No, the engine doesn't stop, it just doesn't keep turning by force of combustion but rather because it is directly connected to the wheels. If you were to put the transmission in neutral while coasting, the ECU would have the fuel injectors pulse enough fuel to keep the engine from stopping dead (i.e. having it run on idle rpm), but as long as you keep it in gear there is nothing that stops it from completely cutting fuel. When you reach the end of the slope, engine rpm will drop to a level where the ECU will start injecting fuel again to keep the engine from stalling.

I don't know if you are speaking from explicit knowledge of how this actually works , but the problem I have with this theory is that if there is no fuel being injected to create energy to turn the engine over (and it does take energy to do this!), then the energy has to come from the motion of the vehicle. If this was the case the car would quickly slow down on anything other than a very steep incline.

In practice this doesn't happen, so there must be at least as much fuel injected as is necessary to keep the engine turning at whatever rpm it is turning at for the road speed. This will surely be twice as much (approximately) at 2,000rpm (70mph in our Scout Tdi) as at 1,000rpm. This is pretty much in line with my observations.

So I'm still not convinced that coasting doesn't save the amount of fuel that it seems to do! But thanks for your input on this.

I don't know if you are speaking from explicit knowledge of how this actually works , but the problem I have with this theory is that if there is no fuel being injected to create energy to turn the engine over (and it does take energy to do this!), then the energy has to come from the motion of the vehicle. If this was the case the car would quickly slow down on anything other than a very steep incline.

In practice this doesn't happen, so there must be at least as much fuel injected as is necessary to keep the engine turning at whatever rpm it is turning at for the road speed. This will surely be twice as much (approximately) at 2,000rpm (70mph in our Scout Tdi) as at 1,000rpm. This is pretty much in line with my observations.

So I'm still not convinced that coasting doesn't save the amount of fuel that it seems to do! But thanks for your input on this.

Admittedly, my knowledge on this matter is purely theoretical. Nevertheless, I think you may be basing your conclusions a little too much on gut feeling.

Sure, energy is needed to turn the engine over. But not as much as you may think.. try attaching a wrench to the flywheel nut or some other part that's directly connected to the crankshaft. It turns with very little resistance. Most of the energy that's lost when coasting is lost to wind resistance and friction between tyres and road, and yes, coasting will effectively cause the speed to decrease unless the slope is steep enough (it doesn't have to be that steep, mind you). That's why, when the speed has decreased to the point where the engine rpm falls below idle, the injectors will resume function and start pulsing fuel into the cylinders again.

I'm talking about the situation where, at least initially, inertia along with the difference in potential energy are enough to keep the car going - in this situation there's no need for fuel to pass through the cylinders, so the ECU shuts down the injectors. You'll notice if you check the display that sometimes it says 0.0 l/100 km, and when the road flattens out and your speed drops it will slowly increase: 0.7, 1.0, 2.0, etc. as load increases (of course, you won't see those exact numbers, but I don't know how the display represents "no gas used" in mpg unless it can display the inifinity sign? )

Besides, in a four-stroke engine, each cylinder is injected with fuel only once per four rotations anyway, the difference isn't all that much removing the fourth too

One more note: you can't assume that twice as much gas is used at 2000 rpms as at 1000. There are so many more parameters that the ECU checks before instruction the injection system on how long it should pulse (there are a heap and a half of lookup tables for engine rpm, engine load, exhaust data, engine temperature, air temperature, to mention a few, crammed into a chip somewhere. It is this chip you replace if you want to "trim" the engine).

Sure, energy is needed to turn the engine over. But not as much as you may think.. try attaching a wrench to the flywheel nut or some other part that's directly connected to the crankshaft. It turns with very little resistance. Most of the energy that's lost when coasting is lost to wind resistance and friction between tyres and road, and yes, coasting will effectively cause the speed to decrease unless the slope is steep enough (it doesn't have to be that steep, mind you). That's why, when the speed has decreased to the point where the engine rpm falls below idle, the injectors will resume function and start pulsing fuel into the cylinders again.

