The 56 mph myth
Discussion
SMGB said:
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 
Isn't that more to do with underspecced components than having to thrash it? I.e. it's just chucking fuel away to keep things cool rather than an inherent thermodynamic efficiency issueSMGB 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, 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?
I have been pondering all this stuff this week. I've always thought that assuming you need 20bhp to maintain a speed, making that in a lower gear, at higher RPM and with less throttle was more fuel efficient than using a higher gear, lower RPM and more throttle.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?
Starting to think that is actually wrong, at lower RPM the pumping losses are less and the VE is higher, so the fuel economy should be higher. I would do an experiment with my car using the two different styles of driving, but since the missus drives it more than I do, and frankly her driving style baffles me, it wouldn't be all that enlightening.
The Wookie said:
Isn't that more to do with underspecced components than having to thrash it? I.e. it's just chucking fuel away to keep things cool rather than an inherent thermodynamic efficiency issue
I just threw that in to say smaller engine does not = better MPG, its optimum engine. Usually a mid range engine in a cars line up will give you enough go and best economy. Fiats multiair is desinged to give you good cylinder filling across a wide rev range but every graph on this subject will still nead south at some point.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.
Also, at peak torque, most engines are having to run at < Lambda 1, ie using more fuel.
think back to the pre-cat days of lean burn engines....
Edited by Scuffers on Friday 12th October 10:14
People often say "a car's is most efficient when driven at peak torque rpm" - just wrong!
Often quoted, but not true.
- Peak torque is where the engine produces power most efficiently at full throttle.
- When cruising at a steady speed the throttle will be only say 20% open, and in these conditions the energy losses due to heat, and energy to keep the engine internals moving become much more important.
E.g. on my last car peak torque is at 4000rpm, but compared to 2000rpm, 4 times as much fuel is used to keep it spinning at that speed, as well as all the extra energy (fuel) lost through extra heat due to extra friction of the parts moving quicker. (and in addition if I drive at the same constant speed and change gears between running at 4000rpm and 2000rpm the trip computer confirms that 2000rpm is way more economical)
Another graph:
Often quoted, but not true.
- Peak torque is where the engine produces power most efficiently at full throttle.
- When cruising at a steady speed the throttle will be only say 20% open, and in these conditions the energy losses due to heat, and energy to keep the engine internals moving become much more important.
E.g. on my last car peak torque is at 4000rpm, but compared to 2000rpm, 4 times as much fuel is used to keep it spinning at that speed, as well as all the extra energy (fuel) lost through extra heat due to extra friction of the parts moving quicker. (and in addition if I drive at the same constant speed and change gears between running at 4000rpm and 2000rpm the trip computer confirms that 2000rpm is way more economical)
Another graph:
Kozy said:
kambites said:
I assume 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.
BSFC vs MPG? I.e. not the same?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.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.
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.
Sorry but the OP is correct.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
The reason 56mph is deemed significant by many ordinary motorists is because that was the speed at which fuekl economy used to be measured (90kph).
Not sure where you got 55 from - it has nothing to do with US speed limits or the 1970s oil crisis.
It's exactly the same reason acceleration is measured as 0-62.
underphil said:
So are you saying that a Civic Type R driving along at 50mph will be more efficient with the engine spinning at 6000rpm (peak torque) rather than in 6th gear at ~2500rpm (no-where near peak torque) ??
No, he is saying fuel efficiency (BSFC) is best at peak torque. As has just been discussed, fuel efficiency (BSFC) and fuel economy (MPG) are not the same thing.GroundEffect said:
Kozy said:
No, he is saying fuel efficiency (BSFC) is best at peak torque. As has just been discussed, fuel efficiency (BSFC) and fuel economy (MPG) are not the same thing.
Best BSFC is usually just before peak torque. Certainly from the engine calibration work I've seen usually to get to peak torque, your running richer than ideal
Kozy said:
No, he is saying fuel efficiency (BSFC) is best at peak torque. As has just been discussed, fuel efficiency (BSFC) and fuel economy (MPG) are not the same thing.
yes, but in a topic regarding what speed will give the best mpg, talking about BDFC is a little pointless and distractingandrew said:
the eu originally quoted 90kmh, 120kmh and urban cycle figures
there was no particular significance to these speeds, other than to demonstrate that broadly speeking, the higher the cruising speed, the higher the consumption
I think that 90kph was a fairy typical main road speed limit and 120kph was a typical motorway speed limit - probably nthing more than that.there was no particular significance to these speeds, other than to demonstrate that broadly speeking, the higher the cruising speed, the higher the consumption
oyster said:
Sorry but the OP is correct.
The reason 56mph is deemed significant by many ordinary motorists is because that was the speed at which fuekl economy used to be measured (90kph).
Not sure where you got 55 from - it has nothing to do with US speed limits or the 1970s oil crisis.
It's exactly the same reason acceleration is measured as 0-62.
Fair enough, I had no idea that figures used to be quoted at specific speeds, it must have been before my time.The reason 56mph is deemed significant by many ordinary motorists is because that was the speed at which fuekl economy used to be measured (90kph).
Not sure where you got 55 from - it has nothing to do with US speed limits or the 1970s oil crisis.
It's exactly the same reason acceleration is measured as 0-62.
The 55 limit in the US was put in place during the oil crisis on the belief that cars were most fuel efficient around that speed though
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