Graph of car fuel consumption versus speed

[Dave Baker]

Well it's present at any speed and where we determine it becomes significant is actually a function of how large the drag force is compared to the rolling resistance one which stays pretty much a constant at all speeds. For an average modern streamlined but heavy car like my Focus the drag force is fairly immaterial below 30 mph. At 45 mph the engine power being used to overcome drag is about the same as that being used to overcome rolling resistance. At 65 mph it's twice as much and at 80 mph three times as much.

However pick a light but less streamlined car like an old Fiesta or Golf from the 80's and the picture changes. At 2000 lbs with driver rather than the 3000 of the Focus but 15% to 20% higher drag the relationship is very different. The power being used to overcome drag is the same as that being used to overcome RR by 35 mph. At 60 mph it's three times as much and at 80 mph five times as much.

Light cars therefore benefit from going slower much more than heavy ones in terms of fuel consumption. This is exacerbated by the fact that the large engines of large heavy cars, as with all petrol engines, only work efficiently at reasonable throttle openings and at low speed on a whisper of throttle the cylinders are not filling well. A small light car with a small engine is doing much better in all respects here. You want a diesel for good efficiency at any power output.

What this translates into is my Focus doesn't really get much different fuel consumption whether I do 60 mph or 80 mph. It seems to peak at about 38 mpg and I just can't improve on that. However an old Fiesta XR2i I used to run which gave 34 mpg on average once achieved 51.5 mpg when I stuck to 40 mph on A roads and 50 mph on motorways during the fuel shortage in 2000. If I gave the Focus the same treatment I doubt I'd beat 40 mpg by much. It's just too heavy.

The formula for good fuel economy is very simple. Small, light, aerodynamic cars with small low friction diesel engines. The 100 mpg practical car is a very easy thing to design but we'll only see them when fuel prices bite even harder. I've designed one in basic terms which would take me from London to Aberdeen, which I do fairly regularly, for 1/3 the fuel cost of the Focus. At present a round trip costs me £150. That's a big chunk of a holiday cost when you can fly abroad with Sleasyjet for a tenner. At £50 it wouldn't be much of a burden.

[Adrian]

gurgled happily, sounding much like they were saying:

They haven't been urban/56/75 since the mid 1990s! The current urban/ extra-urban figures are done across a variety of speeds and loads, including acceleration and deceleration.

[dennis]

You get better MPG in higher gears which gives less acceleration.

You also get better mpg by using just enough gas to give the acceleration needed, not the maximum acceleration you can get. You need to be able to drive to know what acceleration is needed rather than just putting your foot down and this is where most come unstuck. I seldom need more than a quarter throttle to match motorway speeds on any of the slip roads I use. I have had to use the hard shoulder when some prat has decided to stop because he can't get into a gap. Using the hard shoulder is the correct way of doing it of course.

[dennis]

That depends on the torque curves and old long stroke engines had their torque low down so it was true. My wife's corsa has the torque low down since I had the engine map changed. Its far better to have low end torque than a few extra bhp. Its pretty stupid to have high bhp at the expense of torque with 70 mph speed limits.

[Man at B&Q]

That's what my father always said (max torque).

MBQ

[Grimly Curmudgeon]

We were somewhere around Barstow, on the edge of the desert, when the drugs began to take hold. I remember The Natural Philosopher saying something like:

Useter be peak of the torque curve and probably still is.

I agree that slowcoach acceleration is pointless and wasteful of everbody's time, as is boot to the floor (but godammit, it's fun), so when I'm driving economically it's always a compromise, usually about half boot to get up to speed and a whiff of throttle to stay there. On all sort of vehicles and engine types I've found this pays off without holding anybody up.

I digress... Years ago I had to accompany a rider on a clapped-out 175cc bike on a motorway trip of around 120miles. I was riding one of my GS850s and of necessity had to accelerate gently and limit my top speed to ~60mph.

At the end of the trip I was astounded to find I'd done at least 60mpg on a bike that normally returned 38mpg.

[The Natural Philosopher]

In an ideal word, railways make the most sense for medium-long haul. typically miles and up..to the limit of the continent!

What holds it back chiefly is the lack of efficient container loading/unloading.

And no interest in providing it.

[The Natural Philosopher]

Yup. We've been playing with 'sport' mode on the auto, and it seems to use less..goes higher in revs on the deezil..

