Does a car consume fuel under engine braking?

[mattpegher]

Originally posted by: JulesMaximus

Originally posted by: mattpegher
Given a hill sufficent to shift into neutral for any apreciable time the engine will return to idle which is usually less than the RPMs of coasting in high gear. Not that this makes it worth it unless you find a really long hill

Wrong. The engine is just spinning over in gear, the rpms don't matter because the engine is not pushing the drivetrain, the drivetrain is essentially driving the engine. There is little to no fuel going into the engine during engine braking.

False the feul pump never stops fuel supply to the engine. It is the lowest possible to avoid stalling but without feul supply the engine will stall.

 

[DivideBYZero]

Originally posted by: randay
This is interesting. I am not sure of any of this myself, and I am not convinced yet for either arguement, anybody here that can explain this in detail?

A fuel injected car shut's off the fuel supply when there is no throttle input and the RPM's are above idle.

/Thread.

 

[DivideBYZero]

Originally posted by: mattpegher

Originally posted by: JulesMaximus

Originally posted by: mattpegher
Given a hill sufficent to shift into neutral for any apreciable time the engine will return to idle which is usually less than the RPMs of coasting in high gear. Not that this makes it worth it unless you find a really long hill

Wrong. The engine is just spinning over in gear, the rpms don't matter because the engine is not pushing the drivetrain, the drivetrain is essentially driving the engine. There is little to no fuel going into the engine during engine braking.

False the feul pump never stops fuel supply to the engine. It is the lowest possible to avoid stalling but without feul supply the engine will stall.

SHUT UP! SHUT UP! SHUT UP! :|

 

[Vic]

Originally posted by: randay
This is interesting. I am not sure of any of this myself, and I am not convinced yet for either arguement, anybody here that can explain this in detail?

It's simple. A modern engine only uses as much fuel as is necessary to produce the power needed (and occasionally to prevent damage to the engine from pre-detonation or "knock"). It takes very little power to turn the engine over in neutral and at idle. It takes no power whatsoever from the engine to coast down a hill in gear (quite the opposite, the wheels are forcing the engine to turn over).

 

[randay]

Originally posted by: DivideBYZero

Originally posted by: randay
This is interesting. I am not sure of any of this myself, and I am not convinced yet for either arguement, anybody here that can explain this in detail?

A fuel injected car shut's off the fuel supply when there is no throttle input and the RPM's are above idle.

/Thread.

If thats true then yeah /thread. Can you link us please?

 

[FoBoT]

the difference is insignificant
forget about it

 

[Demon-Xanth]

Originally posted by: mattpegher
False the feul pump never stops fuel supply to the engine. It is the lowest possible to avoid stalling but without feul supply the engine will stall.

The fuel INJECTORS will only squirt as much fuel as the engine computer dictates they should squirt. Having fuel pressure in the rails doesn't mean you're using it. The fuel pump is driving fuel to the engine, but it's not using it. The fuel just goes back into the tank from the return line.

 

[JulesMaximus]

Originally posted by: mugs

Originally posted by: Serp86

Originally posted by: Vic

Does a car consume fuel under engine braking?

With modern cars, no. However, modern cars also use very little fuel while idling.

You should never coast in neutral. I'm not going to get into the ridiculous "OMG it's illegal!" garbage, I'm just going to say that it's a stupid and improper way to drive.

This subject has been beat to death on the internet many times in the past.

I live in malta - a very small island with lots of twisty roads and hilly terrain, hence the thread

I never heard of such a law here , however i would think reaction time under neutral coasting would be slightly faster, if you keep the foot on the brake like i do. Under an emergency all you have to do is just push one pedal (brake) as opposed to lifting foot off gas and pushing brake + clutch (not pushing clutch will kill engine + brakes + power steering). Now if you keep the foo of the brake, it can turn into a situation where you push the wrong pedal, but that can also happen when in gear - so whats the point?

Then again my car has never exceeded 90mph due to lack of straight roads so there might be something i'm missing

I think the reasoning behind not ever coasting in neutral is that you could be in a situation where you have to accelerate quickly to avoid an accident. You'd have to really stretch to find a situation where that is true though. And even if you leave the car in gear, you'd have to be in the RIGHT gear or you'd be better off in neutral - so every time you come to a stop you'd have to downshift through every gear.

To me, the slim chance of being in a situation where I'm slowing down and suddenly have to speed up is not worth all that. Call me stupid if you will, I don't care.

No you wouldn't. I drove a stick for 10 years and habitually downshifted when approaching a stop through one or two gears but usually just dropped it into neutral before going into second or first since I was going fairly slowly by that point anyway.

I never coasted any of my manual transmission cars in neutral for any length of time. There is no reason to. If you live in very hilly areas you should always have the car in gear and not ride the brakes.

 

[Serp86]

Another thought:

Engine braking can be VERY strong e.g. going 40mph and selecting 2nd for engine braking - everyone's heads jerk forward like crazy (Done it once when i just recieved it and inserted 2nd instead of 4th ).

I dont think the engine is so hard to turn as in the case of drivetrain turning engine. I mean - if it were, then all the engine would do would be to overcome its own resistance.

Might it be possible that engines, when they sense a no throttle/ over than 1000rpm situation, time the injections just so that they SLOW the engine down (effectively a reverse force on the pistons to slow drivetrain?).

It's not about an extra MPG, but i love how those things work, and the reasoning behind them. But an extra MPG would help

 

[randay]

Originally posted by: Vic

Originally posted by: randay
This is interesting. I am not sure of any of this myself, and I am not convinced yet for either arguement, anybody here that can explain this in detail?

It's simple. A modern engine only uses as much fuel as is necessary to produce the power needed (and occasionally to prevent damage to the engine from pre-detonation or "knock"). It takes very little power to turn the engine over in neutral and at idle. It takes no power whatsoever from the engine to coast down a hill in gear (quite the opposite, the wheels are forcing the engine to turn over).

That much is already established, the issue is whether the engine actually uses fuel in the latter case.

 

[Ausm]

Originally posted by: KLin
As long as the engine is running, it's consuming fuel.

:thumbsup:
Ausm

 

[JulesMaximus]

Originally posted by: mattpegher

Originally posted by: JulesMaximus

Originally posted by: mattpegher
Given a hill sufficent to shift into neutral for any apreciable time the engine will return to idle which is usually less than the RPMs of coasting in high gear. Not that this makes it worth it unless you find a really long hill

Wrong. The engine is just spinning over in gear, the rpms don't matter because the engine is not pushing the drivetrain, the drivetrain is essentially driving the engine. There is little to no fuel going into the engine during engine braking.

False the feul pump never stops fuel supply to the engine. It is the lowest possible to avoid stalling but without feul supply the engine will stall.

Not under engine braking it won't.

FoBoT is right though, the difference is immeasurable and not worth arguing about. My point is more that you should use the engine to slow the car and not coast in neutral. Especially in hilly areas.

 

[Demon-Xanth]

Originally posted by: Serp86
Another thought:

Engine braking can be VERY strong e.g. going 40mph and selecting 2nd for engine braking - everyone's heads jerk forward like crazy (Done it once when i just recieved it and inserted 2nd instead of 4th ).

I dont think the engine is so hard to turn as in the case of drivetrain turning engine. I mean - if it were, then all the engine would do would be to overcome its own resistance.

Might it be possible that engines, when they sense a no throttle/ over than 1000rpm situation, time the injections just so that they SLOW the engine down (effectively a reverse force on the pistons to slow drivetrain?).

It's not about an extra MPG, but i love how those things work, and the reasoning behind them. But an extra MPG would help

The injectors don't spray fuel because the force is what is turning the engine (like the starter) and the engine braking is caused by the fact that the air is still going through the compression strokes and it's still pumping quite a bit of air. Against the closed throttlebody, it's also creating a rather signifigant vaccuum. (which helps with vaccuum assist brakes that are on most vehicles)

 

[Don Vito Corleone]

Originally posted by: Ausm

Originally posted by: KLin
As long as the engine is running, it's consuming fuel.

:thumbsup:
Ausm

Wrong. I'm not sure why people find this so hard to grasp. When the car is coasting the drivetrain turns the engine, rather than the other way around. This is why you can start a manual-transmission car by popping the clutch while the car is going downhill.

 

[DivideBYZero]

Originally posted by: randay

Originally posted by: DivideBYZero

Originally posted by: randay
This is interesting. I am not sure of any of this myself, and I am not convinced yet for either arguement, anybody here that can explain this in detail?

A fuel injected car shut's off the fuel supply when there is no throttle input and the RPM's are above idle.

/Thread.

If thats true then yeah /thread. Can you link us please?

Page 5 - Throttle position sensor section.

 

[FoBoT]

is this on a treadmill? how fast is the treadmill spinning when you start braking?

 

[PricklyPete]

Originally posted by: Serp86
Another thought:

Engine braking can be VERY strong e.g. going 40mph and selecting 2nd for engine braking - everyone's heads jerk forward like crazy (Done it once when i just recieved it and inserted 2nd instead of 4th ).

