[AlanHo]

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..................

[Phil №❶]

I'm not sure I can agree with the Institute of Advanced (Age)  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. 

[AlanHo]

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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.

[Ace Demon]

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.

[druggist]

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?

[AlanHo]

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.

[constipated]

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)?

[mjt57]

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..

[Ultralights]

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.

[Phil №❶]

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.

[AlanHo]

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............

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.

[Ultralights]

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.

[Phil №❶]

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.

[Ace Demon]

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.

[Critta Inch]

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.

[Critta Inch]

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. 

[constipated]

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. 

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.

[agentr31]

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. 

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.