Understanding LPG fuel systems

Any LPG fuel system, from any LPG fuel system manufacturer, including factory fitted systems, can be classed as belonging to one of six design type categories. To clarify, we don’t mean that LPG systems in the same category but produced by different manufacturers use the same parts (although some do), what we mean here is that each category will describe the main mechanical methods of getting the LPG fuel into the engine and how the quantity of LPG is controlled. With exceptions, it is possible to fit most types of LPG system on most vehicles, and indeed the vehicle would then run on LPG, but there can be serious problems in fitting the incorrect type of system on a particular vehicle.


To understand which type or types of LPG system will be suited to a particular vehicle and which won’t requires a little knowledge about the petrol fuel system on that engine. Each type of LPG fuel system is designed primarily to work with one type of petrol fuel system. We have covered Fuel System Basics  and Types Of Petrol Fuel Systems on other pages on this website. This page explains the different types of LPG fuel systems and which type of petrol fuel system each is usually fitted to.  


There are several main types of LPG fuel systems and many different manufacturers producing systems of each type. .


Before we get into explaining the different types of LPG system you need to understand a few terms that frequently pop up, such as ‘open loop’ and ‘closed loop’, ‘single point’, ‘multipoint’, ‘sequential’. Open loop means the fuel system gives the engine a fixed amount of fuel for any particular operating condition. Carburettors and early fuel injection systems work in this manner all the time. Closed loop systems monitor exhaust emissions and from this they can tell if the air / fuel mixture is correct and adjust the fuelling until the mixture is correct. By keeping the mixture correct performance, economy and emissions are all optimised. With a closed loop system, if you move your foot quickly on the accelerator it will immediately give a certain set quantity of fuel to the engine that is approximately correct, but will very quickly adapt to the new accelerator position and will continually correct the air/fuel mixture to then keep it at the optimum. Single point systems have all the LPG entering the engine at the same point or one point per air intake / carburettor. Multipoint systems have a separate point for each engine cylinder (just like with multipoint petrol injection systems). Referring only to some types of multipoint LPG system, sequential means each LPG injector injects fuel in the same sequence as the engine cylinders fire, in a sequence one after the other. Below is a description of each of the main basic types of LPG fuel system in the order in which they were developed. The further down the list, the more advanced and suitable for modern vehicles.


Single point (or mixer) open loop systems


Suitable for carburettor and some early fuel injection engines. These are the most basic type and have been around since the 1970’s, although they have improved since then! Here’s a pic of some of the main parts from one manufacturer (not to scale).


Single point (or mixer) closed loop type systems


Suitable for most fuel injected vehicles produced up to around year 2000 and a smaller proportion of more recent vehicles. These systems can also be fitted to carburettor vehicles if a further inexpensive part (Lambda probe) is fitted in the exhaust. These systems use most of the same parts as the single point open loop systems described above, but have a few additional and different parts. The manual control valve is replaced with an electronically controlled LPG control valve which is controlled by an electronic unit that has a computer. The advantages over the single point open loop system are that the computer continually monitors the exhaust emissions and adjusts the electronic control valve to keep the air/fuel mixture correct with all the associated advantages such as optimised fuel economy, emissions and preventing catalytic converters being burned out. In addition, when you put your foot down on the accelerator they can be set up to give a richer mixture for more power - this is what carburettors try to do and fuel injection does. They can cut off all fuel when you are on the ‘overrun’ (foot off accelerator, maybe coasting downhill), so further save fuel, and again this is what fuel injection does. They are completely independent of the petrol fuel system, so as long as the ignition system on the car works the engine will run even if the petrol fuel injection system is completely broken. They have some of the same disadvantages as the single point open loop system above, in that a backfire is possible because the whole inlet manifold will contain a combustible mixture (again as with carburettors). This system is suitable for many fuel injected vehicles, usually those produced up to around the year 2000 but in some cases later vehicles too. When compared to fully sequential LPG systems described below, they cannot control the mixture as quickly or accurately but on suitable vehicles this makes very little difference in real terms. On most petrol injected vehicles (most petrol injection systems are closed loop), the advantage described above about these systems being independent of the petrol fuel system can also be a disadvantage. This is because the petrol system will always be running in the background, thinking it is controlling the engine fuelling when actually of course the LPG system is. Because the petrol injection also has it’s own closed loop system, the petrol system itself continuously tries to correct the mixture, but of course since the petrol system is not actually in control of the mixture - it only thinks it is still controlling the mixture -  the fuel trims of the petrol injection system can drift to the extreme range of adjustment. What this means is that when you then switch back to petrol the vehicle can be hesitant until the petrol system retunes itself. On some petrol injection vehicles (up to around year 2000) the petrol system won’t illuminate the engine warning light on the dashboard (or go into an error mode) because it can tell that the mixture is around the correct level even though it reaches the limits of it’s adjustment capabilities. Most later vehicles will see that the petrol adjustments have gone to the limits, will illuminate the engine warning light and may go into an error mode. This reason, and the potential for backfiring, is why these systems are only suitable for certain models of fuel injected vehicle, usually older vehicles and those without plastic inlet manifolds.

