Pumping losses during no load closed throttle on Fi engines.

[dootdootdoot]

Given the assumptions (which may or may not be correct that a modern FI car with a TB has it closed during most "no throttle" deceleration, no fuel is being injected during this time, and most gasoline engines approach maximum efficiency at a reasonable rpm range depending on the gearing, and at ~2/3-3/4 throttle...

I was wondering if it would be more fuel efficient to cruise at some speed with the engine operating at half, or more than half of it's maximum efficiency. Or, provided the driver was able to accelerate appropriately, would periodically accelerating in the maximum BSFC area, then coasting in gear, with the same average speed, result in the same, better, or worse mileage?

In other words, does the efficiency gained by accelerating for ~1/4-1/8th of the time with proportionally more power and the approximate halving in BSFC get canceled out by the engine pumping air for the remainder of the time? I'm guessing this also may depend on CR, gearing, and displacement... among other things.

Also, would having the TB wide open with no fuel being injected (assuming one could "trick" the ECU into thinking it's closed) during a coast reduce the losses compared to TB closed coasting?

 

[crysta1c1ear]

I believe the Volkswagen cigar car, that got 285 miles per - UK I think - gallon worked by running the engine where it was most fuel efficient for a period, and then turning it off and coasting until the speed dropped sufficiently; then restarting the engine and repeating.

I know that isn't quite the same as coasting with the engine running, but the idea is similar.

 

[dootdootdoot]

That sounds right. Iirc the 3L lupo disengages the transmission when coasting as well. I'm just wondering if there are pumping losses when the TB is closed and no fuel is being burnt. I.e. when coasting does the vehicle slow down faster because of losses in the transmission/differential/engine spinning, or does the engine have significant pumping losses even when it's not burning fuel.

 

[MWPC]

Truckers have Jake Brakes to increase the CI engine resistance braking thus pumping losses must be much greater with the butterfly closed.

 

[Fabrico]

Turn the key off and try it. There is not a significant difference. Is pumping in a vacuum supposed to be harder than pumping in open air? If you block a household fan or shop vacuum cleaner, it immediately speeds up.

MWPC, a Jake Brake is not a very good example for this discussion beacuse it works by radically altering valve operation.

 

[dootdootdoot]

With an automatic transmission, I can keep the car in gear, turn off the engine, and coast without any problems? If that's true then I can just do a couple runs coasting in gear with the engine on/off and TB closed/opn and compare that to coasting in nuetral to see if the majority of losses come from the engine pumping air in/out of the exhaust when coasting, or from the engine/trans/diff spinning.

 

[dootdootdoot]

Ah, I gotcha with the vacuum analogy. But since the pressure in the cylinder when the exhaust valve opens is probably less than the pressure in the exhaust, wouldn't that result in some air being sucked into the cylinder only to be pushed out again, with the remaining air being proportional to the compression ratio at whatever pressure + exhaust heat, is outside?

 

[GregLocock]

I tried it in my car. The deceleration time, in gear, with the ignition off, was identical whether I had the throttle open or closed.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[dootdootdoot]

Sweet, thanks for the info! So this probably means that accelerating up to some speed in the minimum BSFC throttle/rpm area, and coasting down to a lower speed, with some average speed, is more efficient than just driving with minimal load/maximal BSFC at the same average speed. Obviously the coast can be extended by shutting off the engine and taking the tranny out of gear to net an even larger efficiency gain, but anyway... Thanks again!

 

[Fabrico]

By the way, this is for EFI only. It is not recommended if you have a carburetor, unless you just happen to be on your way to the muffler shop already.

 

[GregLocock]

Well, I am trying to blow my car up.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[patprimmer]

What has your poor car done to incur your wrath.

Is the slight gains from coasting worth the hassle in a normal car.

Can it be detrimental to automatic transmissions.


Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[affleck]

the situation is a little worse than you imagine; at closed throttle over-run your efi car is injecting fuel. It`s maint aining the 14.7 A.F.ratio with it`s O2 sensor and closed loop adjustments; this is to keep the CAT alive. The rationale for mpg is that you spend very little time in this mode. I build aftermarket efi controllers that do cut off fuel on over-run. These are non-smog applications without CATS. The difference in MPG is only noticed in special cases; such as here in Hawaii, where if you`re not going uphill, you`re going downhill. A recent conversion from stock on a 1979 2L efi VW van changed from 20 to 21.5 MPG. There`s one and only one correct engineering answer to the closed/open engine braking question; the open throttle produces more braking effect. The reference to the truck JACOBS braking system is false. The Jake brake holds the exhaust valves open continuously in order to increase "THROTTLE LOSSES", the correct engineering term. It was at one time appllied to gasolene truck engines, and is not linked to the Diesel engine functionly, as is commonly believed.

 

[Fabrico]

affleck

The reference to the truck JACOBS braking system is false. The Jake brake holds the exhaust valves open continuously in order to increase "THROTTLE LOSSES", the correct engineering term. It was at one time appllied to gasolene truck engines, and is not linked to the Diesel engine functionly, as is commonly believed.

Are you saying the Jake brake was geared more for gas engines and not commonly used on Diesels?

 

[mattsooty]

"the situation is a little worse than you imagine; at closed throttle over-run your efi car is injecting fuel. It`s maint aining the 14.7 A.F.ratio with it`s O2 sensor and closed loop adjustments; this is to keep the CAT alive. The rationale for mpg is that you spend very little time in this mode"

That's not true.

Modern EFI engines will cut fuel on overrun. On fuel reinstatement there is a large rich excursion to get the reduction reactions within the catalyst started again.
Obviously, only oxidizing reactions can occur with the small amount of shipping air that has been passing through them during the fuel cut.


MS

 

[automotivebreath]

diamondlarry at mpg research uses a specific driving technique to reduce fuel consumption. I'm not familiar with the details, I’m sure he would be happy to provide; he consistently reports mileage in the 50 - 60 MPG range.

http://mpgresearch.info/profile.php?mode=viewprofile&u=10

 

[GregLocock]

Incidentally Heywood has graphs relevant for the original query, p720 or thereabouts in the single volume paperback.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.