19-07-2021, 08:28 PM
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.
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.