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LPG Reducers - aka Vaporisers

LPG Gas Conversion

The reducer / vaporiser / pressure regulator, normally called the vaporiser has two major functions.

The first is to transform the liquid LPG to LPG vapour. For this heat is necessary which is normally provided by the coolant from the engine. If this is not possible, the liquid LPG has to be pre-heated. This can be done with an exhaust heat exchanger where the necessary heat is taken from the hot exhaust gases.

The second is to regulate the amount of LPG that goes to the engine. Just like a carburettor it tries to I keep the mixture of LPG and air at the optimum mixture. The vaporiser must also deliver a stable LPG pressure to the engine.

The main problem is the changing of the pressure in the tank. The pressure depends on the outside temperature and the amount of LPG left in the tank (4bar up to 27bar). Manufactures of vaporisers solved this problem by lowering the pressure in several stages.

The vaporisers can be classified by their construction: two-stage vaporisers, two-stage tandem vaporisers, three-stage vaporisers.

The two stage vaporisers are the most common. They lower the tank pressure in two stages from approx. 10 bar to operating pressure. The advantages are: they are small and can be cheap. The disadvantages are: they have a limited power range (0.5L to 1.5L or 1.0L to 2.5L or 2.0L to 4.0L engine size).

The two-stage tandem vaporisers work in the same way as the normal two stage vaporisers. The main difference is that the second stage is divided into a small accurate stage which operates when only small quantities of LPG are required by the engine, a large stage which is brought into operation as soon as the required amount of LPG exceeds the capacity of the small stage. This large stage is capable of dosing large quantities of LPG. (Advantage: Efficient delivery of LPG for all engine capacities from very small to large.(0.5L up to 7L). Disadvantages: Large size, expensive).

LPG flow shut off.

All vaporisers shut off the LPG flow as soon as the engine stops. This can be done in two ways:

  • By vacuum control
  • By electronic control

In the vacuum controlled version, the vacuum in the inlet manifold of the running motor is used to hold the shut-off valve open. As soon as the engine starts the vacuum in the inlet manifold opens the shut-off valve in the vaporiser and the vaporiser delivers LPG to the engine. When the engine stops the vacuum in the inlet manifold drops and the shut-off valve stops the LPG flow to the engine. To be able to start using LPG, a solenoid is installed in the vaporiser which allows the shutoff valve to be opened manually. This makes it possible to pre-inject LPG. It is also possible for the driver to enrich the mixture in the inlet manifold to assist with a cold start. A button for this purpose is usually installed in the cabin to operate the vaporiser solenoid.

With an electronically controlled vaporiser the shut-off valve is a solenoid valve which is controlled by an induction relay. As soon as the ignition system is working (i.e. sparks are detected), the solenoid valve is opened allowing the LPG to flow. To make starting easier this solenoid valve is also opened for a few seconds when the ignition is switched on meaning LPG will be injected in the inlet manifold before the engine is started giving a richer mixture.

When the temperature is around 0 degrees or lower it is possible to enrich the mixture further (choke effect) by switching the ignition off and on a few times before starting the engine. As soon as the engine stops the relay closes the solenoid on the vaporiser and the LPG flow is shut-off.

Membranes in the vaporiser.

In the vaporiser membranes are used to control the pressure-reducing valves. The more flexible these membranes are, the faster and more accurately the vaporiser will react to changes in the vacuum signal from the mixer. This will cause engine behaviour at partial load to improve, the venturi of the mixer can he made larger and the fuel consumption at partial load will decrease.

Compensated vaporisers are vaporisers that have no direct contact with the open air. Instead of an air opening, a connection tube is fitted on the rear cover of the vaporiser. A hose can be attached to this tube which goes into the air inlet path, just before the throttle valve. The vaporiser regulates the amount of LPG according to the air pressure in the inlet system. With turbo-charged this kind of compensation is always required. The air pressure compensating hose must be mounted at a point where the air speed is minimal e.g. a "quiet" corner in the air-filter box. The compensated vaporisers are ideally suited for off-road vehicles which are frequently in and under the water.

Position of the vaporiser.

Normally the vaporiser has to be installed with its membranes vertical and in the longitudinal direction. This is to prevent too much or too little LPG getting through in case of acceleration or deceleration. This not only has a negative influence on the consumption but can also make the engine stall or cause backfiring.

The warm water supply must always be connected to the under side of the vaporiser and the return to the top side, this prevents air locks in the vaporiser and makes it self-bleeding. The chance of an air-lock in the vaporiser can be reduced further by placing the vaporiser as low as possible. Also here, all parts of the system have to be at least 10 cm from the exhaust or be protected by a heat shield.

Vaporiser thermostat.

Vaporiser thermostats stabilise the LPG vapour temperature. The temperature of the cooling water in the vaporiser is usually adjusted to 50°C. The advantages given by the thermostat are:

  • This temperature is better for the longevity of the vaporiser than the 90°C engine temperature.
  • The vaporiser reaches its operating temperature much earlier than the engine does.
  • The density of the LPG is lower, meaning more LPG can flow through the pipe.
  • The temperature of the LPG /air mixture is lower so the engine power will be higher.
  • Less chance of stalling when running at idle after high load.
  • Less cooling water has to go through the vaporiser thermostat meaning the interior heater will function better in winter.

The vaporiser thermostat gives best results with non-electronic regulated LPG systems.

Diameter of the low pressure LPG system.

The low pressure part of the LPG system runs by hose, from the vaporiser to the mixer via the main adjuster. The inside diameter of this system, and the sensitivity of the vaporiser, determine the maximum bore of the venturi in the mixer. The smaller the diameter of the low pressure LPG system is, the larger the vacuum signal of the mixer venturi has to be. The result of reducing the venturi size is power loss. If a mixer with a large venturi is required, then the low pressure system must have an internal diameter that is as large as possible.

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