Multipoint non sequential systems


Suitable for most fuel injected vehicles produced up to around year 2000 and a smaller proportion of more recent vehicles. Most of these systems can also be fitted to carburettor vehicles if a further inexpensive part (Lambda probe) is fitted in the exhaust. There are actually a few different subtypes of these systems but they work in mostly the same way. A reducer supplies LPG under slight pressure to an electronically controlled metering unit. From the metering unit the LPG goes to a distribution box, which is essentially a hollow box which splits the LPG supply into a number of small outlets, one outlet for each engine cylinder. From each outlet a smaller pipe runs to an LPG injector, the LPG injectors are placed close to the vehicle’s original petrol injectors and continually inject LPG (as opposed to the later sequential types that inject pulses of LPG). These systems usually use an engine manifold pressure sensor and monitor engine revs to estimate the amount of LPG that should be injected based on a few settings set by the installer, and are closed loop so further keep the mixture correct by monitoring the emissions. They offer many of the same advantages as the single point closed loop systems. In addition there is no resistance on the air entering the engine, so peak power is unaffected. As they inject LPG very near to the petrol injectors the inlet manifold only contains air so the risk of backfires in eliminated. The main disadvantages are the same as for the single point closed loop systems (except the air resistance and backfire problems that these systems solve) in that most of them work independently of any petrol injection system or adjust the air/flow mixture relatively slowly, allowing the petrol system to get out of tune until you have run on petrol again for a while. The main points to remember about these systems are that they eliminate any potential backfires, don’t effect peak power, but in most cases are only suitable for the same vehicle models as the above single point closed loop systems. These systems are now very uncommonly fitted and are seen by most installers as a now redundant stopgap system between the single point closed loop systems and the fully sequential systems.

Multipoint systems that use individual injectors


Suitable for most fuel injected vehicles produced up to around year 2000 and a smaller proportion of more recent vehicles. Most of these systems can also be fitted to carburettor vehicles if a further inexpensive part (Lambda probe) is fitted in the exhaust. The parts on these systems are very similar to the latest fully sequential systems. They use nearly all the same parts, except the ECU works very differently. Where they differ from the latest sequential type is that, as with all the systems described above, they don’t use any actual inputs from the petrol injection systems so the petrol fuel systems can still become out of tune. They work in the same way as the multipoint non sequential systems but may work non-sequentially or sequentially. The advantages are the same as for the multipoint non sequential systems above, and they improve on these systems because the amount of fuel injected can be adjusted immediately - the amount of fuel injected is proportional to the amount of time the LPG injector is held open, which can be very different for each individual revolution of the engine. This means that the air/fuel mixture can be controlled very precisely. They can sometimes be set to work open or closed loop. If you had a carburettor or early (non EFI) injection engine and wanted it to run on LPG, then some of these systems are able to effectively convert your engine to run as a fully electronic fuel injected engine on LPG, but they are more expensive than the single point systems and even more uncommon than the multipoint non sequential systems. Again, not suitable for a more recent fuel injected vehicle because they work independently of the petrol system.

