i30 Owners Club
FUEL ISSUES & ECONOMY => DIESEL => Topic started by: constipated on March 21, 2012, 12:40:32
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I've been doing a bit of an experiment with my diesel i30.
Up until recently I adopted the approach of trying to get into the highest gear possible (that the car would comfortably do) for whatever given speed. This is what had worked best with my previous petrol car to give the best fuel economy. This meant 4th gear for 60km/hr, 5th gear for 70km/hr, and 6th gear for 80km/hr. A lot of the time I'd be keeping the revs about 1600rpm, the lower limit of where the car feels comfortable and is able to pull if needed.
Using this strategy I've been getting about 5.6-5.7 L/100km on the trip computer and in reality 5.8- 5.9L/100km on calculation. Average speed around 38 km/hr.
However, I've been wondering if it would be better to keep revs closer to the lower limit of the flat peak torque curve, say 1900rpm.
For the whole tank, I've been conscious to try to keep revs around 1900. Holding a gear till 2300rpm or so, so that the higher gear falls down to around 1900rpm. So using 4th gear up to 70km/hr sometimes, and sticking to 5th gear for 80km/hr, and only using 6th if 90km/hr or above. Average speed still 37-38km/hr for this tank, but 350km into it, the trip computer is saying 5.2L/100km.
Could it possibly be better to keep revs closer to 1900rpm than 1600rpm for economy?
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constipated,
I guess the proof will be in the pudding. I think I can feel the turbo cut in at around 17-1800 rpm, so it's interesting that your experiment is in fact using the turbo boost a little. My understanding of torque is the engine's ability to pull or move an object. If you stay in that band, I guess the engine operates at maximum efficiency and could use less juice doing it. The interrelationship between torque and power is a very interesting one, perhaps Alan might like to write something about it, having an engineering background. :neutral:
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That's very interesting.
Could it be that you've let it labour too much..?
To me it makes sense that the engine delivers more power when the turbo is helping just a little bit.
As I have the old 5 speed version, my car is doing approx 1800 revs at 80 km/h in 5. gear, but I also shift gear at 2200-2300 revs most times.
I reset my L/100 when my car reached the 150.000 km. For the last 1600 km I have done 5.0 L/100 km :happydance:
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Hi Con,
I would try running just a couple of hundred revs higher and see what it does to the economy.. :winker:
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I always try to get into 5th as fast as I can and shift around the ~2000rpm mark (yeah...I do have a little bit of a lead foot as well) but also one thing to remember is the fuel cut off when you don't apply any throttle so I tend to coast in a lower gear rather than clutch coasting or having the engine attempt to pull the revs up. Seems to work for me since I get 4.3L/100km :happydance: But I also have a tuning box and K&N apollo CAI which could alter fuel efficiency compared to those without one. Also, do remember that fuel economy also depends on the tyre pressure where when I used to use 32 all round, I got around 5.7L/100km but with 38 front and 40 rear I get 4.3L/100km.
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I always try to get into 5th as fast as I can and shift around the ~2000rpm mark (yeah...I do have a little bit of a lead foot as well) but also one thing to remember is the fuel cut off when you don't apply any throttle so I tend to coast in a lower gear rather than clutch coasting or having the engine attempt to pull the revs up. Seems to work for me since I get 4.3L/100km :happydance: But I also have a tuning box and K&N apollo CAI which could alter fuel efficiency compared to those without one. Also, do remember that fuel economy also depends on the tyre pressure where when I used to use 32 all round, I got around 5.7L/100km but with 38 front and 40 rear I get 4.3L/100km.
You' ve left out one piece of important information. What's your average speed per tank?
My tyre pressures are generally 38 psi all round.
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im averaging about 5.1, tyres at 38 PSI, but my average sped commuting is around 50Kph. i dire to keep revs no higher than 2000 rpm, and also having a ultraguage, i try to keep boost below 4PSI.
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Now 730km into the tank, and trip computer consumption for this tank down to 5.1 L/100km. Average speed still 37km/h. True reading would probably be about 5.3 calculated based on previous experience
This is the best economy I've gotten since before my 1000km service when I was driving it this way (ie sticking to lower gears because at that time the car was so rough with low revs). After that I tried to keep revs as low as possible and most of my economy was around 5.6-5.7 on the computer and 5.8-5.9 calculated with same average speed.
