engines, speed and economy....

engines, speed and economy....

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Discussion

E38Ross

Original Poster:

35,301 posts

215 months

Tuesday 28th February 2012
quotequote all
a friend of mine and i were talking earlier about speed and economy, and how it obviously changed depending on what speed you go; as well as engine rpm, gearbox ratios, engine torque and power at said rpm etc etc.

he was of the belief that basically the slower you go, the more economical the car; which i don't think is quite right. if that were so, my car would be more economical at a cruise at 20mph than it is at 55-60mph, and it's not. i know it must have something to do with ideal engine speed etc because obviously, the faster you go there is more air resistance, rolling resistance etc so engines are more efficient at certain speeds.

anyone have any comments?

this question came up after i commented that the (not entirely accurate, i'm sure) consumption computer in my car had the economy at 80 the same as at 70mph (the car does between 2000 and 2100rpm at 70mph)

cheers

ross

mwcr85

152 posts

152 months

Tuesday 28th February 2012
quotequote all
Any engine will be most efficient at a certain rpm (2000 ish generally) , whatever speed this is achieved (due to gearing) at is the most economical speed. It is more complicated than this, but im tired!

Ozzie Osmond

21,189 posts

249 months

Tuesday 28th February 2012
quotequote all
The biggest factor by far in fuel economy is wind resistance. Wind resistance rises at roughly the square of speed, so double the speed = four times the wind resistance.

Once an engine is running quickly enough and suitably geared to be efficient the slower you drive the better your mpg.

sparks_E39

12,738 posts

216 months

Tuesday 28th February 2012
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At 60 mph my E39 528 is doing 40-45 mpg on a straight flat road, at 80 about 30-33.

busta

4,504 posts

236 months

Tuesday 28th February 2012
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On a speed awareness course the instructor insisted that the slower you go, the better the fuel economy but I can't see it being as simple as that. 10mph at 2krpm in 1st can't possibly be more economical that 45mph at 2krpm in 4th can it?

StottyZr

6,860 posts

166 months

Tuesday 28th February 2012
quotequote all
As a rule of thumb, top gear just above labouring revs works pretty well. So around 50mph for me. I once reset my mpg and drove up a sliproad onto a motorway with roadworks. I maintained a very steady 50mph for ~7miles and averaged a very respectable 80.7mpg yes

ZeeTacoe

5,444 posts

225 months

Tuesday 28th February 2012
quotequote all


For example

matthias73

2,883 posts

153 months

Tuesday 28th February 2012
quotequote all
StottyZr said:
As a rule of thumb, top gear just above labouring revs works pretty well. So around 50mph for me. I once reset my mpg and drove up a sliproad onto a motorway with roadworks. I maintained a very steady 50mph for ~7miles and averaged a very respectable 80.7mpg yes
Can I ask what you are driving?

I managed well over 55mpg from tamworth to leicester once. All the petrol stations were asleep so I had no choice. The problem with this type of driving, is that it is really, really, really boring.

McSam

6,753 posts

178 months

Tuesday 28th February 2012
quotequote all
It is almost impossible to give general advice as all cars will behave differently - there are far too many variables.

However, one thing you should consider is that an engine will give you the best brake specific fuel consumption at about 90% load and 25-35% maximum engine speed. That is to say, under those conditions, you are producing power most efficiently, so this would be the best way to accelerate, for example.

Fuel consumption over a longer period, though, is all about balancing the brake specific fuel consumption characteristics of the engine against the other variables - the aerodynamic drag, rolling resistance, gearing, road surface - that determine how much power the engine actually needs at a given time. It is thus unbelievably hard to quantify.

davepoth

29,395 posts

202 months

Tuesday 28th February 2012
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busta said:
On a speed awareness course the instructor insisted that the slower you go, the better the fuel economy but I can't see it being as simple as that. 10mph at 2krpm in 1st can't possibly be more economical that 45mph at 2krpm in 4th can it?
Depends on the car. But as above, minimum possible revs in top gear will generally provide very good economy.

StottyZr

6,860 posts

166 months

Tuesday 28th February 2012
quotequote all
matthias73 said:
Can I ask what you are driving?

