[LINK] Fuel consumption

[David Boxall]

On 27/05/2011 8:07 PM, Ivan Trundle wrote:
>
> 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.
>...

Evidently, not a simple question. I'd add that it depends on the type of 
transmission. Old-fashioned hydraulic automatics were notorious 
fuel-guzzlers. Modern centre-over or dual-clutch electronics are better, 
but even the best only emulate a mediocre driver with poor situational 
awareness. With an adept, alert driver, a good manual can consume less 
fuel than any automatic.

I'd further add that safety should override fuel consumption. Driving 
for extremes can leave the engine with too little flexibility in reserve 
for the unexpected. Saving fuel is little consolation to the dead and 
injured.

I do a lot of driving on poor, unsealed roads. In automatics, the lack 
of control over torque to the wheels feels extremely unsafe to me. It's 
necessary to drive with the engine in the best part of the power band 
and to stay alert to conditions (ahead, to the sides and sometimes to 
the rear).

-- 
David Boxall                         | "Cheer up" they said.
                                     | "Things could be worse."
http://david.boxall.id.au            | So I cheered up and,
                                     | Sure enough, things got worse.
                                     |              --Murphy's musing