If you travel at 60 mph instead of 70, you'll spend 16% longer time covering a given stretch, will the NOx emission rate per mile be reduced sufficiently to have much overall effect on the total emitted over a 4.5 mile stretch?
Fig 5 of this seems to show under a 10% factor on the NOx between the relevant speeds
I am not sure that a study of American cars is particularly relevant fo UK roads. Many petrol V8s and not very aerodynamic body shapes.
Maybe not, but it was what I could find.
It's well known that cars give better mpg at 60 than 70, so an improvement in total NOx output can be expected.
Of course speed isn't the only factor. Rate of acceleration & gear use count too.
NT
No its not.
It shouldn't surprise me to hear you would continually accelerate and decelerate on roads where everyone else, as per the OP, is doing 60 or
70 at a constant speed.
The main reason is that drag varies with the square of speed. So going only a little slower causes a significant reduction in fuel consumption.
I suspect that the question is related to a 60mph limit on 4 stretches of UK motorways that is going to be imposed 24/365 to reduce local air pollution. I suspect that it will not work because in my (limited) experience of one of those sections traffic cannot travel at 70mph or even 60 mph currently during much of the day.
But a continuous 60 limit should stop all the accelerating back up to 70 when the gaps open up. I'd still expect a net benefit, even if not the "theoretical" number.
Fuel usage at high speeds is dominated by drag forces which scale as the square of the speed. 16% longer time but burning less fuel.
The optimum cruise speed for a particular vehicle shape can be determined experimentally if you do a regular particular run and have a trip computer. For my car it is about 55mph.
A bit like the "smart" motorway signs that say 40mph when you are stuck in a stationary traffic queue moving slower than walking speed the posted speed limit becomes irrelevant if you cannot hope to reach it.
But varies by gear. In mine for sixth ("top") an indicated 61 returns the highest mpg. However around 50 in fith returns an even higher MPG. Trouble is 50 on a motorway is seriously tedious and concentration is difficult to maintain. Is there still 30+ miles of average speed cameras set to 50 on the M6 south of Stoke for smart motorway work?
It might not be that well known by the general public but it is correct. Drag goes up with the square of the windspeed and from about 50mph onwards it becomes the dominant factor in fuel usage.
The people who drive with a foot hard down on one or other of brake or accelerator are the ones who get worst possible mpg and most pollution.
They are usually to be found in the outside lane <2m behind the bumper of another vehicle. It is reps with this behaviour that make electric hybrids a tax dodge rather than an effective measure to save the planet.
For *most* cars it's around 55mph. Not an accident (hence the US speed limit in the 70s)
The optimum cruise speed is determined by the power envelope for the engine under any given load and translated through RPM and gears.
I know the RB211s that British Gas used for compressors in the high pressure grid had modified power envelopes to maximise efficiency when running. The software that calculated network flow took them as a parameter when optimising the flow.
There is quite a long bit of 50 mph on the M4 (Port Talbot and west).
I am OK with that when I can set adaptive cruise control and not have to keep checking the speedo.
Noticeably high mpg on that section. As it is auto, gear changes are rare - usually goes to 7 and stays there.
With all due respect wouldn't driving <2m behind the bumper of the car in front considerably reduce air resistance and thus raise MPG, certainly in comparison to the car in front ? Being "sucked along" is, I believe, the relevant phrase.
Maybe it isn't all just testosterone in action after all; but penny pinching by drivers who are nevertheless in a bit of a hurry.
michael adams
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.In article <rjv7s7$2af$ snipped-for-privacy@dont-email.me, michael adams snipped-for-privacy@ukonline.co.uk> scribeth thus
Great idea! We can call it a train;! ... Hang on ....
Behind a large lorry, possibly - behind anotehr car, unlikely
Dunno, I'll be sure to check or avoid it if I'm going that way, the last time I went up the M6 it was a horrid journey.
+1
and I usually have to navigate the M25 first before getting to the M6 :( The joys of living in the wrong part of Essex where the last 50 miles to home can take almost the same amount of time as driving the previous 150 miles!
While not an expert, who'd have guessed, surely the air displaced by the first car wouldn't all have returned before the second car arrived. Maybe at 20 mph but surely not at 70 mph.
I'm not advocating any of this, but if according to the OP they're already doing it anyway....
michael adams
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As ever, there's an excellent Mythbusters about saving fuel by driving in the slipstream ...
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