Petrol consumption question

Canis Lupus

A question thats bugged me for a bit but one that I've never asked. Maybe it's obvious I dunno but:

A way to explain my question is if yur driving up a steep incline (taking into account this is hypothetical). u have foot to the floor but because of the incline yur revs stay at say 3000RPM as yur car struggles up the hill. now is the car still burning the same amount of fuel as if i was on the flat at the same RPM in the same gear but with my foot just on the accelerator?

Basically I wanna know is petrol consumption based on the actual RPM's or does "putting the foot down" (even if putting foot down doesnt equate to higher revs) result in more petrol burn.

Obviously I know that pressing the accelerator= more petrol injected into the engine= more petrol burned, but is RPM inpedendant from actual petrol burned? In the scenario above is there a major difference in petrol burned? We all know revving the hell outta the car in 1st/2nd/3rd gears is one way to throw petrol away but is the same situation above just as wasteful or does it waste petrol but not really that much to worry about?

and dont laff if my question is fairly obvious, ive never looked at what makes a car tick, i just know the very very basics of engines.

mike65

No, revs do not relate directly to fuel consumption, after all you can be flying along a motorway with 3000 rpm showing but with the throttle bearly open.

Equally under short bursts of heavy acceleration the revs may never reach 3000 rpm but the throttle will have opened up wide.

I think I have that right!

Mike.

Canis Lupus

ok.... but do u burn alot of fuel by putting yur foot down in say... 5th gear? seeing as putting yur foot down doesnt equate to major acceleration are u wasting loads of petrol or not really all that much?

mike65

I guess you'd need to have a good trip computer to test that out but in a typical petrol engined car I'd say you're using more fuel putting your foot down in 5th when at moderate to high speed to overtake than you would by dropping to 4th the rapid revs rise will mean you get past quicker as the engine hits max torque.

Mike.

BabyEater

A way to explain my question is if yur driving up a steep incline (taking into account this is hypothetical). u have foot to the floor but because of the incline yur revs stay at say 3000RPM as yur car struggles up the hill. now is the car still burning the same amount of fuel as if i was on the flat at the same RPM in the same gear but with my foot just on the accelerator?

Your car will be burning more fuel going up the hill at 3000RPM with your foot to the board than on the flat as you have the throttle open which means the engine is getting more fuel into it to burn. Its down to RPM and how much the boot is to the board.

unkel

Canis Lupus wrote:

Basically I wanna know is petrol consumption based on the actual RPM's or does "putting the foot down" (even if putting foot down doesnt equate to higher revs) result in more petrol burn

You would have answered your own question by asking yourself what petrol consumption would be like going down a steep hill in 2nd gear @ 40mph doing, say 4000rpm, letting the engine do the braking. Where is your foot?

Furp

Your definitely burning more fuel as you need more energy to keep the car at the same speed going up the hill as when the car is on a level road.

Think about this way, if you where on a mountain bike with the pedals as your RPM gauge, Say your cycling in 5th gear on a level surface you then go up a hill but instead of changing down gear you stay in the same gear and pedal at the same RPM, you need to put more energy or throttle into peddling to keep the same RPM.

Incidentally on www.howstuffworks.com they say that in modern injection engines (i.e. not old carburetor engine's), throttle doesn't actually control the amount of fuel going into your engine but controls the amount of air entering and the engine's ECU match's this by adding more fuel. http://auto.howstuffworks.com/fuel-injection2.htm