Engine efficiency

[pete c]

Originally Posted by drmiller100

it would melt and you might remember to breathe.

Nah, I'm usually OK with that breathin' thing, 'cept when I get a nasty head cold. If I'm stuffed up bad and somebody hands me one of those 12 lb hershey bars, it can get ugly fast.

[Ken Fry]

Originally Posted by Ken Fry

I think you'll find the two engines very close in BSFC at that HP.

You could then convert from BSFC to % efficiency, and plug that number into this site's mileage calculator, remembering that only one line in the chart will be accurate, because for all other lines the BSFC will be different.

I played around a little to get a car that required about 22 hp at 70.

Calculator

This would be in between a 2004 Civic and 2004 Accord (which I use because I have the latter, so know its fuel efficiency). With average tires (.010), .35 Cd, 20 sq ft, 3200 lb, and .3 engine efficiency (which I guessed at right now, rather than actually looking at the Saturn chart) the MPG at 70 is 38.6. My accord will do 34. My accord is fairly immune to variation in cruise speed. I've cruised at 80 for long stints, even with a few hills, and it still gets 34 or close.

This points up a gotcha in the calculator chart, that being that as speed drops, engine efficiency drops too, so the figures at 80 (31.5) and 60 (47.9) are invalid: one too low, the other too high.

Your car will use significantly less than 22 hp at 70, of course.

[drmiller100]

Hey Ken,
I'm trying to wrap my head around some things as you know. So we all "know" a smaller engine is more efficient then a big engine, right?

But, if take a 500cc engine, in theory we could find an RPM and run it at WOT to get our 22 horsepower.

In theory, we could take a 1500cc engine, and run it at 1/3 the RPM, at WOT, and get our 22 horsepower. So, given this example, out 1500 cc engine would be as efficient as the 500cc engine, right????

of course the bigger engine would be a lot heavier, as would the rest of the drive train.

The CX/VX honda engines were designed for this I believe - 2 valve per cylinder, teeny intake and exhaust, and higher compression.

Hey. Another question. Back to the monster diesels sort of. If I have a 1500cc engine at low rpm, will I lose less power to the cylinder walls? Or does the extra time because of the low rpm cancel that out?

maybe I'm over analyzing this - just looking for opportunities.

[UFO]

A smaller engine is not necessarily more efficient than a large one. But for a certain fixed load, if the small engine can support it with all else equal, it will be more efficient.

[Diesel_Dave]

IMO smaller engines are thought of as more efficient because in order to get the same power out of the smaller engine, you have to run at a higher %load. That's where the higher engine efficiency is coming from--not from the smaller engine itself. If the larger engine were at the same %load, it would be more efficient.

Another thing to remember about running lower rpms is that both friction and pumping losses decrease with speed.

[user removed]

Originally Posted by drmiller100

Hey Ken,
I'm trying to wrap my head around some things as you know. So we all "know" a smaller engine is more efficient then a big engine, right?

But, if take a 500cc engine, in theory we could find an RPM and run it at WOT to get our 22 horsepower.

In theory, we could take a 1500cc engine, and run it at 1/3 the RPM, at WOT, and get our 22 horsepower. So, given this example, out 1500 cc engine would be as efficient as the 500cc engine, right????

of course the bigger engine would be a lot heavier, as would the rest of the drive train.

The CX/VX honda engines were designed for this I believe - 2 valve per cylinder, teeny intake and exhaust, and higher compression.

Hey. Another question. Back to the monster diesels sort of. If I have a 1500cc engine at low rpm, will I lose less power to the cylinder walls? Or does the extra time because of the low rpm cancel that out?

maybe I'm over analyzing this - just looking for opportunities.

Larger engines are more efficient because they have less cylinder wall surface area for the same displacement.

The larger engine would be less efficient at 22 HP than the smaller engine due to the fact that 22 HP would be low compared to the best bsfc map of the larger engine. The smaller engine even though it would be less efficient than the larger engine would be more efficient because it would be closer to it's ideal torque output. The larger engine would need to be at a higher HP load to reach it's best efficiency.

