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I've had a discussion with my father today, about the fuel usage of a vehicle at the same rpm, but a different gear.

He claims that the following situations have the same fuel usage:

Gear: 2
rpm: 2000
fuel usage: 5.1l/100km

Gear: 5
rpm: 2000
fuel usage: 5.1l/100km

I say it should be something along the lines of:

Gear: 2
rpm: 2000
fuel usage: 4.1l/100km

Gear: 5
rpm: 2000
fuel usage: 5.4l/100km

Who is right and why? He is also claims that the force to maintain the speed will be the same across different gears with the same rpm.

By the way, I'm not sure if it should be on physics, but it fitted best here.

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I'm very likely wrong, but doesn't the fuel consumption depend on the RPM and the load on the engine? Is there an implicit assumption on the engine load based on the gear setting? –  Willie Wong Jul 10 '11 at 15:10
    
Because a higher gear means the gear is bigger, thus you can have the same rpm. I'm also talking about the rpm displayed on your dashboard. @ Willie, I'm talking about rpm and fuel only in my first question, not the actual force needed for a certain speed or rpm. –  Simon Verbeke Jul 10 '11 at 15:18
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1 Answer

up vote 4 down vote accepted

I've had a discussion with my father today, about the fuel usage of a vehicle at the same rpm, but a different gear.

The fuel injector system does not always inject a constant amount of fuel into the engine.

If you are driving down a hill and remove your foot from the gas pedal (a little) the engine will stay the same rpm but with less fuel passing thou the system.

And if you are driving up a hill you need to press down the gas pedal to maintain the same rpm and speed, more or less you need to push more fuel into the engine.

This is why you will not have a constant fuel consumption at a given rpm.

Then when you remove some load, let's say you are on the top of the hill, you will have a small surplus of "energy released" inside the engine that will push the rpm higher until you remove your foot from the gas pedal and reducing the amount of fuel sent into the engine.

He is also claims that the force to maintain the speed will be the same across different gears with the same rpm.

The faster you travel the more wind resistance you will have, therefore you need more power to overcome this resistance. And to get more power out from a engine you need to send in more fuel (or use the amount have in there in a more efficient way).


But why do we need to switch gears up and down then, why can't we just inject more fuel into the engine?

This is where we go into chemistry since you can only make gas burn within a specified rate between fuel and air, and the volume inside the engine has a fix size and therefore has a maximum volume of gas that you can burn per rotation. Send in more fuel than this and the engine will drop in efficiency.

And when you then switch down a gear you will increase the rpm at this speed and that way increase the amount of fuel you can burn in the engine within this timeframe and you will in most of the cases more power out from the engine.

But at the end of the day there is a lot of other constraints in a combustion engine that will limit how it works beside this simplified version I just describe here.

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But with higher gear you travel longer distance per revolution. So it's mandatory to discuss this effect according to load. –  Crowley Jul 11 '11 at 21:37
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