Techniek - LPG-installatie

Veel mensen vragen zich af hoe hun LPG-installtie nu eigenlijk werkt en wat de onderdelen zijn die er onder je motorkap zorgen voor een goede verbranding van het gas. Op internet vonden we een - engelstalig - artikeltje met een heldere uitleg van alle componenten van een moderne en ouderwetste LPG-installatie.

What is LPG ?

LPG stands for Liquefied Petroleum Gas. LPG is a mixture of propane and butane which is a by-product of crude oil. The mixture is adjusted according to the time of the year, In summer it's 30% propane with 70% butane and in winter 50% propane en 50% butane or sometimes even 70% propane and 30% butane. Those differences are necessary to maintain the LPG pressure. The most important reasons for using LPG as fuel are:

• LPG is an environmental friendly fuel

• The internal engine pollution is less

• LPG is a "waste" product which otherwise would be burned off

• The knock resistance is very high (LPG ± 100 octane regular-grade petrol ± 95 octane and premium-grade petrol ± 98 octane)

• LPG is easy to store as a liquid at ± 6 bar

The components of a LPG-system

The basic components of a LPG-system are:

• A LPG storage tank

• A tank valve to shut off the tank when the engine stops

• A vaporiser / regulator to vaporise the liquid and regulate the amount of LPG to the engine

• A main flow adjuster to adjust the LPG-system to the engine

• A mixer to mix the LPG with the air which enters the engine

• A petrol shut off system

LPG tanks

The LPG tanks can be classified by their shape :

• Cylindrical tanks: the most common straight round tanks (cheap)

• The duo-tank: flattened tank cylindrical to reduce height

• Twin-tank: 2 cylindrical tanks are welded together

• Ring-tank: donut-shape tank with a hole in the middle which can be installed in the spare wheel well of the boot

• U.F.O.-tank: a round pill-shape tank which can be installed in the spare wheel well of the boot

The LPG tank incorporates safety valves :

• Filling-hose connection with a 80% shut off

• Overpressure relief valve which opens at ±30 bar

• A shut off to close the tank by hand or electronically (solenoid)

• Fuel gauge which gives you an indication of the amount of LPG in the tank

• The valves must be covered by an air-tight box with an atmospheric vent to avoid LPG entering the car

Those safety devices can be gathered in one valve. This valve is called a multi-valve and can only be installed in a single-hole-tank. The multi-hole-tank is the most common tank. In a multi-hole-tank are separate valves installed. The installation of the tank is much easier than a single hole-tank because there is more space between the components. Filling up the tank can be done much faster because of the larger connection between the tank and the filler connection (specially in winter).

LPG shut off solenoid. Under the bonnet is a solenoid installed close to the vaporiser. The length of the LPG-drain-pipe must be less than 30 cm. The function of this solenoid is to close the system when the engine stops or when the engine is running on petrol. On modern cars this is done electronic by a relay connected to the ignition system.

Vaporiser / regulator. The vaporiser/ regulator, mostly called vaporiser, has two major functions. The first is to heat up the liquid LPG so the vaporisers wont freeze when the LPG becomes a vapour. Normally this is done by making a connection to the car's water cooling circuit. Some engines have an air cooling system. In that case the liquid LPG has to be preheated before it goes into the vaporiser. This can be done with a special heat exchanger installed in the exhaust system . The second is to regulate the amount of LPG that goes to the engine. Just like a carburettor it tries to keep the mixture of LPG and air at the optimum proportion. Therefor the vaporiser must deliver a stable LPG pressure to the engine. A 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 30bar). The manufactures of vaporisers solved this problem by lowering the pressure in several stages. The vaporisers can be classified in the way they are built :

• Two-stage vaporisers. The two stage vaporisers are the most common. They lower the pressure in two stages. The first stage lowers the tank pressure from ± 10 bar to ± 2 bar. The second lowers the operating pressure to ± 0,7 bar so the mixer can cope with it. The advantages are: they are small and can be cheap. Disadvantages are: they have a limited range (0,5L up to 1,5L or 1,0L up to 2,5L or 2,0L up to 4.0L engine size).

