Engine Braking and Fuel Economy

[RandallNeighbour]

So someone please show me that engine braking does not use any more fuel than putting the car in neutral and applying the brakes.

I tried to find something definitive on the web, but I only found debates both ways on it.

Can anyone point me to some empirical data on it (vs. a simple opinion based on one's current logic)?

My thinking is that if one is not depressing the gas pedal and the car's inertia is causing the high RPM's, the fuel tank is not being drained by engine braking. But if hard evidence proves me wrong, I'm happy to change my position.

[jonnycool]

i think i heard that some modern cars fuel supply is cut off under engine braking, i'm sure clarkson mentions it on top gear when he's testing the audi A8 diesel to see if he can get from edingbugh to london on one tank of fuel. I'm always knocking my truck out of gear to save fuel and use the brakes to slow down, with a gross weight of 44 tonnes i can roll approx 35 miles total on my usual 240 mile trip using various hills and slight gradiants.

[SoCalKen]

My first thought is that in an older carburated engine the verturi effect would be enough to get some fuel to tag along with the air flow but with efi, the injectors have vlaves that just don't open therefore not letting any fuel into the air stream.
Ken

[Lil bastard]

The accelerated wear to the clutch and synchros is gonna cost you 10 times the cost of replacing the brakes - use the brakes !

Neither method is gonna give any fuel saving !

[insite]

Originally Posted by Lil bastard

The accelerated wear to the clutch and synchros is gonna cost you 10 times the cost of replacing the brakes - use the brakes !

Neither method is gonna give any fuel saving !

if done properly, a blip-throttle downshift doesn't really use the synchros or wear the clutch. the important thing is not to slow the car by slowly releasing the clutch (using clutch slip to slow down). this applys reverse torque to the clutch friction surface & will cause accelerated wear.

as for fuel consumption, you use no more fuel than you would at idle when under engine braking. this is because the throttle position sensor (TPS) senses zero percent throttle & acts accordingly.

[RandallNeighbour]

Thanks for all this input. I fully realize that engine braking, when not done right with throttle blips, will wear out synchros and the clutch.

However, throttle blips use fuel.

I'm really focusing only on whether downshifting in and of itself uses as much, more, or less fuel than moving the stick to neutral and using the brakes.

From what has been posted thus far, it would appear that it does not use more fuel due to the computers shutting off the injectors.

[Samson]

Originally Posted by jonnycool

i think i heard that some modern cars fuel supply is cut off under engine braking.

This is true of pretty much all newer cars. No fuel is needed to keep the engine spinning when you are coasting down in gear. When putting it in neutral and coasting, fuel is needed to keep the engine running.
So, you use less fuel if you just coast down to the lights in gear, plus it saves your brakes. Downshifting during this process isn't really necessary on mostly flat ground.

[Lil bastard]

Everyone is sorta dancing around the actual cause here.

It's not result of engine braking which reduces fuel consumption.

It's the fact that the TPS senses that the throttle is closed which matters. At a closed TPS position, the DME only injects fuel sufficiently to keep the engine from stalling. Another downside to engine braking is that you can over time have a greater buildup of carbon in the valve train because of the lack of fuel being injected (this fuel partly aids in cooling the valves).

Practice lifting off the throttle and coasting to bleed off energy before applying the Brakes so you are dissipating the kinetic energy through rolling resistance instead of through the clutch and frictional forces in the drivetrain.

Agreed, you can engine brake efficiently and minimize excessive wear to the drivetrain. But, this requires doing it right each and every time. Most people, even after extensive practice, cannot do this consistently, partly because no two stops are ever exactly the same.

But, with coasting, you're usually off the throttle much sooner than with engine braking and so will probably save more fuel using this technique than engine braking over the long run.

[insite]

either way, we're splitting hairs here. modifying your driving one way or the other would probably result in near immeasurable differences in fuel consumption.

wanna save fuel? drive with your top up, inflate your tires to 36psi, go slow on the freeway, and accelerate at a boring rate. me? i bought a porsche precisely because i don't like to do ANY of those things (except keep the tire pressures proper)

[BoxsterLewis]

Are you guys really wanting to save a couple dollars of gas here? It seems to take more work to attempt to do all the things then just to drive freely. Gas has gone up 1.00 in the past 6 months and thats another 16 dollars per fillup, i think you guys make enough $$$

[Wret]

Originally Posted by insite

either way, we're splitting hairs here. modifying your driving one way or the other would probably result in near immeasurable differences in fuel consumption.

wanna save fuel? drive with your top up, inflate your tires to 36psi, go slow on the freeway, and accelerate at a boring rate. me? i bought a porsche precisely because i don't like to do ANY of those things (except keep the tire pressures proper)

Good point.

If you want your paint to last longer, don't wash it.

If you want your car to last longer, don't drive it.

If you want fuel economy, find an old Geo Metro.

If you want to have fun, step on all the pedals all you want.

[SoCalKen]

Originally Posted by Samson

This is true of pretty much all newer cars. No fuel is needed to keep the engine spinning when you are coasting down in gear. When putting it in neutral and coasting, fuel is needed to keep the engine running.
So, you use less fuel if you just coast down to the lights in gear, plus it saves your brakes. Downshifting during this process isn't really necessary on mostly flat ground.

That wuld make sense to me. If the computer reads the tps fully off it would send enough fuel to keep the engine at idle. If the RPMs are already higher than what it's supposed to be, then it would give less and less fuel until the RPM comes down to where it should be. Alas if you are idling in nuetral the engine needs fuel.
Ken