mpg experiment

[Fume troll]

RT106 said:

Fume troll said:

True, but engine braking wastes a lot more energy than using some fuel to maintain an idle speed. So which is more efficient depends on how much you want to slow down. If you don't want to slow down, coasting in neutral with the engine idling is more efficient than just taking your foot off the accelerator.

Could you explain that to me, please? I can't see how using no fuel could be less efficient than using some fuel...?

Think about it in terms of energy use rather than just fuel. Coasting along in gear with the engine at say 2500 rpm means that the energy required to keep the engine spinning at 2500 rpm is coming from the car's kinetic energy, which slows the car. This is more energy that is required for the engine to idle - the energy requirement increases exponentially with engine speed due to friction / pumping losses in the engine. So leaving the car in gear will slow the car more than coasting out of gear, meaning you have to accelerate more / sooner to get back to the speed you wanted to be at.

Therefore, if you want to slow down backing off the throttle in gear will be most efficient, but if you want to maintain your speed, or slow down less, coasting out of gear would be more efficient. Clearly at some point there is a crossover, where this is would depend on quite a lot of factors!

http://www.metrompg.com/posts/pulse-and-glide.htm

Cheers,

FT.

[Kozy]

Fume troll said:

Think about it in terms of energy use rather than just fuel. Coasting along in gear with the engine at say 2500 rpm means that the energy required to keep the engine spinning at 2500 rpm is coming from the car's kinetic energy, which slows the car. This is more energy that is required for the engine to idle - the energy requirement increases exponentially with engine speed due to friction / pumping losses in the engine. So leaving the car in gear will slow the car more than coasting out of gear, meaning you have to accelerate more / sooner to get back to the speed you wanted to be at.

Therefore, if you want to slow down backing off the throttle in gear will be most efficient, but if you want to maintain your speed, or slow down less, coasting out of gear would be more efficient. Clearly at some point there is a crossover, where this is would depend on quite a lot of factors!

Is this not just common sense?

[mej023]

I was the author of the original post and my experiences with coasting are this:-

(Using the word coast = out of gear, and roll = in gear but no accelerator press)

There are certain parts of my route home that I found easiest to coast. They generally
involve slight hills and gentle curves. If I am at the top of one hill in particular,
I can coast all the way to the first set of traffic lights in town, as long as I
hit the top of the hill at 60mph.

If I try and roll the same distance (about a mile) then there's no chance. The friction/engine
braking/whatever you want to call it slows the car down much sooner and I end up accelerating
again to get to the lights.

Safety and legality aside, I suspect that coasting on idle for this mile uses less fuel
than rolling and then accelerating again, overrun or not.

Michael.

[bigcat]

One other to add, try to plan trips, I know friends that will use the car to go to the bank and then use it again to go to the shops. Do the pair in one trip avoiding as many cold starts as possible. Also try to restrict use of air con, in my diesel it takes about 4 mpg off.

[Sharief]

Overrun? What does this mean?

[Mr POD]

mej023 said:

But it is sooo boring ! Given this distances
I drive, the difference between this and my normal 34mpg driving adds up to
about an extra £15 a month. Perhaps it's worth paying this to enjoy the driving though.

Michael.

I decided this in 1993.

I was working with a guy who got 80 mpg regularly out of an astra mk3 diesel, and I thought I'd try it for one week. Yes my fuel consumption went down, from maybe 28 mpg to 32 mpg. But feck me if I wasn't ready to top myself.

[Kozy]

Sharief said:

Overrun? What does this mean?

Off throttle where the momentum of the car is effectively driving the engine.

[Inny]

mej023 said:

Yes my fuel consumption went down, from maybe 28 mpg to 32 mpg. But feck me if I wasn't ready to top myself.

Edited for formatting error

Edited by Inny on Thursday 17th July 14:21

[filski666]

why do people think 50mph is "best" for fuel economy? - what absolute toss!

[Fume troll]

Kozy said:

Fume troll said:

Think about it in terms of energy use rather than just fuel. Coasting along in gear with the engine at say 2500 rpm means that the energy required to keep the engine spinning at 2500 rpm is coming from the car's kinetic energy, which slows the car. This is more energy that is required for the engine to idle - the energy requirement increases exponentially with engine speed due to friction / pumping losses in the engine. So leaving the car in gear will slow the car more than coasting out of gear, meaning you have to accelerate more / sooner to get back to the speed you wanted to be at.

Therefore, if you want to slow down backing off the throttle in gear will be most efficient, but if you want to maintain your speed, or slow down less, coasting out of gear would be more efficient. Clearly at some point there is a crossover, where this is would depend on quite a lot of factors!

Is this not just common sense?

Apparently not..

Cheers,

FT.

[Mr POD]

Inny said:

MR POD said:

Yes my fuel consumption went down, from maybe 28 mpg to 32 mpg. But feck me if I wasn't ready to top myself.

Edited for formatting error

Edited by Inny on Thursday 17th July 14:21

Edited cause it was me that typed it.

[filski666]

why is this topic wider than the average thread? - I don't normally have to use my bottom scrollbar!

[Mr POD]

filski666 said:

why do people think 50mph is "best" for fuel economy? - what absolute toss!

