General Instantaneous Miles per Gallon

[Sweetsixteen]

Picking up on JR's point, I have experimented with closed throttle going downhill (useful if you need to scrub off speed), and coasting in neutral. As an alternative. I have found that the latter gives considerably more mpg in hilly country. It helps a lot if you know the roads, because you can then plan ahead as you drive.
The argument seems to rest on the 'fact' that many modern engines cut off the fuel supply on a closed throttle (and I query that, because I have been informed that this only applies above idle revs), while the engine has to keep burning fuel when you are coasting.

So here's a hypothesis:
If you calculate your top gear speed at idle revs (in one of my vehicles it corresponds exactly to 20mph) then as long as you are coasting in neutral above that speed you should be saving fuel. An added bonus is that you often reach the base of the next incline with more momentum, which gives you greater initial speed attacking the incline.

 

[jrkitching]

Picking up on JR's point, I have experimented with closed throttle going downhill (useful if you need to scrub off speed), and coasting in neutral. As an alternative. I have found that the latter gives considerably more mpg in hilly country. It helps a lot if you know the roads, because you can then plan ahead as you drive.
The argument seems to rest on the 'fact' that many modern engines cut off the fuel supply on a closed throttle (and I query that, because I have been informed that this only applies above idle revs), while the engine has to keep burning fuel when you are coasting.

So here's a hypothesis:
If you calculate your top gear speed at idle revs (in one of my vehicles it corresponds exactly to 20mph) then as long as you are coasting in neutral above that speed you should be saving fuel. An added bonus is that you often reach the base of the next incline with more momentum, which gives you greater initial speed attacking the incline.

On my 1.2 500, with the throttle fully closed and in gear, the ECU will begin injecting fuel at about 1500 rpm to smooth out the engine. You can demonstrate this quite easily by decelerating on a flat road with your feet off the pedals from about 40-45mph in 5th. You'll feel a point at which the rate of deceleration slows noticably as the fuel flow starts - as I say, it's about 1400-1500 rpm on my car.

Basically this may mean that in some circumstances you save more fuel than you thought by coasting out of gear (when you're sure it's safe to do so) - but it only helps if you can use the extra speed you get without having to brake it off.

If you get a nice descent where you can coast out of gear at around 50mph, with a hill or long straight at the end, you can save a measurable quantity of fuel by going into neutral. You burn a bit of fuel at idle on the descent, but you get more than that back from the extra kinetic energy you have at the bottom.

 

[Sweetsixteen]

?... but it only helps if you can use the extra speed you get without having to brake it off. .

Agree with this general concept except for the above remark. It's not the coasting speed that produces max economy - as long as you're coasting above engine idle speed, that is - it's the distance you coast that gives the saving. The longer the distance, the greater the saving. Having to brake at the end of a long coast does not necessarily offset the saving obtained.
Reading the road is all part of the technique, yes?
All good fun, and the bonus is the savings on tyres, brakes, gearbox, drive- train and engine. After all, why buy a car for economy and not max it?

 

[Mick F]

The main point of coasting and not having to brake, is not the economy gained in the coasting, but the economy gained afterwards.

I cycle a great deal. Hills abound here, and if I read the road well enough, I can stop pedalling just at the right time to coast so that I don't have to brake for a corner.

If I did brake, it could've meant I was going too fast originally, also if I can get round the bend without braking, I can exit the bend at a better speed and not have wasted any effort with pedalling worthlessly originally.

I read somewhere a while back, that braking wastes fuel. No point in going too fast if you have to brake to slow down to speed up gain.

Greetings from Cornwall,
Mick.

 

[Sweetsixteen]

The main point of coasting and not having to brake, is not the economy gained in the coasting, but the economy gained afterwards.



I cycle a great deal. Hills abound here, and if I read the road well enough, I can stop pedalling just at the right time to coast so that I don't have to brake for a corner.



