"The Natural Philosopher" wrote in message
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Arfa Daily wrote:
"The Natural Philosopher" wrote in message
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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.

You get better MPG in higher gears which gives less acceleration.

You also get better mpg by using just enough gas to give the acceleration
needed, not the maximum acceleration you can get.
You need to be able to drive to know what acceleration is needed rather than
just putting your foot down and this is where most come unstuck.
I seldom need more than a quarter throttle to match motorway speeds on any
of the slip roads I use. I have had to use the hard shoulder when some prat
has decided to stop because he can't get into a gap. Using the hard shoulder
is the correct way of doing it of course.