Depends on the car I suppose. There must be one car where this is optium ?

That said I once drove an MG metro Turdo at a steady 70 MPH all the way from Birmingham to Glasgow and was astonished to find I'd got 40 mpg.

My usual consumption comuting with the waste gate fluttering along the back roads from Solihul to Longbridge was closer to 20 Mpg.

[filski666]

Mr POD said:

filski666 said:

why do people think 50mph is "best" for fuel economy? - what absolute toss!

Depends on the car I suppose. There must be one car where this is optium ?

That said I once drove an MG metro Turdo at a steady 70 MPH all the way from Birmingham to Glasgow and was astonished to find I'd got 40 mpg.

My usual consumption comuting with the waste gate fluttering along the back roads from Solihul to Longbridge was closer to 20 Mpg.

maybe there is, but with the wide range of gearing, engine style, aerodynamics, etc there is never going to be one all ecompassing optimum speed for economy!

[filski666]

Mr POD said:

filski666 said:

why do people think 50mph is "best" for fuel economy? - what absolute toss!

Depends on the car I suppose. There must be one car where this is optium ?

That said I once drove an MG metro Turdo at a steady 70 MPH all the way from Birmingham to Glasgow and was astonished to find I'd got 40 mpg.

My usual consumption comuting with the waste gate fluttering along the back roads from Solihul to Longbridge was closer to 20 Mpg.

did you mean to write "Turdo" - cos if you did :

[Pigeon]

I think it comes from the 56mph set speed in fuel consumption test figures and the suspicion that manufacturers optimise their cars for best economy at the standard test speeds.

(Jules2477... Your long long rows of asterisks make it necessary to scroll sideways )

[Jules2477]

Pigeon said:

I think it comes from the 56mph set speed in fuel consumption test figures and the suspicion that manufacturers optimise their cars for best economy at the standard test speeds.

(Jules2477... Your long long rows of asterisks make it necessary to scroll sideways )

Noted about the asterisks thanks. Re the speed issue - basic physics is the faster you go the more power required but in general terms it is a fact that most cars deliver best results around the 50 - 60 mph in top gear. Too fast or too slow brings other factors into play - all far to boring to go into without sending everyone to sleep. One thing you can sure of is that the guy who drives every where at the ton and swears it is the most economic is from another planet - we have all met em at some stage.

This has Proved to be an interesting thread - Plenty of quotes about why you need power to control a car which I certainly do not disagree with but many completely overlook the almost freewheel effect on overrun in old auto's, and many overgeared Eurocans, and of course descending steep hills in any car is about the last place you are going to apply power so the normal cornering or swerving dynamics are completely reversed. However, vehicles go down them daily without crashing so is freewheeling as dangerous as some contributors would have you believe ?

Anyway cutting to the chase, Perhaps it is time to wrap up this thread lest we start going round in ever decreasing circles and lose the will to live ! The main heading is Advanced Driving and not Economy Driving- using the correct gear for the road speed, is and remains best practice for optimum car control if we are going to stick to road craft/IAM standards. Apart from a few advanced techniques (some ahead of the basic IAM standard) which may be used in certain low speed situations I do not know of any examiners that are going to be happy passing any student using prolonged periods of deliberate freewheeling. The other irony is that advanced driving is economy driving in its own right but not ultimate economy driving in the same way that is not racing or excessively fast driving. The latter being a common myth spread by the uninformed.

Edited by Jules2477 on Sunday 20th July 18:51

[Pigeon]

Jules2477 said:

Noted about the asterisks thanks.

Jules2477 said:

Re the speed issue - basic physics is the faster you go the more power required but in general terms it is a fact that most cars deliver best results around the 50 - 60 mph in top gear.

That is indeed the perception, but I'm not sure it necessarily corresponds with reality. Power required rises with the cube of speed - ie. pretty fast - but bsfc vs. power output curves are a fair bit flatter except at the extremes; in simplistic terms a car is most economical as soon as it's "happy" in top gear, which is generally a fair bit slower than 50-60mph. Remember the adverts for "the 80mpg Metro"? That figure was at 30mph in top gear, or it might have been 40, and it caused a cufuffle because it wasn't measured at one of the standard test speeds and so bore even less relation to real-world mpg than the advertised figures usually do

So what is "happy" in top gear? You could go on for ages expounding an engineering definition but it seems to me that in practical terms there is no need to add any extra economy-based tweaks to the usual algorithm for "selecting the appropriate gear for speed and conditions". If you would already say that top gear is an "appropriate" gear for cruising at a steady speed of whatever the speed is that you have chosen, when you do not anticipate the need to make sudden manouevres for which the greater control of being in a lower gear might be appropriate, then in engineering terms the car is "happy". To select speed for economy, then, you're looking at the lower end of the speed range in which top is appropriate for cruising.

Jules2477 said:

Plenty of quotes about why you need power to control a car which I certainly do not disagree with but many completely overlook the almost freewheel effect on overrun in old auto's, and many overgeared Eurocans, and of course descending steep hills in any car is about the last place you are going to apply power so the normal cornering or swerving dynamics are completely reversed. However, vehicles go down them daily without crashing so is freewheeling as dangerous as some contributors would have you believe ?