If I did brake, it could've meant I was going too fast originally, also if I can get round the bend without braking, I can exit the bend at a better speed and not have wasted any effort with pedalling worthlessly originally.



I read somewhere a while back, that braking wastes fuel. No point in going too fast if you have to brake to slow down to speed up gain.



Greetings from Cornwall,

Mick.

My instructor, many centuries ago, told me that if you brake when there is no absolute, unavoidable necessity, it is a defeat for you.
Always put an egg on each of the two r.h.s. pedals (in your mind, that is), he said.

 

[ahmett]

The latter.

It takes less energy to turn the engine at idle rpm that to turn it at cruising revs with the throttle plate shut. Now you have the challenge of finding an on-road situation where it's practical, and more importantly safe, to make use of that information. Hint: it serves no constructive purpose to arrive at the bottom of a hill with kinetic energy (=speed to normal folk)) which you can't safely use.

An engine out of gear and switched off uses less energy that both of the above, but the opportunity to make use of this safely occurs very rarely in real world driving.

I'm not giving away all my secrets or someone might beat my figures!

haha. i think the main reason you are so good with economy is you keep the speed low. I can match fiats advertised figures if i drive carefully but if i go anywhere over 50 mph forget it = ) what an aerodynamically awful car!

 

[jrkitching]

haha. i think the main reason you are so good with economy is you keep the speed low. I can match fiats advertised figures if i drive carefully but if i go anywhere over 50 mph forget it = ) what an aerodynamically awful car!

Keeping the speed low is indeed an important part of ecodriving; that said, I know lots of folks who keep the speed low and get abysmal mpg.

They also get through more brake pads than I do.

 

[jrkitching]

Agree with this general concept except for the above remark. It's not the coasting speed that produces max economy - as long as you're coasting above engine idle speed, that is - it's the distance you coast that gives the saving. The longer the distance, the greater the saving.

You're absolutely correct in saying it's the distance you coast that matters. What I'm saying is that if you coast down any given hill, you'll be going faster at the bottom, and therefore coast further afterwards, if you go down the hill out of gear. You'll use a little more fuel going down, as the engine is idling, but you'll likely save more fuel than you used from the increased coasting distance you get as a result of the extra speed you have at the bottom. If you can't use that extra speed without braking, then you'd be better off coasting down the hill in gear and burning no fuel at all.

The technique works well if the gradient is just right to allow you to coast in neutral at your normal cruising speed; in my case, 50-55mph. If you leave the car in 5th on the same gradient and speed, you'll need more fuel than you'd use if you just left it idling.

It's that total energy equation again. You're turning the engine whether you idle it in neutral or coast it on the overrun; there's less engine friction at idle than at whatever rpm you get coasting in gear, so less energy needed in total and less fuel burnt.

If the hill is so steep that you can't safely coast down the other side out of gear, then you'll never save as much fuel going down as you burnt going up. There are some routes I drive in which the layout of the hills makes ecodriving easier in one direction than the other. Generally you want to take the steep parts in ascent, and the easier gradients in descent - something you'll quickly learn if you're a cyclist. And if you start and finish your journey at different altitudes, you'll usually get better figures in whichever direction is more 'downhill'.

It all adds interest to what might otherwise me a rather boring way of driving.

With the 1.2, 50mpg is straightforward & just needs reasonable driving technique and a cruise speed below 60mph; 55mpg should be achieveable without resorting to these sort of tricks, but routinely getting past 60mpg in the UK climate is hard work and needs dedication!

Interestingly, given the thread topic, the instant mpg readout is, IMO, the least useful tool on the display and I almost never look at it.

 

[Sweetsixteen]

Interestingly, given the thread topic, the instant mpg readout is, IMO, the least useful tool on the display and I almost never look at it.

Playing with this feature I've found that - for instance - driving up a long constant incline and watching the instant mpg readout, I can often gently back off the throttle and maintain my rate of climb (sounds like an F15 !) and see the fuel consumption improve.
Otherwise I never use the feature - I use either the overall mpg figure, or the clock! I leave my mpg figure untouched for at least every 1,000 miles, unless there's a specific trip I need to monitor.
I also tow a light trailer occasionally, and have found no meaningful difference in my economy, believe it or not.

