LPG Autogas

LPG is colourless, odourless and heavier than air. A chemical is added to give it a smell, so that even a very small leak can be easily detected. LPG burns easily in air and has an energy content similar to petrol. LPG Autogas is stored in a liquid form as it is held under pressure (approx 800kPa) inside a cylindrical container generally placed in the boot of the car. A float arrangement similar in principle to a carburettor float, shuts off the inlet valve when the container is 80% full. The reason the tank cannot be filled past this level is strictly for safety reasons as the liquid expands when heated and needs the free space for expansion. An electric shut-off solenoid valve is fitted as close as possible to the outlet valve and is energised when the ignition switch is turned on. Incorporated into the valve is a safety shut-off that detects excess flow. This is a vital safety device, in case the fuel line is ruptured, as in an accident. The valve will detect that excess flow exists and shut off the valve. From this valve copper or steel tubing is used to deliver the liquid to another solenoid valve fitted in the engine bay. This valve is a standard open/close valve fitted just before the converter. The converter is where the conversion from liquid to gas takes place. The liquid expands some 270 times when it is converted to a gas. The conversion takes place because the high pressure the liquid is held under is reduced as it enters the converter. The converter also controls the amount of gas entering the engine according to demand.

LPG is cooler than petrol and has a denser charge, therefore can provide better volumetric efficiency  in the cylinder and this translates into more power. Also since the octane rating of LPG is higher and it also burns slower, more timing advance is required to gain optimum performance and economy and here lies the problem for duel fuel conversions. Adjusting the advance curve to suit LPG is detrimental to the engine when running on fuel because the engine will detonate with the timing so far advanced. In duel fuel conversions there is not much choice except to leave the standard advance settings so that no damage is caused to the engine when running on fuel. See LPG Tuning in tuning section for details on changes required.

The fuel economy of Autogas-powered vehicles is worse and depends on a range of factors but on average, early model carburetted vehicles will use 20% more LPG than petrol. However if the engine is modified and tuned for gas use only, then the economy may be bettered by up to 5% or so by fine tuning of the ignition advance.  In the 80's this 20% difference was more than offset by the price of autogas being less than half the cost of fuel. Nowadays the difference in price between the two fuels only makes converting to LPG viable if annual kilometres are in excess of 60,000 or more. Below is a link to the Autogas site.

 This now brings me to talk about EFI conversions to LPG. I am afraid I cannot offer much insight into conversions on EFI equipped vehicles but I can make observations of what I think is required. I have read on other sites that EFI vehicles can use up to 30% more fuel which after I gave it some thought, tend to agree with and this greatly depends on the system fitted. In the early days of EFI, the systems that were fitted were nothing more than modified carburettor systems. These systems would not be able to deliver a precise enough control of gas flow to the engine to achieve best fuel economy. In other words the LPG system is not controlled by a computer and therefore lacks the control required for a modern engine. In late model vehicles however, the LPG systems nowadays are very high tech. The introduction of improved performance Autogas injection systems will definitely see the economy increase to only about 10 - 15% more than fuel, this is an educated guess of course on the assumption that technology is much more advanced today than 20 years ago. Just added 9th Jul 09, What a lucky guess that figure was, I just went to the site above and read about the new technologies and that is the figure they quoted. As og 2022 I am sure the figures have improved greatly.

If you are considering converting a new EFI vehicle, I would recommend you ask around the reputable installers and check on what economy figures they are claiming can be achieved. Today's systems must be controlled by an ECU and integrated into the existing fuel and ignition management ECU on the vehicle. The LPG ECU will determine the precise amount of gas needed according to the signals it receives from the engine sensors. The ignition advance and idle control can be controlled by either ECU, depending how well the system is designed and integrated.

In conclusion, most vehicles will run well on Autogas but you must make sure that the system installed is suitable for your engine. Autogas systems and installations adhere to very strict Australian standards governing the safety of such systems and is the most stringent in the world. Any conversions or repairs should only be performed by a licensed installer.
This is very important. Unless the mechanic repairing your LPG system is licensed, they are not permitted to break into any LPG lines. They can only make mixture adjustments for tuning purposes.

I made a calculator to show you how much you can save by converting to LPG, but unfortunately it stopped working after Win XP 32bit. There is one on the LPG site link I believe.