I thought I was pretty good at driving economically: I accelerate moderately rather than using maximum power - unless I have to pull out into the only available gap in a long stream of traffic - and I anticipate when I'll need to slow down, lifting off the power early and letting friction and air resistance do the job, rather than staying on the power till the last moment and then braking hard.
I remember that a couple were in then news in 2005 or so, when they drove a diesel Pug 306 (not sure which engine) around the country and achieved an average of 90 mpg. But details of the run were very scarce: were they generally keeping close to speed limits so as to keep up with the general flow of traffic, or were they driving a lot slower, maybe at some optimum speed for the gearing and engine so as to reduce air resistance and run the engine at its most efficient.
I never know with a diesel whether it it more efficient to drive in a higher gear with the engine straining a bit more or a lower gear with less engine torque being needed. There are often conditions where I could choose either 5th or 6th. I tend to use the highest gear that works, but I wonder whether sometimes it would be better to change down and let the engine rev slightly faster but straining less hard.
I am relatively confused by my diesel. It *seems* to be most efficient at 70mph. But things have been muddied by the fact the garage identified a sticking slide pin in a rear brake (or brakes) (despite the fact I specifically asked them about that a year ago, and they said it was 'fine' and its barely done any miles since). So any time I don't use the brakes at all gives better economy than when I do....
But for a normally aspirated diesel the best efficiency is generally light throttle low RPM.
It's more complicated when you strap turbos on. But the manufacturers will generally optimise economy for steady state at the test speed, which is a constant 56mph IIRC. In most IC engines you can trade power for efficiency so it tends to be that any rapid acceleration will increase fuel consumption.
I don't seem to suffer much degradation up to around 85ph or down to below 50mph, but its hard to say because 50mph generally means a twisty road or villages with stop-starts.
And its stop starts that destroy my economy. I try and coast to a halt whenever possible, but any application of brakes is a loss of energy I wont get back.
Firstly I don't give a lot of conscious thought to driving economically and even driven fast I return 66MPG. When the car was new (daughter's company lease then) I am sure I got 80mpg on one occasion. At the time I dealt with the firm I worked for's fuel cards and we had two pug 206s the other chap commuted 80 miles in to work and back and averaged 44mpg and I did 45 miles in and back all but 5 miles motorway and consistently managed over 70.
A long time ago I used to pull things with tractors and would allow them to really bog down before de clutching and it didn't seem to be bad, this fiesta won't allow lugging like that and cannot pull 5th at below
35mph and even then I must change down to accelerate whereas the petrol car I drive will pull away from 20mph in 5th.
Mine is a 1.6 econetic (the greenest ford at the time :-)) and only 89hp which is about 65kW. It's definitely not something you overtake with unless on a long straight road.
Thanks to single lane roadworks, and one incident with a tractor, I know my wife's car gives better economy the slower I go. Down to 32MPH. (I pressed reset on the "average fuel consumption" setting on the trip computer)
I'm not prepared to drive that slowly on a regular basis.
I dont do enough steady runs, but endless cruising at 70 seems to get me about 44mph, and 40 is the best I got in a long 40mph roadwork section. Being stuck in traffic through Cambridge the other day was around 15mpg
And 20mph doesn't work either.
As I said, it is very dependent on a lot of variables like engine size, aerodynamics. how many fixed losses through steering pumps, alternators compressors and fans there are... Dumping a viscous fan for electric normally is a HUGE improvement
And what the engines best efficiency point is. Your 2000rpm in top gear may net you 40mph. Mine nets me over 70...
The one thing I do know is that stop start in a car that doesn't either switch off or store kinetic energy is the very worst consumption of all, which is why car hating councils put in traffic lights, speed bumps and chicanes.
Yes, but not much. A turbo increases power output & reduces efficiency a bit. For max mpg stay below the rpm where the turbo kicks in noticeably.
At 56 in 5th a diesel is typically too high rpm for best efficiency. The gearing is more chosen for acceptable acelleration at speed. Higher gears would improve mpg, but more gears costs. This is partly why the 60s saw trucks with a 12 speed box.
Add engine braking when you don't have all day to coast to stops. Fuel use zeroes during, for an electronically controlled engine.
The petrol pump always reads zero. because it is a diesel
I dunno. I am not interested in the absolute mpg, only the relative mpg for different driving styles.
According to the Interwebby thing, I have a 15 gallon tank, which generally nets me about 400-480 miles, but I never let it run out. Once the engine nearly cut going downhill. That's as close as I wanted to take it.
450 mile range would be around 30mpg, which is my worst case figure for short trips and town traffic.
But I don't fill it up when the engine cuts. In general I put in about
60 litres after about 440 miles, which agrees fairly well with the sort of MPG displayed in my normal driving - about 32-34.
So I think the onboard mpg meter is not too bad.