• Two-stage-tandem- vaporisers. The two-stage-tandem- vaporisers are functioning in almost the same way as the two stage vaporisers. The biggest difference is the second stage. The second stage is regulated in two. One accurate second stage regulates small amounts and a second big stage delivers high quantity's of LPG. The small stage is operating when small quantities of LPG are required by the engine. Because of the small internal size of the valve the LPG release pressure is very stable. The big stage will be opened by the small stage as soon as the required amount of LPG is reaching above the capacity of the small stage. The big stage is capable of dosing large quantities of LPG. Advantages: for small and big engines (0.5L up to 7L). Disadvantages: the large size, expensive.

• Three-stage- vaporisers. This type of vaporiser lowers the tank pressure in three stages after each other. Because of the three stages the vaporiser is extremely stable. Advantages: they can cope with large engines (4L up to 8L) Disadvantages: the large size, difficulties with delivering low quantities of LPG, expensive. All vaporisers shut off LPG flow as soon as the engine stops. This is done in two ways: - by vacuum - by electronics

By vacuum. If the closing of the vaporiser can be done by vacuum there is a connection made from the vaporiser to the air intake manifold. As soon as the engine runs the vacuum in the air intake opens a valve in the vaporiser. The vaporiser will now deliver LPG to the engine. When the engine stops and the vacuum drops the valve will shut down the LPG flow to the engine. Starting on LPG is impossible with those vaporisers, unless there is a solenoid installed on the vaporiser which can be opened manually for pre-injecting LPG. A button must be installed in the cabin to operate the vaporiser solenoid.

By electronic circuit. If the vaporiser closes electronic there is a connection made from the vaporiser to an electronic control relay. This relay gets a signal from the ignition system when the engine is running and opens the vaporiser solenoid. As soon as the engine stops the relay closes the solenoid on the vaporiser and the LPG flow will stop. To start the engine you have to switch the ignition on. At that moment a relay activates the vaporiser solenoid for some seconds. A small amount of LPG will be injected in the air intake manifold before the engine is started. When the temperature is around 0 degrees or lower it might be recommendable to enrich the mixture by switching on the ignition twice before starting the engine. This procedure gives some additional LPG to the engine (choke).

Main LPG adjuster. The main LPG adjuster is installed between the vaporiser and the mixer. ItÕs function is to adjust the LPG system to the engine. The LPG system must create a LPG/air mixture of a constant ratio. If the mixture is a little too rich the engine uses more LPG, the temperature in the combustion chamber stays lower and the engine delivers its maximum power. This lowering of the temperature is desirable for older engines. If the mixture is too lean the performance of the engine drops dramatically, it is using more LPG as necessary, the pollution is rising and the temperature in the combustion chamber might rise dramatically. The high temperature might damage the engine. There are 3 different types of main LPG adjusters:

• a fixed main flow adjuster; The fixed main flow adjuster can't interact with the engine. The result is a mixture which is too rich or too lean. Adjusting the main flow adjusters is easy. The manufacturer gives a Co value (mostly between 05% and 1.0%) at a particular amount of revolutions (mostly 3000 rpm). This type of main flow adjuster ignores the performance delivered by the engine. Advantages: cheap, trouble free and easy adjustable.

• a vacuum controlled main flow adjuster; The vacuum controlled main flow adjusters have two settings. The first setting must be adjusted in accordance with the specifications given by the manufacturer of vaporisers (Co usually between 0.5% and 1.0% mostly at 3000 rpm). Before you can start adjusting you have to disconnect the vacuum hose and blank it off. The second setting has to be done at the same rpm. but the vacuum hose must be re-connected. Make the mixture as lean as possible. Therefore you have to look at the HC-scale and close the second setting until the HC reaches the lowest level (on LPG this will be ±150 HC). The result will be a very economic LPG consumption at partial load and the engine is still capable of giving maximum power at full load. If the system is installed in an older engine the vacuum controlled main flow adjuster is capable to lower the combustion temperature by enriching the mixture even more than strictly necessary. Advantages: cheap, easy adjustable, might give a fuel consumption drop between 5% tot 15% compared with a fixed main flow adjuster and less chance of getting a back-fire. Disadvantages: none

• an electronic controlled main flow adjuster; The electronic controlled main flow adjusters (stappen motors) are operating by computer. The computer constantly gets information about the combustion from the lambda probe which is installed in the exhaust system. The electronic controlled main flow adjusters are capable to reach good economics and (when required) maximum power. The advantages are: self learning, fuel consumption drops between 10% tot 20% compared with a fixed main flow adjuster. The disadvantages are: pre adjusting must be done by an installer by computer.

