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  • General
  • Regulated and unregulated LPG systems
  • LPG and gas tank
  • Filling connection
  • gas valve
  • Fuel shut-off valve
  • Switching from petrol to gas
  • Operation of the evaporator
  • Stepper motor system with dry gas hose (AMS)
  • Vapor gas injection (VSI/EGI)
  • Operation of the EGI evaporator
  • Liquefied Gas Injection (LPi)
  • Coupling Block (LPi)
  • Injectors (LPi)

LPG is used worldwide on a small scale as fuel for passenger car engines. There are (in 2013) approximately 700.000 vehicles running on this fuel. This number may fall, because the road tax benefit for old-timers younger than 40 years has been abolished. The tax rate for these older cars is the same as the rate for a younger car. When the LPG system is removed (and of course re-inspected), then if the vehicle is between 26 and 40 years old, you will be able to use the tax benefit again.

LPG is better for the environment than, for example, petrol or diesel fuels. The exhaust gases are cleaner. The fuel itself is also cheaper per liter than petrol. The consumption is often slightly higher with LPG, but the turning point is low. The engine power decreases slightly with LPG compared to petrol, with the exception of the LPi system. More information about this is explained at the bottom of this page.

There are 3 different types of LPG systems. These systems are explained in detail on this page:

  1. System with a stepper motor in the drying gas hose (AMS) (Single point injection before the throttle valve)
  2. Vapor Gas Injection (VSI/EGI) (Multipoint Intake Valve Injection)
  3. Liquefied Gas Injection (LPi) (Multipoint Intake Valve Injection)

Often the term G2 or G3 is used:
G2 installations use a gas venturi system or a vapor gas injection. A catalytic converter with a lambda sensor may be present on the car and be similar in equipment to a G3 installation. Despite this, they may not be covered by the tax benefit of a G3 installation, because the vehicle does not meet the ECE94-12 emission standards, or because the vehicle has not been tested by a recognized inspection body. G3 installations use the fuel injector drive times calculated by the engine management system. These times are converted to control times for the gas injectors.

Regulated and unregulated LPG systems:
In old cars (oldtimers) without an engine management system, i.e. without a catalytic converter and lambda control, an unregulated LPG system is used. This conventional system was used until 1990, when environmental requirements became stricter. There were also more problems with backfires with the unregulated system. A controlled system, as it is still used today, is provided with an electronic control unit. With the help of the lambda sensor, a more accurate amount of gas can be injected. The catalytic converter converts harmful exhaust gases into less harmful ones.

LPG and gas tank:
The composition of LPG varies in summer between 30% propane and 70% butane, and in winter up to 70% propane and 30% butane. Butane does not leave the tank at a temperature of -10 degrees because the vapor pressure is too low, so the percentage must be lower in winter than in summer. This is done automatically at the gas stations. If the car is driven very little, there is a chance that fuel problems will arise because the composition in the tank was from a warmer period.

The liquefied vehicle gas is stored in the tank. The gas is under a maximum working pressure of 2500 kPa (25 bar).

A tank with liquefied vehicle gas may never be filled to 100%, because otherwise there is insufficient space for the gas to expand when it is heated up. The gas tank is designed in such a way that it can only be filled to 80%. The liquefied vehicle gas leaves the tank via the electromagnetic tap, which opens when the engine is started. In that case, the liquid vehicle gas flows through the pipeline to the gas valve. More about this later on this page.
After the tank has been manufactured, the date of manufacture is stamped into the tank. The tank will be found to be in order for the next 10 years. Gas tanks are pressure tested at a pressure of 3000 kPa (30 bar). The bursting pressure of a gas tank is 10.000 kPa (100 bar). A gas-tight box has been installed around the appendages, which is called the appendage box. The fitting box is connected to the outside air by means of a vent hose. In the event of leakage, the purpose of the fitting box is to discharge the present leakage gases to the outside air. These leak gases must absolutely not end up in the interior.
Gas tanks are attached to a steel base frame with pull straps. This steel lower frame is screwed to the body of the car. Plastic strips are fitted between the tank and the pull straps for protection. The gas tank must not be connected to the body in any other way!

