The Choice of Fuel diesel, petrol, or LPG?

FOR many, choosing between diesel and petrol causes sleepless nights, but in reality the choice is fairly simple. Diesel and petrol engines behave differently. You simply choose whichever best matches your intended usage.

Compared with diesels, petrol engines are cheaper, smoother, quieter, and friendlier to drive in town. On the down-side, they require more service, are more complex, and use up to 40% more fuel on road and far more off-road.

They have little "turning effort" (torque) at low engine speeds, relying upon the gear-box (or the aptly named torque converter in automatics) to multiply the engine's turning effort. As a result a heavily laden petrol-engined vehicle may be unable to hold top gear on even minor hills or into strong headwinds. This results in more gear changing (whether automatic or manual) and greater fuel consumption.

Liquefied Petroleum Gas (LPG)

Because LPG, a mixture of propane and butane, is currently less than half the cost of diesel and petrol, converting a petrol engine to run on LPG can cut fuel bills. But LPG is only cheap because, the ubiquitous GST apart, it is not otherwise taxed.

If it were, the cost benefit would be lost. It is also quite expensive in many regional centres (e.g. as high as 80 cents/litre in the Northern Territory and Western Australia). Running on LPG is claimed to provide further benefits such as smoother running, less wear and tear, and lower emissions. Reliability is good - as proven by taxi fleets, many of which run on LPG.

Conversion costs $2000.$2500, so you need to cover 35,000.50,000 km before you break even. Fuel consumption is 15.20% greater than with petrol. Torque and power reduce by about 10% but users say they only notice this on hills.

The gas is available in most major Australian towns and cities, and the major routes between, but rarely otherwise. Information on gas availability can be obtained from the various State Energy Centres, and also from automobile institutions such as the NRMA.

Converters usually retain the existing petrol system. They add a dashboard control to switch between gas and petrol. Changing between gas and petrol can be done whilst driving. Do it at least twice each month to keep the mechanism in good working order.

Liquid Petroleum Gas works best in engines designed for unleaded petrol. Its use otherwise may burn valves and valve seats unless they are replaced by equivalents meant for unleaded fuel. Converters claim this problem can be overcome by running on leaded petrol for a day or two each week. But as leaded fuel is being phased out this is only a short-term solution. LPG conversion is a specialised business. Have the work done only by experienced installers.

Diesel Engines

Compared with petrol engines of similar size, diesels have substantially greater torque (pulling power) at lower engine speeds. This enables them to keep pulling at engine speeds and loads that stall petrol engines. Many diesel-engined vehicles will continue to move slowly and surely, even up hills, with one's foot off the accelerator pedal.

For any given road speed, diesel engines usually turn over more slowly, resulting in more "restful" progress. This also aids engine longevity (maybe the driver's too). Confirming the former at least, many diesel-engined vehicles have five-year warranties whereas most petrol-engined vehicles are warranted for three.

Because diesels lack high voltage electrics, they are relatively immune to water - unless it's ingested through the air intake, when it instantly (and often terminally) becomes very bad for them indeed.

Turbo charged Diesel

A turbocharger increases the density of the air drawn into the engine's cylinders. This increases torque and efficiency, and decreases fuel consumption by 5.10%.

Turbo charging can be very effective, in fact one writer suggests that turbochargers and diesel engines are like marriages made in heaven. An increasing number of diesel 4WDs and most large trucks now have turbochargers as original equipment.

A turbocharger can be retrofitted, but many installers charge extra for motorhomes, citing lack of engine accessibility as justification. If building your own vehicle, have the turbo installed at chassis stage.

A minor reduction in compression ratio (usually via a thicker head gasket) may be required. The engine may also benefit from a larger or freer flowing exhaust system.

An inbuilt safety mechanism such as a waste gate (a valve that automatically limits pressure) safeguards engine components.

Intercoolers

A turbocharger heats the air it compresses. This is not desirable. Because hot air is less dense than cold air, less of it is pumped into the cylinders. (Technically-minded readers will recognise this is an example of Charles' Law.)

