not 7 specific - fuelling/freewheeling

[JonE]

Can anyone give definitive advice as to which uses more fuel in a modern EFI engine; freewheeling out of gear at tickover, or engine braking with no foot on accelerator (where the revs are higher than tickover, if going downhill)?
I'm trying to determine if there are other factors other than revs.
And whether the answer is different for older carburettored engine?

[AustinWood]

All modern car engines have over-run cut-off. If the car is in gear & engine turning then the fuel is cut off completely if the accelerator is released. Coasting on tick-over means that the engine is being fueled the same as if the car was stationary. Some carburetors had over-run cutoff from early 1990s.

[Chris KC]

Since the advent of ECU's few people know what an engine will be doing at any given moment, and then only by cross-referencing a list of parameters against software. Abandon hope all ye who enter here.

[JonE]

ha! yes, indeed. Thankyou Jim though. A friend uses freewheeling and I thought it couldn't improve his mpg, as well as being dangerous. He does seem to get mpg improvement but I guess the same would be so if he just made sure he wasn't on the accelerator!

[Tony Griffiths]

It's an old chestnut that coasting is illegal - it is not,many cars have been fitted with this system and, when coasting, the dynamics of a car are not compromised in any way. My current new car, when in "Eco" mode, coasts automatically when the accelerator is released. In gear, on the overrun, the computer mpg readout goes to zero, when coasting it shows 185 m.p.g. Of course, some older cars allowed you to switch off the engine when coasting and, if vacuum-assisted brakes were involved, that might have been entertaining. One of my schoolmasters had an early Mini - his going-home coast, engine off, was from Bents Green to almost the bottom of Ecclesall Road, about 2.3 miles. My best was in Switzerland; 12.5 miles giving a 150 mpg readout - though by the time I arrived home 800 miles later, this had dropped to 64 mpg.

[andrew34ruby]

The Highway Code used to strongly say you should not coast out of gear, but this has now been changed. It says you should be aware of various things, but no longer forbids coasting.

I felt that coasting in a modern car would not save anything as the fuel supply would be cut off anyway, but my 7 year old modern does use less petrol if coasting. Hills always reduce the mpg, but coasting out of gear does help to conterbalance this. Of course I'm just going by the digital readout on the dash. I have compared that to a true 'brim to brim' mpg, and measured the distance with motorway distance posts. When new the readout was about 7 mpg low compared to the true figure. A recent check was more like 4 or 5 mpg low.

With care I can coax 52 mpg out of the 1.4 petrol over a 50 mile distance.

[Tony Griffiths]

I agree, coasting does work. Modern cars are also very heavy and fitted, generally, with low rolling resistance tyres. I know that my present car coasts far more readily than previous ones; on a test stretch that I use, I now don't have to engaged drive to cover the flat part before the next decent. I find my Chummy reluctant to coast - but the heavier Ruby quite willing.

[John Cornforth]

Hi JonE

To give one example, my modern has a 1.4 litre VW TSi engine. Fuel on the overrun is cut off completely above 1400 RPM, and the instantaneous mpg readout maxes out at 99.9 but would read higher if it could.  So during the descent of a hill it uses a little bit less fuel than if one were to freewheel with the engine ticking over. If at the bottom of the hill there is a red light/slower car/speed limit etc then staying in gear is an mpg winner.

On the other hand, if the downhill section is immediately followed by a clear uphill one, the (scarily ?) greater speed achieved at the bottom of the hill by coasting without engine braking would reduce fuel usage at the beginning of the uphill bit, and this might be enough to tip the overall balance the other way.

Another economy technique is to alternate say 10 seconds of gentle acceleration with lift off deceleration, and allow your speed to see-saw up and down a little.  The idea is that the engine is more efficient when being used to deliver a moderate amount of power part of the time, rather than a small amount all the time.  Cylinder de-activation is the modern equivalent.

The trouble with all these techniques is that they aren't always compatible with road safety, and traffic density is often a lot more than those brave souls encountered on the "Mobil Economy Runs" of the Fifties and Sixties.

[Bob Culver]

Several older cars such as Rovers were fitted with freewheel. Before power brakes drivers reported them as very disconcerting to inexperienced as the car seemed to accelerate when back off.
As above engines are theoretically most efficent flat out at mediaum revs. Full throttle enrichment cuts in so generally best at not quite full throttle. In freak economy runs extreme mpg was acheived by nobbling enrichment devices and accelerating hard then coasting with the engine off.
High overrun revs are undesirable with Sevens. The huge net upward inertia load on the crank is not countered by any strongly firing piston. 
Curiously old cars with weak idle pop on the overrun??

[Steve kay]

Sadly I understand little of the modern techno speak. The Saab 96 owned many years ago had free wheel. Living in Scotland, when venturing into the more mountainous parts, the fixed was always engaged particularly if four up. I have often thought that fuel consumption is only one effect of free wheeling. A well known Hereford 2CV wizard who has a number of Seven owning customers says he notes a significantly lower brake wear in drivers used to old cars, modern drivers wear out pads and shoes distinctly more.