Increasing fuel savings

How about this: while the amount of time you're accelerating is small, you do burn about fifty times as much fuel when doing so (depending on how much and how fast you depress the gas pedal). So, even when you do so only 2% of the time, that would double your gas consumption. You cannot avoid accelerating at all, but you can make it more efficient.

But when you do that at 3000 rpm, it would (roughly) only cost half the amount of fuel burned than when you do so at 6000 rpm. But the amount of acceleration would be less as well. That is, if the ratio of fuel injected stays the same, which it doesn't.

As long as we don't have good graphs about the actual difference, it's hard to say how big that is (as you burn a bit less fuel per explosion with higher rpm for the same amount of power produced, but need a bit more at the same time for the increased pressure needed for the faster rotation), although it's a given that the efficiency decreases when you increase the rpm.
 
DiGuru said:
How about this: while the amount of time you're accelerating is small, you do burn about fifty times as much fuel when doing so (depending on how much and how fast you depress the gas pedal). So, even when you do so only 2% of the time, that would double your gas consumption. You cannot avoid accelerating at all, but you can make it more efficient.
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Well, I don't think you should be throwing that 50x number around. It doesn't really say anything, because it's a comparison between cruising and fast acceleration, which isn't the comparison of relevance here.

But when you do that at 3000 rpm, it would (roughly) only cost half the amount of fuel burned than when you do so at 6000 rpm. But the amount of acceleration would be less as well. That is, if the ratio of fuel injected stays the same, which it doesn't.
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I wouldn't say that. Depending upon the engine, it may be possible to get much more air to the engine per unit time at 6k rpm's than at 3k rpm's, which would mean that you can get more power out of it for the same amount of fuel burned.

Granted, I do think that typically going at 3k rpm's will be better (as this is going to be near the start of the 'power plateau'), but you have to be sure not to give the engine too much gas.

as you burn a bit less fuel per explosion with higher rpm for the same amount of power produced, but need a bit more at the same time for the increased pressure needed for the faster rotation
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The pressure stays the same: it has to. The question is whether 6k rpm is too fast for the cylinder to become saturated with air at some given amount of pressure. If it is much too fast for a given engine, then that engine won't be able to extract any more power (I think I'm convinced now that if you look at those power curves, the point at which air can no longer fill the cylinder fast enough is going to be very close to the peak power).

I think you may be mistaking "increased pressure at high rpm" with "increasing the pressure allows you to go to higher rpm before power starts to fall off."
 
Chalnoth said:
Increase what pressure?
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The pressure created by the explosions. If you increase that, the engine will start to rotate faster.

And that also means, that you cannot just look at the amount of air (and fuel, indirectly) needed to be able to supply the power needed and divide that by the explosions per minute.
 
DiGuru said:
The pressure created by the explosions. If you increase that, the engine will start to rotate faster.

And that also means, that you cannot just look at the amount of air (and fuel, indirectly) needed to be able to supply the power needed and divide that by the explosions per minute.
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I think that's pretty much exactly what it means. Of course, you have to take friction into account too. But increasing the explosion pressure is merely a matter of adding more fuel, and as long as you're not yet to the point of saturation, it isn't a big deal.
 
_xxx_ said:
More fuel AND air.
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Not necessarily. There's an optimal ratio of air/fuel where with much less fuel, you gain no significant amount of combustion efficiency, and with much more fuel you don't gain much more power. So as long as you're below this threshold, your acceleration isn't costing you much.
 
I'm talking about real acceleration, kickdown if you wish. It'll surely require more fuel and air. There surely are different mixes and all, but in the end you'll still burn more of both.

EDIT: not related to rpm, just the absolute consumption
 
Well, sure. And if you have more air, it won't make much difference for overall fuel consumption. The problem comes in when you're combustion becomes starved for air, which has to happen if you really try to accelerate fast.
 
Chalnoth said:
Not necessarily. There's an optimal ratio of air/fuel where with much less fuel, you gain no significant amount of combustion efficiency, and with much more fuel you don't gain much more power. So as long as you're below this threshold, your acceleration isn't costing you much.
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But, the Engine Control Module (computer) isn't trying to supply the amount of fuel needed for optimal combustion in any case! It can supply the minimal amount needed, the maximum amount or something in between, related to the air intake, if you're accelerating or coasting, the rpm and the gear you're in. A variation of between 1 and 9 percent (depending) of fuel to the air volume.