I was thinking the same. The way I see it is that it's just another aspect of planning your driving so that the control inputs you will need to make do not exceed those which are appropriate to the conditions, where "conditions" includes the current state of the engine and drivetrain as well as the road conditions themselves. My car is a big lazy old auto and frequently puts its engine and drivetrain into a state equivalent to freewheeling for long periods - at any speed which is at the low end of its range for selecting top, the throttle feels so disconnected from the rear wheels that it's more or less effectively freewheeling even under power; you can't apply power to assist a manoeuvre unless you manually select second first, because otherwise nothing happens until it kicks down by which time you've done whatever it is you were going to do. So it's just a case of planning so as not to need any manouevres which aren't straightforward under no-drive conditions.

Jules2477 said:

The other irony is that advanced driving is economy driving in its own right but not ultimate economy driving in the same way that is not racing or excessively fast driving. The latter being a common myth spread by the uninformed.

Smoothness and avoidance of unnecessary braking and acceleration do more for economy than anything else. It's changing speed which sips...

[Jules2477]

Pigeon said:

Jules2477 said:

Noted about the asterisks thanks.

Jules2477 said:

Re the speed issue - basic physics is the faster you go the more power required but in general terms it is a fact that most cars deliver best results around the 50 - 60 mph in top gear.

That is indeed the perception, but I'm not sure it necessarily corresponds with reality. Power required rises with the cube of speed - ie. pretty fast - but bsfc vs. power output curves are a fair bit flatter except at the extremes; in simplistic terms a car is most economical as soon as it's "happy" in top gear, which is generally a fair bit slower than 50-60mph. Remember the adverts for "the 80mpg Metro"? That figure was at 30mph in top gear, or it might have been 40, and it caused a cufuffle because it wasn't measured at one of the standard test speeds and so bore even less relation to real-world mpg than the advertised figures usually do


The above is the bit that i fully understand but did not want to get too technical ! That 80mpg was of course achieved on a rolling road hence no driver would ever stand a hope in hell of matching it. - A complete con by BL.

I entirely agree with your views about 'happy in top' but I think the reason that most drivers will usually yield best results with 50ish as guide is because you can use the cars inertia more effectively to cope with road undulations. Whereas, at slower speeds (possibly the cars most economic speed on a dead level road) you may need to change down and use engine power to replace the loss of inertia. Cube squared in reverse if you like !

If there is nothing behind me and I am not in any hurry, I will often allow physics to provide the rollercoaster effect to be in play on a switchback road. Many of these types of road can be covered with minimal use of the loud pedal by allowing speed to vary between say the the sixties and the forties but get behind a slow driver, brakes, gear changes and engine power all equal unnnecessarly increased fuel consumption as you waste inertia by converting it to heat instead of useful energy.

Anyway the pro's and cons of using 50ish as a guide could go on for ever as there are so many variables but I can see that we are of similar thought. It is also the case that the new technologies will see more economic low speed rural driving than a standard infernal combustion engine set up can deliver. Good exchanging view points with you.

Edited by Jules2477 on Monday 21st July 04:20

[peterperkins]

mej023 said:

Hi,

I read some article in the paper a few weeks ago about hypermiling - some
thing (apparently) started in the USA about people using techniques to get
better mpg out of their vehicles. Typically they use either a Prius or a Honda
Insight and there is one guy who's had over 100mpg out of his insight. But
they claim to be able to get better mileage out of anything, even a crappy US
gas guzzler.

So, I decided to try some of the things in my car (A 1999 Honda Accord 1.8) Namely:-

1) Tyres are all 3psi over inflated, for less rolling resistance.

2) Roll (or coast) up to an stop situation as soon as you know
theres no actual need to acclerate (or keep constant speed) up
to it. Cons: Annoys drivers behind.

3) Coast down any hill that's around, or any downwards semi-slope.
(I can coast about 7% of my average trip home from work)

4) Never drive above 60mph (This one was hard for a whole tankful)
Cons: Really annoys drivers behind, esp if you do the best mpg 50mph
in a 60 zone. But it was a science experiment !!

5) Keep the revs low. I rarely went above 3000 for the test tankful -
which is a shocking waste of a good VTEC.

6) No hard braking or acceleration - basically 2) and 5) combined.

The best I've ever had from this Accord was 34 mpg as I remember, and
the worst was about 25, which comes from driving it around in the vtec
zone all of the time.

Speaking as Mr UK/PH "Insight" Man I can concur with all that, and it all makes a big difference. I don't do coasting though as it's a faff, and I use the energy from descents etc to re-charge the Insight Battery. 100mpg is easy peasy in an Insight applying the techniques above. Since I bought my car with 60,000 on the clock I have averaged 94.2mpg for the last 39,000 miles It's now just about to roll over the 100,000 mile mark.

102.4mpg at the moment on last tankful of Shell's finest. Yes I drive like a snail a lot of the time but so what. I'm laughing all the way to the pumps!

The same techniques do work with most vehicles in my experience.

Edited by peterperkins on Monday 21st July 03:59