Theoretically According to Road and Track..

[Fleeingpepper]

http://www.metrompg.com/posts/xfi-pulse-and-glide.htm

In the thing from Road and Track, in the paragraph that starts out "It derives from Lesson Two"

In there it says that "Spark-ignition engines are most efficient when run at either full throttle or turned off." So am I correct to say that theoretically it is more fuel efficient to floor it?

Am I reading this correctly?

I guess RPMs would come into account and drop it some but still..

[Addicted To Boost]

I remember reading that when I had my 99 Metro..

 

Supposedly, it is more efficient to go for full throttle for short periods than to accelerate slowly up to speed over a very long distance.

[slick]

The engine does run more efficiently at full throttle than at, say 30% throttle, but you sure as hell aren't going to get great gas mileage with a lead foot.

 

However, the fuel is burnt more efficiently at full throttle, so it could be said that proportionally, you are using less fuel at full throttle than at 30% throttle.

[Fleeingpepper]

I see..

And obviously going higher in the rpm range will lower mileage as well. Oh well..I still should drive like a granny when money gets tight.

[Z34-5speed]

I read some study that some big university (UCLA or Cal I believe) tested the "floor and coast" technique. Basically they would floor the car up to a certain speed then shut it off and then restart it with the clutch and repeat, IIRC they got over 100mpg out of a brand new but completely stock early 90s Geo Metro XFI and right at or below 100 on an early 90s Civic (economy edition or somthing like that).

 

Lemme try and dig around and find the article.

[Addicted To Boost]

The XFI cars were awesome. I think just driving them normal they got like 60 MPG.

[Night Fury]

They were the only car to A)be the most fuel-efficient car in America every year it was made and the only non-hybrid car to win said honor. I want one.

[Z34-5speed]

My regular 92 Metro, with low compression on #1 cylinder got 45 in town no matter how i drove it. There is a guy on teamswift.net that makes either performance head and cam packages or economy head and cam packages. With his economy packager people have gained horsepower and about 6-9 mpg overall on their XFIs.

[Schurkey]

it says that "Spark-ignition engines are most efficient when run at either full throttle or turned off."

Spark ignition engines are throttled to control the amount of power produced. Partial power means a partial charge of fuel/air mix. By comparison, Diesel engines are power-limited by adjusting the amount of fuel sprayed into the cylinder. Diesels always get a full charge of air, but the lack of fuel limits the power produced.

 

The pistons in a diesel don't have to "fight" against manifold vacuum when they go down on the intake stroke. Think about how much force it would take to pull a piston down--at high speed--when there's ~15 or more inches of vacuum above the piston. The power required to pull the piston down against that vacuum is subtracted from the engine power; it's wasted power.

 

SO: If you can make a spark-ignition engine run without manifold vacuum--you've eliminated one source of wasted energy. Result is better efficiency. And the way to run a spark-ignition engine without manifold vacuum (or at least with very little manifold vacuum) is to whack the throttle wide-open. I term this "running at WFO".

 

Here's some of the tradeoffs:

1. At WFO--which produces little or no manifold vacuum (duh!) the fuel mix is adjusted extra-rich by the carburetor or fuel-injection mapping. The carb or computer tuner expects that you've put the pedal on the carpet BECAUSE YOU WANT MORE POWER, not because you want extra mileage. Now you're offsetting the benefit of not fighting manifold vacuum with a fuel-heavy mixture.

 

2. At WFO you could expect the engine RPM to increase. Increased RPM decreases fuel mileage and in virtually every car you're gonna be a prime candidate for a speeding ticket. Hense the WFO--shut off--WFO--shut off cycle. The coasting is to keep the vehicle speed in check.

 

What's actually needed is a cylinder-deactivation scheme, perhaps coupled to a totally separate engine. You use the "normal" engine for acceleration, trailer towing, etc. Then there's a very small engine--let's say the equivalent of a two-cylinder motorcycle engine of 250 or 350 cc. The little engine is run WFO but doesn't have the power to accelerate the vehicle--but it could maintain highway speed on level ground. It's mixture is set on the lean side (but not so lean that it holes the pistons!) The small engine has less parasitic drag (friction) than the regular engine; friction is another source of wasted power.

 

The obvious problem is that not all driving is done on level ground--and--what if you want to maintain 75 mph with four passengers instead of 60 mph and just a driver? The engine becomes so specialized that it's only good for one speed, one type of terrain, and with a certain vehicle load. So even your "economy" teeny-tiny engine needs to have "excess" power, then when the target speed is achieved, you cancel fuel flow to the cylinders to maintain that speed. Kind of like an RPM limiter, but on the fuel system rather than the ignition system or the air-flow system.

 

GM and Chrysler use cylinder deactivation on some of the LS-series and the Hemi V-8s. No need for the auxilliary engine; but you're still dragging all 8 pistons up 'n' down the bores even if there's no fuel being sprayed into them. They're "dead" but still have some friction so it's not a totally free ride.