Fully sequential multipoint LPG systems


These systems are suitable for the vast majority of electronically controlled fuel injected vehicles - most vehicles on the road today. We install more of this type of system than any other. If you have a modern fuel injected vehicle you want to convert to LPG it is likely that a fully sequential system is the only way to do it to avoid problems. By definition, a sequential system is multipoint (sequential meaning it opens each LPG injector in turn in the same order as the vehicle’s firing order, if it opens each injector in turn it must have more than one injector, if it has more than one injector it must be multipoint). A reducer supplies LPG at a set pressure to the LPG injectors, the injectors may be in a block together and mounted remotely of the inlet manifold, individual and mounted remotely of the inlet manifold, or individual and mounted directly on the inlet manifold. If they are in a block, a separate pipe runs from each outlet on the LPG injector rail to an individual outlet on the inlet manifold. When running on LPG the system electronically disconnects the petrol injectors but monitors the amount of time the petrol injection computer thinks it is opening the petrol injectors for. The brain of the LPG system uses this information to calculate how long to keep the LPG injectors open. This isn’t as straightforward as you might think, because petrol is injected as a liquid which cannot be compressed and is hardly affected by changes in temperature. LPG is injected as a gas, gasses can be compressed and their pressure or volume is affected by temperature. The electronic injectors fitted to modern petrol vehicles and on sequential LPG systems work by using electro-magnets to open and close a mechanical valve which lets the fuel through and into the engine. The properties of petrol injectors and LPG injectors are very different in that they take a different amount of time to open and close (or to start and stop the flow of fuel) and for any set time they are open the amount of fuel that can flow through them is different to petrol injectors, not least because one type of fuel is a liquid the other a gas. The LPG brain must compensate for all these factors and must be calibrated to do this properly. In simple terms the LPG brain processes the petrol injection signals and turns them into suitable signals to drive the LPG injectors - It does an initial calculation on the petrol injection times to compensate for the differences in injector properties, then applies calibration and compensation factors programmed by the installer. The petrol injection brain still provides the closed loop system because if the petrol brain sees the engine needs a little more fuel to keep the mixture correct it will send a longer injector opening signal, which will be intercepted by the LPG brain and hence a longer LPG injector opening signal will be sent. The main differences between these systems from the various manufacturers is in the ways they are able to be calibrated, the overall quality of the components, and how much LPG they are capable of supplying the engine with (which limits the size or power of engine they can be fitted on). It is because this type of system is now the most common fitted and there are so many different manufacturers making them, all hyping their own systems with half truths and techno jumbo, that there is so much misinformation regarding them.

