Following on from modern car using no fuel when off gas...
Discussion
I knew that a car doesnt use any fuel when zero throttle applied with forward momentum. I really did learn a lot from Max Torques posts regarding the ECU programming though etc.
So I have a further question that hopefully won't mark me out as one of the mentals not understanding anything on the other thread.
Here goes:
If I am accelerating say from 30mph to 60mph on a totally flat piece of road is it better (more fuel efficient) to do it in the correct gear for that speed of your car so you are in the peak of the power band? Say maybe 2nd or 3rd. This way you dont have to open the throttle all the way but make brisk progress up to 60. You then change up to 5th and maintain a constant throttle to stay at 60mph.
Or is it more fuel efficient to stay in too high a gear for 30, say 4th or 5th when you are out of peak power but apply full throttle (while the car struggles to accelerate to 60) until you get to 60mph at which point you can then reduce throttle and maintain your constant speed.
Hope that makes sense...
So I have a further question that hopefully won't mark me out as one of the mentals not understanding anything on the other thread.
Here goes:
If I am accelerating say from 30mph to 60mph on a totally flat piece of road is it better (more fuel efficient) to do it in the correct gear for that speed of your car so you are in the peak of the power band? Say maybe 2nd or 3rd. This way you dont have to open the throttle all the way but make brisk progress up to 60. You then change up to 5th and maintain a constant throttle to stay at 60mph.
Or is it more fuel efficient to stay in too high a gear for 30, say 4th or 5th when you are out of peak power but apply full throttle (while the car struggles to accelerate to 60) until you get to 60mph at which point you can then reduce throttle and maintain your constant speed.
Hope that makes sense...
A wide open throttle in a petrol engine is actually one of it's most effiecent modes, keep it around peak torque with the throttle wide open then quickly up into the highest gear to minimise internal friction, sorted.
It does depend on the engine though, some (RX-8, high boost turbo's) throw quite a lot more extra fuel in at WOT to keep the engine cool, so 70-80% throttle may well be better in some instances.
It does depend on the engine though, some (RX-8, high boost turbo's) throw quite a lot more extra fuel in at WOT to keep the engine cool, so 70-80% throttle may well be better in some instances.
Edited by PhillipM on Wednesday 26th January 22:41
I was actually commended by both my driving instructor and examiner (I got a very rare combination of an instructor who had a V8 muscle car as well [so he knew what he was doing], and a decent examiner who wasn't obsessed with SPEED KILLS and ENVIRONMENT etc) for block shifting - the test route I took led me from a 30mph zone into a 50mph zone. What I used to do was accelerate smoothly in third gear up to 50 (the car was doing 3500-4000rpm) and then change straight to fifth (top gear). I was told it was a good idea to accelerate smoothly, efficiently, and safely, and this was a display of progress that (the examiner) would be expecting after considerable time behind the wheel.
FWIW I just thought it was a nice way to hear the little 3-cylinder Aygo sing.
FWIW I just thought it was a nice way to hear the little 3-cylinder Aygo sing.
So same sort of question if you are going up a steady incline, is it more fuel efficient to be in top gear with a lot of throttle, or a lower gear with less throttle to maintain a constant speed.
I guess the bit i dont understand and perhaps Max Torque could clarify is what are the injectors doing when you have your foot buried into the carpet when in top gear trying to accelerate? Do you use less fuel as the engine is turning over less when in 5th, or do you use less with the engine turning over faster but with less throttle in a lower gear?
I guess the bit i dont understand and perhaps Max Torque could clarify is what are the injectors doing when you have your foot buried into the carpet when in top gear trying to accelerate? Do you use less fuel as the engine is turning over less when in 5th, or do you use less with the engine turning over faster but with less throttle in a lower gear?
I think accelerating from 30-60 in most cars, you'll want to do it as slowly as possible in whatever gear you would use for cruising at 30 because most cars are at their most economical at around 30mph (in other words it's best if you accelerate infinitely slowly and just remain at 30mph 
). Accelerating from 10-30, you probably want to do reasonably briskly.
