Do I use less petrol at lower revs?

[swee'pea99]

Having started one pointless car-related thread that's spun completely out of whack, I thought I'd take a crack at a second, so...

I try to drive fuel-efficiently. I mean, I'm not obsessive or anything, but other things being equal I tend to try to avoid hard acceleration, coast rather than brake, and so on. I was musing idly, as I do, about whether it saves fuel to be in arguably too high a gear at any given speed, on account of lower revs must mean fewer explosions, which presumably means less fuel used. In a way, it feels counter-intuitive, in that you can feel the engine is 'having to work harder' - like trying to cycle up a hill in a higher gear. Do car engines have an optimum cadence? Or is my original idea correct, ie, one rev = a precise amount of fuel, therefore lower revs means less fuel?

 

[welsh dragon]

Having started one pointless car-related thread that's spun completely out of whack, I thought I'd take a crack at a second, so...

I try to drive fuel-efficiently. I mean, I'm not obsessive or anything, but other things being equal I tend to try to avoid hard acceleration, coast rather than brake, and so on. I was musing idly, as I do, about whether it saves fuel to be in arguably too high a gear at any given speed, on account of lower revs must mean fewer explosions, which presumably means less fuel used. In a way, it feels counter-intuitive, in that you can feel the engine is 'having to work harder' - like trying to cycle up a hill in a higher gear. Do car engines have an optimum cadence? Or is my original idea correct, ie, one rev = a precise amount of fuel, therefore lower revs means less fuel?

No. It depends on a lot of things. It cannot be easily quantified.

 

[XRHYSX]

you can use just as much fuel being in to higher gear as being in to lower gear

 

[threebikesmcginty]

Just get an automatic, you don't have to think about that sort of stuff then.

 

[XRHYSX]

also coasting down hill uses more fuel than leaving it in gear and using engine braking

 

[welsh dragon]

No. It depends on a lot of things. It cannot be easily quantified.

You are much better off talking about the best and optimum speed for fuel efficiency.

 

[welsh dragon]

Just get an automatic, you don't have to think about that sort of stuff then.

Snap

 

[XRHYSX]

Just get an automatic, you don't have to think about that sort of stuff then.

most autos use more fuel then their manual counter parts, as they have to rev higher before up shifting.

Speed plays a big part in fuel efficiency when I'm driving my lorry, I use less fuel driving at 52mph than I do driving at 50 or 56mph (when loaded) its all to do with how you work the engine and how/when you change gear

 

[welsh dragon]

most autos use more fuel then their manual counter parts, as they have to rev higher before up shifting.

Speed plays a big part in fuel efficiency when I'm driving my lorry, I use less fuel driving at 52mph than I do driving at 50 or 56mph (when loaded) its all to do with how you work the engine and how/when you change gear

 

[nickyboy]

It seems logical that travelling at 30mph in 5th gear @ 1500rpm will use less petrol that 30mph in 2nd gear @4000rpm

But assuming that more petrol is consumed in 2nd gear and the resultant work done in propelling the car forward is the same, where is the extra energy lost? Is it in the mechanics of the engine (having to fire and thus move more frequently)? Perhaps an engineer could enlighten me

 

[welsh dragon]

[QUOTE 2775257, member: 45"]...and how much energy is lost in the auto transmission compared with a standard gearbox?[/quote]

I think it is swings and roundabouts. You may use a bit more because it is auto, but gear changing can be and is smoother. It is all relative and evens itself out.

 

[donnydave]

It seems logical that travelling at 30mph in 5th gear @ 1500rpm will use less petrol that 30mph in 2nd gear @4000rpm

But assuming that more petrol is consumed in 2nd gear and the resultant work done in propelling the car forward is the same, where is the extra energy lost? Is it in the mechanics of the engine (having to fire and thus move more frequently)? Perhaps an engineer could enlighten me

There will be losses associated with higher revs, pumping losses, friction and other stuff. It probably depends how old your engine is as to what's best, a lot of new petrol cars I've driven recently are mapped to behave like diesels with the power lower down the revs and nothing up top, possibly so people perceive them as being quicker and because it seems to me less and less people have any clue about what gear to be in. Lots of people accelerate to overtake me on my bike (after waiting patiently behind for an appropriate gap, of course!) and you can hear them labouring away at 25mph in 5th in a petrol car hardly moving at all.

When I say mapped I also mean what the timing is like on inlet/exhaust. I only really know about older engines where you have massive overlap where inlet and exhaust are open at the same time which helps things along at higher revs but you're literally throwing unburned fuel straight down the exhaust at lower revs, which was fine with the emissions regs of the day

 

[Deleted member 26715]

[QUOTE 2775257, member: 45"]...and how much energy is lost in the auto transmission compared with a standard gearbox?[/quote]
LOTS unless it is of a DSG type, autos are known for inefficient use of fuel due to the 'slushbox' nature of the torque convertor as there is no direct drive from engine to wheel.

Alan...

 

[sazzaa]

also coasting down hill uses more fuel than leaving it in gear and using engine braking

I've never understood this.

 

[donnydave]

I've never understood this.

With the clutch in the engine has to power itself at idle to keep turning over . If you leave it in gear a modern engine can turn the fuel down to a dribble and the movement of the car keeps the engine turning round.

Or something like that.