I definitely believe keeping revs up higher is the reason.
Anyone else care to give out a try over a tank, particularly if up till now you've been adopting the low revs strategy.
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I've just had a fill and a reset and done a small 135 km trip today. Most of the trip was showing between 3.9 and 4.0 and finished up at 4.1 l/100km when I hit the traffic again near home. Average speed was 53 km/h.
I practice the lowest is best idea and while I've not tried recently I discovered earlier on when I got the hang of the diesel that higher revs only increased the consumption for me. And I mean only marginally, I don't think I've ever hit 4k at all yet.
Having said that there are also some moments when I need to press a little harder to pass or just for fun on a hill.
Interesting experiment though.
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Pip,
Have you calculated manually to verify the results :question:
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Now 730km into the tank, and trip computer consumption for this tank down to 5.1 L/100km. Average speed still 37km/h. True reading would probably be about 5.3 calculated based on previous experience
This is the best economy I've gotten since before my 1000km service when I was driving it this way (ie sticking to lower gears because at that time the car was so rough with low revs). After that I tried to keep revs as low as possible and most of my economy was around 5.6-5.7 on the computer and 5.8-5.9 calculated with same average speed.
I definitely believe keeping revs up higher is the reason.
Anyone else care to give out a try over a tank, particularly if up till now you've been adopting the low revs strategy.
I believe the key is to stay pretty much within the max torque range and don't let the revs drop that much that things start to get rough.
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I believe the key is to stay pretty much within the max torque range and don't let the revs drop that much that things start to get rough.
That is what is interesting. Having the car at 1900rpm in 5th at 80km/hr seems to be more economical than 6th at 1600rpm at 80km/hr.
Even in 6th, the car is not labouring. But it is certainly zippier and more responsive in 5th just at the start of peak torque range.
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Pip,
Have you calculated manually to verify the results :question:
No, only in the sense that I've driven over 1200 on a tank. Even if my trippy figures are in error I'm happy that at least they will be consistent so that if I get 3.9 l/100km now and again (never been less on any reasonable run) but other times it's up to 4.7 l/100km I can put it down to my driving/conditions/fuel etc, and be happy. Even 5.n is good enough and sometimes I get that when I've not had a chance to be economical.
I've never seen 6.n though, even on the way home from picking up new, IIRC it was around 5.9 and I watched that slowly go down over the next few weeks. I remember being excited to see 5.2, I thought I had arrived, but it just kept tumbling. I'm certain it is now going the best it ever has, and maybe will, at just on 50,000 km.
There are a lot of variables but I'd also like to think I ran it in well even though I did use the "baby it" method. I read some very convincing arguments from some on here that the rings will bed better if put under some pressure from more a spirited break-in. I wondered if I'd perhaps failed to keep up with the latest practice. Whether or not my method was lacking the engine runs beautifully and uses zero oil between changes - at least I can't measure it on the dipstick.
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Hey Pip
Amazing low fuel consumption you got there. :goodjob:
I do envy you that.
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Yes Pip,
Regardless of the calculations, no one can argue @ 1200 kms / tank, that's great. :goodjob2:
Whoever built your engine must have been a perfectionist. :D
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:whsaid: That's great.
I've never got that much in mine :)
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Getting back to the original topic... my gearshift points when not in traffic (I don't like to hold anyone up really) are:
1 -> 2 generally at 1500 (just to get a roll really - and if already moving never ever reselect 1st. Start in 2nd on even the smallest decline)
2 -> 3 at 2500 (there's a bit of a step up to 3rd)
3 -> 4 at 2000
4 -> 5 at 1600+ on highway (generally hold 4th around town)
If ever I find myself in unsympathetic traffic I bump these by only 500 rpm or so and just push the accelerator a bit more.
The real measure I use is not how much I press the accelerator nor where I swap gears but rather how much or how little I use the brake. I judge all slow-downs so that I either coast in neutral or in gear if I need to wash speed at the same time. This includes downhill on highways at speed - a practice not accepted by some who believe it unsafe. There are plenty of "advanced" techniques that are unsafe. :confused:
My economy driving will try to waste as little momentum as possible by avoiding slowing on the over-run or worse with the brakes.
My brake pads last forever. My last car did over 200,000 km and I can't remember now whether I did or didn't change the pads... must have but probably only once.
