[LINK] Fuel consumption

[Ivan Trundle]

On 27/05/2011, at 5:49 PM, Darryl (Dassa) Lynch wrote:
>  
> |> BTW, there is another strategy that is quite useful. When coming up to
> |> lights or a roundabout, change down and let the car drive the engine.
> |> Fuel consumption drops to zero. That's because an idling engine
> |> requires
> |> fuel.  If you let the inertial of the car provide the energy, you
> |> don't
> |> use fuel for idling.
> |> 
> |> My car can show instantaneous fuel consumption. I've noticed that
> |> changing from 5th to 4th makes no difference to the consumption, but
> |> the
> |> changing down strategy makes a significant difference. It also reduces
> |> wear on the brakes.
> 
> I'd have to say that you do use fuel idling, while ever the engine is
> running you are using fuel, you will use less fuel if you coast in either
> neutral or with the engine off.

Not entirely true with modern cars: fuel flow is reduced to practically zero in engine braking activities - the compression of the engine cylinders is driven by the vehicle wheels, but no fuel enters the combustion chamber.

> My car has an overdrive gear that kicks in at about 98 Km, if I run at 100 I
> get far better fuel economy than if I run at 95. In the order of 3-4 Km/L.
> When the overdrive kicks in the revs drop so I can travel the same distance
> in a shorter time meaning the engine is running for less but at the same
> revs or lower, hence saving fuel.

...but again, it depends on the 'sweet spot' of the engine. Holden performed studies on their Commodore platform to try to determine if a 4-cylinder engine would be more economical, and determined that due to the lack of comparative power, it used more fuel at highway speeds. Some engines are optimised for 100kmh, others at lower and higher speeds.

Generally though, and back to the original question, an engine revving at a higher speed is more likely to consume more fuel, but this does not mean that a very low revving engine will be more 'efficient', since if the engine speed drops below the power band, no saving is made.

iT

--
Ivan Trundle
http://itrundle.com ivan at itrundle.com ph 0418 244 259