Pumping losses

[theholycow]

Originally Posted by dosco

Interesting. 2200 is right about the rpm for my Camry cruising at 60mph on level ground in top (4th) gear.

60mph = ~2700rpm (IIRC) in 5th in my VW. I don't remember exactly. I do remember this exactly: 70mph = 3000rpm.

In my truck it's something more like 1600 or 1800 rpm at 60mph.

[dosco]

Originally Posted by theholycow

60mph = ~2700rpm (IIRC) in 5th in my VW. I don't remember exactly. I do remember this exactly: 70mph = 3000rpm.

In my truck it's something more like 1600 or 1800 rpm at 60mph.

Actually, the more I think about it, I think 2,200 is about 65 mph. Will have to make a note of it tonight on the drive home.

[R.I.D.E.]

EPA claims you can improve mileage by 80% with no change of the engine, through better powertrain.

holycow, is exactly right, he needs about 3 more gears to lower his engine speed at higher vehicle speeds. I wish I had a 6th gear in my VX.

Aero means less sustained average power is needed to maintain a given speed. Better aero combined with much higher top gear ratios (import autos are now up to 8 speeds) mean you can capitalize on better aero by using lower engine speeds at higher loads.

It may be the same rpm as idle but it will always be at a greater load, therefore with a much better BSFC than idle. Cylinder filling combustion events are always most efficient (lowest vacuum), but WOT at 1000 rpm is overkill.

Vacuum is the thermometer of efficiency but the old belief that sustained lower vacuum operation equals economy is basically flawed.

If you want an efficiency guage take the vacuum guage and read it
backwards. Your pulse should be very low (to no) vacuum. If EOC is utilized, its no vacuum. When idling its high vacuum.

Most efficient is low vac
High vac is least efficient
EOC is no energy consumption

I found driving automatics using P&G to be much harder to get dramatic improvements, unless you can fool the tranny into staying in higher gear with a decent load applied.

In the old days you could adjust the kick down cable to get the desired results. Maybe the tranny shift points could be changed through reporgramming the ecu.

regards
gary

[1993CivicVX]

I can't see how anyone would want a 6th gear in their VX. I would constantly need to downshift at the slightest hills with such a setup. Seems the gains would be very slim and not worth the constant shifting, unless you keep the final gear ratio the same and make 1st, 2nd and 3rd closer. . Also, on fe.gov a 2004 4 speed automatic Celica gets 33hwy 25 city and 5 speed manual gets 30hwy and 23 city... how does that work? The site makes no indication the engines are different.

[monroe74]

Originally Posted by R.I.D.E.

Here is my thought.

You invited me to ask you a specific question. So I did. You responded by posting another 1,000 words, or so, none of which answer my question. If the answer is hidden in there somewhere, maybe you can tell me where to look, because I can't find it.

[monroe74]

Originally Posted by dosco

By maximizing my car's time in throttle, I'm using less fuel but I am operating my engine very inefficiently.

I assume you're talking about a steady cruise, where you use less fuel per instant, but you end up using more fuel per mile. In P&G, during the pulse you're using a lot of fuel per instant. But during the glide you get a very favorable return on investment.

Imagine taking this case to the extreme, where drag is reduced to the point where highway cruise could be maintained while only using idle rpm. The engine would be horribly inefficient, however the mpg would be astronomical due to the low FC.

OK, now I think I see what you mean. I'm sorry I misunderstood. Yes, if you magically modified your car to have zero drag and zero weight, you could still get great mpg even if you were operating the motor inefficiently. That's because even though you're paying a lot (in fuel units) for each unit of power produced, you are demanding, overall, a small number of units of power. Nevertheless, however, you're still missing a chance for yet more mpg, if you operated the engine more efficiently. For example, if idle rpm is inefficient, but also is enough to drive the car at highway speed, you are not obligated to drive it this way. You still have an opportunity to pulse at a higher rpm (if that's more efficient, for whatever reason), and then do EOC while you use the stored momentum.

But I still don't really understand your example, because the low rpm would tend to be an advantage, not a disadvantage. Idle is only a wasteful state if the engine isn't moving the car forward. If your engine idles at 800 rpm, and can also propel the car at 60 mph, at 800 rpm, then the latter state isn't properly called idling. And the key question is something you haven't specified: what's the load?

Originally Posted by dosco

I also have a hard time believing that effective P&G could be had with revs up to 4,000 rpm.

It's possible to imagine such a circumstance, but it would be unusual. High revs in a low gear is unavoidable if a higher gear just doesn't have the power you need to fit your needs, in the moment. This problem is more likely to happen in a car with wide gear ratios (maybe an old 3-speed). And it really could be a situation where you do need to pull that heavy trailer up that steep hill, and you need to do it quickly. Given those parameters, WOT at 4,000 rpm in, say, 2nd gear, could be the most efficient way to get the job done. Maybe you can't shift yet because 3rd gear is so tall that it just doesn't have the power you need.

