does coasting save petrol

tbourner

I think it's worth mentioning that coasting/overrun is going to net you limited gains whichever way you think it will go.

Easily the biggest thing is realising that every time you touch the brake pedal, you're turning on a fuel heater in each wheel arch that is run straight from your fuel tank!

Tobster86

I concur, any gains either way are likely to be insignificant compared to other factors that would govern fuel usage. The error in the fact that no one will drive identically, in non-identical weather, traffic and other conditions between fuel tanks is probably greater than any savings from proposed overrun or coasting.

The premise of the anti-coasting argument is that there is just no benefit to it; it's plain and simply bad driving. It is driving in a state of reduced control and potentially increased brake wear, and in the case of most fuel injection systems, increased fuel consumption.

DCodd

There is another factor to remember. Most cars are not designed to coast in neutral be they manual or automatic. It has taken Jaguar years to design a prototype gearbox that is designed not to be damaged by coasting.

mikey72

Coasting v overrun

I bought an Elm327, and paired it to an old Android phone with Torque. So reading live data via the ODB2 interface.

It's on a drive by wire, 1.8 petrol vauxhall engine. Urban, 30 to 50 limit, but good runs between the lights, islands etc, so ideal to coast, or just lift off.

Level though, so the car will decelerate, and eventually stop.



Torque is set to show MAF, throttle position, and MPG.

On overrun,

MAF and throttle show about 5.

MPG varies between 60 and 40, dropping with speed.



Coasting,

MAF is 3, throttle drops to 4.

MPG is between 175, dropping to about 150, before eventually reading 0 when the car stops.

So

on overrun, the MAF is reading higher, as the engine is pulling in more air, and the rpm is higher.

the throttle is slightly more open, either held there by the ecu, or simply pulled open by the greater air flow.

Fuel cut off never happens, shown as the Elm calculates mpg based on injector pulses, and duty cycle, and it always registers a reading of 40 to 60mpg

So it injects enough fuel to keep up with the higher airflow at the higher rpm.

Coasting, the engine uses just enough fuel to keep the engine idling, with the minimum airflow, which is less then overrun



Now I need to find a steep hill, that the car will accelerate down on overrun, and see if cut off is achieved.