I'm talking about the situation where, at least initially, inertia along with the difference in potential energy are enough to keep the car going - in this situation there's no need for fuel to pass through the cylinders, so the ECU shuts down the injectors. You'll notice if you check the display that sometimes it says 0.0 l/100 km, and when the road flattens out and your speed drops it will slowly increase: 0.7, 1.0, 2.0, etc. as load increases (of course, you won't see those exact numbers, but I don't know how the display represents "no gas used" in mpg unless it can display the inifinity sign? )

Besides, in a four-stroke engine, each cylinder is injected with fuel only once per four rotations anyway, the difference isn't all that much removing the fourth too

One more note: you can't assume that twice as much gas is used at 2000 rpms as at 1000. There are so many more parameters that the ECU checks before instruction the injection system on how long it should pulse (there are a heap and a half of lookup tables for engine rpm, engine load, exhaust data, engine temperature, air temperature, to mention a few, crammed into a chip somewhere. It is this chip you replace if you want to "trim" the engine).

Well there's quite a lot I could say in response to the above!

Firstly, there will be a difference between coasting with the drivetrain in neutral and with it engaged, and that's what I'm trying to address. If the drivetrain is engaged it is my contention that at the appropriate rpm for the road speed the engine will be consuming more fuel than it would at idle, ie when the drivetrain is not engaged. This appears to be borne out in my observations.

Secondly, you suggest that I can't assume that the engine will consume twice as much fuel at 2,000rpm as it will at 1,000rpm because of parameters like load etc. But surely when the drivetrain is disengaged the engine must by definition be using the least amount of fuel possible. There is no load on it at all (apart from internal friction and the flywheel), not even the gearbox. If the drivetrain is engaged at the same road speed, however, even on the overrun, there will be more load and the effect of friction will be increased at higher engine speeds because this is (IIRC) an exponential effect. Plus the engine will be running slightly hotter at a higher speed so there will be more thermal losses, although this will probably be slight.

So I would imagine that it would be AT LEAST twice as much fuel at 2,000rpm as at 1,000rpm. You mention the parameters the ECU will be working on, but all of these will be neutral between the two states or beneficial to the situation at idle with the drivetrain disengaged.

But all of the above is still theory rather than from knowledge so I totally admit I might be wrong because I haven't taken something else into account. I am just about to go out so I will look carefully at the instantaneous figures while driving downhill with the drivetrain both engaged and disengaged to see the difference in the light of this discussion.

Well there's quite a lot I could say in response to the above!

Firstly, there will be a difference between coasting with the drivetrain in neutral and with it engaged, and that's what I'm trying to address. If the drivetrain is engaged it is my contention that at the appropriate rpm for the road speed the engine will be consuming more fuel than it would at idle, ie when the drivetrain is not engaged. This appears to be borne out in my observations.

Secondly, you suggest that I can't assume that the engine will consume twice as much fuel at 2,000rpm as it will at 1,000rpm because of parameters like load etc. But surely when the drivetrain is disengaged the engine must by definition be using the least amount of fuel possible. There is no load on it at all (apart from internal friction and the flywheel), not even the gearbox. If the drivetrain is engaged at the same road speed, however, even on the overrun, there will be more load and the effect of friction will be increased at higher engine speeds because this is (IIRC) an exponential effect. Plus the engine will be running slightly hotter at a higher speed so there will be more thermal losses, although this will probably be slight.

So I would imagine that it would be AT LEAST twice as much fuel at 2,000rpm as at 1,000rpm. You mention the parameters the ECU will be working on, but all of these will be neutral between the two states or beneficial to the situation at idle with the drivetrain disengaged.

But all of the above is still theory rather than from knowledge so I totally admit I might be wrong because I haven't taken something else into account. I am just about to go out so I will look carefully at the instantaneous figures while driving downhill with the drivetrain both engaged and disengaged to see the difference in the light of this discussion.

Yes, but I was also talking about the drivetrain being engaged or disengaged. If it is disengaged, i.e. in neutral, then yes there is a need for fuel to keep the engine turning (otherwise it would eventually come to a halt). When on the other hand the drivetrain is engaged, which it is if you simply lift your foot off the throttle (and disengage the cruise control) when approaching a downhill slope, then there is no need for fuel to keep the engine turning, so fuel injection is turned off. However, if the slope isn't steep enough, or infinitely long, then sooner or later tyre friction and air resistance will make the car start to slow down. As it does, so will the engine, and eventually the rpms will drop low enough for the ECU to decide it's time to resume fuel injection.

Well.. I think you should try going on a straight stretch of road at 1000 rpm in top gear. Then try gearing down while maintaining road speed so rpms hit 2000. I doubt you'll find that the display halves your mpg figure.. I'd guess most car engines are tuned so they perform significantly better at 2000 rpm than 1000, thus wasting less fuel (relatively speaking).