Its a BIG engine. It will have lots of fixed losses - friction, water pump, alternator, power steering, air con - and it wont normally shift into top gear much below 60mph.
What seems to be best is top gear around 65-80mph. I think that's around
TBH I am not that bothered. I am just glad that at highway speeds a
280bhp twin turbodiesel uses less diesel than my old Freelander did. All about the aero..
And that does indeed make a big difference. All I wanted to say in this thread was that apples are not oranges, and the best mpg speed for one car is not the best mpg speed for another. Back in the day the aerodynamic bricks we all used to drive were probably best at a steady 45mph, which curiously used to be a fairly common speed limit in those days. Not seen it for years..
prior to turbos diesels worked best at ultra low revs and power. Commercial diesels were tuned for the cruise and going up hills they 'rolled coal' . Turbo diesels can run at very high boost pressures and generate a lot more power without smoke. But still its not normal or needed to make them revvers.
I think mine is redlined at 4500, and it never goes over 4000 because its designed not to .
I actually looked at the rev counter today. 1500rpm was around 60mph in
Good. That's how I tend to drive: better in 3rd on a roundabout with foot to the floor rather than having to rev-match down to 2nd gear with less throttle. Shame that my wife's 1.6 turbo-diesel Honda CR-V hasn't got the torque to do that (heavier car, maybe more reliance on turbo boost) and needs one or even two gears lower than my Pug would be happy with. I wonder if it's running out of turbo boost: approach hazard in 6th with no throttle (and so no exhaust beyond the inlet air going out of the exhaust) and so no turbo boost when I call for power. Drop a couple of gears, and the engine burns fuel, produces exhaust and thus spins the turbo and starts accelerating the car.
It's rare that I've had mine above 4000, and only briefly when accelerating from rest or near-rest to pull out of a junction into heavy traffic and haven't had chance to change up.
That's very high geared. My Pug 308 (1.6 HDi) does about 2000 at 60 mph. But that is still a *lot* less than the 1.8 petrol 306 that I was loaned by a garage when my car went for servicing. That did about 4500 rpm in top (6th?) gear, and at that engine speed the car had bugger-all acceleration. If I got behind a lorry at 50 on the motorway and then went to overtake, the car was utterly gutless in getting up to 70, no matter whether I tried 4th, 5th or
6th gear. My diesel Pug manages it moderately well in 6th and with a real kick up the backside in 5th. For me, 40-60 or 50-70 acceleration is far more important than the much-touted 0-60 time ;-)
One question. What does the "panel" think about this symptom of my Pug, which has only developed in the last thousand miles or so: slow down in (for example) 5th gear from 40 to 30. Change to 4th as I level out at 30. Press the accelerator slightly to increase speed a smidgen (eg 25 to 30 mph) and the engine falters and stutters, before starting to accelerate. It's never done this before in the 190,000 miles I've had the car. Otherwise, the engine still has shit-hot acceleration: it's only very gentle around-town acceleration where it happens. The car had been running for some time with an air filter than needed changing (due to a misunderstanding with another garage as to what was being replaced at each service). When that was replaced by a different garage who were investigating the 30 mph stutter, the 30 mph symptom was not fixed, but the general acceleration 40-60 etc on country lanes was dramatically improved. So "raw acceleration" was being a bit starved of air, but curing that hasn't helped with the stutter. It's the
1.6 110 hp engine, running on normal diesel (as opposed to premium) - often supermarket brand but sometimes Jet or Esso fuel. It seems to happen equally often with a cold engine or after driving for a long time when the temp gauge is reading normal.
Is it likely to be a sign of a turbo that needs to be changed? The DPF and cat were changed at about 155,000 miles.
I know *exactly* what happens if the turbo hose comes off and the engine runs normally-aspirated, because a garage once forgot to retighten the jubilee clip on the turbo-output/engine-inlet hose (after doing work which required its removal) and the hose came off, and then the same thing happened again a few thousand miles ago when a grommet perished so the hose would not grip onto the turbo even with the jubilee as tight as I could get it. That was a total lack of acceleration and inability to go above about 40 mph: the stutter lasts a second or so, and is nowhere near as bad as the "no turbo assistance" fault ;-)
Diesels don't suffer the induction losses a petrol does at part throttle. Therefore the gain is minimal and you may get a better efficiency at higher revs. Plus your engine and dual mass flywheel wear will be significantly higher when labouring your engine.
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A stutter in an HDI engine implies no fuel. Do you have access to an OBD tool where you can view pressures and demand as well as throttle position?
If the oil is changed regularly then I see no impending reason why the turbo should be changed.
I'll still be surprised if the decrease in energy required to push the mass through the air more slowly (remember, air drag is proportional to the square of the speed) is really outweighed by a drop in engine efficiency.
Your most economical speed may well be different to my wife's little car
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