The 56 mph myth
Discussion
Forgive me if this has been mentioned before but I keep coming across non motoring folk saying that 56mph is the scientifically proven most economical speed at which to drive. Whilst it certainly explains how many A Road queues are heade by a Micra, an old Mondeo and a Rover 25 maintaining 56mph , with the cars often optimising their economy by running in a tight packed bunch 10 ft or less apart, I doubt the 'science'.I think it is founded solely on the fact that the EU economy figures used to quote consumption at 90km /h and 120 km/h- respectively 56mph and 75 mph.Am I right?
coppice said:
Forgive me if this has been mentioned before but I keep coming across non motoring folk saying that 56mph is the scientifically proven most economical speed at which to drive. Whilst it certainly explains how many A Road queues are heade by a Micra, an old Mondeo and a Rover 25 maintaining 56mph , with the cars often optimising their economy by running in a tight packed bunch 10 ft or less apart, I doubt the 'science'.I think it is founded solely on the fact that the EU economy figures used to quote consumption at 90km /h and 120 km/h- respectively 56mph and 75 mph.Am I right?
and that manufacturers optimised their engine and transmission to achieve the best figures at those speeds?
Nope. Firstly EU figures are done on a drivecycle which consists of acceleration and deceleration as well as fixed speeds (although it is very simple and is now being used despite being outside of its original scope, i.e. it's now irrelevant).
Secondly the 55mph thing probably stems back to the 70's oil crisis that resulted in the 55mph speed limit in the US, and thus is probably based on some old V8 yank tank and is thus completely irrelevant.
Really, the slowest you can run while operating the engine in an efficient region of its range is the most efficient way to travel, and it's probably between 30 and 40mph constant speed in most cars, although I believe 55 is commonly seen as a reasonable compromise between fuel consumption and not clawing your own eyes out with boredom or, as many seem to forget, dieing of old age because it takes twelvety million years to get their Micra from their bungalow to the local country stores.
Secondly the 55mph thing probably stems back to the 70's oil crisis that resulted in the 55mph speed limit in the US, and thus is probably based on some old V8 yank tank and is thus completely irrelevant.
Really, the slowest you can run while operating the engine in an efficient region of its range is the most efficient way to travel, and it's probably between 30 and 40mph constant speed in most cars, although I believe 55 is commonly seen as a reasonable compromise between fuel consumption and not clawing your own eyes out with boredom or, as many seem to forget, dieing of old age because it takes twelvety million years to get their Micra from their bungalow to the local country stores.
Edited by The Wookie on Friday 12th October 08:55
MarJay said:
Certainly two out of the five or so cars I've owned were most efficient at 55mph. I suspect that was because they were both designed for the American market...
Interesting. I'd have thought it would be almost impossible to get the peak efficiency to be that fast without intentionally hobbling something at lower speeds. Here are a couple of not very interesting plots for you:
Basically all of the smaller engined stuff looks to peak at 30ish mph and have a bit of a come back around 50 (the peaks are most likely due to gearing) whereas broadly speaking the big V8 stuff gradually gets better and better as it dethrottles up to a point where the drag starts costing too much.
Basically all of the smaller engined stuff looks to peak at 30ish mph and have a bit of a come back around 50 (the peaks are most likely due to gearing) whereas broadly speaking the big V8 stuff gradually gets better and better as it dethrottles up to a point where the drag starts costing too much.
The Wookie said:
Basically all of the smaller engined stuff looks to peak at 30ish mph and have a bit of a come back around 50 (the peaks are most likely due to gearing) whereas broadly speaking the big V8 stuff gradually gets better and better as it dethrottles up to a point where the drag starts costing too much.
I suppose that makes sense, since the smaller engines will be operating closer to peak efficiency at lower speeds than the bigger ones. I'm very surprised that even the big V8s manage to get that fast before they start to lose the war with drag, though. I guess it just shows quite how inefficient these engines are at very low loadings.
kambites said:
I suppose that makes sense, since the smaller engines will be operating closer to peak efficiency at lower speeds than the bigger ones.
I'm very surprised that even the big V8s manage to get that fast before they start to lose the war with drag, though. I guess it just shows quite how inefficient these engines are at very low loadings.
I don't think it's that they're particularly inefficient compared to other engines, it's more that they're running at very low load at low speeds which is the worst region for efficiency for any engine. A smaller engine is never going to be running with that little throttle, but a massive engine ticking along at low speed has now choice.I'm very surprised that even the big V8s manage to get that fast before they start to lose the war with drag, though. I guess it just shows quite how inefficient these engines are at very low loadings.
In fact, it could be more due to the fact that Yank V8's typically have superb peak efficiency, and it's just about getting into that region at higher speed.
The Wookie said:
I don't think it's that they're particularly inefficient compared to other engines, it's more that they're running at very low load at low speeds which is the worst region for efficiency for any engine. A smaller engine is never going to be running with that little throttle, but a massive engine ticking along at low speed has now choice.
That's what I said, isn't it? The Wookie said:
kambites said:
That's what I said, isn't it?
Sorry, I read it that bigger engines were less efficient at low loadings, I was trying to say that as a percentage of maximum load they're actually little different to smaller engines!saaby93 said:
Isnt mpg is going to go down with mph any way? You're doing more miles.
Is it better to divide one by the other to see gallons per hour at various speeds?
Its all quite simple once you disentngle the engine and drag sides of it in your head. Is it better to divide one by the other to see gallons per hour at various speeds?
Any ic engine is most efficient at the rpm where you get peak torque, below this you dont get optimum cylinder filling due to dynamic aitflow effects, like valve overlap and the scavenge effect, above this torque drops off due to pumping losses. The power curve still goes up because of more bangs per minute. Thermodynamics means efficincy is a function of effective compresion ratio and so the temperature difference the engine works between. Volumetric efficiency is a term you may have heard.
You can see this in practice, the real world economy of the Fiat 500 Twinair is terrible. because you just cant help running it in the high pumping loss region
Then there is the non engine related drag, where aerodynamics goes up as the square of speed and some others are linear.
So you can have the engine getting more efficient as you get faster, but its working harder against the drag.
We all know big engines will never give the same economy as small ones, partly this is extra friction and windage internally, but more importatly on the whisker of throttle they need to loaf along you are not letting much air into the cylinders and so volumetric efficiency is low, some cars, the 300C V8 Chrylers for example have variable displacement to mitigate this. when you do not need all the capabilty it turns into a 4 cylinder. The valves remain shut on the unused cylinders to aviod pumping losses.
Sorry to go on a bit but does that help to read those excellent graphs above?
Gassing Station | General Gassing | Top of Page | What's New | My Stuff