LPG mixer

The LPG-mixer (also called mixer) as two functions.

• Giving a vacuum signal to the vaporiser; The vacuum signal must be as constant as possible compared with the amount of air that is going trough the venturie of the mixer. To achieve this the mixer must be carefully designed. One of the most important components is the venturi. The LPG/air exit of the venturi must have an ± 8 degrees angle to achieve a constant ratio of LPG with air. One of the problems of this mixer will be his length ( ± 30 CM.), which makes it almost impossible to install. Theoretically this mixer won't produce any swirl, so that the gases wonÕt mix with etch other.

• Mixing up the LPG and air; To get a fast and almost complete carbonation the LPG and air must not only be in the right proportion but also be proper mixed. Therefor it's necessary that the gases LPG / air) swirl. To combine those two demands is almost impossible. Therefor a mixer will always be a compromise. The manufacturer of mixers has to work out the best mixer for every single car. The result mostly is that the mixer gives a right mixture in the partial load and a lean mixture at full load. Not only the shape of the mixer is important but also the size of the venturi. The smaller the diameter, the higher the vacuum signal to the vaporiser, the more accurate the LPG flow. The disadvantage is that the engine gets less mixture because of the small diameter. This means it looks as if your engine only can drive half throttle. Especially cars with injection or big carburettors will have a high power loss. (up to 20%). Therefore we recommend that :
  - The venturies must be at least 75% of the superficial measure of the carburettors venturie or butterfly valve if its an EFI engine.
  - The venturies superficial measure must be 7.5MM2 and if possible 10 MM2 for every HP. If you have a 3.5 Lr 200Hp engine you need a mixer with a superficial measure of 200X7.7MM=1500MM2. You need a mixer with a venturie diameter of at least 43 MM. If you have a 3.5 Lr 200Hp engine you the need a mixer with a superficial measure of 200X10MM=2000MM2. You need a mixer with a venturie diameter of 50 MM for the best performance. The manufacture or importer can advise you whit a chose of the mixer.

Simulator

Modern cars are filled up with electronics. Those electronics provide the driver with information when one of the measurements are out of tolerance. When a LPG system is installed the motor management system detects that the fuel injection has stopped, the lambda signal has changed etc. etc. The modern LPG system has to cope with those problems. According to the car there will be installed: - injection simulator - lambda simulator - auto petrol start Injection simulation The injection simulator has 3 functions:

• shut down the petrol injection by blocking off the signals to the injectors

• tricking out the motor management system by giving a similar signal as the injectors to the motor management system

• avoiding the chances of getting a back-fire by delaying the shutting down of the petrol injection by several seconds after activating the LPG system Lambda simulator

If an engine has a constant learning motor management system a lambda simulator has to be installed This constant learning motor management system will try to reduce the amount of fuel constantly until it gets a low read out of the lambda probe. Because the LPG computer is regulating the mixture the lambda will give a lambda "1" read out. The motor management system will lower the injected amount of petrol and still get a lambda "1". This will result in lowering and lowering the amount of petrol until the system is completely closed. Sometimes this will result in activating an alarm. At the moment the engine has to run on petrol again the motor management system is completely out of its proper working range. This means that the car can't run on petrol any more and needs some time to reprogram itself. Sometimes starting is impossible because the engine has to start on petrol before switching over to LPG. To start the engine the motor management system has to be reset by the dealer.

Back-fire safety protection

There are two ways of back-fire protection.

