Engine theroy regarding power and economy.

Didn't top gear run a v12 BMW and a prius on a track at 85mph. The BMW had higher mpg due to the engine not working as hard as the prius. My prius gets 13 mpg on rallycross.

Flight Service wrote:

codrus wrote: Pumping losses would only be different if the engines were being throttled down to less than max power. Since they're running at > 100% VE, we can assume the throttle is wide open, intake manifold is at ambient pressure, and there will be no difference in the pumping losses between the engines.

can't make that assumption. Most engines don't run at 100% VE at WOT, especially these days of fat powerbands. Peak combustion efficiency doesn't happen at WOT. which is essentially what you are talking about.

According to the chart you just posted, engines ARE the most efficient at WOT. Lower Y-axis means less fuel needed for a given output.

In the real world, you'll get best efficiency at somewhat less than WOT due to fuel enrichment at WOT. But closer to WOT does mean less pumping losses.

rslifkin wrote: In the real world, you'll get best efficiency at somewhat less than WOT due to fuel enrichment at WOT. But closer to WOT does mean less pumping losses.

Which is why EGR increases fuel efficiency. Putting exhaust gasses into the intake manifold dilutes the intake air, which means you can open the throttle wider while making the same cruise-maintaining horsepower, thus fewer pumping losses.

stan_d wrote: Didn't top gear run a v12 BMW and a prius on a track at 85mph. The BMW had higher mpg due to the engine not working as hard as the prius. My prius gets 13 mpg on rallycross.

I'm going to break into the technical discussion and say it is berkeleying AWESOME that you RallyCross your Prius.

Now back to your regularly scheduled discussion.

rslifkin wrote: In the real world, you'll get best efficiency at somewhat less than WOT due to fuel enrichment at WOT. But closer to WOT does mean less pumping losses.

In the real world, many engines are still knock limited at loads near wot, too. Another reason why EGR can be a real increase of fuel economy.

On that note- a lot of people like to look at pumping efficiency mostly in their examination, not remembering that it's only part of the full combustion efficiency equation.

I worked with one group trying to get the best of the best fuel economy- so the engine ran at WOT at low speeds. Except that looking at the pressure trace- combustion was terrible- it was both terribly knock limited and unstable (too low of speed). Making the exact same power 500 rpm higher with EGR was significantly better.

rslifkin wrote: In the real world, you'll get best efficiency at somewhat less than WOT due to fuel enrichment at WOT. But closer to WOT does mean less pumping losses.

The parasitic losses are also minimized, in the sense that they end up being a smaller percentage of engine output. The oil pump always takes X horsepower at Y rpm, the waterpump always takes Z, the accessory drive takes W, regardless of the actual load on the engine. If you're just barely above neutral throttle, then most of your engine power is going towards feeding those losses. If you're loading the engine down heavily, more of the combustion pressure/temperature is making it out to the flywheel.

So. If you go to electric P/S to eliminate a major source of accessory loss, you have 50-80% of the cars on the market today. Electric water pump and variable displacement oil pump, another growing percentage. Build the alternator into the flywheel and run an electric A/C compressor so you can eliminate the accessory belt altogether and you have most hybrids...

This is a lot of why I like hybrids. In one fell swoop they eliminate a lot of the things that are inefficient about the gasoline-engine automobile. And this is before considering that the drive arrangement permits you to design the engine to be maximally efficient in a very specific and narrow range, because engine speed and load is no longer so directly linked to vehicle power requirements. It's like the world's most badass CVT that also lets you store excess energy to expend later.

And this brings me to rotaries, which can get BSFC down into the Diesel realm at WOT (bridge and peripheral port engines will touch .4 lb/hp/hr at 100% load) but are hideously inefficient at light load. I wonder if Audi is still working on a series hybrid that uses a small Wankel in the trunk as the APU. Perfect application for the little bugger.

I was hoping to keep each cylinder the same and let the displacment be the varible. Guess its just not that simple. Good disscussion everyone.

something else I thought of earlier... involving friction and VE

a 4 valve engine spinning at high speeds is much more efficient than spinning at low speeds.. but has a lot more frictional surfaces

a 2 valve engine is more efficient at lower engine speeds, but has less friction

4 valve engines also generally have significantly lighter valvesprings and a lot less valve lift, so frictionally it is probably a wash. Certainly if there were an efficiency advantage to 2-valve engines, we wouldn't have practically every engine on the market be a 4-valve arrangement.

Because there is so much more available valve area, the 4 valve engines need less left and also less duration, so 4 valve is an advantage at low speeds as well.

In reply to Gearheadotaku: A different consideration is durability.. All over the road semi engines of great durability (think a million miles or more) are in-line six cylinders.. due to balance issues. Detroit Diesel made a powerful 4 cylinder that was wonderfully fuel efficient but relatively short lived.. Caterpillars 3406 (in line 6) was one of those million mile motors while it's 3208 (V8) was a short lived disposable (cost more to rebuild than simply buying a new one) diesel..

In short six in a row is the way to go......

frenchyd wrote: In reply to Gearheadotaku: Caterpillars 3406 (in line 6) was one of those million mile motors while it's 3208 (V8) was a short lived disposable (cost more to rebuild than simply buying a new one) diesel..

That was by design though. The 3406 is a wet liner engine meant to be easily rebuilt indefinitely. The 3208 isn't, it's a standard bore in block setup like a typical gas engine. That's the cause of the reduced rebuildability. Durability wise, the 3208 wasn't as good, but it certainly wasn't bad. The "throwaway" name comes from not being wet liner, not an actual knock on its durability.