One thing I discovered a long time ago is that it's nigh on impossible to drive at a constant speed on motorways unless you're prepared to ignore the speed limit, or sit behind a truck at 50-something mph. 70 mph is a myth. It's impossible. 50, yes. 85+, yes. In between? Try it sometime.
Lane 1 is for convoys of trucks at 50-55 mph, lane 3 is a constant procession of 80-90 mph tailgaters, leaving lane 2 the only refuge for those wishing to make progress within the law. The problem is, lane 2 is also where the trucks doing 55 mph suddenly swerve out to overtake those doing 54 mph, and take a mile to get past, forcing even the most observant motorist to either slow down, or accelerate into the lane 3 stream to get past. Braking and acceleration = poor economy as AE says.
I can believe the difference in economy the OP reports. I've found similar in every car I've owned, and it's purely down to the constant acceleration / deceleration.
Until they put snipers on motorway bridges to punish shoddy lorry driving, and of course people who don't understand the concept of keeping left unless overtaking, it will remain this way.
Edited by DP on 18/08/2009 at 15:00
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Take into account the difference in height of both destinations and the prevailing winds at the time. It makes a significant difference.
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With most diesels lower revs means better MPG.
My 207 SW displays current and overall fuel consumption. I was surprised on a recent trip from Cheshire to Fife (about 250 miles via Edinburgh) to achieve more than 70mpg - M6, A701, M90, A91 - doing 60-65 on the M'way and 50-55 on the A roads. I have always understood that the best mpg is obtained somewhere around peak torque, which is about 2000 rpm, or about 60mph in 5th on my car.
My impression is that doing 70 instead of 60 wouldn't make a big difference, but I very much doubt that it would be 'better'.
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I understood that diesels were most economical when running at their maximum torque revs.
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I understood that diesels were most economical when running at their maximum torque revs.
The engine might be most efficient, but if that equates to (say) 80mph, might it be more economical going at 70mph where the engine is less efficient but you're wasting less fuel overcoming air resistance?
Edited by Focus {P} on 18/08/2009 at 16:17
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I understood that diesels were most economical when running at their maximum torque revs.
I am not an engineer but logically this cannot be possible, an engine cannot burn less fuel at 2000 rpm than it does at 1000 rpm. What I can believe is that peak torque is where you get the most power for your litre of fuel.
Somebody technical want to put me right?
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an engine cannot burn less fuel at 2000 rpm than it does at 1000 rpm.
No, but you're going twice as fast, so if it's burning less than twice as much fuel it's more efficient.
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My Focus 2.0 TDCI runs at 2000rpm at 70mph in 6th gear, spot on max torque, maybe the design engineers know what they are doing!
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>>What I can believe is that peak torque is where you get the most power for your litre of fuel.
So, if you're travelling at a speed where you could be in any of a number of gears, the gear which puts you nearer peak torque will not be a bad bet to get best economy. This may not necessarily be the gear which makes the engine turn the slowest [although at motorway speeds, it probably is].
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So if you're travelling at a speed where you could be in any of a number of gears the gear which puts you nearer peak torque will not be a bad bet to get best economy. This may not necessarily be the gear which makes the engine turn the slowest [although at motorway speeds it probably is].
Correct - no different to riding a bike.
You wouldn't ride a 10 speed bike in 10th all the time as you'd expend too much energy getting it going.
You ride a bike in the gear which means you are comfortable using the pedals, not too fast and not too slow as both mean wasted energy.
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so what happens to the fuel economy if the engine is ran at either partial turbo boost or no turbo boost ?
Wouldn't this use more fuel ?
I do not have a proper trip computer on my car so I cannot tell how much the difference is.
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Fuel economy will increase with turbo boost, but there won't be much needed at a constant speed, only accelerating.
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Fuel economy will increase with turbo boost
Efficiency will, but to what extent is that offset by increased drag?
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Good blog, and just remember the faster you go the less economical the car is as drag increases and more energy is used.
Most economical is about 45- 50 mph in top/ 2000 rpm, anything above that starts to get more drag.
For ultimate economy you have to go SLOW.
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Dont give people an excuse to mimse!
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And drag will increase by the square of the speed.
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I do find my old-ish 2.2TID Saab seems to give of its best at around 70 mph. Going slower seems to put the consumption up, but maybe it's all in the mind
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veryoldbear, that's what I suspect is happening in my case.
To me, the car 'feels' more economical at 70mph than 60mph
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I believe that the slower the engine is turning, the less fuel it is using, turbo or no turbo.
