Arfa Daily wrote:
"The Natural Philosopher" wrote in message
...
Chris wrote:
I would like to make a graph of car fuel consumption versus speed.
My driving is not smooth enough to gather the raw data myself.
Do you know of any reliable figures, or graphs?
I'm interested in relative values, rather than those for any particular
car.
I think that frictional losses including the rolling resistance are pretty
much linear with speed, but aerodynamic drag is the cube of velocity. Or
it might be the square.

Hence economy driving broadly falls into these categories

To reduce frictional losses - otherwise approximately constant per mile -
lighten the car and pump the tyres up. And fit eco-tyres. You can probably
get 3-4% this way

- keep speeds below 60 mph at which point aero losses start to mount
sharply. This is significant. On cars with consumption meters 50-70
represents about 10% increase in fuel consumption, over that it goes up
massively.

- strip all external junk like roof racks and the like. There is probably
at 70mph a couple of percent to be had here.

- try and drive at a gear and speed where the engine is most efficient.
For a diesel that is at the lowest throttle setting IIRC where the
fuel-air ration is leanest. That possibly means use revs and less welly to
get acceleration and power, not slogging in a low gear at higher throttle
settings. For a petrol it may well be the other way around I am not sure.
This can net you about 5% from typical driving styles.

- reduce acceleration and braking to a minimum by anticipating the road.
Braking represents a net loss of energy that is never recoverable. This is
as great a contributions as speed reduction. Especially in towns.

There's been a lot about this on the radio in recent months, with people
wanting to cut their fuel consumption because of the price of it (now that
oil is back to $85 a barrel, why is petrol still £1.09 at the pumps?) and
the consensus is that the greatest savings to be had are by using gentle
acceleration. Fair enough. Unfortunately, a lot of people seem to have heard
this, and have taken it to heart without any thought. When joining a
motorway, or dual carriageway, your boot should be on the floor, especially
where it is an uphill slip road. You need to get your vehicle up to at least
the speed of traffic on the inside lane, so that you can make the judgement
to slip in behind or in front of any vehicle near you in that lane, without
causing any problem to them.


I m not convinced that slow acceleration is as effective as its made out
to be.

the energy needed to get a car up to speed is the same. Its just whether
the engine is operting more efficiently at high or low power outputs.

Now very high power outputs ARE inefficient, that's without doubt. But
whether 'tickover plus one' is more efficient than - say - half throttle
- is a really moot point.

We know, that at idle, producing no actual acceleration, the powertrain
is necessarily 0% efficient.

WE suspect that, at full power, its less efficient than part power. So
the curve of efficiency is definitely sort of parabolic. Where IS the
most efficient part?


Intelligent guesswork suggests its not close to idle at all. There
frictional loses in the engine will be nearly all the losses.


My gut feel is that a petrol engine does best at about half RPM and half
throttle. A diesel possibly at somewhat higher RPM and somewhat less
throttle