 

[jrkitching]

I leave my mpg figure untouched for at least every 1,000 miles, unless there's a specific trip I need to monitor.

Forgive me if I'm telling you something you know already, but are you aware the 500 has two trip computers - a feature I find quite useful.

You may have to turn the second one on via the menu.

Once they're both enabled, a long press of the button on the stalk will reset whichever one happens to be displaying at the time.

 

[Mick F]

.......... If the hill is so steep that you can't safely coast down the other side out of gear, then you'll never save as much fuel going down as you burnt going up. There are some routes I drive in which the layout of the hills makes ecodriving easier in one direction than the other. Generally you want to take the steep parts in ascent, and the easier gradients in descent - something you'll quickly learn if you're a cyclist.

Yes, it's being a cyclist more than a driver that you see economy of effort. If I'm fit and the ride is short, I'm happy to burn my fuel, but if the ride is long and arduous, I'll conserve as much as possible.

A little story from some years back regarding our Clio .....
When we'd not long bought it, my mother-in-law happened to be not very well. We live in Cornwall and she was in Lancashire 300miles north. We drove Up North most weekends and attempted to get as much MPG as we could. The Clio would happily do 50mpg providing I kept off the GO pedal and kept the speed around 60mph.

Going north A30, M5, M6 we invariably managed 52mpg or slightly more, but coming back south the economy wasn't so good - maybe 48mpg. I always tried my hardest to keep as economical as possible, but try as I might, coming back south was never ever ever more economical as driving north.

We did that journey dozens of times over a period of maybe six months and then some more over the next year too, so I knew the road quite well.

I put it down to hill profile, like you suggest. Some of those gradients on the A30 through Devon are a long long slog.

Interestingly, given the thread topic, the instant mpg readout is, IMO, the least useful tool on the display and I almost never look at it.

Yes, rather useless, but I found it interesting, hence my original post.

Regards,
Mick

 

[Sweetsixteen]

... I put it down to hill profile, like you suggest. Some of those gradients on the A30 through Devon are a long long slog.



Regards,

Mick

Nah ... it's the headwinds - designed to keep foreigners out of Cornwall!
That's also why you get charged at the bridge to enter Wales, but you can get out free!

 

[Mick F]

Tolls for vehicles to cross the River Tamar are levied leaving Cornwall entering England.

Kernow bys vyken!

 

[UFI]

The ultimate energy management machine is probably a glider

Gliders kind of works on the BSFC principle, tow/ winch into the air and glide the rest of the way (I know thermals/ updrafts have a bit to do with it...)

The huge gaping hole in UFI's argument is that coasting in gear with your feet off the pedals is hugely wasteful in energy because of the significant pumping losses and mechanical drag of the windmilling engine; going downhill in gear with your foot off the gas isn't a zero energy solution because of engine braking effects.

The arguments might have some validity if you coasted downhill out of gear with the engine off, but that's hardly a safe or practical way of driving.

I wouldn't argue otherwise. Engine braking isn't 100% efficient (I pointed this out earlier). You cost yourself momentum, but it lets you keep the engine and ancillaries running. On the flat, I'd more or less agree completely (I've seen a test where pulse and glide [in gear using DFCO] resulted in a 5% economy gain. For the record I've tried this with UFI and it doesn't work, the car is too light (not enough stored momentum) and not very aerodynamic (glide lenghts are too short). You can search youtube for pulse and glide resulting in 120mph US on the highway. The theory is sound, but the 500 is not the vehicle to take advantage of it based on my experience.

Switch it over to gliding down hills, and DFCO is better than using fuel down hill and then braking. The engine braking argument only applies to very shallow down grades. Bare in mind the the thread was started in relation to a TA, no throttle plate pumping losses, and only half the frictional engine losses of a four cylinder. In a heavy vehicle like my Transporter, putting it in 6th only slows the vehicle as much as turning the A/C on in a small car.