You should not jump multiple gears until the car has appropriately slowed down and you should be rev matching to make sure that you enter the gear close to the RPM it will be running at based on car speed.

 

[Vic]

Originally posted by: randay

Originally posted by: Vic

Originally posted by: randay
This is interesting. I am not sure of any of this myself, and I am not convinced yet for either arguement, anybody here that can explain this in detail?

It's simple. A modern engine only uses as much fuel as is necessary to produce the power needed (and occasionally to prevent damage to the engine from pre-detonation or "knock"). It takes very little power to turn the engine over in neutral and at idle. It takes no power whatsoever from the engine to coast down a hill in gear (quite the opposite, the wheels are forcing the engine to turn over).

That much is already established, the issue is whether the engine actually uses fuel in the latter case.

It does not. Not that modern engines use a lot of fuel at idle either. A couple of winters ago, I got stuck in a CalTrans "chain-check" line before going over the Siskiyou Mts. on I-5. We waited for 6 hours, mostly at idle, creeping forward a few feet at a time. The effect of that 6 hour wait on gas mileage over a full tank (~300 miles)? NONE. I'm serious.

 

[randay]

Also on a related note, would it be more economical to engine brake more then regular brake, or regular brake more then engine brake. Meaning would coasting in neutral offset engine wear, gas vs maintenance? cost wise.

 

[Demon-Xanth]

Originally posted by: PricklyPete

Originally posted by: Serp86
Another thought:

Engine braking can be VERY strong e.g. going 40mph and selecting 2nd for engine braking - everyone's heads jerk forward like crazy (Done it once when i just recieved it and inserted 2nd instead of 4th ).

You should not jump multiple gears until the car has appropriately slowed down and you should be rev matching to make sure that you enter the gear close to the RPM it will be running at based on car speed.

Aww, but it's fun to dump the clutch in 2nd or 3rd gear when someone's riding your ass!

 

[JulesMaximus]

Originally posted by: DonVito

Originally posted by: Ausm

Originally posted by: KLin
As long as the engine is running, it's consuming fuel.

:thumbsup:
Ausm

Wrong. I'm not sure why people find this so hard to grasp. When the car is coasting the drivetrain turns the engine, rather than the other way around. This is why you can start a manual-transmission car by popping the clutch while the car is going downhill.

Is there an echo in here? :laugh:

 

[gsellis]

It depends on the fuel management system. As stated, carbed, burns some fuel. Some fuel inj engines burn fuel. Other good ones don't. F1 engine burns fuel (to avoid throttle lag from fuel flow inertia of on/off, F1 engine management justs dumps it to the exhaust. Which is why the big flames out the tailpipe under braking).

 

[PricklyPete]

Originally posted by: Demon-Xanth

Originally posted by: PricklyPete

Originally posted by: Serp86
Another thought:

Engine braking can be VERY strong e.g. going 40mph and selecting 2nd for engine braking - everyone's heads jerk forward like crazy (Done it once when i just recieved it and inserted 2nd instead of 4th ).

You should not jump multiple gears until the car has appropriately slowed down and you should be rev matching to make sure that you enter the gear close to the RPM it will be running at based on car speed.

Aww, but it's fun to dump the clutch in 2nd or 3rd gear when someone's riding your ass!

Of course there are always exceptions!

 

[Vic]

Originally posted by: PricklyPete

Originally posted by: Serp86
Another thought:

Engine braking can be VERY strong e.g. going 40mph and selecting 2nd for engine braking - everyone's heads jerk forward like crazy (Done it once when i just recieved it and inserted 2nd instead of 4th ).

You should not jump multiple gears until the car has appropriately slowed down and you should be rev matching to make sure that you enter the gear close to the RPM it will be running at based on car speed.

Driver error. Proper engine braking involves proper rev-matching. Proper rev-matching means you can literally dump the clutch out while downshifting and still be smooth. I can downshift at very high rpm with very little jerking (I won't say no jerking because I only do that kind of thing under VERY spirited driving, and that tends to be a little less than perfectly smooth ).

 

[Demon-Xanth]

Originally posted by: Vic
It does not. Not that modern engines use a lot of fuel at idle either. A couple of winters ago, I got stuck in a CalTrans "chain-check" line before going over the Siskiyou Mts. on I-5. We waited for 6 hours, mostly at idle, creeping forward a few feet at a time. The effect of that 6 hour wait on gas mileage over a full tank (~300 miles)? NONE. I'm serious.

From what I've heard, alot of the people that work at oil facilities in northern alaska leave thier engines running while they're at work. It keeps them from freezing, makes for less wear and tear (very cold starts), and nobody there is gonna steal your car without leaving thier own behind.