Best driving style for fuel efficiency

[Andres125sx]

Hi all,

Some time ago my brother told me he read an article where it was stated for best fuel efficiency it is better to accelerate strongly... but only to the point you get enough momentum to reach next corner, stop, etc., contrary to the general idea that it´s better to not accelerate (hit the throttle) too much.

He told me the article said once a petrol engine is consuming fuel, the difference is not that dramatic between hitting the throtle a 10% or 100%, so it´s better to take advantage of the available power to build some speed quickly and then release the throttle and use the big inertia of our heavy cars. Basically the idea is it´s more efficient to accelerate at 100% for 10% of the time/space than accelerating a 10% for 100% of the time/space. Obviously when I say accelerating at 100% I mean using all the power our engine can provide revving it up, not hitting the throttle to the end but doing an upshift at 3000rpm

It made sense to me, but I´d like to read a bit about that. He can´t remind where did he read it, anyone?

Does anyone know some graph for production cars wich shows fuel efficiency at different rpm? I guess that must be the key factor. When it´s said petrol engines efficiency is below 30% or even 20% I guess that´s reffering to its best regime, but it must be even lower at lower revs so if we want to be fuel efficient, we must use all the available power for as little time as possible


As you can see I´m saying all this from memory, and my memory sucks, so fell free to correct, add or laugh about this

[Jolle]

Well, first of all, (hard) acceleration is one of the biggest fuel guzzlers, so driving with an egg under your foot is key. So is braking, because... after braking you have to accelerate again. High corner speed is very very green!

There is no simple graph for fuel consumption because there are to many factors, but generally, the lower (within reason) revs, the lower the fuel consumption because of less friction. Where most people get confused by is efficiency of a petrol engine, which is generally around 70% load. This is why small engines in the same car are more fuel efficient then big engines. To perform the same task (or output) they are more likely to be closer to their max efficiency. A big engine in 6th gear that has 3000 rmp has for instance a load of 30% while, with the extra friction and revs, in 5th gear at 4000 rpm it could be 35%, but with a slightly better efficiency but not enough to counterbalance the extra load.

to safe fuel: take your time to get up to speed and then don't loose that speed again.

[netoperek]

I did one "efficiency" test a while ago, eco-driving through a city at 3 am, trying different styles and techniques. According to my Mitsubishi Lancer's average fuel consumption meter, hard acceleration (almost full throttle) at second gear at most to about 60 kph, then switching fast to 4th gear and adjusting throttle just so to minimally loose the speed (like 1kph each 200 meters or so) up until about 40kph then repeat. When i had to stop or decelerate i used almost only engine braking with fast gear reducion. I managed to achieve 3.1 l / 100km average on 7 km route with that technique. My usual average is about 9 - 10 l/100.
I'd have to admit though, that i wouldn't like to drive behind myself that night

[rscsr]

Hi all,

Some time ago my brother told me he read an article where it was stated for best fuel efficiency it is better to accelerate strongly... but only to the point you get enough momentum to reach next corner, stop, etc., contrary to the general idea that it´s better to not accelerate (hit the throttle) too much.

He told me the article said once a petrol engine is consuming fuel, the difference is not that dramatic between hitting the throtle a 10% or 100%, so it´s better to take advantage of the available power to build some speed quickly and then release the throttle and use the big inertia of our heavy cars. Basically the idea is it´s more efficient to accelerate at 100% for 10% of the time/space than accelerating a 10% for 100% of the time/space. Obviously when I say accelerating at 100% I mean using all the power our engine can provide revving it up, not hitting the throttle to the end but doing an upshift at 3000rpm

It made sense to me, but I´d like to read a bit about that. He can´t remind where did he read it, anyone?

Does anyone know some graph for production cars wich shows fuel efficiency at different rpm? I guess that must be the key factor. When it´s said petrol engines efficiency is below 30% or even 20% I guess that´s reffering to its best regime, but it must be even lower at lower revs so if we want to be fuel efficient, we must use all the available power for as little time as possible


As you can see I´m saying all this from memory, and my memory sucks, so fell free to correct, add or laugh about this

Well that is completely true. Accelerate hard until you hit the speed you need and then just lift off and let the car roll (with highest possible gear engaged to limit the engine resistance). That is way more efficient than driving at a constant speed. You can even try it for yourself when your car has a average fuel consumption display. Of course you have to drive at the same average speed to spend the same time on the straight.
You can even view such a behaviour on optimum control systems, where you usually make the first push as strongly and as short as possible and decelerate in the same way when you reached your goal.

Such a diagram is called consumption map: Wikipedia
But you will find way better charts when you search for "Verbrauchskennfeld" or "Muscheldiagramm".
They show the max torque curve over rpm with isolines for specific consumption (usually in g/kWh) or efficiency.

Check this chart out for example:


It shows additionally curves with constant power. And you can see the line which shows thr minimum consumption line (Linie minimalen Kraftstoffverbrauchs). That line is quite close to the maximum torque curve (for pretty much every ICE)

[Greg Locock]

In English that is called a brake specific fuel consumption (BSFC) map. Burn and coast is usually the most efficient, that is accelerate hard along the minimalen line in that map, then knock it into neutral and switch the engine off (if you are serious). The choice of gear(s) and the two end point speeds are not entirely straightforward.

As you can see the minimalen line more or less tracks full throttle (Vollastkurve).

Unless you have regen then braking is very bad, whether using the brakes or the engine.

If you look at how a Prius operates it sits on that minimalen curve.

[toraabe]

Accelerate quick and keep the engine within it's optimum tourqe curve. Not below and not above.

[J.A.W.]

Some of this is due to 4T SI engine 'throttle' characteristics, & fuel delivery sophistication..

