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Escalating fuel costs are now an increasingly major part of our travelling
budgets. Whilst there is little one can do about the price, understanding
the factors that affect fuel consumption can enable usage to be reduced.The
first essential is that the engine be in good mechanical condition, and
tuned correctly. This is particularly true of older petrol-engined vehicles.
Having the engine checked and tuned for economy by a competent mechanic
who has electronic engine testing equipment, and knows how to use it,
is a good start. Diesel engined vehicles are less critical, but compression
must be to the maker’s specifications, fuel pump pressure correctly
set, and the injectors cleaned and checked at least every 100,000 km.
Particularly with petrol units, engine performance is the result of design
compromises between power and economy. It is possible to increase one
at the expense of the other, but not both.
Innumerable devices claim to improve both power and economy but (wearing
my ex- vehicle research engineer hat) I have never come across any after-market
method or device that does so except at the expense of one parameter
or the other.
There is a long-standing myth that the cooler the engine runs the better:
some even remove thermostats to achieve this end. The opposite is closer
to reality.Cool running wastes fuel. Further, engine wear is increased.
An engine should run at least at 80 degrees C (rising to about 85 degrees
C when working hard). Some are designed to run hotter than that.
Here, one modification worth considering is replacing the belt-driven
cooling fan by a thermostatically controlled electric equivalent. This
way the fan runs only when required (primarily when moving slowly in
heavy traffic, and when climbing hills). Further fuel is saved because
the electric fan motor has less energy loss than a V-belt drive. The
vehicle is also quieter.
Given an engine in good condition, the main fuel consumption determinants
are component friction, mass, rolling resistance (of tyres) and air resistance.
Component friction can be reduced by using the lightest multi-grade engine
oil that the vehicle maker recommends, but this is not advisable with
older engines. Some people swear by friction-reducing additives. One
such product (Nulon) was independently proven to enable an engine to
be run for surprisingly long distances (Sydney to Melbourne) without
any oil, and with no measurable harm. But it cannot necessarily be inferred
from that test that such additives materially assist economy.
Below 80 km/h, mass and tyre rolling resistance are the major determinants
of fuel consumption and are inter-related. As a very rough guide a diesel-engined
motorhome with a laden weight of 4-5 tonne typically consumes 12-14 litres/100
km (at 70-80 km/h). Consumption increases by roughly 1 litre/100 km for
every 1000 kg above that base line. Weight should thus be kept to the
absolute minimum.
There’s no point in travelling with full water tanks except where
there’s little water. Get rid of everything in the vehicle (except
essential spares) that has not been used in the past 12 months. Be aware
that books, especially hardbacks, are very heavy.
If self-building, bear weight very much in mind: it’s only too
easy to add thousands of kilos of excess weight. One vehicle that I know
of was intended to be eight tonne. It ended up at close to 15 tonne -
and drank accordingly.
The above figures are likely to be 10% or so ‘better’ for
turbo-engined diesel powered vehicles (especially if intercooled) as
long as they are driven as before: i.e. that the extra power now also
available is not used. Adding an intercooler is likely to improve consumption
by a further 3%-5%.
Above 70-80 km/h increasing air resistance causes fuel consumption to
increase by a minimum of the square of the speed. Thus at 100 km/h the
consumption of the above 4-5 tonne vehicle is likely to be about 35%
higher for the 20% increase in speed.
There is however a minimum speed below which fuel consumption rises once
again. This is because even when driving very slowly, the engine’s
heat losses are considerable. Thus, because the engine runs for longer
to travel a similar distance at very low speeds, the vehicle becomes
less a means of going some-where – and more a low-speed petrol
or diesel-powered heater! The optimum in terms of economy is likely to
be 65-70 km/h but it is unfair to others to drive this slowly on main
routes.
Where possible plan major routes to avoid ploughing into strong headwinds.
On my frequent trips from Broome to the east coast the prevailing 15-20
km/h dry season wind causes the OKA to use 20% more fuel in the east-travelling
direction than when I am west-bound a couple of weeks later – then
it uses 20% less.
Within reasonable limits engine size is irrelevant to consumption. For
any given load, a small engine must work harder than a larger one – and
may well use more fuel than the larger one in doing so. Small-engined
cars typically use less fuel than large engined cars, but this is mostly
because the latter are larger and have greater frontal areas (and hence
air drag), are heavier and are often driven faster.
Again within limits, gearing has only a minor effect. There is marginally
more friction loss at higher engine revs, but unless your vehicle has
an exceptionally (numerically) high differential ratio (as have some
Coasters intended for city use) there will not be any worthwhile fuel
saving by changing the diff ratio. For optimum economy climb hills in
whatever gear enables you to drive at a speed that corresponds to the
peak of the engine’s torque curve. For a typical diesel-engined
vehicle this is typically 2000-2250 rpm.
The corresponding speed in top gear is also likely to be the most economical
for cruising. The torque curve peak can usually be established from the
maker’s engine data.
Tyre rolling resistance is a serious fuel gobbler. Whilst I was well
aware of this previously its extent was brought home practically many
decades ago when my big 1937 Railton broke a piston some 300 km from
home. All that I had available to retrieve the (two tonne) beast was
a 1926 Bull nose Morris Oxford that, at best, developed 18 brake horse
power.
At first, the Morris could only tow the Railton on a flat road in second
gear. I increased the tyre pressures on both vehicles to the tyre maker’s
permitted maximum (about 60 psi): then, whilst acceleration was still
barely perceptible, the Morris pulled the Railton at 50-55 km/h in top
gear, but needed full throttle to do so.
Interestingly, the Morris’s fuel consumption over the 300 km was
almost exactly what the combined consumption would have been for both
cars running at the same speed over the same distance. The Morris returned
about 9 mpg instead of its typical 30 mpg – vividly illustrating
the effect of a small engine working hard.
The above is an extreme example, not least because the then cross-ply
tyres had greater drag than today’s radial plies, but nevertheless
dramatically illustrates the effect of tyre pressure on rolling resistance.
If not already running at maximum pressure, increasing it by 10%-20%
above that recommended for the vehicle’s weight can only assist.
By reducing unintentional throttle movement, a cruise control can marginally
assist consumption on flat roads. However, by attempting to maintain
the set speed, cruise control may gobble fuel in hilly going.
The power required to keep a 4-5 tonne vehicle rolling at 70-80 km/h
on a flat road is about 20–25 kW. If you have 1000 watts or so
of solar available (and some RV's do), consider charging only from solar
whilst travelling because generating that amount of energy via an alternator
requires close to 2000 watts (50% or so is lost via the drive belt).
The fuel saving is likely to exceed 5%.
Collyn Rivers, W8054
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