And it does favour the extremes.
 
Sure, but the amount of air that enters the engine is basically set by the rpm's. And the amount of gas is still controlled by the user. So there's still a window there where you get fairly equivalent fuel economy, before the mixture gets too enriched.

Exactly how big is that window? Well, I can't say that I know, but I'm hoping that it ends at the point where the power curve starts to fall off. That is, I notice different amounts of increased power depending upon how hard I push on the pedal, and there is a distinct point where adding more pressure on the pedal leads to less additional acceleration, dependent upon the rpm's.
 
Well, the amount of air is controlled by the user, the ECM adds the amount of fuel it thinks the user wants.
 
DiGuru said:
Well, the amount of air is controlled by the user, the ECM adds the amount of fuel it thinks the user wants.
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How does the user control the amount of air? That's set by the amount of time the valve is open, and the air pressure. Without a turbocharger, why wouldn't you just always give the cylinder the maximum amount of air possible?
 
Chalnoth said:
How does the user control the amount of air? That's set by the amount of time the valve is open, and the air pressure. Without a turbocharger, why wouldn't you just always give the cylinder the maximum amount of air possible?
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In most gasoline cars, the amount of air is still controlled by the gas pedal. The ECM determines most other things.

With diesel engines, the amount of air is more or less constant, and the gas pedal controls the amount of fuel injected (indirectly, as the ECM still tries to inject the amount it thinks the user wants).
 
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DiGuru said:
In most gasoline cars, the amount of air is still controlled by the gas pedal. The ECM determines most other things.
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Okay, but I was wondering as to the mechanism this happens, because it didn't make any sense to me.

Anyway, I found this in Wikipedia about fuel injection cars. According to the numbers they give, the fuel/air ratio within the cylinder is nearly constant when compared between maximal acceleration and idle, even though the dynamic range in fuel consumption is still on the order of 50, as you've been stating.

If this information is correct, I think I'm going to have to reverse my argument, and state that fast accelerations aren't going to significantly impact your fuel economy, except in the case where a large portion of the fuel consumed is during those accelerations (which is probably the case for city driving).

According to the numbers in that article, the fuel/air mixture looks like it will vary by as much as about 30%, which is roughly the difference that the drivers in the article sparked this thread found as the difference between very aggressive driving and "granny" driving.

But given that at idle, the engine is only obtaining a fraction of the air it can possibly intake (which makes a bit more sense now that I think about it: pushing the air out of the engine takes work, so intaking too much air will hurt fuel economy), there's going to be a fair range on that gas pedal where you're still running at optimal fuel/air ratios. I guess I have to hope that this ends at the point where the gas pedal becomes "soft," that is, the additional acceleration starts to drop off.

This makes some sense when thinking about the engine as operating in this way, as it makes sense for the engine to keep the fuel/air mixture constant until the valves are open for the maximal amount of time, and then start enriching the fuel mixture once that occurs (to some extent: there may be some mixing based on other factors to obtain maximal fuel economy). It would make sense for there to be an observable difference in how the car accelerates between when you're giving the car more air, and when the computer is just giving it more gas at the same amount of air.
 
epicstruggle said:
Check out their methodology, and winner between jetta and a prius in fuel economy.
http://www.autoweek.com/apps/pbcs.dll/article?AID=/20060424/FREE/60417021&SearchID=73244466514443
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That makes sense. Overall, going hybrid or going diesel from a gasoline engine nets about the same improvement in fuel economy. But hybrids are better in town (due to the ability to charge the batteries on braking), and diesels are better on the highway (when driving for long distances, hybrids have to rely more on their gasoline engines).

Anyway, I really like this statement at the bottom of the article:
Imagine a Prius-like hybrid that ran on biodiesel instead of gasoline. We may not be there yet, and adapting diesels to use the cylinder-cutoff technology found in the Jeep and Honda might be a tough task, but look how far we’ve come already.
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I think also adding in the instant-feedback on mileage that the prius has to all cars would provide a significant improvement in overall fuel economy.
 
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