Most efficient way to accelerate in a petrol automatic car?
Discussion
As per the title, if I want to attain a given road speed what is the most efficient way to accelerate in a large capacity petrol engine automatic? Does one gently waft up to speed keeping the revs low (<2k rpm) or is it actually more efficient to use a medium rev band of (2k-3k rpm) and a decent engine load and get there quicker? Presumably flooring it and revving to the redline is not the answer.
As an aside, my engine is a Hemi which has a multi-displacement system where it runs on 4 cylinders under certain conditions (1k-3k rpm, low load, up to temperature) so how would this affect things? It might be possible to run on 4 cylinders under acceleration but would it actually be more efficient to use all 8 to get going and then spend longer in the 4-cylinder lean cruise state?
The official teachings would have you believe that gentle acceleration is the way forward but my understanding is that an engine is more efficient at converting fuel in to motion under a medium load than at nearly idle so perhaps this only applies to cars with small engines that need lots of revving just to make them move at all?
Any empirical observations or actual data would be appreciated.
As an aside, my engine is a Hemi which has a multi-displacement system where it runs on 4 cylinders under certain conditions (1k-3k rpm, low load, up to temperature) so how would this affect things? It might be possible to run on 4 cylinders under acceleration but would it actually be more efficient to use all 8 to get going and then spend longer in the 4-cylinder lean cruise state?
The official teachings would have you believe that gentle acceleration is the way forward but my understanding is that an engine is more efficient at converting fuel in to motion under a medium load than at nearly idle so perhaps this only applies to cars with small engines that need lots of revving just to make them move at all?
Any empirical observations or actual data would be appreciated.
chris182 said:
As per the title, if I want to attain a given road speed what is the most efficient way to accelerate in a large capacity petrol engine automatic? Does one gently waft up to speed keeping the revs low (<2k rpm) or is it actually more efficient to use a medium rev band of (2k-3k rpm) and a decent engine load and get there quicker? Presumably flooring it and revving to the redline is not the answer.
As an aside, my engine is a Hemi which has a multi-displacement system where it runs on 4 cylinders under certain conditions (1k-3k rpm, low load, up to temperature) so how would this affect things? It might be possible to run on 4 cylinders under acceleration but would it actually be more efficient to use all 8 to get going and then spend longer in the 4-cylinder lean cruise state?
The official teachings would have you believe that gentle acceleration is the way forward but my understanding is that an engine is more efficient at converting fuel in to motion under a medium load than at nearly idle so perhaps this only applies to cars with small engines that need lots of revving just to make them move at all?
Any empirical observations or actual data would be appreciated.
With an auto you won't really have much choice tbh, not unless you are continually faffing about with the gear selector.As an aside, my engine is a Hemi which has a multi-displacement system where it runs on 4 cylinders under certain conditions (1k-3k rpm, low load, up to temperature) so how would this affect things? It might be possible to run on 4 cylinders under acceleration but would it actually be more efficient to use all 8 to get going and then spend longer in the 4-cylinder lean cruise state?
The official teachings would have you believe that gentle acceleration is the way forward but my understanding is that an engine is more efficient at converting fuel in to motion under a medium load than at nearly idle so perhaps this only applies to cars with small engines that need lots of revving just to make them move at all?
Any empirical observations or actual data would be appreciated.
And if I'm honest, why bother. If you are only going to tippy toe about then you might as well have bought a diesel version, as you'd be able to drive it harder, quicker and still see better mpg.
Think its more applicable to manuals but BMW used to suggest full throttle acceleration but change up at lowish revs in their handbooks.
I'm not sure if its still true with modern ECUs but engines are generally more efficient at load (but low revs). All within reason of course as you don't want to be labouring the engine.
I'm not sure if its still true with modern ECUs but engines are generally more efficient at load (but low revs). All within reason of course as you don't want to be labouring the engine.
300bhp/ton said:
With an auto you won't really have much choice tbh, not unless you are continually faffing about with the gear selector.
And if I'm honest, why bother. If you are only going to tippy toe about then you might as well have bought a diesel version, as you'd be able to drive it harder, quicker and still see better mpg.
Well you do have a choice because depending on how much throttle you apply the drivetrain shifts gears at different points. If I hold the throttle at one position it changes up at roughly 2k rpm, if I hold it at another it changes up at roughly 3k rpm. And if I'm honest, why bother. If you are only going to tippy toe about then you might as well have bought a diesel version, as you'd be able to drive it harder, quicker and still see better mpg.
As for why bother, when there is space then one can enjoy the performance but a lot of the time I'm just accelerating towards the next red light or pensioner doing 30mph so there is no benefit to thundering off as fast as possible. In that situation one might as well use as little fuel as possible so one can use it properly later on!
I'm driving a diesel auto but i guess similar principles apply. I find a balance of swiftly but not full on throttle in getting to my cruising speed to be the most efficient.
Cruising/coasting and braking as little as possible seems to be when any car is most efficient, acceleration the least so I spent as little time accelerating as possible.
Cruising/coasting and braking as little as possible seems to be when any car is most efficient, acceleration the least so I spent as little time accelerating as possible.
Trial and error will probably be your best option. Although I'm sure someone here could do the maths and work out the pumping loses and such.
Personally I've found a light throttle and smooth actions normally work well. Don't let the engine labour and revs are fine assuming you aren't heavy on the pedal.
Low rpm may not give the best mpg, my V8 auto Camaro is more economical at 80mph (just under 2000rpm) than it is at 60mph (just under 1500rpm) in top.
I don't know for sure, but I suspect revving to 3000rpm but with the throttle position that only lets you rev to 2000rpm would be the more economical route, but to do this you'll need to manually hold it in gear.
