BUYING ADVICE

IS LPG A BETTER BET THAN DIESEL

An around-Australia trip on LPG fuel is possible - on bitumen.

Until diesel climbed above the $2/litre mark in some bush servos there wasn’t a huge amount of interest in the LPG alternative, but now, for those diesel-savvy readers who don’t know much about LPG, here’s a brief history. 

The writing’s been on the dunny wall for years: increased demand for fossil fuel will continue to push up prices.

Australia produces shed loads of liquefied petroleum gas (LPG), which we export very cheaply and used to sell to motorists relatively cheaply – around half the price of petrol and as little as a third the price of diesel. These proportions changed in 2011, when the Federal Government introduced an LPG excise that added 2.5 cents a litre a year until it was capped in 2016 at 13.2 cents per litre.

That price hike spelled the beginning of the end for LPG as a bush-touring fuel. The final nail in the coffin for LPG in Australia was new engine development that wasn't LPG-friendly.

 

What is LPG

LPG is a mixture of propane and butane, although the butane proportion is rising steadily as propane demand approaches supply. These gases are heavier than air and become liquid under relatively low pressure.

There is no possibility of Australia running out of LPG in the foreseeable future and, with nearly 4000 retail outlets for LPG, an around-Australia trip on gas fuel was easily done. However, the number of LPG outlets is shrinking on a weekly basis and there's no LPG on any gravel road we know of.

To enjoy the cost savings of LPG you had to pay for an LPG kit. It’s not possible to fit quality components and do a professional LPG installation job for less than about $2500 and the most efficient sequential vapour injection designs were up around the $4000 mark.

‘Payback time’ – the period of use necessary to justify the LPG investment - varied greatly, but the LPG Association website had a payback calculator that did the job accurately.

The much-discussed LPG-fitment-rebate scheme - $1000 cash-back for buying an LPG-powered vehicle or $2000 for converting a petrol machine - was of no value to commercial vehicle owners and buyers, because it didn’t apply to business vehicles or those bought on novated leases.

 

Engine Compatibility

LPG burns more cleanly than petrol, with slightly less output per litre, but when that efficiency loss is measured against its lower purchase price, the attraction of LPG was obvious.

The negative aspects of LPG on engine life are the fuel's dry vapour and hotter burning nature. This hotter combustion can cause valve seat recession problems in engines that aren’t designed for it.

However, many petrol 4WD vehicle engines were warranted for use with LPG. For example, Ford produced the Territory with a dedicated LPG-only E-Gas engine.

Holden had a warranted dual-fuel conversion for the Alloytec V6 that powered the first-model Colorado ute. It was a modern sequential vapour injection system that cost $3900 and made very little difference to petrol power and torque figures.

 

From fumigation to gas then liquid injection

The original LPG system design ‘gassed’ the incoming air charge in the inlet manifold, in the same way a carburettor did. The petrol to gas changeover was simple, with one fuel mixture replacing the other in the manifold.

When electronic petrol injection took over from carburettors petrol engines gave much better fuel consumption. However, old-style LPG fumigation systems couldn’t ‘plug into’ the engine’s computerised electronic control and there was a marked drop in fuel efficiency when running on LPG.

Making this type of dual-fuel engine operate well on petrol and LPG was tricky, which is why Ford and Mitsubishi opted for dedicated-LPG Falcons and Magnas.

The latest sequential vapour injection LPG systems were ‘plug and play’ types that use the engine’s ECU to control their LPG injectors, so there was very little difference when running on petrol or LPG.

Sequential vapour injection was expensive, but was by far the best route for a dual-fuel system. The latest turbocharged petrol engines with direct-injection fuel systems can also be converted to LPG operation, but these LPDI (liquid phase direct injection) systems are even more expensive. Without LPG cost incentives the take-up of LPDI has been tiny in Australia.

 

LPG vs Diesel

Modern light commercial diesels are more thermally efficient than petrol or LPG engines, thanks to compression ignition, turbo-charging, intercooling and high-pressure common-rail injection.

The modern diesel engine uses less fuel than a similar-output fuel injected petrol engine under heavy engine load conditions, such as when towing heavy trailers or driving in soft sand.

However that economy advantage is distorted by pump prices that can see petrol some 20 percent cheaper in metro areas and LPG around three-quarters the price of diesel.

But even if a LPG conversion seems tempting, there's a lack of suitable petrol engines in 4WDs these days. Nearly all utes are diesel-only and very few 4WD wagons offer petrol power.

From 2020 onwards the shift from straight petrol and diesel vehicles will be into hybrids, where the potential LPG tank space on their chassis will be devoted to batteries. LPG may well be finished as a 4WD fuel Down Under.

 

 

 

 


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