How does the engine gears affect fuel economy?

[Anonymous]

Im a little dense when it comes to engines so bear with me. -
Id like to know whether it uses more or less petrol, keeping the accelerator slightly pressed, doing high revs in a low gear, or whether it uses less keeping the accelerator at the same level in a high gear doing low revs. is  petrol usage always determined by accelerator depression, or do the gears change the amount of petrol being fed, in relation to how much the accelerator is pressed.  I want to know, because if it makes no difference i will always keep it in a low gear, so the performance is there, and i dont have to change down.
 thanks for any answers my dunce hat has been implemented

[SimonC_Here]

I think I'm right in saying that you will use the same amount of petrol over a specified time with the pedle in the same place. However, in a higher gear you will have gone further in the same time.

Simon

[Anonymous]

If you think of it as putting a teaspoon of fuel into a cylinder each cycle of the engine (if shouldn't change as the accelerator is in the same place for both situations) then putting 1 tsp in 1500 times a minute is going to use much less fuel than putting in 1sp 4000 times a minute.  However, the lower gear gives you a greater mechanical leverage (hence the better acceleration), so what really happens is to maintain a constant speed, the accelerator is pressed further in the higher gear than the lower gear, even if it is just a little.

You put the accelerator down further in the higher gear to maintain the same leverage pushing you forward to overcome friction, air resistance etc.  In reality there isn't much in it.  The lower gear will be slightly worse due to increased engine frictions, but not much.  Of course, if you keep using that extra acceleration you have by being in the lower gear then you'll be using significantly more fuel.

IMHO, of course   s;-) ;-) s;-)

[Anonymous]

Basically, if you want to get around by getting up to speed quickly, but not that bothered by high top speed, then low gears and foot flat on the floor is always going to use more fuel than  getting there slower, but using same top speed in a higher gear. Basically because the engine is not spinning as quickly, therefor not using any extra fuel..........

It all sorts of evens itself out over your normal driving patterns. If you hoon the thing around, whereever you go, like you stole it, then you are by definition going to use more fuel because your foot is planted more to the floor.

Basically, higher revs means higher fuel consumption.

[Tem]

I think I'm right in saying that you will use the same amount of petrol over a specified time with the pedle in the same place.

That's true, if the engine is off the car, or you're just revving for fun in the parking lot  s;) s;)

Once you start loading the engine, things get more complicated as you can have different airflow into the engine with the same rpm, which obviously means change in fuel amount as well.

[SimonC_Here]

I think I read it as same RPM rather than same position of foot.   s:oops: :oops: s:oops:  

As MrBarney said. engine will be spinning faster/using more fuel in a lower gear.

Simon

[Anonymous]

Ok thanks for that,- i kind of understand, - if the revs are high, more petrol is being used, even though im not going any quicker.

[Anonymous]

Ok thanks for that,- i kind of understand, - if the revs are high, more petrol is being used, even though I'm not going any quicker.

It's not even that simple, I'm afraid.

You see, in a lower gear, you need less power because you have better leverage.  So, you use less of the accelerator pedal to maintain that speed, so less fuel per revolution is being consumed.

Real world, yes you use more fuel in the lower gear, but not much.

I'm not explaining this very well, am I?   s:? :? s:?

[Tem]

Real world, yes you use more fuel in the lower gear, but not much.

I'm not explaining this very well, am I?   s:? :? s:?

There are too many things happening in the real world to explain this in detail within the character limit of one post  s;) s;)  Not that I even could do that...but here's the skin of what I understand  s:? :? s:?

Basically any given engine has a certain rpm where it makes the most power with same amount of fuel called max volumetric efficiency. So you'd want to keep the engine at those revs for best fuel economy.

Then you have the transmission, where you have least power loss with the straightest gear (closest to 1.000:1), so concerning the gears you'd want to be there.

Then you have the diff, axles, joints etc, where generally every spinning part loses more energy the faster they spin, so you'd want to spin them as slow as possible.

Tyres are most efficient at some speed, which varies a lot from tyre to tyre. Generally performance tyres are designed for higher speeds, while generic ones are happiest getting groceries. You'll be lucky to get this info from the manufacturer. Naturally tyre pressures also play a big role in this, with some given pressure being the best.

And since we're actually moving in a non void place, air resistance has a role as well.


Best fuel economy is a compromise of all those (and surely has even more points to consider, if you really want to count them all). I've heard a rule of thumb that a max torque rpm/2 would be a good guess in real life...