Modern fuel injection engines won't overfuel when the engine is labouring as the ECU will only allow enough fuel through to burn, except on the initial acceleration surge as it enriches the fuel. At a constant speed, less revs = less fuel IMO.
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kith - have a nosy at this, which I posted in answer to this question the last time it came up
The aim for efficiency is to operate near "the centre of the onion" in terms of brake specific fuel consumption - in this case about 280 g/kWh
----------------8<------------
Figure 2.4 on this page
www.agen.ufl.edu/~tburks/Handouts/CH2_Performance%...f
helps to make the jump between engine characteristics and vehicle economy.
It's quite a busy graph, so, I'll try to explain it bit by bit.
Along the x axis is engine speed, with engine torque on the y - in this respect, it's a standard torque curve, with the engine's maximum output being represented by the line towards the top of the graph, dropping off steeply at 2400 rpm (the low engine speeds sugges that this must be a commercial vehicle engine)
The solid contours are contours of equal brake specific fuel consumption, with the minimum probably at about 80% load, and 1500rpm.
The dotted lines are lines of constant power.
When you are travelling at constant speed, you need a set amount of power to overcome drag in all its forms, so, each of those dotted lines representing power could, for a specific vehicle be re-labelled as different speeds.
For reasonable speeds, there may be a number of possible gear ratios. These gear ratios at a given vehicle speed will give a known engine rpm, and therefore a known intersection with a BSFC contour.
As an example, if the vehicle is doing a speed which needs 25kW, and the engine is doing 1350 rpm, then, changing up a gear is likely to produce worse fuel economy. Whereas if the engine were doing 2100 rpm, then changing up would bring the engine, and vehicle into a more economic region.
The problem is that these performance maps are not routinely made available. Perhaps they should be made available?
----------------8<------------
I hope this helps to explain why lowest engine speed does not always give best fuel economy - it depends where on the map you begin, and where you move to after changing gear.
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kith - have a nosy at this which I posted in answer to this question the last time it came up
NC - isn't this just talking about engine efficiency, and not considering how economy might be affected by increased drag at higher speeds?
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>>NC - isn't this just talking about engine efficiency
Yes and no.
Look at the dashed lines on the graph - these are constant power lines.
With a knowledge of your car's drag, you could relabel these dashed lines in terms of the car's speed as there's a one to one relationship between speed and total drag power, so, you can use these performance maps to see how speed would affect consumption.
If you had access to the controller, a cvt would allow one to roam at will around the performance map, while for manual transmissions, only discrete jumps around the map are allowed.
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i think HJ did something about this in the paper a few years ago. it depends on what gearing your car has. 70mph in top would make sense for best 'practical' mpg.
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A lot of confusion between engine efficiency and fuel efficiency.
Generally lower RPM = better fuel efficiency but it is VERY important not to run too low. Between 1.5k and 1.8k RPM gives me the best economy on my toyota 2.2 180bhp engine.
An extended fair test over a hilly dual carriageway confirmed the following
@ 60mph with cruise control
4th = 41.6mpg / 2.6k RPM
5th = 46.3 mpg / 2k RPM
6th = 51mpg / 1.6k RPM
Running too low can actually use more fuel as the engine over fuels to try and generate enough power to keep the mass moving.
So you now know what RPM gives the best fuel economy, you have to get up to speed first. This is where engine efficiency comes in. What you need to do is work out when your engine is going to produce enough power for the least amount of fuel used.
You want to change gear whilst the engine's mean piston speed reaches 1200-1500. This is when the engine is running low frictional horsepower loss and high volumetric efficiency. In other words, working most efficiently and is the very best range to accelerate in as the engine generates power for the least amount of fuel that is used. I worked this out as a measure of RPM*engine stroke length(inches)/6. Which on the t180 works out at 2k RPM to 2.5k RPM, as the range to accelerate and shift in.
Another important factor especially with VNT diesel engines is to accelerate as smoothly as possible to ovoid over fuelling. Even better if you do it within the engines mean pistol speed band. Thats why i like to sit just under the band at about 1.7 / 1.8k RPM if i know i have to accelerate.
The only thing i can think that would make 60mph worse on MPG is if you were in 5th at higher RPM's or you had a head wind or going gradually uphill. OR as i said before you were in 6th at such a low RPM that the car was injecting more fuel because it was labouring.
Edited by OldSkoOL on 18/08/2009 at 20:59
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