I don't have my car here right now so can't check myself, but it'll be interesting to hear what you've found when you return

One point nobody has mentioned is the temperature...

for example I travelled the 125miles to my parents - Trip computer read 70mpg Av. Temp. 14oC - Return Trip computer read 59.5mpg Av. Temp. 2oC (My trip computer exaggerates a little!)

I find any temp. below 10oC economy drops off then below 5oC it drops off a lot!

I've averaged about 52mpg (rim to rim) myself over this cold period but I will expect to see 60mpg+ when the temperatures pass 10oC average.

I don't think any of your figures are bad! Just let the engines bed in and wait for warmer weather

I agree about the outside temperature, the problem being 14oC on tyneside is a hot summers day and we dont get many of them.

With most cars, you get optimal fuel economy driving between 2000 and 2500 rpm. That's why car makers used to quote fuel consumption at a steady 56 mph. It is true that a small engine at higher speed will use more fuel than a larger engine running well within its speed range. My husband and me both drive Vauxhall Astras at present, we have similar driving styles and the main difference between the cars are:

1. Engine size: Mine is the 1.4 16v and the other is a 1.6 16v. The gearing on both cars is very similar.

2. Tyres: The two cars use marginally different tyre sizes and brands.

Both cars deliver around 40 mph on our normal driving, but the 1.6 is more fuel efficient at 70 mph.

I suspect the reason why the fuel consumption of the 1.4 80 bhp engine is less predictable on mpg is the fact that it does need to be pushed harder to get the maximum performance whereas the 1.9 has more oomph and doesn't need to be driven so hard. If you have to floor the gas pedal, the mpg will inevitably suffer.

Try 60mph instead of 70 mph on the motorway and your mpg figures will improve dramatically and unless your travelling at a very quiet time of day, your journey time will be very similar. The car will just purr along quietly and it's generally a much more relaxed drive.

It will interesting to see how my new Skoda 1.4 16v fares when I get it, compared to the Astras.

Jo

I agree about the outside temperature, the problem being 14oC on tyneside is a hot summers day and we dont get many of them.

That's very interesting as in theory the cooler the air temp, the more dense the air. This normally equates to a better fuel/air mix and therefore better combustion?

During the summer your mpg should actually drop off and increase during the winter.

Being a pilot when we fly (piston engine) any temp above 15 degrees C (standard ISA), will effect the engines performance, increase the take off roll and slow the rate of climb :eek:

Gavin.

I agree it doesn't make much sense on car engines especially Tdis colder denser air over the intercooler & intake = more power with less trottle, surely!? I think I read somewhere that engines are set to optimum at 10oC (UKs yearly average temperature) then any thing below richen the mixture/retards the timing - If we could drop that closer to zero we would get better economy all year round...But there may be an unacceptable slowing of throttle {Hesitancy} response (speculation on my part).

You also need to take into account that the diesel supplied during the winter months has extra additives added to stop it waxing and these adversely affect the cetane rating which would lessen your mpg.

I am getting my 1.4 TDI later this week and this a very informative thread. I haved used her Cooper the last few days which only has 3000 and i drive it like a go-kart and it must return about 45 mpg. If I ever put the active mpg display up it goes from 30-70mpg in a given moment and even on the motorway seems to always indicate a greater mpg. Food for thought. My outgoing A160CDI A class automatic with 31000 is good for 50mpg, of that I am sure and on a motorway may well be doing 60 plus, yet it has only 75 bhp. Many of the 80bhp diesel returns from the contibuters does not seem so good

In a former life in the Police we were taught to miss gears out going 1-3-5 or 2-4 which is the way I tend to drive and also dependant on conditions, this often returned good mpg on the larger powered vehicles, so I concur that driving style has much to do with it, as does the total mileage of your car.

Finally with high tech running in the optimum mpg as stated should surely be reached within a few thousand miles or at least a figure within 10% of that quoted. In 1998 I had the manual 1.9 TD Seat and that even driven hard was good for 45-55 mpg, in fact one of the best cars i have ever had but looking at relative figures it seems we have hardly moved forward at all. Tell me i am wrong?

Sorry for not rereading the entire post. Yet you don't have the DPF spec-ed do you? As that would be understandable then!

That's very interesting as in theory the cooler the air temp, the more dense the air. This normally equates to a better fuel/air mix and therefore better combustion?

During the summer your mpg should actually drop off and increase during the winter.

Being a pilot when we fly (piston engine) any temp above 15 degrees C (standard ISA), will effect the engines performance, increase the take off roll and slow the rate of climb :eek:

Gavin.