Above left is a mixer ring, on the right a pressure reducer (or vaporiser). There are many more parts that are fitted but these are the main parts. The mixer goes just before the throttle housing on the engine, so all air entering the engine passes through it (note there are different types of mixer which can look completely different but they all work in the same way). An LPG feed from the reducer is connected to the mixer and the LPG is literally sucked out of the reducer and mixed with the air entering the engine. Here’s how it works - The little holes on the inside of the mixer are where the LPG comes out. Because the mixer is designed to create a slight resistance to the airflow above the holes, the holes are partially exposed to engine vacuum which sucks out the LPG into the flow of air that is going into the engine. Mixers come in many different sizes and have interchangeable venturi’s (the middle part which causes the air restriction and has the holes). In addition to a manual control valve fitted in the LPG feed pipe, the size of the venturi controls how much LPG is sucked into the engine. These systems work in much the same way as a petrol carburettor. The advantages are that they are the cheapest form of LPG system, very simple, not much to go wrong, and can be the best or only option for a carburettor or older fuel injected engine. The disadvantages are that the mixer creates a slight restriction to the air flow into the engine so can lower the peak power of the engine slightly. Backfiring is a possibility, but no more so than with petrol carburettors with a properly set up system. Bare in mind that a backfire could destroy a plastic engine inlet manifold, air filter, or if fitted to a fuel injected engine a backfire could damage an air flow sensor mounted in the air inlet stream, but if your vehicle has a carburettor it will have a metal inlet manifold for the same reason. As there is no closed loop control of the air/fuel mixture there may be certain driving conditions where the air/fuel mixture isn’t optimised, so economy and drivability might not be quite as good as with a closed loop system. We wouldn’t fit this type of system on a vehicle with a catalytic convertor, because an incorrect mixture can burn catalytic convertors out (carb’ vehicles won’t have a catalytic convertor). But again, this is just the same as a carburettor. Carburettors wear out quite quickly, LPG mixers don’t wear. You may have more power and get more mpg from one of these systems than you do with an old carburettor. Of course, it will be cheaper to run on LPG in any case as LPG is much cheaper to buy. When converting any carb vehicle to LPG, the system cannot be made to change between fuels instantly because there is no way of instantly stopping the flow of fuel from the carburettor. The changeover switch for carb vehicles has three modes, to switch from petrol to LPG you press the button once, which cuts off the fuel supply to the carb but doesn’t yet turn on the LPG. When the fuel that is remaining in the carb starts to run out the engine will hesitate, then you press the button again and the system switches to LPG mode and carries on running. When changing from LPG to petrol you press the button once and the system allows petrol to the carburettor but keeps the engine running on LPG. When the carb fills with enough petrol the engine will start to hesitate as it is now getting both petrol and LPG so will have a very rich mixture. When it hesitates you press the button again and the LPG is turned off leaving the system running only on petrol. Usually carb vehicles will start happily on LPG so there is no need to start on petrol and LPG is used all the time.

Liquid LPG injection systems


These are suitable for use on the same vehicles as the fully sequential multipoint LPG injection systems (above), meaning they are suitable for most electronic fuel injected vehicles. These systems are also fully sequential multipoint LPG injection systems but instead of injecting LPG in gaseous form, they inject LPG in liquid form in much the same way as a petrol injection system. These systems are nowhere near as common as the gaseous injection systems but are fitted by certain vehicle manufacturers on some models of vehicle as factory fitted LPG systems, rather than by aftermarket installers such as ourselves. The advantage to injecting LPG in liquid form for vehicle manufacturers is that no separate LPG brain is required - the normal petrol ECU can control the liquid LPG injectors directly. This is because the main role of the LPG brain on gaseous LPG injection systems is to compensate for the varying LPG pressure and temperature, and to allow for the different flow rate of gaeous fuel rather than liquid fuel. As liquid LPG fuel systems don’t need to allow for these factors none of these factors apply, so the petrol brain is capable of driving the LPG injectors. Another advantage for vehicle manufacturers of installing a liquid LPG injection system is that they don’t have to consider the mounting location of a vaporiser / reducer (the part that turns high pressure liquid LPG into lower pressure gaseous LPG), additional water piping to heat the reducer, or need to consider the wiring loom and ancillary components that a gaseous LPG injection needs such as pressure and temperature sensors. The big downside to liquid LPG injection systems is that they require a high pressure pump to be mounted in the LPG tank, because the liquid LPG injectors need a pressure that is higher and more constant than the pressure in the LPG tank. The high pressure LPG pump mounted in the tank is a very expensive part to replace at around £1000 and they are not too reliable! We have done a lot of work for insurance companies that cover warranty plans on vehicles, where the insurance company has preferred us to completely remove liquid injection systems and replace them with a vapour sequential injection system instead. The insurance companies would rather us completely replace the system with a new one that will be more reliable and in any case is covered by our warranty than run the risk of the same or another expensive part on the liquid LPG injection system breaking down again. We don’t currently offer liquid gas injection systems because of the issues with reliability, and because there is no way of calibrating for the slightly different properties of LPG compared to petrol.