Personally I've found a light throttle and smooth actions normally work well. Don't let the engine labour and revs are fine assuming you aren't heavy on the pedal.
Low rpm may not give the best mpg, my V8 auto Camaro is more economical at 80mph (just under 2000rpm) than it is at 60mph (just under 1500rpm) in top.
I don't know for sure, but I suspect revving to 3000rpm but with the throttle position that only lets you rev to 2000rpm would be the more economical route, but to do this you'll need to manually hold it in gear.
I guess the only real way to find out is to do the same trip with the different driving styles and brim the tank between them to see the difference. I have a trip computer which tells me mpg but currently I think it's telling me lies.
I'm not to clear on the efficiency profiles of automatic gearboxes either, I know they lose power pumping fluid around and by slipping the torque converter but is this loss roughly constant or does it increase/decrease with rpm or torque?
I'm not to clear on the efficiency profiles of automatic gearboxes either, I know they lose power pumping fluid around and by slipping the torque converter but is this loss roughly constant or does it increase/decrease with rpm or torque?
chris182 said:
I guess the only real way to find out is to do the same trip with the different driving styles and brim the tank between them to see the difference. I have a trip computer which tells me mpg but currently I think it's telling me lies.
I'm not to clear on the efficiency profiles of automatic gearboxes either, I know they lose power pumping fluid around and by slipping the torque converter but is this loss roughly constant or does it increase/decrease with rpm or torque?
I'm not so familiar with the latest breed of auto boxes. But yes the TC will cause drag, however most modern TC's can 'lock up', which effectively removes the slip, however they main only do this at certain speeds and gears.I'm not to clear on the efficiency profiles of automatic gearboxes either, I know they lose power pumping fluid around and by slipping the torque converter but is this loss roughly constant or does it increase/decrease with rpm or torque?
As for the onboard computer, I guess it might be using US gallons (approx 3.78 litres) instead of Imperial gallons (4.546 litres).
300bhp/ton said:
As for the onboard computer, I guess it might be using US gallons (approx 3.78 litres) instead of Imperial gallons (4.546 litres).
I had thought of this but that should mean it displays worse than reality, whereas it was telling me 20.9mpg when the petrol pump was telling me 18.6mpg. I am keeping track of it so I can verify accurately over several tanks of fuel what the real figure is.As for torque converter lockup, I know this happens under cruising conditions but I don't know if it does under acceleration or not. If not then that would be another argument for accelerating quickly to spend less time in a slipping state. I shall keep a close eye on my rpm vs road speed and see if this is discernible.
trickywoo said:
Think its more applicable to manuals but BMW used to suggest full throttle acceleration but change up at lowish revs in their handbooks.
I'm not sure if its still true with modern ECUs but engines are generally more efficient at load (but low revs). All within reason of course as you don't want to be labouring the engine.
Generally aim for this - open the throttle wider to reduce pumping losses, but try and get the autobox to change up at ~3k.I'm not sure if its still true with modern ECUs but engines are generally more efficient at load (but low revs). All within reason of course as you don't want to be labouring the engine.
Krikkit said:
trickywoo said:
Think its more applicable to manuals but BMW used to suggest full throttle acceleration but change up at lowish revs in their handbooks.
I'm not sure if its still true with modern ECUs but engines are generally more efficient at load (but low revs). All within reason of course as you don't want to be labouring the engine.
Generally aim for this - open the throttle wider to reduce pumping losses, but try and get the autobox to change up at ~3k.I'm not sure if its still true with modern ECUs but engines are generally more efficient at load (but low revs). All within reason of course as you don't want to be labouring the engine.
It also has a sport mode which does the same kinda thing but to a more exaggerated extent than in regular drive mide.
So all cars are different and there is no definitive answer to your question really.
Glad to have been so helpful
jon- said:
Isn't an engine most efficient at WOT and at the peak power point in the graph, so in this auto example keeping the throttle pinned to the intended speed would be most efficient?
I believe an engine is most efficient at WOT though I'm not sure about the peak power bit, I thought it was roughly around peak torque. Does the engine producing power in its most efficient state necessarily equate to that being the most efficient way to accelerate the whole vehicle though? If this was the case then surely manufacturers would recommend that we accelerate at full throttle to minimise fuel consumption which certainly isn't the case.In my daily E38 4.4 V8 I accelerate gently the gearbox ECU starts off in 2nd unless it's in sport & changes up as early as it can without labouring and I get 23-25 mpg UK City and 30 plus at my-way speeds.
Currently driving a 2.0 Mitsi Lancer with what I think has a CVT gearbox and it's behaviour under normal driving conditions is hilarious. Returning 44 mpg US on highway and 32 mpg US city. Pressing the accelerator to the floor results in max revs for the same increase in momentum as gentle pressure. Going by the mpg indicator it's not very fuel efficient when driven like that.
Weirdest damn gearbox behaviour I have ever seen haven't found a quiet road I can check out the speeds possible in reverse yet
Currently driving a 2.0 Mitsi Lancer with what I think has a CVT gearbox and it's behaviour under normal driving conditions is hilarious. Returning 44 mpg US on highway and 32 mpg US city. Pressing the accelerator to the floor results in max revs for the same increase in momentum as gentle pressure. Going by the mpg indicator it's not very fuel efficient when driven like that.
Weirdest damn gearbox behaviour I have ever seen haven't found a quiet road I can check out the speeds possible in reverse yet
Edited by B'stard Child on Wednesday 20th November 15:43
chris182 said:
Does the engine producing power in its most efficient state necessarily equate to that being the most efficient way to accelerate the whole vehicle though?
Not an expert here, but no. Most efficient way to produce power != most efficient way to use fuel.Brake-specific fuel consumption is what you're after. Generally best at a holy trinity of WOT, peak torque and low revs.
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