Pumping losses during no load closed throttle on Fi engines.
Pumping losses during no load closed throttle on Fi engines.
(OP)
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?
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?





RE: Pumping losses during no load closed throttle on Fi engines.
I know that isn't quite the same as coasting with the engine running, but the idea is similar.
RE: Pumping losses during no load closed throttle on Fi engines.
RE: Pumping losses during no load closed throttle on Fi engines.
RE: Pumping losses during no load closed throttle on Fi engines.
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.
RE: Pumping losses during no load closed throttle on Fi engines.
RE: Pumping losses during no load closed throttle on Fi engines.
RE: Pumping losses during no load closed throttle on Fi engines.
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: Pumping losses during no load closed throttle on Fi engines.
RE: Pumping losses during no load closed throttle on Fi engines.
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.
RE: Pumping losses during no load closed throttle on Fi engines.
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: Pumping losses during no load closed throttle on Fi engines.
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.
RE: Pumping losses during no load closed throttle on Fi engines.
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.
RE: Pumping losses during no load closed throttle on Fi engines.
Are you saying the Jake brake was geared more for gas engines and not commonly used on Diesels?
RE: Pumping losses during no load closed throttle on Fi engines.
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
RE: Pumping losses during no load closed throttle on Fi engines.
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RE: Pumping losses during no load closed throttle on Fi engines.
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.