Graph of car fuel consumption versus speed

[The Natural Philosopher]

I think its more to do with having to meet targets set at 56mph speeds..

You can in theory optimize design for any speed..and I suspect thats the one thats chosen. I.e. they have set the motor to be geared optimally for 56mph on a flat road..

I used to notice similar on the last car I had with a consumption meter. There was little to be gained sub 50mph.

My camper seems to be relatively unaffected whether I drive at 50 or 60 either, but does show a marked worsening at 70mph.

I am scratching my memory for where engines are efficient..I am fairly sure with a diesel its actually lightly loaded but at reasonably high RPM..the friction loses in a slow revving diesel are low anyway, so you can go for the weakest mixture and the most complete combustion at the highest temp. Thats at lowish throttle settings.

For petrol cars, with injection, I am less sure where they are optimal. It is certainly wasteful to rev to the limit. But whether flat out at

1500 RPM or half pedal at 3000 is better or worse I couldn't say.

You might even find that periods of acceleration and periods of coasting is in fact a better way to drive.

[Adrian]

Nige Danton gurgled happily, sounding much like they were saying:

There's a clue in my email address...

[Man at B&Q]

It's all to do with the trans-dimensional (in)stability of the Yorkie Bar.

MBQ

[AJH]

I'm sure you are right, the engine is probably the first thing optimised for this speed, below the optimum the thermodynamic efficiency tails off significantly with a petrol engine, less so with a diesel.

A smaller engine in the same car should move this part of the graph toward higher mpg. I think this is one of the reasons the original hybrids appeared poor in open road conditions they had smaller engines and did best at about 45mph on the open road.

With an average drag I think the rolling resistance equals the wind resistance at less than 20mph (12mph for a cyclist) and intuitively this point is the best combination of energy efficiency and timeliness.

AJH

[The Natural Philosopher]

Highly debatable.

Light skinny cars with skinny wheels and small engines probably peak there, but heavier cars with fatter tyres and bigger engines probably peak - albeit at far worse consumption figures - somewhere in the 50-60 mph mark.

[Martin Bonner]

I *think* that the force exerted by drag goes as the /square/ of the speed. That means the energy (and hence fuel consumption) per unit distance will also go as the square. I suspect what you are thinking of is that the /power/ (energy per unit time) is force * distance / time which is force * speed, and hence the power used by drag goes as the cube of speed.

[Martin Bonner]

The optimum speed for heavy duty trucks depends on quite a number of things:

- fuel consumption

- using the (expensive) truck as much as possible

- urgency of the cargo

- reaching the destination before the driver has to take a break.

Certainly some North American fleets limit their trucks to below the legal maximum to improve fuel economy.

[The Natural Philosopher]

I think we all agree. I have to often estimate drag in power terms in my hobby*..and thats where the confusion came in.

• model aircraft,where someone always wants one that goes faster..which ultimately becomes how much power you can cram in without the thing needing a catapult or a mile of runway to get off the ground. Getting beyond 200mph is pretty hard for a powered one (though jets are up there), though the current unpowered speed record is about 300mph I think. Don't ask how...google 'dynamic soaring' and take out a loan with Northern Rock if interested.

[The Natural Philosopher]

Add stability and safety to that. Very heavy loads on an articulated truck are not things to stop in a hurry.

And thats generally why the convoy each other as well. Up to 25% less drag in a 'train'

[Mat C]

This link might help in understanding the effects of wind drag

's an episode of MythBusters, where they tested whether driving behind a large truck saved fuel.

Very interesting results..

Mat

[The Natural Philosopher]

A bit better than I expected, actually. but something every racing driver knows,and anyone who has driven anything resembling a commercial vehicle, and something that has been proven time and again in analysis of train efficiencies.

[AJH]

Increasing the weight does move it up and my 20mph figure may be low for an average car.

I did find a calculator where you can put in parameters for your car and read off when both drag and air resistance are about 50%.

It also calculates the theoretical mileage at this speed.

know how that compares with real life!

AJH

[Dave Baker]

The equations and sample data are on my website.

[AJH]

It makes a fuel efficiency case for allowing long distance trucks to couple up!

AJH

[The Natural Philosopher]

It's called a railway...

[Rod]

Start with two, and by the end of the journey you have the two big ones and a little one as well?

Isn't this (coupling up) effectively what happens with the lorry and dangler arrangements?

[The Natural Philosopher]

fairly close. and no more than 35 needed for 70mph.

However it does seem to be a bit conservative - many cars exceed 60mpg at 56mph, and also the fact of the matter is that so long as the big heavy cars aero losses are well below the frictional losses it doesn't matter a damn what speed it goes at. I.e. my old Jaguar would never ever turn in better than 27mpg no matter how slow it was driven. Typically it did 19-20mg. To get 27mpgh required it to be trundled at - yas - 56mph constantly. One stop would ruin it!

It does show that there is plenty of room fr improvement though. Low friction small turbo charged engine, and lighter more aerodynamic cars and skinnier tyres could still have the speed. motorcycle has the speed but not the fuel consumption, after all.

[AJH]

Railways may be fuel efficient but being able to choose when to leave the train could also be resource efficient. OK it's a long way off but intelligently controlled traffic management may be on its way, in which case the chief benefit of a train, in fuel effectiveness due to low frontal area to payload, may come to roads.

AJH

[<me9]

[snip]

The 'official' tests for fuel consumtion were for 56mph. It's no surprise that the manufacturers optimised consumption at the 'official' speed.

[Roger]

The message from The Natural Philosopher contains these words:

It is more than 40 years since I did any thermodynamics and we didn't do much on IC engines anyway but through the mists of time ISTR that an IC engine is most efficient at max bmep (brake mean effective pressure IIRC) and max bmep equates very closely to max torque.

One of the reasons I remember this at all is because it seems to conflict with the age old advice that the way to save fuel is to get into as high a gear as possible as early as possible.