Engine braking with stick shift

[desura]

When the RPMs spin up with engine braking, does this use up gasoline?

What I'm thinking happens is that if you're not hitting the accelerator, then the engine is just using up as much fuel as it would normally in idle. This is because if I am in a parking lot and am in 1st or 2nd gear and I just ease up on the clutch without hitting the accelerator, the car moves forward.

Or does it use up gasoline? It seems that neutral coasting doesn't really save fuel at all.

 

[Jumpem]

No, it does not. Coasting in neutral uses more fuel.

 

[Zenmervolt]

Modern cars have a deceleration fuel cut-off. If the engine is above a certain RPM and the throttle is closed, the injectors stop firing and the engine uses no fuel.

The injectors start firing once RPM drops below a set threshold (usually between 1,200 and 1,500 RPM for a warm engine) so that the transition back to idling is smooth.

That said, the amount of fuel saved through this method is very small; most modern engines can idle for something like an hour on a half-gallon of fuel (once they're warm).

ZV

 

[nerp]

DFCO helps you conserve gas while downshifting but as Zemmervolt said, the gains aren't large and only occur at certain RPMs.

 

[LTC8K6]

The newer Chrysler products actually indicate when DFCO is active.

 

[Insomniator]

Whats modern though? Would a 2004 mini shut off the injectors?

 

[thomsbrain]

Whats modern though? Would a 2004 mini shut off the injectors?

Almost certainly.

 

[JulesMaximus]

Whats modern though? Would a 2004 mini shut off the injectors?

Pretty much any fuel injected engine would shut off the injectors completely on decel in gear.

 

[Don Vito Corleone]

I think any EFI engine would (i.e., nearly any fuel-injected engine made in about the last 30 years), but I don't know about older mechanically fuel injected cars. A 2004 Mini, most definitely.

 

[LTC8K6]

Whats modern though? Would a 2004 mini shut off the injectors?

My 1995 Taurus did, as did a 1996 Lumina and a 1997 Z24.

Get a Scangauge and you can see it going on.

This is why you get that ridiculously high instant mileage reading when decelerating.

 

[Zenmervolt]

Whats modern though? Would a 2004 mini shut off the injectors?

It was a feature of Bosch Motronic from introduction and was used in L-Jet as well. So "modern" in this sense means anything with EFI from the mid-late 1970s to present.

I'm sure there are a few late 1970s/early 1980s EFI systems that didn't incorporate it, but by the late 1980s DFCO should basically be universal in EFI engines.

ZV

 

[overst33r]

Modern cars have a deceleration fuel cut-off. If the engine is above a certain RPM and the throttle is closed, the injectors stop firing and the engine uses no fuel.

The injectors start firing once RPM drops below a set threshold (usually between 1,200 and 1,500 RPM for a warm engine) so that the transition back to idling is smooth.

That said, the amount of fuel saved through this method is very small; most modern engines can idle for something like an hour on a half-gallon of fuel (once they're warm).

ZV

Just to provide some real world idling numbers (calibrated scan gauge, AC OFF)

1.5L Yaris .12 GPH
1.8L Civic .16 GPH
2.2L S2000 .34 GPH
5.7L Z06 .64 GPH

 

[Mark R]

Overrun fuel cut off is pretty much universal these days.

The only catch is that on more modern cars (after about 2005), the cut off is only temporary. During a long period of overrun (e.g. when descending a long hill) the ECU will restart the fuel flow to prevent the catalytic converter from overloading with oxygen and cooling down.

 

[Zenmervolt]

Just to provide some real world idling numbers (calibrated scan gauge, AC OFF)

1.5L Yaris .12 GPH
1.8L Civic .16 GPH
2.2L S2000 .34 GPH
5.7L Z06 .64 GPH

Yup. The '87 924S I used to have had an MPG/GPH gauge (two scales, would switch over from GPH to MPG above ~10 mph) that would sit a bit below 0.5 GPH at warm idle for its 2.5 litre 4-cylinder. The gauge was driven off of the injector duty cycle signal from the ECU (and the vehicle speed signal when it was in MPG display).

When you get right down to it, it's rather amazing how little fuel it takes to make an engine idle (or even start) when it's warm.

ZV

 

[natto fire]

When you get right down to it, it's rather amazing how little fuel it takes to make an engine idle (or even start) when it's warm.

ZV

Or on the other side of the coin, how much fuel it takes to overcome frictional and pumping losses when the engine is doing little to no effective work. I guess some newer GM (maybe other makes too) eliminate this through shutting the engine down at extended stops. There are the hybrids that seem to all feature this as well. It doesn't seem like much, but when you multiply it by the number of cars wasting fuel idling on choked roadways it will help.

 

[leeb2013]

The Ford Falcon over here in Oz, which uses the 5.4 V8 F150 engine up to 2006, doesn't shut off the fuel on deccel, but the Renault scenic launched 10 years earlier does. So it varies from car to car.

The reason why the car moves forward on idle is because the engine is producing enough power to do so, also modern engines control the idle speed, so as it drops when you engage the clutch, it brings the idle back up.

 

[iamwiz82]

In most stick shifts you can tell when the injectors kick back in when decelerating. There will be a slight vibration and the tach will move a hair. It's quite subtle but it's there.

 

[Zenmervolt]

The reason why the car moves forward on idle is because the engine is producing enough power to do so, also modern engines control the idle speed, so as it drops when you engage the clutch, it brings the idle back up.

It takes remarkably little power to move a car forward in 1st gear. The starter motor alone can do it and those typically have 1.5 horsepower or less.

With a torque converter the power required is even less since the converter's built-in slippage has a torque multiplying effect.

ZV

 

[JulesMaximus]

It takes remarkably little power to move a car forward in 1st gear. The starter motor alone can do it and those typically have 1.5 horsepower or less.

With a torque converter the power required is even less since the converter's built-in slippage has a torque multiplying effect.

ZV

True, I had a '73 SAAB with manual transmission as my first car when I was in high school. The clutch cable broke when I was at work one evening so I got home by starting it in gear and then shifted using rev matching. Ever traffic light I would turn the engine off as I was coming up to a light, put it in gear and start the car in 1st to get going. Worked like a charm! Can't do that with modern cars though. :biggrin:

 

[Zenmervolt]

Can't do that with modern cars though. :biggrin:

Sure you can. Just have to jumper the switch that detects whether the clutch pedal is engaged.

ZV

 

[skyking]

I use DFCO on the way to work quite a bit. There are a couple of nice downhills where I coast in neutral, then use the minimal engine braking in 5th and 4th to time the lights out.
The TDI idles a bit less than 0.10 GPH, so coasting in neutral is also good.
I'll know in a couple of weeks how I am doing.