The 56 mph myth
Discussion
underphil said:
kambites said:
I assume, as you say, that it's a miss-quote of the statement "an engine is most efficient at peak torque" which is generally true, or very close to it.long time lurker said:
My 1.8t Passat does 33mpg @50mph and 39mpg @ 70mph.... both in 5th!! I guess that has something to do with it being petrol turbo? (1800rpm is 50mph and when the turbo starts spinning proper)
In that case I suspect it's either very highly geared and would return still better economy at 50mph in 4th, or it's broken. 
Scuffers said:
SMGB said:
Its all quite simple once you disentngle the engine and drag sides of it in your head.
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,
first problem is that this is just not correct..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,
Peak torque is the point of peak BMEP, this may well also be the point of peak volumetric efficiency, but NOT fuel efficiency.
it may also be the point of max HP vs. fuel flow, (unlikely but possible) but for a car that's not the point.
unless you have an engine who's peak torque provides the exact amount of power to equilibrium the power required to keep a car at a set economical speed.... ie, a tiny one.
alangla said:
Sounds like a situation where a parallel hybrid might make sense - a piddly little engine to maintain cruising speed and an electric motor to assist in accelerating and regenerating under braking. Obviously you're then into extra weight of the battery/motor & logistics of charging the batts etc.
Or better still a range extender hybrid, where the piddly engine has two states - "charging the batteries at peak efficiency" and "off". The Wookie said:
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.
They are now, but they didn't used to be.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.
Years ago the figures were quoted at a steady 56 and steady 75. (And I think a steady 30 to give "town mpg although not certain on that one).
Hence, as the OP states, this myth about 56mph being "the most economical speed".
alangla said:
Sounds like a situation where a parallel hybrid might make sense - a piddly little engine to maintain cruising speed and an electric motor to assist in accelerating and regenerating under braking. Obviously you're then into extra weight of the battery/motor & logistics of charging the batts etc.
The torgque and BHP curves you see published are test bench numbers done at max throttle opening. In the real world you are using a fraction of the BMEP the engine is capable of and so a low effective compresion ratio. Using peak torque as a guide to where the engine breaths best will let you operate your car economically. You are right that it is best specific fuel consumption, so you may use less fuel absolutley at lower RPM than peak torque, I kept it simple. And yes, the hybrid approach gives you exactly the plusses and minus above. Research has been done on storing braking energy in a flywheel to help out when you need more go than the "cruising" engine can give. I remember that one program came to an end when gyroscopic effects caused the flywheel to leave via the side of the bus based test vehicle on a hump back bridge.
long time lurker said:
My 1.8t Passat does 33mpg @50mph and 39mpg @ 70mph.... both in 5th!! I guess that has something to do with it being petrol turbo? (1800rpm is 50mph and when the turbo starts spinning proper)
Sounds beleivable, as the forced induction kicks in your effective compresion ratio goes up. Ari said:
They are now, but they didn't used to be.
Years ago the figures were quoted at a steady 56 and steady 75. (And I think a steady 30 to give "town mpg although not certain on that one).
Hence, as the OP states, this myth about 56mph being "the most economical speed".
Agreed, I remember it in the back of my Rover 220 turbo manual. 56mph was the most efficient speed..........Years ago the figures were quoted at a steady 56 and steady 75. (And I think a steady 30 to give "town mpg although not certain on that one).
Hence, as the OP states, this myth about 56mph being "the most economical speed".
That was printed in the manual. Source of the myth I guess.
Ari said:
Years ago the figures were quoted at a steady 56 and steady 75. (And I think a steady 30 to give "town mpg although not certain on that one).
Correct, but the urban figure was for 'simulated' urban driving:EU Test regs (Section 3.1) http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?...
Puddenchucker said:
Ari said:
Years ago the figures were quoted at a steady 56 and steady 75. (And I think a steady 30 to give "town mpg although not certain on that one).
Correct, but the urban figure was for 'simulated' urban driving:EU Test regs (Section 3.1) http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?...
in short, the cars never went anywhere near a road !
GroundEffect said:
You will get best fuel economy at the intersect point between engine loading decreasing and drag increasing.
No one has mentioned the cost of the time saved by driving faster yet.Losing a couple of mpg is trivial compared to the cost of the time
saved, even at national minimum wage.
Move on up to a tenner a hour, not an outrageous rate of
pay by any means, and the sums are even more emphatic.
Worked example: Let's assume I am making a hypothetical journey
of 100 miles and my car does 30 mpg at 70 mph and 28 mpg at 100 mph.
We assume petrol is £1.35 per UK gallon and I'm a £10 an hour worker.
At 30 mpg petrol cost is £20.43. Same 100 miles at 28 mpg is £21.89.
Time cost at 70 mph is £14.29. Time cost at 100 mph is £10.00
So I save £4.29 in time and it costs me £1.46 extra in petrol.