If the inherent 'pumping losses' in high-throttle opening/sub-optimal torque band situations are minimised, thats good..
& one reason why current EFI mills which 'cut-off' fuel delivery with closed throttle - respond to the O.P. technique..

Older, carb-moteurs may do better with the 'treading on an egg' - (unless they have a CV/CD vacuum response inlet),
since less air in, means less air to carry fuel to wastefully pump..

[MadMatt]

In English that is called a brake specific fuel consumption (BSFC) map. Burn and coast is usually the most efficient, that is accelerate hard along the minimalen line in that map, then knock it into neutral and switch the engine off (if you are serious). The choice of gear(s) and the two end point speeds are not entirely straightforward.

As you can see the minimalen line more or less tracks full throttle (Vollastkurve).

Unless you have regen then braking is very bad, whether using the brakes or the engine.

If you look at how a Prius operates it sits on that minimalen curve.

One has to consider extra fuel needed to restart the engine, tho. And if you wanna go even further, cost of battery and starter motor wearing quicker. But yea that would probably be the most economical way of driving, purely based on fuel consumption.

[Andres125sx]

Great replies, some very instructive, thanks mates

[Belatti]

I save fuel consumption average (long-term because short-term it can be tricky) in the car displays and always compare fuel consumption, its a mania I have
I use less fuel than my wife, my dad or even friends/colleagues when we share driving at holidays or work trips.

I dont use 100% throttle but instead a firm 50-70% and once in the desired speed I coast or keep constant if the next stop is far away. One of the keys in traffic is plan ahead. Sometimes if you know you have to stop 100m ahead then just go slowly.

Here in Argentina, traffic lights usually have countdown in seconds, so if you are 100m away from a red light but there are 10 secs left, you just make sure you coast in a way you dont have to stop to 0km/h and move again.... you get used to it and it saves a lot.

And of course, "high speed" cornering as long as its reasonably safe. For example: my wife always complains and tell me I reckless at the wheel, I drive like Im racing... blahblahblahh... the truth is Im not: if Im doing, let´s say 40km/h and I have to turn a street corner I just do it, constant speed (as long as I have enough visibility and I know Im reasonably far from adhesion limits) without loosing much more than 3 or 4km/h. For her this is turning too fast! For me, its just efficient driving

[NL_Fer]

That is basicly how i drive, pretty quick of the line at 70-80% throttle 1500-2300 rpm (diesel) and it works. And try to keep momentum, no braking. It works for my car.

Also once i reach my desired speed i can shift to a very high gear and run the engine with almost no throttle at 1400 rpm. That is efficient. Even a few power takeoffs, shift at redline, have almost no influence on the consumption. As long as i keep the same constant speeds as always.

[Tommy Cookers]

regarding the consumption map shown in rscsr's post ...... IMO .....

the line of minimal fuel consumption is falsely named
clearly the full throttle line would give lower fuel consumption accelerating hard from eg 800-1500 rpm
the line of minimal fuel consumption is actually to meet the expectations of 'ordinary' drivers, accelerating gently if using low rpm
presumably internal 'fly-by-wire' throttling or VVT is done, the engine follows this (or lower lines) unless the driver is using full accelerator travel

the so-called 'full throttle' line is clearly internally throttled or VVT'd or wastegated or ignition-retarded between 1500 and 4500 rpm
otherwise CR/MAP combinations intolerable to the engine could occur

no doubt the car is very nice when as is usual it's being driven on or below the minimal fuel consumption line
the full throttle line is probably only obtained during economy and emissions certification


regarding talk of eg '70% throttle' ..... IMO ......

the engine cannot benefit from eg 70% throttle unless its air consumption is already quite close to 70% (of the maximum possible ie peak bhp)
a few hundred million CD/CV carburettors proved this (even if the accelerator was at 100% travel)
of course 'proper' drivers know this and so don't usually use 100% accelerator travel - because it wouldn't improve anything

[Powy]

Nice discussion! It reminds me of the Shell Eco Marathon competition.

From http://tufast-eco.de/de/competitions/

Travel a certain distance within a certain amount of time
–
and use as little energy as possible, doing so.

I've always wondered what the optimum speed and strategy is.

-----

@rscsr
You mentioned optimal control systems. Are they already employed in standard cars?

[Greg Locock]

(Re edit) . The minimale line is a smoothed line of minimum BSFC at a given rpm. If you have a CVT you then multiply the engine speed by the torque of that line in the right units to give power, and now you have the curve of demand power vs rpm vs throttle opening that will more or less minimise fuel consumption for a given demand power.


In a fixed gear car driving on that line will result in slightly better fuel consumption when accelerating than using full throttle, but of course gives less torque, so your acceleration is slower. the difference between minimale and WOT is fairly small in fuel consumption terms so if you are't competing for fuel economy you might as well floor it. That's my excuse anyway.

[ojlopez]

Well, first of all, (hard) acceleration is one of the biggest fuel guzzlers, so driving with an egg under your foot is key. So is braking, because... after braking you have to accelerate again. High corner speed is very very green!

There is no simple graph for fuel consumption because there are to many factors, but generally, the lower (within reason) revs, the lower the fuel consumption because of less friction. Where most people get confused by is efficiency of a petrol engine, which is generally around 70% load. This is why small engines in the same car are more fuel efficient then big engines. To perform the same task (or output) they are more likely to be closer to their max efficiency. A big engine in 6th gear that has 3000 rmp has for instance a load of 30% while, with the extra friction and revs, in 5th gear at 4000 rpm it could be 35%, but with a slightly better efficiency but not enough to counterbalance the extra load.

to safe fuel: take your time to get up to speed and then don't loose that speed again.

I have always wondered what this means: if you can't press the accelerator pedal more than 50% or if you have to apply it slowly?