The optimal seems to be aroudn 2000-3000 RPM. Turbochargers make the torque peak a lot lower in most engines.

[Grimly Curmudgeon]

We were somewhere around Barstow, on the edge of the desert, when the drugs began to take hold. I remember The Natural Philosopher saying something like:

Christ, no. I was in a car once being driven like that and I couldn't wait to get out of it - the prat was doing it all along the motorway. I've also been behind prats doing it - total fuckwits.

[Andrew Gabriel]

They aren't fuel efficient. Whilst efficiency of many modes of transport (such as cars) have been a consideration for many years and resulted in many improvements over those years, efficiency of trains (at least in the UK) was completely ignored until very recently and is miles behind.

[The Natural Philosopher]

same as me i my old Triumph spitfire. Surrey to Cambridge used to be around 38mpg..till I had to do it in 6" of snow. With wheelspin at anything more than a gentle prod, and a top speed of more than 50mph impossible, I recorded over 50mpg on that run..

[The Natural Philosopher]

Oh, its a nightmare to sit in, but it might be fuel efficient, if bloody dangerous.

[The Natural Philosopher]

I think you will find its always been miles ahead, and is slowly having its advantage eroded actually.

I am pretty sure a 4500 bhp loco can pull 50 35 ton containers. At very sensible speeds.

I don't think a 90 bhp truck could do the same for one 35 ton container. with any adequacy.

[Harry Bloomfield]

Nige Danton laid this down on his screen :

I fancy it will be a trade off in a car between the gear in use and aerodynamic friction. Going just fast enough to be able to stay in top gear will be the most economic speed in most cars, which will be slightly below 60mph.

[Clint Sharp]

In message , Nige Danton writes

What sort of advantage does wearing lycra so tight that we can tell what religion you are play or is that, as I have long suspected, just a fetish thing?

BTW, if you tell me you shave your legs for aerodynamic advantage like my neighbour does, nothing in the world will persuade me the lycra isn't a fetish thing.

[Clint Sharp]

In message , The Natural Philosopher writes

Under heavy acceleration the engine management system will go open loop and injects more fuel than is strictly necessary for very good reasons, the excess fuel is burned in the cat and wasted so slow acceleration will use less fuel even if the eventual speed is the same.

Of course, but unfortunately real life internal combustion engines don't run best with theoretical energy inputs so even though the amount of energy required is the same you'll find that a heavy foot will still use more fuel.

[andrew]

Me too, that's why it should be better to get to your target speed by keeping the revs in the max torque band as you change gears, which implies full throttle but I suspect the fuel map is such that its best to back off from this a bit.

AJH

[The Real Doctor]

Only for a constant drag coefficient. Cd itself, however, depends on the flow pattern around the car and that in turn depends on the speed

- it's often expressed in term of the Reynolds number. As speed increases the flow becomes more turbulent and drag increases enormously. Conversely, when the speed is low the flow is (largely) laminar and drag is low.

Ian

[The Real Doctor]

40mph. Highway Code, section 123.

[The Natural Philosopher]

Yes, I know that, but the real point is where is the optimal throttle setting for efficient acceleration. It ain't idle, and as you have pointed out, it ain't full welly either.

But a mouse foot may be just as bad. That's the point.

If what the engine is optimised for is top gear cruise at 56mph, thats medium revs and maybe 1/3rd throttle.

Possibly that's where we should be accelerating?

I've been dong research. Its fascinating. some facts seem to be well agreed.

1/. Full throttle is not efficient.It tends to cause over rich mixture. 2/. Max RPM is inefficient. It wastes power in engine friction. 3/. On other than racing engines, peak torque is arrived at at lower than max revs. This is generally true even with a turbo, as the sort of breathing or boosting that will make an engine develop peak torque at peak RPM is totally intractable in a road car.

So that definitely means we want to not use full throttle or max RPM when accelerating..

Down at the bottom end, it seems to be the case that wide throttle on a very low RPM is not efficient either.

Surprisingly, this area of car engine performance seems ill understood by almost all the sources I can find. Or its simply ignored.

Interestingly, one poster on a US group said that sub 50mh, he needed to drop a gear to get his mpg back. Thats just cruising along.

I can't be sure, but I get the distinct impression that medium light throttle and medium revs works best on my petrol machine, and in fact the turbodiesel as well.