Total saving £2.83 per 100 miles, by driving at 100 mph. QED
I rather suspect all those folks worrying about saving small
quantities in fuel consumption either haven't done their sums
or are retired and so have all the time in the world.
Puddenchucker said:
I'll be amazed if there is any car out there that does only 2mpg less for a 43% increase in speed.
I thought someone would say that.Velocity isn't acceleration, which is precisely why such figures are
achievable in practice with any number of mainstream cars.
Have a reality check about why the town mpg figure is always less, sometimes
much less, than the out of town mpg figure.
For example, 25 mpg in town, but 35 mpg out on the open road, might
be typical figures.
dcb said:
No one has mentioned the cost of the time saved by driving faster yet.
Losing a couple of mpg is trivial compared to the cost of the time
saved, even at national minimum wage.
Move on up to a tenner a hour, not an outrageous rate of
pay by any means, and the sums are even more emphatic.
Worked example: Let's assume I am making a hypothetical journey
of 100 miles and my car does 30 mpg at 70 mph and 28 mpg at 100 mph.
We assume petrol is £1.35 per UK gallon and I'm a £10 an hour worker.
At 30 mpg petrol cost is £20.43. Same 100 miles at 28 mpg is £21.89.
Time cost at 70 mph is £14.29. Time cost at 100 mph is £10.00
So I save £4.29 in time and it costs me £1.46 extra in petrol.
Total saving £2.83 per 100 miles, by driving at 100 mph. QED
I rather suspect all those folks worrying about saving small
quantities in fuel consumption either haven't done their sums
or are retired and so have all the time in the world.
Leave earlier and drive at the most efficient speed would yield even more money, no? I fit perfectly into your example of a £10/hour worker. Your example surely only works if you spent all your free time travelling to and from work, so to minimise it would allow you to do more work. I can't imagine this is true for anyone?Losing a couple of mpg is trivial compared to the cost of the time
saved, even at national minimum wage.
Move on up to a tenner a hour, not an outrageous rate of
pay by any means, and the sums are even more emphatic.
Worked example: Let's assume I am making a hypothetical journey
of 100 miles and my car does 30 mpg at 70 mph and 28 mpg at 100 mph.
We assume petrol is £1.35 per UK gallon and I'm a £10 an hour worker.
At 30 mpg petrol cost is £20.43. Same 100 miles at 28 mpg is £21.89.
Time cost at 70 mph is £14.29. Time cost at 100 mph is £10.00
So I save £4.29 in time and it costs me £1.46 extra in petrol.
Total saving £2.83 per 100 miles, by driving at 100 mph. QED
I rather suspect all those folks worrying about saving small
quantities in fuel consumption either haven't done their sums
or are retired and so have all the time in the world.
Please forgive me if I have misunderstood your point?
highest mpg in the golf is definitely at around 50-60 in a queue of steady traffic, because you seem to get pulled along in the slipstream.
It doesn't like motorways as much, loses around 10% at a steady 70-80, even when too close behind a van or something.
Porsches seemed to like 70mph, any other speed would result in an mpg around 15% lower.
Haven't paid attention in the Mustangs, it'd be too depressing.
It doesn't like motorways as much, loses around 10% at a steady 70-80, even when too close behind a van or something.
Porsches seemed to like 70mph, any other speed would result in an mpg around 15% lower.
Haven't paid attention in the Mustangs, it'd be too depressing.
dcb said:
I thought someone would say that.
Velocity isn't acceleration, which is precisely why such figures are
achievable in practice with any number of mainstream cars.
Have a reality check about why the town mpg figure is always less, sometimes
much less, than the out of town mpg figure.
For example, 25 mpg in town, but 35 mpg out on the open road, might
be typical figures.
Because town figures presumably include things like stopping at traffic lights where you are burning fuel and not travelling anywhere, meaning you're doing 0MPG which brings down the average.Velocity isn't acceleration, which is precisely why such figures are
achievable in practice with any number of mainstream cars.
Have a reality check about why the town mpg figure is always less, sometimes
much less, than the out of town mpg figure.
For example, 25 mpg in town, but 35 mpg out on the open road, might
be typical figures.
One thing I'd like to ask though, does the throttle have a completely linear direct relation towards the amount of fuel/air going into the engine? Thus surely the ideal RPM at any speed is the one that requires the least throttle travel to hold that speed? Or is that wrong/too simplistic?
Gooly said:
One thing I'd like to ask though, does the throttle have a completely linear direct relation towards the amount of fuel/air going into the engine?
Someone may correct me, but I don't see how it can, the ECU surely controls that using the throttle position as a factor, as well as certain other things.SMGB said:
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.
Toyotas 1.8 was known as a high mph motorway slogger in its Avensis.Did it come as any surprise when Toyota updated the Prius 3 years ago that they increased the engine from 1.5 to their trusty 1.8 ( modified to run an Atkinson cycle)?
There are so many things which come together to make the right engine in a particular car
Gassing Station | General Gassing | Top of Page | What's New | My Stuff