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What is the difference between Natural Gas and Liquified Petroleum Gas?

Liquified Petroleum Gas (LPG) fuel is a flammable hydrocarbon gas fuel including propane, butane and mixtures of these gases. It is produced from the processing of natural gas (the gas used in many homes for heating) and oil refinement. In other countries, Natural Gas (CNG) is different from LPG. It is predominantly methane and requires very high pressures to compress it into a liquid form. CNG fuel can not be used in an LPG-equipped vehicle without significant modifications which can only be made by a licensed specialist and cost many thousands of dollars to perform.

Types of LPG Systems

There are four types of conversion systems for LPG to be used in motor vehicles:

1. Converter and mixer systems - many early systems used this technology, and most "dual fuel" aftermarket conversions. The liquid fuel is converted into vapour and then mixed with air before going into the intake manifold. 

2. Vapour phase injection (VPI) - use a converter and mixer system, but the gas exits the converter under pressure and is injected into the intake manifold. 

3. Liquid phase injection (LPi) - these systems inject liquid directly into the intake manifold without the use of a converter, where it vaporises. The fuel vaporising in the intake manifold cools and increases the density of the intake air, substantially increasing power output, improving fuel economy, whilst also resulting in lower emissions.

4. Liquid phase direct injection (LPDI) - in these systems, the LPG instantly vaporises which cools the combustion chamber fuel-air mixture, resulting in further improvements in performance, economy, and emissions

Benefits of LPG Systems 

Ford vehicles that run on LPG or LPi may have the following benefits when compared to their Unleaded Petrol equivalents, such as:

  • Significantly reduced running costs (LPG costs about 50% of Regular Unleaded Petrol, but note that it does consume up to 20% more fuel in volumetric terms to cover the same distance);
  • More power and a different delivery curve where that power is available at some more useful RPM ranges (such as overtaking);
  • LPG has a higher Research Octane Number (RON) (anywhere between 100 and 110) than Regular Unleaded (RON of 91).
  • Reduced emissions and exhaust pollution.