This is the secret, not wasting the energy you have put into the car is more important (to economy) than not putting the energy in to start with.
To sum up: learn to drive without using the brakes except for stopping the last few metres at the lights. Have in mind that every time you press the brake pedal you are using your fuel not to progress your car but instead to wear out the pads.
Apologies for preaching. Some here might not have thought about such things however.
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I do the same thing too Pip, last car, I changed the brake pads @ 160K Kms and they still had some 3-4 mills on them. Driving without using the brakes unfortunately tends to inflame the impatient drivers out there, so I sometimes have to sacrifice economy for peace. We have a steady decline 1.3 kms to the off ramp on the freeway to my home. At the top I deselect cc and no juice all the way to the off ramp. I am often passed in the fast lane by other drivers and they, like me are eventually stopped by a red light, they gain nothing. I still need to apply brakes because the diesel auto just won't slow on its own. :neutral:
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I still need to apply brakes because the diesel auto just won't slow on its own. :neutral:
Not much suck from the manual either I might say. It's a blessing sometimes as you can shut the fuel use with little holdup. But yeah, 3rd offers some resistance... but as you will know if you are into 3rd for the braking then you musta misjudged it. :-[
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Does anyone know what happens on diesel deceleration, is there ANY fuel going to the engine eg idle quantity, or none at all. :question:
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Does anyone know what happens on diesel deceleration, is there ANY fuel going to the engine eg idle quantity, or none at all. :question:
I was once told that the fuel runoff is 100%, ie no fuel is delivered or consumed during decceleration.
I remember testing that theory by resetting my consumption meter while travelling along a motorway (several years ago) and taking my foot off the gas. Can't remember the exact mpg reading but it was in triple figures
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I'm sure any CPU operated engine will use zero fuel on over-run. Only a dumb engine needs to maintain idle fuel.
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Hi Pip
Well, that explains a lot how you can obtain these nice figures.
I do not have the patience to drive like you do, but no doubt, if I did, I would get better fuel economy.
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I judge all slow-downs so that I either coast in neutral or in gear if I need to wash speed at the same time. This includes downhill on highways at speed - a practice not accepted by some who believe it unsafe. There are plenty of "advanced" techniques that are unsafe. :confused:
When you put the car in neutral and coast - the engine uses fuel in order to keep it at idling speed.
When you take your foot off the accelerator with the car in gear - fuel is no longer supplied to the engine - the motion of the car keeps it revolving.
Hence on a long fairly steep downhill slope it is best to leave the car in gear - rather than coast - and no fuel will be used.
If you put the car in neutral you may have to use the brake to limit the speed and the engine will use fuel to keep running.
On less steep declines - the engine will use fuel only when you use the accelerator to maintain speed.
It is a fine balance between when to coast, and when not to, for maximum economy. Any fuel saving would be marginal and I consider safety implications make it not worth coasting at any time.
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I drive semi economically at times then other times i like to give it a few boots so im pumping smoke out the back! hehehe!! love it! Cleans the system hehe. most of the time i change at about 2500RPM. I dont drive for super-economy but i like to get reasonable economy. I normally average about 5.5L/100.
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I judge all slow-downs so that I either coast in neutral or in gear if I need to wash speed at the same time. This includes downhill on highways at speed - a practice not accepted by some who believe it unsafe. There are plenty of "advanced" techniques that are unsafe. :confused:
When you put the car in neutral and coast - the engine uses fuel in order to keep it at idling speed.
Yes
When you take your foot off the accelerator with the car in gear - fuel is no longer supplied to the engine - the motion of the car keeps it revolving.
Yes
Hence on a long fairly steep downhill slope it is best to leave the car in gear - rather than coast - and no fuel will be used.
You've lost me here, to conclude "best" is too simplistic. To save the most fuel you must use the method that uses the least fuel. When you coast in gear you are the wasting fuel you have already spent adding kinetic energy to the "car". The question you must ask is whether the fuel wasted (pushing the engine around) is greater than the fuel you would otherwise use idling it. I would answer a resounding yes!
If you put the car in neutral you may have to use the brake to limit the speed and the engine will use fuel to keep running.
But instead of using the brake you would put it in gear as you already determined is better in this situation because it slows you down and uses zero fuel doing it.
On less steep declines - the engine will use fuel only when you use the accelerator to maintain speed.