I managed well over 55mpg from tamworth to leicester once. All the petrol stations were asleep so I had no choice. The problem with this type of driving, is that it is really, really, really boring.
123d, short gearing so 70+ the economy isn't very good. But as shown with the graph I would have very little wind resistance at 50.

Also, do the same test in a Audi A3 140 tdi and I'd expect you'd get 85+, its not remotely an accurate guide to the cars fuel consumption.

McSam

6,753 posts

178 months

Tuesday 28th February 2012
quotequote all
davepoth said:
Depends on the car. But as above, minimum possible revs in top gear will generally provide very good economy.
Generally, in engine terms you'll want to be well above idle. In most petrols 2000rpm would be reasonable, but probably still a little over. In some cars, though, you may find that the increased drag from going that fast buggers the gain in specific consumption.

crocodile tears

755 posts

149 months

Tuesday 28th February 2012
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ZeeTacoe said:


For example
to add a lot of complication.. and make any use of it.. I'd love to see gear and RPM included hehe

davepoth

29,395 posts

202 months

Tuesday 28th February 2012
quotequote all
McSam said:
davepoth said:
Depends on the car. But as above, minimum possible revs in top gear will generally provide very good economy.
Generally, in engine terms you'll want to be well above idle. In most petrols 2000rpm would be reasonable, but probably still a little over. In some cars, though, you may find that the increased drag from going that fast buggers the gain in specific consumption.
Absolutely. By minimum possible I was thinking of "as low as you can go while not labouring the engine"; most cars will pootle along at 1500rpm on the flat in top gear.

busta

4,504 posts

236 months

Tuesday 28th February 2012
quotequote all
davepoth said:
busta said:
On a speed awareness course the instructor insisted that the slower you go, the better the fuel economy but I can't see it being as simple as that. 10mph at 2krpm in 1st can't possibly be more economical that 45mph at 2krpm in 4th can it?
Depends on the car. But as above, minimum possible revs in top gear will generally provide very good economy.
But as we can sort of see from that beautiful graph, most cars achieve higher MPG in lower gears than they do in top.

I guess the next question is where do you draw the line between saving fuel and wasting time.

cheddar

4,637 posts

177 months

Tuesday 28th February 2012
quotequote all
StottyZr said:
matthias73 said:
Can I ask what you are driving?

I managed well over 55mpg from tamworth to leicester once. All the petrol stations were asleep so I had no choice. The problem with this type of driving, is that it is really, really, really boring.
123d, short gearing so 70+ the economy isn't very good. But as shown with the graph I would have very little wind resistance at 50.

Also, do the same test in a Audi A3 140 tdi and I'd expect you'd get 85+, its not remotely an accurate guide to the cars fuel consumption.
123d's are very economical at 50 - 55mph with a very light throttle application, I've seen a steady 80mpg over 10 miles but you've only got to flex your little toe microscopically to see 80mpg become 45. Similarly, raise the speed to 65 mph and it knocks 25mpg off.


Talksteer

5,025 posts

236 months

Wednesday 29th February 2012
quotequote all
Ozzie Osmond said:
The biggest factor by far in fuel economy is wind resistance. Wind resistance rises at roughly the square of speed, so double the speed = four times the wind resistance.

Once an engine is running quickly enough and suitably geared to be efficient the slower you drive the better your mpg.
Not quite true, one issue is throttle losses. At really low speeds the engine is breathing through a mostly closed throttle which introduces pumping losses.

The drag on a car at low speeds is also really low say you had a car that needed 200bhp to overcome air drag at 150mph. At that speed it would have to apply a force of 2231N. The force the car has to apply to cruise at 10-50mph will be:

10 10N
20 40N
30 89N
40 158N
50 247N

Rolling friction would be 200-400N and is usually speed independent.

The power outputs for cruising below 50 are so low that economy is more down to how efficient the engine is part load and how the gearing works. In real world driving acceleration and climbing hills uses far more petrol than steady state flat road conditions.

If it was just down to weight and drag then a Ferrari would give the same fuel economy on the EURO cycle as VW Golf.

