Explaining the different types of conversions

Single Point

The ‘single point’ conversion is so called because it has one single point of gas entry into the vehicles engine. The basic kit comprises of an electronic means of switching the gas on with a switch and fuel gauge inside the vehicle and a reducer, sometimes called a vapouriser and a mixer. The mixer is installed in to the inlet tract as close to the throttle as possible. It is designed to draw varying amounts of gas vapour into the inlet stream relative to the air flow passing through it into the engine. The mixer is linked to the reducer using rubber fuel hose.

Single Point Kit

Single Point Kit Made By Zavoli

The reducer is mounted inside the engine compartment; this part converts the liquid from the tank into gas vapour in a controlled manner relative to gas demand. Since the boiling point of LPG is approximately – 42ºc, the LPG changes from a liquid state to a gas state inside the reducer. In doing so, the LPG takes energy from its soundings, lowering the temperature of the reducer. To prevent the reducer freezing, a small modification to the engine’s cooling system is made, to allow warm engine coolant to pass through the reducer.

The basic electronic system switches the petrol on or off, along with safety solenoids that are fitted on the tank and in the engine bay. More advanced systems utilise microprocessors to improve engine fuelling accuracy. Fuel enrichment under high demand or finer emission control can be achieved by referencing engine management system sensors, allowing electromechanical adjustment of devices fitted between the reducer and mixer, in order to modify the flow of gas to the engine. Microprocessors can also be used to emulate the petrol injectors and lambda sensor signals to stop ‘Check Engine lights’ and adverse rough running when returning the vehicle back to petrol.

There are a number of drawbacks to these systems, among which power loss and ‘Blow-backs’ are common. Since the mixer acts as a restriction in the induction system, the maximum air-flow can be reduced, reducing the maximum available engine horse power for both petrol and gas. Blow-backs are caused when the premixed air and fuel in the inlet manifold ignites before it has reached the combustion chamber, this creates a powerful shock wave that can cause severe damage to inlet manifolds, air filter housing and MAF sensors.

Multipoint vapour injection

Multipoint systems differ from single point systems as in they introduce the gas into each cylinder individually. These kits comprise of an electronic switch with a gauge, an LPG Electronic Control Unit (ECU), reducer, injectors and nozzles.

The reducer on a multi-point injection system works slightly different to that on single point system. Similarly, it connects to the engine cooling system to prevent freezing, but it also serves to maintain and regulate gas pressure. Additionally, manifold pressure is used to further modify the gas output pressure, ensuring that the difference between the gas pressure and manifold pressure remains constant.

The injector nozzles are installed into the inlet manifold, one into each cylinder port of the engine. These are fitted as close to the inlet valve as possible and, in some cases, straws are inserted into the manifold to better achieve the optimum distance. The injectors are attached to the nozzles via small bore flexible fuel hoses or attached directly to the nozzles themselves, depending on the type of kit used. The injectors are electronically operated by the LPG ECU.

Earlier LPG injectors incorporated a stepper motor design, which allows the flow of gas to be controlled in a relatively linear fashion, much like a tap, but the injection speed and accuracy of this method is limited, mainly due to the time lags associated with opening and closing these type of injectors.

Later LPG injectors are designed much like petrol injectors, with electromagnetic coils acting as solenoids, allowing the injectors to be simply open or closed. The fast-acting injector solenoids are switched on by the LPG ECU, for a precisely measured duration, then an internal spring-loaded mechanism ensures the injector quickly snaps closed at the end of the injection duration, allowing an exact quantity of LPG to be injected at exactly the right moment.

The length of time the injectors are open varies typically from 2 to 25 milliseconds (2 to 25 thousandths of a second), depending on fuel demand, and they are opened at the same frequency as the petrol injectors, i.e. once every second revolution of the engine. The LPG ECU determines the correct injector duration, and therefore the quantity of LPG injected, by monitoring petrol injection durations and, depending on the type of LPG ECU used, signals from a number of engine management sensors.

