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Sequential Vapour Injection

The majority of LPG Conversions are carried out using Seqential Vapour Injection kits. This technology has evolved over the last decade and is now very sophisicated and reliable.See Which Kit


How SGI works...


  • A solenoid valve on the LPG storage tank allows liquid to pass to the engine bay via copper or plastic pipe..
  • When the liquid fuel reaches the engine bay it passes through a filter and lock off solenoid valve to the vaporiser.
  • The vaporiser, also known as a reducer or converter, turns the liquid into vapour at the required pressure (about1.2 bar above manifold pressure). In order to maintain this vaporisation heat is required and that is provided by connections to the engine's hot water system.
  • The gas vapour then passes via a filter to the gas injectors. In a SGI system there is one injector per cylinder, fitted close to the petrol injectors, and they are opened and closed in sequence to allow the correct amount of gas to flow.
  • The ECU (Electronic Control Unit) is the heart of a modern Sequential Injection System. It Controls the Gas Injectors, it controls the change over between the fuels, it monitors the fuel level and pressure and it interacts with the petrol ECU to prevent over or under fueling.
  • Legislation that prevents the interference with emission control equipment on modern cars has rendered the old closed loop systems redundant. They used emulation to fool the cars management systems. Modern systems overcome this by allowing the vehicle systems to perform without using emulation.
  • The ECU intercepts each petrol injector signal and converts it to a signal for the gas injectors (because the petrol signal is intercepted the engine stops running on petrol and now runs on LPG). Hence if the engine is running lean the petrol ECU will increase the petrol signal and therefore the gas signal will increase. If the engine is running rich the opposite will happen. The conversion of the signal is not usually linear so the gas ECU contains a map to correct the conversion for different conditions. The key concept is that the petrol ECU is still in control, it is sending its signals to the gas ECU instead of the petrol injectors.
  • The gas ECU monitors water temperature, gas temperature and pressure together with the state of the change over switch in order to achieve a smooth change of fuels when conditions allow. On some systems the change over is staggered i.e. one cylinder is switch and after it has time to settle down the next cylinder is changed and so on.
  • The gas ECU also controls the illumination of a series of LED's on the change over switch. This indicates which fuel is being used and the level of LPG. Any diagnostic errors are also indicated by flashing LED's.
  • The latest developments involves connecting the gas ECU to the vehicles OBD diagnostic socket. This monitors the long term fuel trim and take corrective action if a drift occurs.
  • The future may well see more Liquid Injection Systems being developed. These will work in a similar manner to SGI except the vaporiser will not be required.