The Honda CX engine was 8 valves. The VX was 16 valves. In lean burn the VX engine had one intake valve that was barely open, which helped to create the swirl in the combustion chamber that was an essential component of a properly functioning lean burn. The turbulence helped to better mix the fuel and air to reduce the potential for certain areas in the mixture to be too rich while other areas were too lean.

The VX engine would pull fine from 1000 RPM. Not sure about the CX engine, but my VX would cruise at 1000 RPM, 30 MPG in 5th gear. For acceleration it would do it but it was better if you downshifted.

To answer the 1500 CC engine question. The ideal RPM is probably between 1300 and 2100 RPM. Above or below those RPM levels and you have losses increase due to different factors. That is a general rule with exceptions that would be due to engine design parameters like stroke, valve timing, weight of the reciprocating masses, intake and exhaust tuning, etc.

regards
Mech

[some_other_dave]

Originally Posted by Diesel_Dave

Another thing to remember about running lower rpms is that both friction and pumping losses decrease with speed.

Huh? That's backw....

Oh, wait--you mean that both of those decrease as engine speed decreases, right?

-soD

[Diesel_Dave]

Originally Posted by some_other_dave

Huh? That's backw....

Oh, wait--you mean that both of those decrease as engine speed decreases, right?

-soD

Correct. Sorry, I worded that very awkwardly. Hear what I think, not what I say!

[sendler]

Very nice engine from Honda that switches cam lobes to choose a low powered Atkinson cycle or high output cycle.
.
Technical Overview of Honda's new R18 i-VTEC Implementation

[Ken Fry]

Originally Posted by drmiller100

But, if take a 500cc engine, in theory we could find an RPM and run it at WOT to get our 22 horsepower.

In theory, we could take a 1500cc engine, and run it at 1/3 the RPM, at WOT, and get our 22 horsepower. So, given this example, out 1500 cc engine would be as efficient as the 500cc engine, right????

By using the Saturn chart for both, we are effectively saying that the engines are identical, even down to cylinder size: one cylinder of a four cylinder engine produces 1/3 the hp at exactly the same efficiency, as the other three. (In real life this is unlikely to happen that you find a single with exactly the same traits as a 4 cylinder.)

In this case, the cylinder volume to cylinder area difference disappears. Then the issue that arises is the ~ 3:1 difference in rpm to achieve the same hp. Due to acoustics, inertia effects, combustion time vs stroke time available, heat influx vs expansion rate, etc there is a speed at which the engine is most efficient under a particular high load. In the example I'd used, in one engine, the speed is below this point, and in the other it is above. Coincidentally, it's about the same distance above and below, and the engine is pretty flexible (broad torque band) so the actual efficiencies are about the same.

In practice, the general shape of the power band and BSFC map of two engines, one at 500cc and one at 1500cc are likely to be quite different. And the cylinder sizes are likely to be different too: the 500 could be a thumper, or a twin, or could be a four-cylinder motorcycle engine with peak power at 12,000 rpm.

So the reason most basic reason a small engine is typically more efficient is mainly because the applied is closer to the engine torque peak, where BSFC is at a minimum. The effect shows up more dramatically under low loads, and especially dramatically when idling when (if the accessories are not cycled on) the engine is producing no useful work, but is overcoming internal friction. Then, the difference can be pretty close to the difference in displacement: a 500cc engine uses about 1/3 the fuel of a 1500 cc engine when idling.

Going beyond those generalities really requires the specific BSFC maps of the engines in question. After selecting a good fit, a designer would then tweak everything to keep the engine close the sweet spot as much as possible... balanced against marketing demands. It is a rare design in which all the stops are pulled out in the interests of fuel efficiency, with the Prius being the best example of heading in that direction. But even there, it could certainly be much lighter, could have better aero, could have lower frontal area (if comfort can be sacrificed a little), the cargo area could be smaller, etc.

But then the market would likely be a little smaller.