Filling connection:
Thread is provided in the filling connection. An adapter (adapter) can be screwed into this. This may be necessary when refueling abroad. The external filling valve is fitted with a non-return valve, which prevents gas from flowing back after filling. The pump at the gas station will force the pressurized gas through this filling connection. The gas flows through the filling hose to the gas tank via the filling connection.

The fill connection cap prevents dirt from entering the fill connection.)

gas valve:
The gas valve is mounted as close to the evaporator as possible. The gas valve is energized when the ignition is turned on and the fuel selector switch is set to gas. The control unit controls this gas valve. The control is stopped when the engine stalls. The LPG entering the gas valve from the gas tank flows through the filter. When the coil is de-energized, the valve closes off the passage to the evaporator. The LPG then enters the space around and above the valve via bore “A”. Because the autogas presses on the valve, the passage to the evaporator is firmly closed. As soon as the coil is energized, the soft iron core becomes magnetic. The magnetism causes the valve to be pulled upwards. The passage to the evaporator is now open, allowing LPG to flow to the evaporator. As soon as the engine is decelerated, the gas valve temporarily shuts off the gas supply until the driver accelerates again.

Fuel shut-off valve:
When driving on gas, the fuel supply is shut off. At that moment the coil is not energized and the valve closes the passage. The moment the gas is switched back to petrol, the coil is energized and the soft iron core becomes magnetic. This pulls the valve upwards, allowing the gasoline to pass through.

Switching from petrol to gas:
If it was started on petrol and a switch is made to gas, this switch will not take place immediately. The engine temporarily runs on both fuels. This ensures a smooth transition from petrol to gas. This situation is called the “double maturity”.
The control unit determines how long the engine will run on both fuels at the same time. This will be longer with a cold engine than with a warm engine, because the evaporation of a fuel is worse when the outside air is cold. After a few minutes (depending on system and temperatures) the fuel supply is completely turned off via the fuel shut-off valve.

Operation of the evaporator:
To make the operation of the evaporator as clear as possible, the evaporator in the picture has been drawn as simply as possible. Later on this page an explanation will be given about a real (EGI) evaporator, which is a lot more difficult. Therefore, the simple vaporizer is first explained to make the basics clear.

The evaporator's task is to make the liquefied vehicle gas in the tank gaseous. The liquid gas must be vaporized (hence the name evaporator). Heat is needed to evaporate the liquid gas. This heat is extracted from the coolant. This is warmed up by the engine and is therefore around 90 degrees when the engine is at operating temperature. It is important that the evaporator heats up as quickly as possible, so that is why the coolant is drained before the thermostat. This is also possible with the cooling circuit of the stove, because this supply line is also connected before the thermostat.
Since the evaporator needs pure heat, it makes sense that the engine must first be warmed up before the evaporation process can begin. That is also the reason why it is not possible to start directly on gas. On a cold start, the engine will run on petrol for the first few minutes before the system switches to gas.

Theoretical operation of the evaporator:
Room A is from the first stage, room C is the room from the second stage.
The reference pressure prevails in rooms B and D, which in this case is the outside air pressure.

Gas valve open, motor not running:
The liquefied vehicle gas flows from the gas tank past the valve of the 1st stage to chamber A. The vehicle gas hereby changes from the liquid form to the gaseous state.
The autogas builds up a pressure in chamber A. This pressure pushes the membrane of the 1st stage to the left. Spring 1 is hereby compressed, while spring 2 relaxes. When the pressure in chamber A is approximately 135 kPa, the 1st stage diaphragm has moved so far to the left that the 1st stage valve closes. No more LPG flows to chamber A. Spring 3 ensures that the valve of the 2nd stage remains closed in this condition.

Gas valve open, motor running:
When the engine is running, the inlet air creates a negative pressure at the outlet opening of the gas/air mixer. This negative pressure moves through the drying gas hose into room C (the 2nd stage) of the evaporator/pressure regulator. The reference pressure in chamber D now causes the second stage diaphragm to move to the left. Spring 3 is thereby compressed and the valve of the second stage opens. Autogas now flows from room A to room C, and from there to the engine. Because LPG flows from room A to room C, the pressure in room A drops. This will open the valve of the first stage, so that LPG again flows from the tank to room A. The autogas that flows past the valve from the second stage to chamber C builds up a pressure in chamber C. Depending on the fuel requirement of the engine, the diaphragm of the second stage will assume a certain position, so that the passage of the valve of the second stage becomes larger or smaller. The greater the negative pressure at the outflow openings of the gas/air mixer, the more autogas can flow to the engine. An equilibrium situation is created, in which more or less gas flows past the valves of the first and second stage, depending on the underpressure at the outflow openings of the gas/air mixer.

