Increasing fuel savings

[_xxx_]

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.

Thus we have turbos and roots loaders... But I get your point.

EDIT: also, you have to keep the fuel/air ratio within certain boundaries in order not to violate the exhaust limits, so there's only a limited range you can use. Thus you can't just pump as much air or fuel as you'd like to.

 

[_xxx_]

Well, the amount of air is controlled by the user, the ECM adds the amount of fuel it thinks the user wants.

Inside the ECU it's all about torque. Even the control model is officially named torque-controlled engine model. When you press the gas pedal, it knows how much torque you'd like to have and adjusts everything so it can reach that as good as possible. The throttle valve is also electronic and the opening angle doesn't necesserily translate to how much you pressed the pedal.

 

[Simon F]

Aren't all you guys missing one important thing? You aren't driving in a vacuum.

The air resistance goes up with the square of the speed, so if you accelerate quickly (and break quickly) you'll be spending a higher proportion of your journey at a higher speed where you are losing a more energy to moving air around the car.

Similarly, I'd expect that non-linear behaviour will also occur inside the engine (i.e. shifting air/exhaust though the engine) which would only get worse at higher RPM.

 

[KimB]

Aren't all you guys missing one important thing? You aren't driving in a vacuum.

The air resistance goes up with the square of the speed, so if you accelerate quickly (and break quickly) you'll be spending a higher proportion of your journey at a higher speed where you are losing a more energy to moving air around the car.

Sure, but air resistance is only significant above ~40-50mph or so.

Similarly, I'd expect that non-linear behaviour will also occur inside the engine (i.e. shifting air/exhaust though the engine) which would only get worse at higher RPM.

Clearly, but you can drop the rpm's right back down to near idle once you get up to speed (for city driving, of course).

Anyway, I think that these things are liable to result in at most a few percent difference in fuel economy. The big hitter will be if you're really laying on the gas, and accelerate with a less-efficient enriched fuel mixture. I don't think this happens until the pedal is quite a ways down, though. Hard braking will similarly hurt you (better to coast as long as possible before braking, trying to go slow enough at the top of hills that you don't need to brake at the bottom, etc.).

 

[Simon F]

Sure, but air resistance is only significant above ~40-50mph or so.

So that accounts for pottering around urban areas but not for the (motor|free)way where, e.g. in the UK, you'd be doing 70ish.

Clearly, but you can drop the rpm's right back down to near idle once you get up to speed (for city driving, of course).

But surely if you integrate the instaneous fuel consumption over the distance traveled , the non-linearity will cause the high peak to be more significant.

 

[KimB]

So that accounts for pottering around urban areas but not for the (motor|free)way where, e.g. in the UK, you'd be doing 70ish.

Right. But, at least in the US, when people drive on the highway it's usually for significantly longer distances than in the city. And thus the acceleration time becomes less important, and highway driving speed becomes the dominant factor, due to air resistance.

But surely if you integrate the instaneous fuel consumption over the distance traveled , the non-linearity will cause the high peak to be more significant.

I don't think it's that nonlinear, though, not unless you're talking about extreme situations (like, say, redlining the engine at a red light and popping the clutch when it turns green). According to the Wikipedia article on fuel injection engines I posted a little bit ago, the worst air/fuel ratio is only about 30% worse than the optimal ratio, and thus the worst fuel economy drop just from combustion efficiency should be around that (though not quite the full 30%, as you do gain some additional power from enriching the mixture).

You might lose a little bit more from the friction that comes from running at high rpm's, of course, but that won't be all that huge either, as you can't run the engine too high before the power starts to drop off again. So I would hazard to guess that in a worst-case scenario, your heavy fuel consumption situation is going to be about 50% worse in fuel economy when compared to a best-case scenario. But accelerations still aren't going to comprise the majority of your driving time, so that quickly can become a much smaller portion of your average fuel economy.

 

[ShootMyMonkey]

Now this point is just wrong. In the situation of acceleration, the instantaneous fuel economy is unimportant, since acceleration doesn't last long. It's the average that is important, and low gears are what you want for maximal fuel economy for acceleration, as otherwise you'd have to have an exceptionally-rich mixture to get any amount of power out of the engine.

That's not quite what I was talking about -- the point wasn't about what gear you should accelerate in, but about hard acceleration vs. soft acceleration and what gears in which it makes a big difference. At low gear, since the speed is low, hard acceleration vs. soft acceleration makes a bigger difference, and any and all drop in mileage will affect your average depending on how peaky it gets. Also, it depends on what kind of driving you're doing -- in city driving, repeated acceleration and deceleration are the dominant modes of motion; It's only in highway driving that they're the minority (and not quite so much on some highways ).

I don't think it's that nonlinear, though, not unless you're talking about extreme situations (like, say, redlining the engine at a red light and popping the clutch when it turns green). According to the Wikipedia article on fuel injection engines I posted a little bit ago, the worst air/fuel ratio is only about 30% worse than the optimal ratio, and thus the worst fuel economy drop just from combustion efficiency should be around that (though not quite the full 30%, as you do gain some additional power from enriching the mixture).

Except that air/fuel ratio doesn't tell you about total volume of both. It just tells you how the two relate. At WOT, the throttle body valve is as wide open as it gets and depending on the engine, the intake valve at the cylinder head may open longer and lift further, so you'll draw in a hell of a lot more air. And then further bottlenecks result from that which affect the mechanical efficiency of the engine negatively. For instance, drawing in more air probably means that the piston has to work a fair bit harder to actually compress the charge of air and fuel. It also means that if you have exhaust bottlenecks, that will lead to back pressure, and you get a further loss of power because you have to work harder to exhaust spent gases. So your additional power output from enriching could just as well be canceled out (in effect keeping you at about the same net power output to the wheels).

But accelerations still aren't going to comprise the majority of your driving time, so that quickly can become a much smaller portion of your average fuel economy.

You've obviously never witnessed traffic congestion.

 

[Frank]

Chalnoth, did you ever drive a car with instant and current mpg figures?

 

[DemoCoder]

Rule #1 kills it for me. I drive fast, and aggressive (I'd say defensive). If I always drove the speed limit, didn't change lanes, and stayed in the slow lanes, I'd probably fall asleep at the wheel.

 

[KimB]

Chalnoth, did you ever drive a car with instant and current mpg figures?

Yes, but it's been a while. My grandmother's minivan had that feature (she didn't like to drive, so I was elected to that duty fairly often when visiting). I remember mpg's ranging from efficiency in the 1-5 mpg range when moving at very slow speeds and accelerating, to 90mpg or so when coasting at medium/low speeds. But I don't think the feedback was anything near as good as you get on a Prius (it was hard to look at when driving due to the position, and I never got a sense of how the instantaneous related to the average).

 

[KimB]

For instance, drawing in more air probably means that the piston has to work a fair bit harder to actually compress the charge of air and fuel.

This shouldn't of any concern. Specifically, just consider the situation where you let that air in, but there is no explosion and you don't let it out: the cylinder would just bounce, with no loss of mechanical energy.

What would be of concern is the work required to push the air back out of the cylinder after the explosion has taken place. So yes, that's going to eat somewhat into fuel economy, but again, I doubt it's all that significant.

That said, I would like to know exactly what the configuration of the engine is in a stick shift when you take your pedal off of the gas. Specifically, I'd like to know what forces are responsible for engine braking. That may give some insight into the relative power of some of these efficiency concerns.