). Accelerating from 10-30, you probably want to do reasonably briskly. kambites said:
most cars are at their most economical at around 30mph
Really? I always understood it was about 58mph, but am always happy to be corrected. My wifes alfa 156 won't (happily) do 30 in top gear - it would chug liek anything if you came to any sort of hill, so is 30 in 4th still the most economical for the car?chrisga said:
kambites said:
most cars are at their most economical at around 30mph
Really? I always understood it was about 58mph, but am always happy to be corrected. My wifes alfa 156 won't (happily) do 30 in top gear - it would chug liek anything if you came to any sort of hill, so is 30 in 4th still the most economical for the car?Edited by kambites on Thursday 27th January 09:53
PhillipM said:
A wide open throttle in a petrol engine is actually one of it's most effiecent modes, keep it around peak torque with the throttle wide open then quickly up into the highest gear to minimise internal friction, sorted.
It does depend on the engine though, some (RX-8, high boost turbo's) throw quite a lot more extra fuel in at WOT to keep the engine cool, so 70-80% throttle may well be better in some instances.
Most efficient in terms of power produced per unit of fuel burnt, yes, peak power is the best for that. That does not equate to most efficient in terms of miles covered per unit of fuel burnt, though, and that sort of calculation is properly complicated.It does depend on the engine though, some (RX-8, high boost turbo's) throw quite a lot more extra fuel in at WOT to keep the engine cool, so 70-80% throttle may well be better in some instances.
I think the most economical way would, in the vast majority of cars, be to keep above a third of the available rev range but below two-thirds of it - so you have plenty power and aren't labouring the engine, but also aren't spinning the engine too fast and incurring big frictional losses from it.
ETA
kambites said:
There is no particular reason that a car should be most economical in top gear; in fact I very much doubt that any are. 58 is usually quoted as a good compromise between economy and actually getting somewhere, I think.
Actually, there is. A higher gear gives the car a lower effective mass, which automatically makes it more economical for a given engine speed and fuel input. The engine will also be turning more slowly at a given roadspeed, which minimises losses to friction and heat. The absolute most economical speed is usually the lowest speed at which the car is not labouring in its highest gear - so the engine doesn't have to over-fuel to maintain speed, or fight any more drag or rolling resistance than necessary.Edited by McSam on Thursday 27th January 09:58
I read a study years ago that said for best fuel efficiency, acceleration should be done by going to 2/3 of the peak power point and then changing up, repeat until you reach chosen speed, then shift to the highest gear that doesn't labour the engine.
Obviously as other posters have said, every incarnation will be slightly different, but this seems to make sense and regularly taking a V8 to 4/5000 rpm is lovely
Obviously as other posters have said, every incarnation will be slightly different, but this seems to make sense and regularly taking a V8 to 4/5000 rpm is lovely
ok then the boost on my td5 kicks in at about 2k rpm. below that there is almost zero torque which is wierd for a diesel...
so am i better to cruise on the motorway in 5th at just below the point the turbo kicks in, as i assume as well as the extra air being forced in there will be extra fuel needed to keep the mixture right.
Or do i want to be cruising with the turbo on boost
which is likely to be more efficeint...
so am i better to cruise on the motorway in 5th at just below the point the turbo kicks in, as i assume as well as the extra air being forced in there will be extra fuel needed to keep the mixture right.
Or do i want to be cruising with the turbo on boost
which is likely to be more efficeint...
MaximumJed said:
I read a study years ago that said for best fuel efficiency, acceleration should be done by going to 2/3 of the peak power point and then changing up, repeat until you reach chosen speed, then shift to the highest gear that doesn't labour the engine.
Thinking about it that makes sense when I think of the cars I've had. I used to change up in the Leon at about 3.5k and normally go to about 5.5-6k in the RX8.McSam said:
Actually, there is. A higher gear gives the car a lower effective mass, which automatically makes it more economical for a given engine speed and fuel input. The engine will also be turning more slowly at a given roadspeed, which minimises losses to friction and heat. The absolute most economical speed is usually the lowest speed at which the car is not labouring in its highest gear - so the engine doesn't have to over-fuel to maintain speed, or fight any more drag or rolling resistance than necessary.