Stepper Motor System with Dry Gas Hose (AMS):
This is Vialle's AMS system. The tank contains liquid LPG. The evaporator/pressure regulator ensures that the gas evaporates as it comes out of the tank and that the pressure is reduced. The amount of gas leaving the evaporator is controlled by the venturi in the gas/air mixer, which creates a vacuum. The greater the negative pressure, the more LPG is drawn in. The negative pressure depends on the speed and the load on the motor (due to the air speed). So if more revolutions are made, the amount of sucked gas is always greater. However, this is not really accurate. Fine tuning is needed to deliver exactly the amount of gas the engine needs. Calculated the correct mixing ratio using the measurement of the oxygen sensor.

If too little gas is injected, the mixture will be lean (lambda > 1). If there is too much gas, the mixture is too rich (lambda < 1). (The sign > means greater than, and < means less than). The lambda probe will measure this in the exhaust gases. The motor management will therefore recognize the mixture that is too rich or too lean and will control the stepper motor. The stepper motor then makes the gas passage larger or smaller. This stepper motor is usually placed on the evaporator. At a cold start, this stepper motor will be in a neutral position and will not work yet. The motor is still running in an “open loop” situation. This means that the lambda probe signal is not yet used because the cold start enrichment is still active. The disadvantage of the AMS system is that it is single-point injection. The gas is injected before the throttle valve and is distributed with the air over the various cylinders. Due to the large amount of gas in the inlet pipe, there is a strong risk of a backfire.

Vapor Gas Injection (VSI/EGI):
This is the Vapor Sequential Injection (VSI) or Electronic Vapor Gas Injection (EGI). For convenience, it is now just called EGI. The vapor gas injection system is a multipoint injection system which is controlled by means of a control unit. It is now possible to inject per cylinder instead of centrally in front of the gas valve. This can be with a 4 cylinder engine, but also easily with a 6 or 8 cylinder. The gas is injected just before the intake valve. The chance of a backfire is now a lot smaller compared to the AMS system. This type of gas installation must always be started on petrol. After a short time, the gas system will be switched on automatically.

The autogas comes from the evaporator in a gaseous state. The pressure has been reduced by the pressure regulator in the evaporator. The gas then flows to the distribution housing. The distributor housing doses the amount of gas and distributes it to the injectors using the control slots. The injectors inject the vaporous gas into the intake manifold, just in front of the intake valve.

Operation of the EGI evaporator:
The following text relates to the image below.

  • Operation first stage:
    In the unpressurized condition, spring 6 against membrane 7 the lever against spring 8 push down, causing the 1st stage valve 3 is open.
    When the gas at the entry grommet 1 enters, the gas will penetrate the membrane 7 against spring 6 push up. the siphon 4 is now released, and spring 8 pushes the lever up. This closes the valve of the 1st stage 3.

    At the top of membrane 7 the negative pressure of the engine prevails, so that the pressure in the 1st stage also depends on the engine negative pressure. The pressure in the 1st stage can be adjusted by adjusting bolt 5. Pressure 1st stage= Regulated pressure 1st stage – engine negative pressure.

