Does this waste gas?

May 14, 2008 7:57 PM

When I push the gas pedal on my manual car down beyond where it accelerates, am I wasting gas?

If I shift early and end up on the lower end of my engine's power curve, sometimes I instinctively push the gas pedal down -- even if it isn't providing any more power to the wheels. When this happens, I imagine that the throttle actuator is squirting more fuel into the cylindars than is actually being burned, thus wasting some gas and dropping my gas mileage.

Is this true?

For what it's worth, I have a 2005 Jetta.

posted by yellowbkpk to travel & transportation (11 comments total)

Cars inject a certain amount of fuel every revolution. This is a constant amount per revolution.

The pedal you are pushing doesn't increase the amount of fuel, it increases the amount of air. Increasing the amount of air increases the power output, which increases the revs.

To answer your question. No, you are not wasting fuel, you are actually probably being more fuel efficient by shifting early.
posted by Jerub at 8:03 PM on May 14

Wonderful answer! Thanks Jerub.
posted by yellowbkpk at 8:12 PM on May 14

It may seem like a wonderful answer, but it's largely wrong, though the end conclusion is correct.

I've written about it here before, but fuel injected is nearly directly linearly proportional to the amount of air being pulled into the combustion chambers. It's correct that (in a gasoline-burning car) your foot controls the amount of air being allowed in. The harder your foot presses, the more air gets allowed in, and thus the more fuel gets injected along with that air. Excepting special circumstances, it's almost always more frugal to shift at a lower engine speed and use more throttle in order to speed up. Yes, you're using more gas by giving it more gas, but the difference between 75% and 100% throttle is negligible compared to the difference between 2000 and 4000 RPM.
posted by TheNewWazoo at 8:36 PM on May 14

I should also add that, for a given RPM value, an engine is more efficient as the throttle opens wider. That is, the horsepower generated per amount fuel consumed goes up as your foot gets closer to the floor.
posted by TheNewWazoo at 8:38 PM on May 14

"Cars inject a certain amount of fuel every revolution. This is a constant amount per revolution. ... "
posted by Jerub at 11:03 PM on May 14

That's not true for most gasoline fuel injected cars. If it were, they'd constantly be running lean or rich, but they don't. It's just not how fuel injection on modern gasoline engines works, and it is certainly not how carburetors on standard engines ever worked.

When you depress your accelerator pedal further than your engine can respond to, at that point in its power curve, not much happens in a modern engine. The throttle position sensor records your ridiculous demand, and passes it on to the Engine Management Controller (EMC). The EMC laughs it off as the silly human wish that it truly is, and looks at the gear you are in, the reflected load from the transmission, the air pressure around you, the number of revolutions that your engine is turning over, and some miscellaneous factors, like the temperature of your engine coolant, and the amount of air your engine is getting, as reported by the MAPS sensor. In about every 2 milliseconds, after it has thought about all that, and measured, again, some of it, your EMC sends some variable pulse data to your fuel injectors, who, if they are clean and working right, inject your Jetta's cylinders with just the right amount of fuel to maintain current torque, to drive the current load, and, if you are lucky, enough more, that eventually, you will reach the ridiculous, unsafe speeds for which you wish.

But not before you've had plenty of time to think about it...
posted by paulsc at 8:45 PM on May 14 [18 favorites]

Wikipedia on carburetion.

The throttle (accelerator) linkage does not directly control the flow of liquid fuel. Instead, it actuates carburetor mechanisms which meter the flow of air being pulled into the engine. The speed of this flow, and therefore its pressure, determines the amount of fuel drawn into the airstream.

Putting your foot on the accelerator restricts the flow of air into the engine, creating a faster flow over the needle, and a richer mixture. That's why it's called a throttle.
posted by Fiasco da Gama at 8:45 PM on May 14

I take it back. TheNewWazoo is correct.
posted by Fiasco da Gama at 8:53 PM on May 14

It seems I was mislead, obviously the person who explained it to me totally oversimplified. Thanks folks!
posted by Jerub at 9:02 PM on May 14

Favorited the above mostly for:

"...The EMC laughs it off as the silly human wish that it truly is, and looks at the gear you are in..."

Lovely bit of prose, that.
posted by SlyBevel at 9:28 PM on May 14

It may help your understanding a bit to see what a throttle body looks like. The air flows into your engine through the throttle body, and your foot on the accelerator is simply rotating the disc to allow more or less air through it. The key, of course, is that you don't control fuel supply with your foot, you control air flow. Then, as paul describes, the ECU computes how much fuel to inject based on the air flow and a bunch of conditions.

However! Some car ECUs change their behavior at WOT — wide open throttle. The car may switch into an "open loop" mode, where it throws out the idea of maintaining ideal stoichiometric combustion ratio (14.7:1 air:fuel ratio), disregards certain sensor inputs (O2 sensors, for example), and instead throws additional fuel into the mix as a safety measure. Running rich (more fuel than you can possibly burn) keeps the combustion temperatures cooler, and helps to keep you from blowing something up during full throttle acceleration.

Regarding your perception of the throttle response in your car... The reason you notice that there's a point at which you stop getting additional acceleration is the shape of the throttle mechanism itself. The relationship between the angle the disc is rotated and the area of the throttle body that is being restricted is not linear. I don't have the math skills to prove the exact relationship, but if you think about it, you can imagine that you'll get more of a sinusoidal response to throttle rather than a linear one. The value of the sine function starts at zero and quickly races upward, initially. As you approach 90 degrees, the value is still going up, but not nearly as fast, and it's starting to level off. And, the way the throttle is connected, the gas pedal can only explore the first 90 degrees of that curve.
posted by knave at 9:48 PM on May 14 [1 favorite]

It also has to do with the power of your cars engine. If it is at its maximum, then pushing further on the accelerator can't make it go faster.

However, going back to carbureted engines, there is something called the accelerator pump. When you press on the accelerator, it squirts an extra shot of fuel into the engine so that it will accelerate right. (That's also one of the reasons you had to push the gas pedal once or twice before starting older cars- to prime it.)

It's just the nature of gasoline engines.

Anyway, fuel injected cars don't have such a pump. But their computers do the same thing when the throttle position sensor senses movement. Even if the engine can't go any faster, it will try. Because the computer's workflow is to try to make the best guess as far as air-to-fuel ratios go, and then it will correct based on the oxygen sensor readings.

So yes, it wastes gas. But not as much as a carburetor would.
posted by gjc at 7:04 PM on May 15