Which would be a reason to coast in neutral which will use hardly any fuel at all.
It is a fine balance between when to coast, and when not to, for maximum economy.
Any and all coasting in gear will waste fuel and impinge on the economy. This is not the answer!
Any fuel saving would be marginal...
On the contrary Alan, I would say significant.
..and I consider safety implications make it not worth coasting at any time.
An opinion shared by many it seems. :confused:
To achieve greatest economy you should strive to waste as little as possible. By waste in this context I refer to the side of the economy ledger that refers to any fuel used for other than advancing the car - specifically this would be unnecessary or ill-judged coasting in gear (over-run) and braking. By paying attention to this aspect I get great economy and can still put the foot down occasionally to use up some of my "credit".
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The ability to drive for economy is an unfortunate balance between economy and practicality. I always drive for economy as much as possible, but that has to be tempered with the practicalities of mixing with other road users. If everyone else would get off the road, I'd have a lovely time minimising my fuel usage. For example, I like to drive away in my auto by gently depressing the go pedal and holding that position until the car is in top gear. I estimate the acceleration to be about 1g so it's not excessive. In real life, some other user is behind me and I am forced to drive to the requirements of other users, this is the case more often than not. The other problem is no one in my household likes this method of driving so there is little harmony when we drive, but at least the car's nice to drive. :mrgreen:
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I would say that only those who dont know how to drive well and who are foolish coast down a hill in NEUTRAL. VERY DANGEROUS! You have no control over the cars speed other than brakes which heat up and once heated too much..they FAIL. Also, you have better traction when you have your car in gear. I am horrified hearing people do this just to save a PHANTOM amount of fuel.
Ideling uses more fuel than coasting. Coasting uses a very small amount of fuel to keep the engine going as well as the cars forward motion.
My source, My father who has been both a diesel and petrol mechanic for 40+ years. :)
Please dont idle down hills, leave it in gear. Coasting uses less fuel than idle as idle is richer mixture of fuel than coasting.
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I would say that only those who dont know how to drive well and who are foolish coast down a hill in NEUTRAL. VERY DANGEROUS! You have no control over the cars speed other than brakes which heat up and once heated too much..they FAIL. Also, you have better traction when you have your car in gear. I am horrified hearing people do this just to save a PHANTOM amount of fuel.
Ideling uses more fuel than coasting. Coasting uses a very small amount of fuel to keep the engine going as well as the cars forward motion.
My source, My father who has been both a diesel and petrol mechanic for 40+ years. :)
Please dont idle down hills, leave it in gear. Coasting uses less fuel than idle as idle is richer mixture of fuel than coasting.
I'm not an advocate of coasting in neutral either for the reasons you've mentioned. I'm not convinced that in gear or neutral makes any difference at all if you're travelling down hill because in a diesel, you'll end up applying the brakes at some stage. In a diesel, I suspect idle fuel is consumed in gear or in neutral.
As far as a rich mixture at idle, fuel injection & ecu's look after all that. :neutral:
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When you put the car in neutral and coast - the engine uses fuel in order to keep it at idling speed.
When you take your foot off the accelerator with the car in gear - fuel is no longer supplied to the engine - the motion of the car keeps it revolving.
The engine needs power to turn no matter whether it is on overrun or coasting. When on overrun it takes it by reducing the car's potential and kinetic energy. When idling it takes it by using fuel. Overrun requires more power than idling (because of the rev difference).
If you want to reduce KE & PE because you have to slow down then overrun is perfect. But whatever you do, that KE & PE has to be replaced - by putting fuel into the engine.
On balance, overrun is usually likely to be better but it is not something for nothing.
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One of the problems with this thread is that we are each using a different definition of "Coasting"
For me coasting means that the car engine is disconnected from the wheels either by being in neutral or the clutch depressed. In such a circumstance the velocity of the car will be dependent on gravity, wind resistance, connected drive friction and rolling resistance etc. The engine will use fuel to keep it idling.
Overrun is when the car is in gear, the clutch is engaged, and the driver takes his foot off the accelerator to slow down or prevent the speed increasing on a downward slope. In this condition the motion of the car will be driving the engine and no fuel will be used.
The Institute of Advanced drivers in the UK has this to say on the topic :-
Coasting in itself is not an offence in the UK, furthermore on a modern car it actually uses MORE petrol than keeping the gear engaged with the throttle completely shut. Here's why...