The LPG ECU contains a fuel map, allowing adjustments to be made in accordance with engine speed and load, and a connection to the vehicle’s oxygen sensor (or ‘Lambda’ sensor) allows the ECU to control the fuel in a ‘closed loop’ mode of operation. This enables the  fuel mixture to be monitored and controlled, maintaining correct catalytic convertor function, reducing fuel consumption and lowering exhaust emissions.

Sequential fuel injection

Sequential injection is the latest and most sophisticated generation of Multipoint systems. The kits are designed to follow the petrol injection system exactly and allow the engine management system to control the gas system.

The kits comprise of an electronic switch with gauge, an LPG Electronic Control Unit (ECU), reducer injectors and sensors for the engine coolant temperature, LPG temperature and LPG pressure.

Multi-point Sequential Injection Kit

Alisei Sequential Injection Made By Zavoli

These kits are fitted in much the same way as the earlier multipoint systems but, because they provide greater injection timing precision, the later, faster, solenoid type injectors are always used. The quantity of LPG injected is governed by injection duration, much like similar non-sequential multipoint systems, but the timing of each injection is synchronised with the vehicle’s petrol injection pulses, allowing the LPG to be injected independently into each cylinder at precisely the right moment.

Because the gas vapour pressure and temperature alter under normal operating conditions, the kit uses sensors that monitor the changes and adjust the opening times accordingly. If the petrol ECU switches off an injector because a problem is detected, the LPG ECU immediately senses that the injector has stopped operating and responds by ceasing operation of the relevant LPG injector.

Since the LPG ECU closely monitors the petrol injection signals, mirroring the adjustments the vehicle’s engine management system makes, the engine management system remains largely in control of fuelling and emissions and is unaffected by the modifications, ensuring no engine management lights or fault codes are activated.

Direct Injection LPG conversion kits

Direct injection engines have fuel injectors mounted inside the combustion chamber, which creates a new set of challenges for LPG system designers. These injectors are subjected to much greater temperatures and rely on the cooling effect of the petrol flowing through them to prevent the combustion heat from damaging the tips. Fuel pressure is also much higher (and variable) and injection durations are generally shorter and more complex, making LPG system interfacing much more difficult.

To further complicate matters, each direct injection engine management system is a little different, making it extremely difficult to develop a universal LPG system. For this reason, LPG systems for direct injection petrol engines are usually vehicle-specific.

Liquid Injection

A liquid injection system works in a similar way to a petrol-injection system; liquefied fuel is pumped out of the tank to a fuel pressure regulator and to the injectors. The system comprises an LPG fuel tank with an integrated pump, a fuel pressure regulator with a pressure sensor and integral safety valve, liquid gas injectors and an LPG ECU.

Like many petrol injection system, the fuel is pumped from the tank to the regulator with the excess fuel returning back to the tank, creating a steady, pressurised supply of fuel to the injectors. When the injectors open, liquid gas enters the inlet manifold, where it quickly expands and turns to vapour. This has a cooling affect on the charge entering the engine which is beneficial to the combustion process, allowing a more condensed air-fuel mix

Valve-Saver Kits

Flashlube Kit

Flashlube Kit

When running vehicles on LPG, the low carbon content effects how the valve wears in the cylinder head. Vehicles that have hydraulic lifters and self adjusting valve clearances, automatically adjust for this and can run for over 100,000 miles without any issues. Vehicles that can’t self-adjust should be fitted with an additional valve-saver kit to coat the valves and reduce wear.

Flashlube is a delivery system designed to introduce a concentrated lead-replacement additive, formulated to minimise Valve Seat Recession. There are two types of kit available; the first uses a bottle with a micro filter, tap and drip-glass. This is mounted under the bonnet and connected to the engine with a brass jet. The vacuum in the inlet manifold pulls the oil through the system and the flow rate is adjusted via a small tap, using the drip-glass to count and regulate the time between drops.

Electronic Flashlube Kit

Electronic Flashlube Kit

For forced induction engines and high capacity engines there is an electronic system available that uses a bottle and pump with a small electronic control unit that connects to the LPG injector wiring. Depending on inlet manifold design and requirements, the fluid may be introduced via a central point, or multiple points to allow for more even distribution to each cylinder.