GCR: is it possible to make MPG worse by downsizing engines TOO much?

[Duffman]

Makes me think of the top gear fuel economy experiment between a prius and a BMW? on a racetrack. If you want to cherrypick an example you can make any premise correct I guess.

[gone-ot]

Originally Posted by Duffman

Makes me think of the top gear fuel economy experiment between a prius and a BMW? on a racetrack. If you want to cherrypick an example you can make any premise correct I guess.

...only 'works' when "...all other factors..." are equal, ie: weight, gearing, aerodynamics, drivers, etc.

[Duffman]

Originally Posted by Old Tele man

weight, gearing, aerodynamics, drivers, etc.

I dont think the Prius was cheated in any of these, it was cheated in that it was run WOT around the track and the other car was not WOT to keep up cause it was significantly more powerfull.

I dont disagree that the top gear episode was somewhat stupid, the point they were going for was that the driver was the biggest component towards good FE, which it is. But it does also make the case that if you have to run an engine at WOT all the time then it definitely is to small to get optimum FE.

[Duffman]

For those who have not seen it.
Start at 4:20

http://www.youtube.com/watch?v=JmxUsGiGp3w

[serialk11r]

Originally Posted by Duffman

I dont think the Prius was cheated in any of these, it was cheated in that it was run WOT around the track and the other car was not WOT to keep up cause it was significantly more powerfull.

I dont disagree that the top gear episode was somewhat stupid, the point they were going for was that the driver was the biggest component towards good FE, which it is. But it does also make the case that if you have to run an engine at WOT all the time then it definitely is to small to get optimum FE.

Wide open throttle is pretty efficient, come on, you're an ecomodder :P

A Prius is extremely efficient even at maximum output, since it essentially has a CVT. The problem is that it was probably really struggling through turns and braking a lot while the M3 shrugged the corners off at the same speed.

[radioranger]

In The boating world the 2 stroke direct injected outboards are reportedly getting better fuel efficeny than the same size larger four stroke, direct injection and no throttle body might be the ultimate answer.

[radioranger]

I think the guys driving the Ecoboost trucks are using all that torque way too often, killing the fuel mileage, pretty strong motor,

[jamesqf]

Originally Posted by Daox

BSFC charts show that an engine under high load runs more efficient than an engine at low load. This is why smaller engines work more efficiently. You can not 'work an engine too hard'. If we look at a BSFC chart you can see why.

One of us obviously doesn't understand BSFC charts. Is not efficiency represented by the contour lines? So in that chart, peak efficiency would come at about 2500 rpm and 124.7 ft-lb?

[gone-ot]

Originally Posted by jamesqf

Is not efficiency represented by the contour lines? So in that chart, peak efficiency would come at about 2500 rpm and 124.7 ft-lb?

...yep.

...like in these two Prius engine examples:




...and this chart:

[Duffman]

Originally Posted by serialk11r

Wide open throttle is pretty efficient, come on, you're an ecomodder :P

No its not because at WOT you are running the engine on fuel enrichment, you would want to run at the maximum throttle opening that permits stoichiometric air/fuel ratio. Maybe thats 80% or 90%, I really dont know the point, but WOT is not where its at for FE on a stock production gasoline engine.