear too

[Stumanbmx]

If you are coasting in gear no fuel is being added until the RPM's drop down past a certain point. (Most likely different for different engines.)

Correct me if I'm wrong.

[hamradio]

I don't know...if you put it in neutral or clutch in, you're still using up brakes to slow you down. Downshifting, you may or may not use fuel, depending on the vehicle. I think a diesel might give a tad more fuel when you downshift and hit maybe 3k, as the pump is spinning about 3x faster than idle, etc. With a car with CIS, maybe, as the car would be pulling more vacuum and moving the airflow sensor maybe a little more. But with downshifting, you would be using the clutch, too, and potentially putting wear on it. I think it's a trade off, it depends on what vehicle I drive, I think. I'll downshift when driving my friend's ranger, but I never really downshifted to slow down in my rabbit, I just didn't like the way the engine was when I'd downshift to slow down. The last time I had to rely on downshifting to slow down, was when I blew a rear wheel cylinder in the rabbit.

[silver saloon]

or option three is taking it out of gear too

true. i was gonna say that too..you probibly spend more money letting it rev then you would on a new clutch

[aaronmjr]

I put the car in neutral to cause less drama for the engine. I'd much rather replace brake pads at closer intervals than engine/clutch parts regardless of which way is better for fuel!

I know it's just healthy discussion but if someone was that worried about fuel economy to consider this important...they are pinching pennies HARD! lol.

[momo]

Ok thanks guys, i thought it was kinda something that everyone knew so no one talked about it. kinda just picking nits.

[SlimMJS]

higher revs and more throttle both mean more gas
coast to a stop in neutral (clutch in) unless you're not slowing down to a stop
downshift/engine-braking if you are slowing to go again
abuse your brakes more than your engine

[hamradio]

higher revs and more throttle both mean more gas
coast to a stop in neutral (clutch in) unless you're not slowing down to a stop
downshift/engine-braking if you are slowing to go again
abuse your brakes more than your engine

Don't know if I already said it in this thread, but my dad always tells me, "Brakes are cheaper and easier to replace than a clutch or engine.

[bagpipe goatee]

It depends on the fuel management of the car.

An old IDI diesel or a CIS car will keep putting fuel in. On the IDI, fuel is dependent on rpm and throttle position (and boost, if you have a TD). On CIS, it will keep pulling air through, lifting the throttle plate a bit. In these cases, pushing in the clutch would be more economical.

On modern fuel injection cars (my mom's b5 passat, mk4's, my megasquirted 16v) It recognizes the engine braking situation and cuts fuel. In these cases, it's more economical to engine brake, to an extent. In megasquirt I had to set it to do that myself.

The few people I know that learned to drive in Europe engine brake through the gears, that's too much for me. (extra wear to clutch, takes gas to speed up the engine so it's nice to the synchros, etc) I engine brake until a little above idle, then push in the clutch and brake normally.

[Die Frau]

Engines are made to run. Engine braking is not hard on them. And if you rev match, you clutch will see very light use. Truckers use engine braking and those things see hundreds of thousands of miles.

I engine brake all the time.

I would venture a guess that 90% of clutch wear comes from slipping the clutch from a stop. Once the transmission is turning and the car is at speed, there is little wear and tear. I have never heard of someone killing a clutch by braking. 3rd gear burnouts is a different story.

[bagpipe goatee]

Oh, I agree. But revmatching through the gears just right in a rabbit is too much work and doesn't really matter. On a truck, on the other hand...