For electric cars driving at about 30mph is the best way to extend range because their motors tend to be most efficient at lower rev ranges.

http://news.cnet.com/8301-11128_3-10384984-54.html



Edited by Talksteer on Wednesday 29th February 13:12

Nick M

3,624 posts

226 months

Wednesday 29th February 2012
quotequote all
Oddly enough I'd pondered something similar, because my E32 735 *generally* seems to have a higher average fuel economy around 80mph rather than 70mph. This is massively subjective as I've not got data logging and lots of spreadsheets / graphs to back it up, but is based on the average mpg over longer journeys where I've set out to try and maximise fuel consumption (just for something to do as much as anything wink ).

My suspicion is that there is a point where the aero drag plateaus out for a bit (maybe because you've done most of the work punching a hole in the air) before ramping up again as you go faster, and the rpm / throttle opening required to maintain 80mph is an efficient combination as far as the engine is concerned.

However, there's also an element of how you drive at those speeds - constant on/off/on/off the throttle is a killer for the fuel economy on my car, and I find I can generally drive more efficiently than the cruise control (less obvious changes in throttle opening and I can let the car slow down a few mph when going up a hill and then let it build again when going down).

Basically, in my limited experience with my car, there doesn't seem any tangible benefit to going slower, so I don't bother ! biggrin

uk_vette

3,336 posts

207 months

Wednesday 29th February 2012
quotequote all
I see all the talk about petrol engines, and slow rpm.
To be honest, petrol engines and slow rpm do not go "hand in hand"

If your wanting slow rpm, then you need a big 4 cylinder diesel, as the 4 cylinders, asopposed to the 6 clyinders will deveop more tq, at lower rpm.
For example.
A 3 liter diesel Land Cruiser, (newer D4-4) is quite happy to pootle along at 1000 rpm.
It is almost effortless, providing the road is reasonably flat.

My guess is that the economical speed of a vehicle is determined by the following.

1/ where the tq. of the engine is best, say for example from 1400 rpm to 2200 rpm
2/ cross that with where the hp of teh engine is besy, say for example at 4200 rpm
3/ the various resistances, ie, wind and rolling.
4/ Boredom

I guess you need to put them all into the maths, to get any sensible answer.
It will be different for almost every car due more to the resistances holding you back.

For the 3 liter Land Cruiser, I see the economical speed is about 45 to 50 mph, even that will just break the Land Cruiser into the low 30's mpg. Like 32 or 33 mpg.
Up the speed to 55 and you can just get 30 mpg.

vette

McSam

6,753 posts

178 months

Wednesday 29th February 2012
quotequote all
uk_vette said:
I see all the talk about petrol engines, and slow rpm.
To be honest, petrol engines and slow rpm do not go "hand in hand"
...
My guess is that the economical speed of a vehicle is determined by the following.

1/ where the tq. of the engine is best, say for example from 1400 rpm to 2200 rpm
2/ cross that with where the hp of teh engine is besy, say for example at 4200 rpm
...
It's not the case, actually. People don't think it's "right" to run a petrol engine at low speeds, but as I said above, the maximum power output per unit fuel will be achieved at fairly low revs with around 90% of maximum load. This is actually quite true for both petrol and diesel engines, but below is a graph for a 1.8l I4 non-turbocharged engine:



It's a map of engine brake specific fuel consumption, so the lower the number the better. The y axis is brake mean effective pressure, which is directly proportional to torque output (so gives you an idea of how much load for a given speed), and the x is simply engine speed.

As you can see, this engine is happiest in the 2000-3000rpm range, from maybe 75-90% load. That is the place in which you will generate the most power output per unit fuel consumed - so you want to balance all the other factors to fit in with that. The complications for cruising come when you find that increasing your speed by x% improves engine efficiency by y%, but also increases drag by z% - balancing y and z is the hard bit.

Further to that, this graph has other uses: if you wanted 7 bar BMEP (700 on the scale used), which would equate to almost exactly 100Nm for this engine, there's a clear pattern of the best engine speed to use. You can make that much torque at idle, but you're using 20% less fuel at 1600rpm, and less still at 2500rpm. You can go all the way round to 3200rpm (where 100Nm is about 33.5kW or 45bhp) and still not be using a lot more fuel. This then shows the most efficient way to accelerate with this particular engine - but while the numbers may vary slightly, the trends apply to pretty much any engine.