Engine Braking and Fuel Economy

[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 $$$

[Brucelee]

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

Folks are funny around fuel prices.

[Brucelee]

BTW- I asked this question last year.

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

:dance:

[Lil bastard]

Originally Posted by Brucelee

BTW- I asked this question last year.

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

:dance:

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

[RandallNeighbour]

Originally Posted by BoxsterLewis

Are you guys really wanting to save a couple dollars of gas here?

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.)

[Samson]

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/

[Houston C4S]

Randall,

You could just ask me...

- Mark

[SoCalKen]

Originally Posted by RandallNeighbour

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.)

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

[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