Whichever way the theorists want to argue this one, my own long term results are in my sig and you are welcome to improve on them, if you can.

We don't all have a 1.2 driven on your roads. Your 1.2's official combined MPG is 60.1. You're only beating it by <2%.

My Jeep's official MPG is 16 (US), I manage 20.85 on offroad trips (1:1 hills, mud, low range etc). The last tank was 50-50 city highway with lots of short trips and still managed 17.84MPG (I didn't drive the entire tank), that's 11.5% over official.

If we move onto my Daily drivers, both are rated at (coincidentally) 22.15 MPG (US) and they both sit at 30.2MPG (one with only 1500km from new). That's 36% over their official ratings.

2%? You might have the thriftiest 500 on the forum (and possibly Fuelly), but on the hypermiler forums 60% is pretty normal.

 

[306maxi]

as always the problem is that your roads and terrain differ to ours, there is a church near me which I could probably hit with a well thrown stone. The church is referenced in the Domesday book. That should give you an idea of what some of the roads around here were designed for *hint - it's not cars*

 

[UFI]

Picking up on JR's point, I have experimented with closed throttle going downhill (useful if you need to scrub off speed), and coasting in neutral. As an alternative. I have found that the latter gives considerably more mpg in hilly country. It helps a lot if you know the roads, because you can then plan ahead as you drive.

For my hill test run I used DFCO down hills as I felt this was how most people drive. Coasting in neutral does indeed produce better results.

The argument seems to rest on the 'fact' that many modern engines cut off the fuel supply on a closed throttle (and I query that, because I have been informed that this only applies above idle revs), while the engine has to keep burning fuel when you are coasting.

On a low geared car like a 500, you have to be travelling very slowly to be below idle, which the engine won't be happy with anyway..[/QUOTE]

On my 1.2 500, with the throttle fully closed and in gear, the ECU will begin injecting fuel at about 1500 rpm to smooth out the engine. You can demonstrate this quite easily by decelerating on a flat road with your feet off the pedals from about 40-45mph in 5th. You'll feel a point at which the rate of deceleration slows noticably as the fuel flow starts - as I say, it's about 1400-1500 rpm on my car.

The 1.2 has poor DFCO, it restores fuel at 1200rpm in 5th, 1320rpm in 2-4th, and 2000 in first. The TA has much better DFCO (much like a diesel) which works at any time the engine is above idle (maybe 50rpm).

 

[UFI]

as always the problem is that your roads and terrain differ to ours, there is a church near me which I could probably hit with a well thrown stone. The church is referenced in the Domesday book. That should give you an idea of what some of the roads around here were designed for *hint - it's not cars*

If that's the case why the big aversion to DFCO? It should be a godsend. At any rate JR doesn't live in Wales, does he? And again, my Jeep sees hills that can't be climbed in a 500 period. Still beating the official ratings.

 

[306maxi]

I'm not averse to it, it's just that it's not always possible to drive economically here...... *replays broken record*

 

[ahmett]

also cold weather really doesnt help things out = )

 

[jrkitching]

If that's the case why the big aversion to DFCO?

There isn't one - given the topography of the roads round here, it is very often the best you can do; probably even more so where Maxi lives. It will save you fuel on any descent where you would otherwise have to brake.

However the engine braking effects on the 1.2 are significant, and as you have already said, coasting in neutral is more efficient where the terrain is suitable for that technique and, more importantly, it is safe to do so.

The rolling Hampshire countryside gives you more opportunities to use this technique than the mountains of snowdonia - there are several nice straight descents round here with clear runs at the bottom where the 1.2 will coast nicely out of gear at around 50-55mph, depending on the prevailing wind. On one regular jouney I can consistently get another 2-3mpg by freewheeling down the main gradients.

also cold weather really doesnt help things out = )

:yeahthat: