Engine Braking and Fuel Economy
 

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.

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.

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

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.

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.

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.

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.

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)

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 $$$ :cheers:

How much is perrier per gallon, how about a Starbucks latte.

Folks are funny around fuel prices.

:)

BTW- I asked this question last year.

How much SHOULD gas cost and how do you know that?

:dance:

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

According to the API, it costs about $60/bbl. to drill, pump and refine a barrel of crude from on-land and coastal-water rigs and upwards of $90 for deep-sea rigs.

One barrel of crude equals 42 gallons. This will yield 47% (19.7 gal.) gasoline, 23% (10 gal.)heating oil and diesel fuel, 18% (7.5 gal.) is refined to other products including petrochemical feedstock such as polypropylene, 4% (1.7 gal.)propane, 10% (4.2 gal.) jet fuel, and 3% (1.26 gal.) asphalt. (Percentages sum to over 100 because there is approximately a 5% processing gain in refining.) - source: Energy Information Administration - http://tonto.eia.doe.gov/ask/crudeoil_faqs.asp

Not me! I started the thread because a friend of mine is emphatic about coasting with brakes vs. engine braking for fuel economy's sake alone and I completely disagreed with him on the basis of sound logic.

I wanted some empirical data to back up his claim or mine so I could settle the argument.

Still haven't seen any yet. Great discussion and more sound logic to bolster my position in this disagreement, but no empirical data (charts, testing, results, etc.)

You could just buy a ScanGauge II and find out exactly when fuel is and isn't being delivered (along with a bunch of other info).
http://www.scangauge.com/

Randall,

You could just ask me...

- Mark

Having said that I agree with your friend. With your car in neutral you will start coasting sooner resulting in less fuel usage. End result you will probably be off the gas and coast for 3000 yards vs. waiting to engine brake and be of the gas for only 2000 (coasting numbers random) yards. I don't believe it's the actual braking process but the habits associated with it, ie: initiate fuel saving coasting earlier.
Ken

That MAY be the cost of procuring the oil to gas. The second question is how much should it sell to us for?

:D

Great! You've got my email address... shoot me a copy of the empirical data on engine braking and the effects it has on fuel economy in your possession.

Opinions based on sound logic have already been shared, so there's no need for any more. I want a published study by a reputable source. Got one?

Without reading any of these responses I will say that I tested this out the other day.

I filled up my tank and then reset the onboard computer to check my MPG.

It is only 1/2 a mile from the Shell to my house so while driving home I blipped the throttle when downshifting and braking and I watched as the MPG numbers went WAY down. It was 19.2 and dropped to 15.4 by the time I made my final turn into my garage.

When the RPMs go up the MPG goes down.

As a general rule, I've been following the "coast to a stop" philosophy. It seems to make much more sense to me to have the engine at idle as I coast from say 30-40 mph than to have the idle at at any other RPM. I also think its fun to see how well I can use my momentum to use the brakes as little as possible to come to a stop. This requires you to look far ahead as possible which is a trait you need in motorsports so its kind of putting theory into practice.

OK now you open up a whole new can of worms...

Crude Oil is traded as a commodity and therein lies the problem. As a commodity, it's price is based primarily on Investor Confidence - confidence that an Investor can buy it and then later sell it at a profit !

Geo-political environment and events play a much greater role in establishing this Investor Confidence than do Supply and Demand (though these too play a lesser role).

Having it commodity-based allows for much greater manipulation of the price than if it were simply a raw material bought and sold as such. There is no way of setting a price of how much it should cost us... only what we're both willing and able to pay.

Indeed, There is no SHOULD, there is only the point at which supply and demand meet.

coasting in neutral, instead of in gear... we're talking pennies here.

my take on it, leave it in gear, why, because if you don't, its DANGERIOUS! leave it in gear. when you are in neutral, you loose out on engine braking should you need to panic stop, should you need to evade something, you now have to put it back into gear, then accel out from whatever it was that you were about to hit.

FWIW, when I took my driving test years ago, the only point deduction that I got, and I almost failed because of it... I put the car in neutral too far before the stop sign, (about 100') I was already under 10mph. And some of the people here are talking about coasting it at 40mph? wow! i'm guessing you're coasting for a several hundred yards? are you at least leaving one hand on the shifter, so that you can get it back into gear quickly?

reasons I didn't buy a Porsche:

- to save on gas, even though this is my most fuel efficient car right now.
- to save on tires and purchase all-season touring tires

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.