Contrary to popular belief, finding out how much fuel a car engine uses up while the car is idling is not a difficult task. It is even easier if you have an OBDII scanner, which is a scanner than can actually be synced with the car’s engine in order to gain valuable information about its reliability, fuel consumption and other important components.
Normally, in the case of a gasoline engine, it is likely to consume 0.08 to 0.1 gallons of fuel per hour if the engine is completely warmed up and runs at about 650 to 700 rpm which would then mean that if the engine is 2L, it is more likely to consume about 0.16 to 0.2 gallons per hour and this is closely propionate to the engine’s idle speed, 000rpm idle burns ~50% more fuel than 700rpm idle.
If the gas pedal is held down, the engine will basically be revving at about 3000rpm and the fuel consumed would be about 3 to 4 times more than it was while idling normally.
However, the consumption is also likely to be different for numerous kinds of engines for example, for a Scion FR-S (Subaru FA20 engine) the engine is likely to consume 0.16-0.17 gallons per hour at 2 litre displacement which comes out to be a little more than 0.08 gallons per hour per litre. On the other hand, a Honda Pilot (Honda J35 engine, standard Honda V6) would consume about 0.3 gallons per hour while idling.
The kind of fuel is also likely to have an impact on how much the engine will consume. Diesel engines for example, are likely to burn less fuel since there are no complications posed by the throttle and it might be about 0.06 gallons per her per litre displacement.
The fuel consumed also depends on the size of the engine and the speed at which the idle is set at, for example a truck or larger SUV would have a larger engine and would idle at lower speeds. An example could be of a Corvette which idles at around 500rpm instead of 600 (the usual for a V6 SUV) or 700 (the usual for a small economy car). The engines can also have cylinder deactivation. This conserves energy (at idle, the vast majority of power is used to move past the restrictions of the throttle plate.
On the other hand, a motorcycle can idle between 1200 to 2000 rpm and will therefore burn more fuel and if the fuel somehow fails to ignite, it will burn even more. However, sometimes the engine happens to be performance oriented and that is when it burns less fuel while idling.
Over the past years, numerous engines like the Honda Civic VX could idle at 500 rpm and burn less fuel, but this would lead to poorer quality idle. The engine would not run smoothly and would stumble when throttled. The best way to get accurate answers about fuel consumption at various speeds is by getting an OBDII scanner which would give exact figures.