I don't know much about aviation besides the very basic physical laws that govern flight, but wouldn't you be more surprised, really, if the rate of climb didn't slow? I mean, shouldn't denser air mean more lift (more air flowing around the wings than if it was less dense)? I bet in a vacuum you'd experience a very slow rate of climb indeed

When it comes to cars, or rather the internal combustion engines found in them, it's true that cold air is denser and therefore contains more oxygen per unit of volume. That means more fuel can be mixed with the air and you'll get more power. If however the air is warm, it contains less oxygen and more of it needs to be sucked in to the mix, meaning the throttle must be opened further. This reduces resistance (the engine doesn't need to suck as hard, if you'll excuse the pun), which means less power will be wasted doing stuff that won't move the car forward. Effectively, you get better mpg but slightly less power.

If this holds true for turbocharged engines, I don't know. I do remember my SAAB (205 bhp turbocharged) used to get noticeably better mpg during summer, but that was probably due to my switching to 98 RON fuel during the warmer months. I don't know if it would have improved much had I kept running 95 RON.

Should never discount the use of A/C both in summers heat & winter for the Defroster/Defoger... Oh & those of us with Climatronic need to remember to cancel the A/C by pressing the "Econ." button!!!

Hi. The garage were as helpful as they could be. The car diagnostics were checked over but as I feared nothing was amiss. I've just done a round motorway trip of over 600 miles since my first post. I stuck to around 60 mph all the way and still only managed 58 mpg. How the 71 quoted as the test figure is achieved is beyond me. I have done all the usual things to maximise the mpg; no air-con, windows closed, tyres fine, no heavy weights in car, feather-footing on the brakes and throttle....

I'm not sure where to go now. The garage has done its best. I suppose my complaint has to be with Skoda or the Dept. of Transport.

How do they get those figures?

If you're thinking of getting a Fabia 1.4 TDI don't do it because of the fuel economy figures!

Hi. The garage were as helpful as they could be. The car diagnostics were checked over but as I feared nothing was amiss. I've just done a round motorway trip of over 600 miles since my first post. I stuck to around 60 mph all the way and still only managed 58 mpg. How the 71 quoted as the test figure is achieved is beyond me. I have done all the usual things to maximize the mpg; no air-con, windows closed, tyres fine, no heavy weights in car, feather-footing on the brakes and throttle....

I'm not sure where to go now. The garage has done its best. I suppose my complaint has to be with Skoda or the Dept. of Transport.

How do they get those figures?

If you're thinking of getting a Fabia 1.4 TDI don't do it because of the fuel economy figures!

No car is ever really expected to reach these fictitious #'s! Last I looked into it these figures can only be achieved under very controlled conditions on a dyno...:(

I agree with the remark to a degree but with the supposed advances the top of 50's should be the norm surely and circa 50 might be the norm even with a heavy foot. As I have stated I get 50 out of my auto A160 CDI A class Merc and that has to be floored to get going. I hope that your Fabia will improve with age?

No car is ever really expected to reach these fictitious #'s! Last I looked into it these figures can only be achieved under very controlled conditions on a dyno...:(

Found this link for more information on MPG testing:

VCAcarfueldata.org.uk - Fuel Consumption Testing Scheme

It seems that things have changed recently in regards to the testing.

BTW. My wifes Fabia now has 300 miles on the clock and is averaging around 55mpg. This seems fairly reasonable?

No car is ever really expected to reach these fictitious #'s! Last I looked into it these figures can only be achieved under very controlled conditions on a dyno...:(

I have to disagree with this - we are currently showing 46.5mpg overall (for the last 7,000 miles) on our Octavia Scout TDi against an official combined figure of 44.1mpg. Also my Zafira GSi Turbo has given 33.5mpg overall (last 20,000 miles or so) against an official combined figure of something like 28mpg (IIRC).

The 33.5mpg is nowhere near good enough for today's world (I thought it was pretty good back in 2002 when I bought it) and that's why I am changing it, but I can get nearly 40mpg out of it on a long run. So I do sympathise with prudell only getting 58mpg out of the 1.4TDi - that would be very disappointing indeed in a car which had been purchased specifically to be economical. But it's very interesting for me as someone who is actively looking to get a Fabia for that same reason.

BTW, I haven't been able to test out the Octavia when coasting since I last posted (my wife's been using it recently as she has been the one doing the long runs), but in the Zafira it is very clear that when coasting downhill with the drivetrain disengaged the instantaneous mpg figures are at least twice as good as if you do it with the drivetrain engaged.