An intercooler reverses this effect. It is a big strong radiator that cools the turbo-heated air, enabling the engine to accept a denser charge. Adding an intercooler increases power by 10% to 15% and reduces fuel consumption by about half that amount.

Diesel efficiency

A petrol engine's power is regulated by constricting both air and fuel: with the throttle only partially open, air intake is reduced, thus also reducing the effective compression ratio. As shown below, a diesel engine's air intake is unrestricted, enabling the cylinders to take in much the same amount of air at al engine loads, hence the effective compression ratio remains much the same. Fuel consumption of a diesel is thus mainly a function of load.

ABOVE (left/right) The descending piston draws draws in air through the open inlet valve (top left). With both valves closed, the ascending piston compresses the air, which rises in temperature to about 500 degrees. Diesel fuel injected into the cylinder is ignited by the hot air. The burning air/fuel mixture expands exerting a strong downward force on the piston. Picture courtesy CAV/Lucas.

Diesel Reliability

Diesel engines are inherently more reliable than petrol engines. Given adequate compression, and fuel injected in more or less the right amounts at roughly the right times, diesels more or less have to run! Because they are comparatively simple, they are more readily fixed in remote areas. There, and substantially for these reasons, most vehicles are diesels, and diesel fuel is more readily available than petrol in these areas.

The diesel engine's high compression provides invaluable engine braking. This can be enhanced by fitting an exhaust brake. Engine braking saves brake pad wear and, particularly for heavy vehicles, reduces the risk of brake fade.

Late generation diesels use electronic and computer controlled ultra-high-compression injectors. This technology originated with ship and truck engines so reliability is unlikely to be prejudiced.

Diesel Trends

The sulphur content of Australian diesel fuel is being lowered to reduce emissions. This will also extend injector life and engine longevity, but may cause oil seals to deteriorate in older engines.

There's a move in the USA and Europe toward biodegradable diesel fuel. The best-known is made from a methyl ester derivative of soybean oil. The new fuel slashes pollution yet requires no engine modifications. Also in hand are efforts to run diesels on LPG, but conversion is more complex than with petrol engines.

Fuel Consumption

Below 80 km/hr, fuel consumption for a four to five tonne diesel-powered motorhome is typically 10.14 litres/100 km. This increases by about 10% for every 1000 kg thereafter. A turbo-diesel will consume about 10% less. Above 80 km/hr, speed increasingly becomes the major factor.

Fuel costs vary around Australia, depending on government and fuel company greed.

Diesel currently costs about four cents/litre more than petrol in most areas.

Because petrol and diesel vehicles usually have similar size fuel tanks, a diesel vehicle will have a greater range. Seven hundred kilometres is typical. As a result, less supplementary fuel needs to be carried in isolated areas.

The Final Choice?

New diesel-powered vehicles cost more than their petrol equivalents to the extent that one may need to cover 100,000 km before even recovering the cost. Low down torque, more relaxed driving characteristics, reliability, longevity etc., are probably more relevant considerations.

Well-maintained diesels run for well over half a million kilometres, but inadequate maintenance can slash this by half. Diesel engine overhauls are costly, so if buying secondhand have the engine checked over by a specialist before handing over your money.

A petrol engine is cheaper, quieter, more powerful, and more flexible than its current diesel equivalent. These factors cause it to be a generally better proposition for city driving. Further, diesel is not available at all big city service stations.

LPG offers some benefits but the gas is so expensive in the northern and western parts of Australia that it is not an economic proposition for around-Australia travel.

For larger campervans and motorhomes, and any vehicle used extensively in the outback or off-road, I'd pick a (preferably turbo charged) diesel every time.

A diesel engine develops maximum torque over a limited engine speed range. A tachometer (an engine speed indicator) enables this characteristic to be fully exploited, assisting the driver to change gear at the right speeds to optimise torque. This results in smoother, faster, and more economical progress. The engine will also have an easier time.

Back to Book Details...