When the car is moving and you take your foot off the accelerator, the engine will not require fuel to keep turning - the motion of the car will do this.
When the car is in neutral, either because your foot's on the clutch or you've taken it out of gear, there has to be enough fuel going into the engine for it to 'idle' at 700-800rpm, whilst also driving the power steering, alternator, aircon pump etc.
If the car is rolling, in gear and you take your foot off the accelerator, all that is taken care of by the momentum of the car, which is why it slows down on anything but a steep downward slope where gravity exceeds the other deceleration forces.
Here endeth the second lesson.................. :whistler:
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I'm not sure I can agree with the Institute of Advanced (Age) :mrgreen: drivers UK this time. In an old car fitted with a carby, coasting or overrunning consumes the same amount of fuel because the idle throttle position is set by an adjustment screw to maintain idle revs, so although the engine is revving faster than idle in overrun mode, idle fuel is still consumed by the motor.
For diesels & fuel injected petrol, I suspect, but can't prove, that the same scenario exists. My logic regarding this is that I can not feel any discernable difference when the fuel is being either cut off or resupplied to the motor. The only way to prove this would be to put a vehicle on a dyno, get it up to speed & decelerate the vehicle with an exhaust gas analyser & see what comes out. If there's no fuel going in there should be just hot air from the exhaust. I wish manufacturers would cut the fuel, as it would help fuel economy and maybe my diesels might actually slow down on overrun. :neutral:
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I understand and agree with your argument about an old car with a carburettor - but the passage I copied clearly states "Modern" cars - this surely means fuel injected cars - be they petrol or diesel.
My understanding is that one of many reasons why fuel injected cars are so much more economical than carburretor cars is that fuel is not injected on the overrun.
What I do know is that with my car - if the cruise is set and the car meets a slow decline - on some occasions the engine will surge on and off slightly - I assume this is because the speed goes just above the set point and the fuel is cut off, the car decelerates below the set point and the fuel is restored to get back to the set point ad infinitum. When I drive the car manually this does not occur because I subconsciously ignore minor speed deviations and "oscillate" the throttle less. It's difficult to put into words - I hope I have explained myself.
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When the car is moving and you take your foot off the accelerator, the engine will not require fuel to keep turning - the motion of the car will do this.
At risk of repetition, the engine does not draw fuel for the period overrun is occurring. Complete agreement. (And I use the same same definition of coasting and overrun as you do.)
The car's "motion" (PE & KE) comes only from fuel energy. Overrun still requires fuel, except the delivery is displaced in time.
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What I don't understand from this thread is that if no fuel is going into the engine why does the tachometer indicate rpm and how does it start again?
Am I being too literal about the no fuel definition?
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What I don't understand from this thread is that if no fuel is going into the engine why does the tachometer indicate rpm and how does it start again?
Am I being too literal about the no fuel definition?
The tachometer is operating from a speed sensor on the engine crankshaft - it is nothing to do with fuel delivery - just the rotational speed of the engine whether it is being rotated by fuel burnt - or by being pushed round by the motion of the car in gear.
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Great to see the fascinating discussion regarding fuel use whilst slowing down has attracted many to this thread.
To keep things on topic perhaps people could also answer regarding their views re my question, better economy with lowest revs possible (1500-1600) vs keeping revs at the bottom of flat peak torque curve (1900)?
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I do not have the patience to drive like you do, but no doubt, if I did, I would get better fuel economy.
I did try to drive like Pip for one or two tank fills. However, try as I may, it always comes out at around 750km to a tank and about 5.7l/100km.
And when I did fill it (twice) to the brim, best I got was about 850 km to the low fuel light.
So, for me, whether I drive it like a pussy or drive it normally (this may mean "spirited" to some), the fuel economy pretty well stays the same.
The car has 6,000 km on the clock now, and they say that it should improve as it loosens up. Thing is, it doesn't feel tight, likes to rev and so on. So, I don't know. Whatever, 5.5l is a hellava lot better than 14.5l that our other vehicle, which the i30 replaced, gets..