I ve just looked on What car website and the MPG figures on there are:-

Town - 49.6

Out of Town - 70.6

Average - 61.4

At present I do a 7 mile trip to work evry day, which consists of a mixture of 30-60mph and I can get 55mpg without trying. I have been monitoring this situation for a few weeks and it is getting better every week. I think the acid test will be after the first service 10,000 miles. Mind I have had it over 60 on several occasions.

Also on What car the running costs are 29p a mile, I woonder how this compares to other cars, as after all fuel is just a part of the expense of running a car.

VCAcarfueldata.org.uk - Fuel Consumption Testing Scheme

This link only goes on&on to enforce what I was stating!!!

"is carried out in a laboratory at an ambient temperature of 20°C to 30°C on a rolling road from a cold start, i.e. the engine has not run for several hours."

"Maximum speed is 31 mph (50 km/h), average speed 12 mph (19 km/h) and the distance covered is 2.5 miles (4 km)."

"Maximum speed is 75 mph (120 km/h), average speed is 39 mph (63 km/h) and the distance covered is 4.3 miles (7 km)."

"they cannot be fully representative of real life driving conditions."

"only one production car is tested" "and may therefore produce a better or worse result than another similar vehicle."

"there are infinite variations in driving styles and in road, car and weather conditions, all of which can have a bearing on the results achieved."

"For these reasons the fuel consumption achieved on the road will not necessarily be the same as the official test results."

"The testing is carried out either by independent test organisations, by the manufacturers or importers themselves at their own test facilities."

Need I go on? Looks pretty Black&White to me!!!

Obviously any objective test has to have objective criteria and as result it will not deliver the same results as the use of the car in real life. However despite all the caveats the figures SHOULD be comparable to another different car tested with the same methodology.

So if you can get 40mpg from a car that has an official figure of 45mpg then you should get 50mpg from one that has an official figure of 56.25mpg if your driving style in the two vehicles is roughly consistent.

Personally I have found in the past that the combined figure is a surprisingly close indication to the mpg I can attain when driving a car every day in a relatively carefully fashion, and the extra-urban figure corresponds pretty well to what I can get on a long mainly-motorway journey.

All of the above assumes that the results have not been falsely obtained in any way, and certainly the last point you quote does concern me in this respect. How valid can a test be if it is performed by the manufacturer or importer?

Coasting in neutral other than when gearchanging/stopping is not recommended and in some situations is positively dangerous. It is not the best route to better mpg. I'm sure any driving instructors , police drivers or advanced drivers will endorse this.

Looking ahead, slowing down early and good use of the gears will aid economy and make for a much more comfortable journey.

Jo

As a matter of interest, what are the situations where it is positively dangerous? I certainly wouldn't advocate driving in any way that is unsafe, and this applies to a vast range of things you could do in a car in a vast range of situations. It would probably be unsafe to travel at 40mph on a busy motorway, for example.

All of your suggestions will definitely help with economy (and I practice all of them as a matter of course) but in my experience by nothing like the same degree as coasting where it is possible (and safe) to do so.

I have to disagree with this - we are currently showing 46.5mpg overall (for the last 7,000 miles) on our Octavia Scout TDi against an official combined figure of 44.1mpg. Also my Zafira GSi Turbo has given 33.5mpg overall (last 20,000 miles or so) against an official combined figure of something like 28mpg (IIRC).

What do you mean by showing? based on the maxi-dot??? or do you have a running log book with liters filled vs miles traveled?

Used to do it the long hand way before I had Maxi-Dot. Continued it for some time afterward till I determined it was never more than .1 off my figures in the long run... Found it a great way to know if something was about to go amiss!

BTW, I haven't been able to test out the Octavia when coasting since I last posted (my wife's been using it recently as she has been the one doing the long runs), but in the Zafira it is very clear that when coasting downhill with the drivetrain disengaged the instantaneous mpg figures are at least twice as good as if you do it with the drivetrain engaged.

I tried it going home from work yesterday. Going 90 km/h in a downhill slope in 6th gear the maxidot reported consumption as 0.0 l/100 km. I then put it it neutral, at which point the figure rose to 0.8 l/100 km, with the engine speed falling from ~2000 rpm to idle (around 800?). Back in gear, for a few seconds the consumption was back at 0.0, but then the road started leveling out so the road speed decreased to the point where the car geared down and consumption was for a brief moment around 1 l/100 km, before I canceled the experiment.

As you see, my figures are metric. I'm curious about how your maxidots work; how do they report nil fuel consumption? As 99.9 mpg?