I disagree with almost all of that. The mass of the car makes no difference whatsoever to fuel economy at constant speed on the flat, except in so far as it affects the loads on the bearings etc. There WILL be slightly less mechanical drag in a higher gear, but friction increases somewhere near linearly and aerodynamic drag increases with the square of speed, so it's heavily out-weighed above certain speed and that speed is, in my experience, typically very low.
Every car I have ever owned, has been most efficient in a lower gear than top. The Elise seems most efficient in third at about 25mph, although 4th at about 35mph is similar (not particularly easy to tell without a trip computer, mind). The Punto is about 30mph in fourth. The Corrado was about 32 in fourth.
A car which was most efficient in top gear would, in my experience, have really stupid gear ratios. Top gear is usually chosen to give optimal efficiency at the speed people usually cruise at (so somewhere around 70), not to give optimal fuel efficiency at the most efficient speed.
Edited by kambites on Thursday 27th January 10:37
PhillipM said:
A wide open throttle in a petrol engine is actually one of it's most effiecent modes, keep it around peak torque with the throttle wide open then quickly up into the highest gear to minimise internal friction, sorted.
^^^^^^^ This.When it comes to cruising, as opposed to accelerating, you get best mpg around peak torque (as someone has already said) BUT you need to achieve that at a speed where wind resistance, which increases very quickly as speed builds up, does not have any great effect.
Dracoro said:
So I should be driving at 7500 rpm for best mpg then? hmm....
No, you should be driving at 7500rpm with the throttle wide open for best engine efficiency, which is not the same as best mpg. There is an argument that the most efficient way to drive at a certain speed is to put the car in a gear that puts the engine at about the RPM where peak torque is generated, and then alternate between full on (peak efficiency) and fully off (zero fuel usage) the throttle. I suspect this doesn't work very well in practice, though.
kambites said:
McSam said:
Actually, there is. A higher gear gives the car a lower effective mass, which automatically makes it more economical for a given engine speed and fuel input. The engine will also be turning more slowly at a given roadspeed, which minimises losses to friction and heat. The absolute most economical speed is usually the lowest speed at which the car is not labouring in its highest gear - so the engine doesn't have to over-fuel to maintain speed, or fight any more drag or rolling resistance than necessary.
I disagree with almost all of that. The mass of the car makes no difference whatsoever to fuel economy at constant speed on the flat, except in so far as it affects the loads on the bearings etc. There WILL be slightly less mechanical drag in a higher gear, but friction increases somewhere near linearly and aerodynamic drag increases with the square of speed, so it's heavily out-weighed above certain speed and that speed is, in my experience, typically very low.
Every car I have ever owned, has been most efficient in a lower gear than top. The Elise seems most efficient in third at about 25mph, although 4th at about 35mph is similar (not particularly easy to tell without a trip computer, mind). The Punto is about 30mph in fourth. The Corrado was about 32 in fourth.
A car which was most efficient in top gear would, in my experience, have really stupid gear ratios. Top gear is usually chosen to give optimal efficiency at the speed people usually cruise at (so somewhere around 70), not to give optimal fuel efficiency at the most efficient speed.
Edited by kambites on Thursday 27th January 10:37
I think the mechanical losses in the engine are more than you're giving them credit for, but nonetheless using a lower speed in a low gear reduces them. It's an interesting thought, not one I'd really considered, since I was rather too biased towards getting where I needed to go! Time taken to travel has factored into my efficiency
what I should perhaps have said is that the best balance between fuel efficiency and travelling speed will be found quite low down in the highest gear.The effective mass, engine inertia etc. is pretty low by third and certainly by fourth, the more I think about it. Rolling the Audi along at 35mph in 4th feels no more effort than 45mph in 5th, and possibly even less
My thinking is, though, that if the car uses, say, x units of fuel per hour at 35mph in fourth, that's lovely. But if it uses 1.25x units of fuel per hour at 45mph in fifth, I'm using my fuel much more quickly, but I'll still have gone further on the same amount.
Out of interest, how are you measuring the Elise's efficiency at those speeds?
Edited by McSam on Thursday 27th January 10:56
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