Originally Posted by dosco

Unfortunately that's [BSFC] not how we're measuring it, are we? We're trying to figure out ways to maximize miles while minimizing fuel used. Conceptually we agree, though, that this means maximizing work accomplished while simultaneously minimizing energy expenditure.

Yes, we don't normally think in terms of BSFC. But we should. It would help us understand how to get the same results (moving a certain mass over a certain distance in a certain amount of time) for less fuel.

what if I could setup my car to run at highway speeds at idle rpm? Should I really believe that I would achieve horrible mpg, despite the fact that the engine's BSFC would be completely horrible?

As I said, it's a question of whether you look at the situation in relative terms, or absolute terms. You might achieve great mileage as compared with what you're used to, but it would still be horrible, compared with what's possible if you optimize BSFC.

And there's no reason to assume that running idle rpm at highway speeds is inherently inefficient, in BSFC terms. If the load is high (because you've installed a small motor, for example), it's not. Since you haven't specified the load, we don't have a clear picture of the scenario you're imagining.

I'm not convinced that P&G is solely to maximize engine thermodynamic efficiency. It seems like it is more about *both* creating power as efficiently as possible (when you need it) while also minimizing fuel consumption during the glide (hence EOC). Perhaps I'm restating the obvious.

It gets to be a matter of semantics, and it gets to be a question of whether you focus just on one pulse, compared with focusing on an entire trip composed of many P&G cycles. Both perspectives can be helpful. But if we look at the situation broadly, and focus on the entire trip, we can say that P&G maximizes thermodynamic efficiency. It efficiently extracts a certain amount of work from the engine, and that work propels us during both the pulse phase and the glide phase.

[monroe74]

Originally Posted by theholycow

my point was that in an automatic where you don't have the ability to go wide on throttle and tall on gear at the same time, it's still going to be worth it even though you sacrifice some of the pumping savings. It's still better than nothing.

I see what you mean. Thanks for explaining. Good point.

I think, like me, he doesn't have enough gears to have low RPM at highway speed.

I don't think that's it. If you read his posts, he's not talking about cruising at 4000 because he's already in top gear and can't upshift anymore. That's not it. He's talking about hitting 4,000 momentarily, just before he upshifts. That's enough to tell me he needs to shift earlier (unless he's in an unusual situation, like pulling a a trailer up a hill).

By the time I hit 40 mph I'm pretty strongly itching to shift but there's no 6th gear.

Right, I understand the feeling. I love tall gearing. Even in my VX, I'm occasionally doing 75 mph at 3000 rpm and wishing I could upshift again. Because the thing really has a lot of torque at 2000, and even lower.

My point wasn't that it's better to rev high in the pulse, just that it's better to pulse revving high than to not P&G at all.

Good point. I didn't realize that's what you meant. Thanks for explaining.

I rarely reach the 2200 rpm that PaleMelanesian reports until I run out of gears.

Same here. I spend a huge amount of time under 1500. Sounds like you do the same. But every now and then it's fun to rev it up. And believe it or not, Honda actually recommends you do that occasionally. To blow out carbon.

I do remember this exactly: 70mph = 3000rpm.

In a VX, 70mph=2500 rpm.

edit: oops, I was a bit off with those numbers. That should say 70mph=2357 rpm. And above I should say that 75mph= 2525 rpm.

[monroe74]

Originally Posted by dosco

Interesting. 2200 is right about the rpm for my Camry cruising at 60mph on level ground in top (4th) gear.

You'll find that those numbers remain the same even when the ground isn't level!

[monroe74]

Originally Posted by R.I.D.E.

Vacuum is the thermometer of efficiency but the old belief that sustained lower vacuum operation equals economy is basically flawed

You're making an important and correct point, but you're saying it backwards. The old belief promoted high vacuum operation, not low vacuum operation.

I made that point here: http://www.gassavers.org/showpost.ph...7&postcount=22. Click through to see a picture of a Stewart-Warner 'Motor Minder,' which illustrates the point we're talking about.

[monroe74]

Originally Posted by 1993CivicVX

I can't see how anyone would want a 6th gear in their VX. I would constantly need to downshift at the slightest hills with such a setup.

Your statements on another thread (see here: http://www.gassavers.org/showpost.ph...&postcount=36), along with this statement, lead me to believe that your car isn't running quite right. My hunch is that you should check your timing.

on fe.gov a 2004 4 speed automatic Celica gets 33hwy 25 city and 5 speed manual gets 30hwy and 23 city... how does that work?

It could be something about how the shift ratios magically coincide with certain features of the EPA test cycle. Just a guess.