  • Operation second stage:
    The gas in the first stage can initially pass through the released opening through the valve of the 2nd stage 13. The gas then presses against spring 11 and membrane 10, causing the 2nd stage valve 13 by spring 14 close.
    On the underside of membrane 10 the negative pressure of the engine prevails, so that the pressure in the 2nd stage depends on the engine negative pressure. The pressure in the 2nd stage can be adjusted by adjusting bolt 12.
    Pressure 2nd stage= Regulated pressure 2nd stage - engine negative pressure.
  • Overpressure protection 1st stage:
    When the pressure in the 1st stage becomes too high, diaphragm 7 together with membrane plate 19 move up.
    When the diaphragm shaft 18 against adjusting bolt 17 lies, the diaphragm axis 18 no further up. 
    Membrane 7 moves with diaphragm plate 19 further upwards, causing membrane plate 19 at the narrower part of diaphragm shaft 18 comes to lie. An opening is released here, through which the gas from the 1st staircase through space 16, channel 20 and manifold pressure grommet 15 to the engine's intake manifold.
  • Feedback:
    The gas pressure from the 1st stage can be via channel 22 under plunger 23 to come.
    This gas pressure therefore acts on the plunger at the bottom 23, opposite to the gas pressure from the 1st stage on the 2nd stage valve 21.
    Now the gas pressure of the 1st stage will be on the 2nd stage valve 21 no longer affect the opening of the 2nd stage valve 21, because the gas pressure of the 1st stage under the plunger 23 is directed opposite.

Liquefied Gas Injection (LPi)
LPi means: Liquid Propane Injection). With liquid gas injection, LPG is injected as a liquid. So there is no evaporator present in this system.
Because the liquid gas does not have to be evaporated either, you can simply start on gas. The fuel injection system is therefore in effect disabled. This does have the disadvantage that the fuel injection system can get dirty due to little use. It is therefore advisable to occasionally drive on petrol. The LPi system tries to approach the fuel injection system as closely as possible. The liquefied vehicle gas is injected through the injectors on the intake valve (exactly like indirect injection petrol engines).

The evaporator and the gas/air mixer have been replaced by the coupling block and the injectors. A pump is fitted in the tank to circulate the liquid vehicle gas. The fluid injection is controlled from the existing engine management system, which fully retains and utilizes its self-learning properties. The LPi system only uses the signal of the opening time of the petrol injector and translates this to LPG. Liquid LPG can be dosed very accurately. Better than vapor gas.
The LPi system follows the fuel control unit's injection strategy. All options such as fuel cut-off during deceleration, speed limitation, full load enrichment and lambada control are also carried out on LPG. With LPi, the motor has no power loss. This is due to the lack of the air displacement effect, which persists with vapor dosing. Due to the air displacement effect, the engine fill rate decreases by approximately 6%. The liquid injection also provides a cooling effect for the evaporation of the gas in the cylinder. This results in a better filling degree. This also improves engine performance. The fuel consumption is still higher than when the same engine is driven on petrol, because there is less combustion energy per kg of gas than in a kg of petrol.

A high system pressure is required to inject the LPG in liquid form. The system pressure is supplied by the diaphragm pump in the tank. This pumps the LPG via the coupling block to the LPG injectors. The system pressure is regulated by the pressure regulator to 5 bar above the tank pressure.
Due to heating, vapor bubbles could form in the pipes. Vapor is compressible and therefore cannot be injected accurately. By pumping the liquefied vehicle gas under pressure, it heats up and thus prevents any vapor in the pipeline. The pipes are also made of plastic and insulated against the heat.
A filter is also mounted on the return line, which must stop any contaminants and metal particles.

Coupling Block (LPi):
The coupling block is the connection between the tank and the injectors (see picture below). An electromagnetic valve is included in the coupling block, which opens and closes simultaneously with the tap on the tank. The pressure regulator (which was normally included with the evaporator) and the pressure sensor are also mounted in the coupling block. There are 4 connections on the coupling block. The flexible high pressure pipes are attached to the coupling block with a banjo bolt. The connections may not be interchanged due to the flow of the vehicle gas. In the event of a defect, the coupling block must be completely replaced, because it must absolutely not be disassembled.

Injectors (LPi):
To inject the liquefied vehicle gas, “bottom-feed injectors” are used. This type of injector has the advantage (in contrast to top-feed injectors) that the heat from the injector coil does not cause heating of the vehicle gas. There is also almost no stock of LPG in the injector. The injector coil has a resistance of 1,8 Ohms. A filter is mounted in front of the gas inlet of the bottom-feed injector to prevent coarse assembly dirt from entering the injector.

The injectors are placed in a universal injector holder. The seals are provided by O-rings. The injector is held in place by a screwed ring. Depending on the placement on the manifold, the gas is passed through the outlet pipes (see item 9 in the picture).