• Active back-fire protection; Active back-fire protection is a way to minimise the chances of getting a back-fire. This can be done by: - starting automatically on petrol - giving LPG and petrol for several seconds when switching over from petrol to LPG The chances of getting a back-fire are the highest during the starting procedure. This risk can be minimised: - Start on petrol and switch after a little while over to LPG. This can be done by an electronic switch with a time delay, with a vacuum switch which measures the under pressure in the air intake manifold, a ref. counter or a combination of the three. - Starting on petrol and LPG. After deactivating the starter engine of the petrol injection will be shut off.

• Passive back-fire protection; Passive back-fire protection is a way to minimise the damage of a back-fire. This can be done by: - Installing blow-back valves which will open by overpressure in the air intake system. The overpressure can escape and afterwards the valve closes itself. Plastic air filter boxes can't cope with high over pressure and will split open when a back-fire occurs. - Installing an air-flap opener. The airflap will be opened during the time the engine is running on LPG. If a back-fire happens the pressure can pass without damaging the airflap. They can be operated by LPG pressure from the vaporiser or by the under pressure in the air intake manifold. Once the airflap is damaged it has to be replaced. The replacement is very expensive.

SU lifters

Some older engines have carburators where the fuel proportion is controlled by a vacuum valve. The valve has a needle which wears quickly out at the bottom while driving on LPG. The most common are manufactured by SU and Stromburg. To minimise the wear out it's recommendable to install a so called SU-liftset. This device will lift the valve with the needle and hold it in an upper position. This can be done by vacuum or manually. The carburator will last longer once a SU-lift-set is installed.

Power loss

Almost all engines which are running with a dual fuel system have less power than before transformation on petrol. The reasons are: - The lower caloric power of LPG. This is partly compensated by the more complete combustion. - The mixers are blocking the airflow partly. (see mixers)

Higher fuel consumption

LPG engines are using more fuel on LPG than on petrol. This is partly explainable by the lower caloric power of LPG. This is partly compensated by the more complete combustion. There will be a theoretical gap of ±5%. In daily use the gap will be larger . It depends especially on the vaporiser, the mixer and main flow adjuster. If the vaporiser is too small and therefor can't give the proper amount of LPG the consumption of LPG rises. If the mixer is too small for the engine, the engine can't give the needed power and the driver will drive with high refs.. This will also rise the fuel consumption. The fuel consumption may be sometimes over 20% higher compared with petrol. 10% extra fuel consumption is reasonable.

Testing power loss

The most accurate way of testing the power loss is by rolling road. Because this way of testing is not for everyone accessible we give a simple way of testing your car. You have to make 3 test runs. Go to a place where the road is more or less fled. A motorway is recommendable. Speed your car up to 50 MPH. Put the gear lever in 4 gear and don't change gears!!

* Give full throttle and measure the time you need to reach 80 MPH. Do this once on LPG, once on petrol with the LPG system installed and once with the mixer removed.

** To make the measuring more precise make every run 2 or 3 times and middle the time of each run.

* If you have an automatic gearbox put the gear lever in the first position lower than the "D". As soon as you give full throttle put the lever in the "D" position.

** If your car has a LPG-injection system installed don't make the last run. Do not try to remove the injectors!! . The difference between with and without injectors is neglectible.

The differences in time give an indication of the performance of the LPG system. The difference between the first and second run gives an indication of the power loss of the LPG it self. The difference between the second and third run gives an indication of the power loss of the LPG system. The difference between the first and third run gives an indication of the overall loss of power. As indication difference in time between the first and last run is: 3 10% extra time or less is very good 4 15% extra time or less is still good 5 25% extra time or less is acceptable 6 40% extra time or more is worst up to very worst We tested a RANGE ROVER 3.5 EFI with an automatic gearbox with an Iwema LPG system. The system used a 50MM venturie. We measured the next results: Orig. On petrol 18 sec. On petrol with the mixer installed 18.5 sec difference 0.09 % On LPG 20 sec. difference 8,33 % We also tested this car on the rolling road. A 40MM mixer was installed. We measured a loss of 1.3 % at max. toque and 9.4% at max. HP.

Bron: IWEMA enterprise, LandRover Club Holland (www.iwemalpg.com)
Auteur: Hugo van Osch

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