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my results from a quick experiment, i parked my car at the top of a long hill, turned car off, and reset the fuel use part of the trip metre, i started the car , released the clutch, did not touch the throttle and let the car coast down the hill, In gear. approx 2 km long. at the bottom, fuel use still had 4 dashes and a few seconds idling at the bottom, registered 0.1l/100km.
i repeated the steps above, but left the car in neutral, clutch out, and engine idling all the way down the hill. when i got to the bottom, fuel use was 0.3l/100km, and it measured it from the start.
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So does that mean in test 1 that the car was placed in 1st gear from a standstill and rolled at low speed in 1st gear to the bottom, or did you change gears to allow the speed to build up but not touch the throttle.
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my results from a quick experiment, i parked my car at the top of a long hill, turned car off, and reset the fuel use part of the trip metre, i started the car , released the clutch, did not touch the throttle and let the car coast down the hill, In gear. approx 2 km long. at the bottom, fuel use still had 4 dashes and a few seconds idling at the bottom, registered 0.1l/100km.
i repeated the steps above, but left the car in neutral, clutch out, and engine idling all the way down the hill. when i got to the bottom, fuel use was 0.3l/100km, and it measured it from the start.
That's a nice experiment and rather verifies that an i30 overrunning in gear uses no fuel............ :goodjob:
The only suggestion I could make is that it would have been better to zero the trip after the car was rolling and checked the reading before you let it idle at the bottom because you measured the fuel used to start the engine, get the car moving and some idling at the end.
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when in gear, i changed up to top gear and got up to about 80 kph before engine breaking and other forces wouldn't let it accelerate any more, i know the difference was only 0.1l/100km, but the big thing i noticed was the trip meter reading ---- when in gear, and 0.2l/100 when idling.
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Sorry, has to be exhaust gas analysis to be sure. With respect, your test proves that the trip computer senses the gear position, The trip computer displays what it's programming tells it to display. :neutral:
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Great to see the fascinating discussion regarding fuel use whilst slowing down has attracted many to this thread.
To keep things on topic perhaps people could also answer regarding their views re my question, better economy with lowest revs possible (1500-1600) vs keeping revs at the bottom of flat peak torque curve (1900)?
Conventionally, higher revs mean more frictional loss inside the engine so theoretically it is best to use the lowest revs that give the power you want. Anecdotally there is suggestion that this is not the best technique with this engine. I don't concern myself with revs when accelerating up to speed and with clear road use 3-4K through the gears; when conditions change, cruise along down to 1300rpm or so.
Incidentally, the issue about the torque curve does not apply when cruising. It is only when the engine is actually producing maximum torque (when you press the pedal to the floor), that it achieves maximum efficiency. This requires a high instaneous fuel flow and intuitively looks bad but is not wasteful in terms of energy usage.
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Modern day common rail diesels and older diesels infact, turn off fuel flow to the engine while over running. The older injection pumps have either a fuel rack (in line pump) or a control sleeve (rotary pump) that when the accelerator lever is moved to idle, the governor returns the control device to the no fuel position until the spring tension of the idle spring begins to overcome the governor and move the control device to an operating position. This has been carried across to modern day diesels and some fuel injected petrols as there is no need for fuel when the engine is overrunning. This is one of the ways manufacturers are improving fuel economy and emissions.
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Also I shift at at about 2k which drops the revs to 1500 where the turbo kicks in. I average about 4.8L per 100km around town. Low 4s highway. I also over run a lot too. Dont use the brakes as often as I should. :p
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Also I shift at at about 2k which drops the revs to 1500 where the turbo kicks in. I average about 4.8L per 100km around town. Low 4s highway. I also over run a lot too. Dont use the brakes as often as I should. :p
Do you know what your average speed per tank roughly is. Don't know if the GD Actives now have trip computer. Helps to put your town and highway figures into perspective.
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I always try to get into 5th as fast as I can and shift around the ~2000rpm mark
Also I shift at at about 2k which drops the revs to 1500 where the turbo kicks in. I average about 4.8L per 100km around town. Low 4s highway. I also over run a lot too. Dont use the brakes as often as I should. :p
FD the turbo is at about 0~1 PSI at those revs, at 2400 its at 15psi, this would be far more economical, because instead of having to WORK to get air into the cylinders i.e using a cylinder on its power stroke to get a cyl on its intake stroke to 'inhale', its being FORCED in at no cost to the engine.
you would literally be better off reving it to 2700 rpm and having less load on things than labouring the engine and running it under its power band.