Types of petrol fuel system cont.

Not to be confused with multipoint or sequential petrol injection systems which are described below, these systems do not have one petrol injector per engine cylinder. The petrol injectors (usually one or two) are installed just after the throttle valve of the engine, the air and fuel flow together from this point through the inlet manifold to the individual cylinders so the fuel from a single petrol injector might be distributed to all engine cylinders. Throttle body injection systems look very similar to carburettor systems at first glance, the pic shows the throttle body injection system main engine components on the engine inlet manifold. The rest of the system is extremely similar to the multipoint electronic injection systems described below. The disadvantages of these systems compared to multipoint electronic systems are that a backfire is possible because the inlet manifold contains both air and fuel, the injector(s) have to be made bigger to flow more fuel because one injector has to flow the same amount of fuel as several injectors on multipoint systems - so the fuel droplet size will be bigger likely leading to increased emissions and less economy, and drivability will not be as good because the injectors are placed much further from the engine inlet valves. They improve on carburettors, which they are often confused with due to looking similar at first glance, because they have much better control over the amount of fuel that enters the engine and usually have a closed loop mode. The best type of LPG system to fit on engines with this type of petrol fuel system is usually the single point closed loop type but multipoint none sequential types or multipoint sequential systems that use individual injectors can sometimes be fitted. Fully sequential multipoint LPG systems are usually not suitable.



Electronic fuel injection (EFI) - Throttle body injection type


Like mechanical injection systems, these systems also have one fuel injector per cylinder. A way to tell the two types apart is that the injectors on these systems have electrical wires going to them. There are two types of multipoint EFI systems, ‘full group’ and ‘sequential’. Full group systems operate all the injectors at the same time, sequential systems operate each injector individually in the same sequence that each engine cylinder’s inlet valve opens. Sequential systems are more advanced and have the advantage that at low throttles and engine speeds there is a better chance that all the petrol injected will enter the engine cylinder in the form of a vapour and be able to mix with the air properly as described in ’Fuel System Basics’, because the petrol is injected when the inlet valve is open and the flow of air is carrying the petrol into the cylinder. With full group petrol injection systems, because the petrol is injected all at the same time for every cylinder, on some cylinders the inlet valve will be closed at the time of injection so the petrol which was injected in tiny droplet form may have time to condense back into bigger droplets before it enters the engine to burn (this doesn’t happen with LPG because it is already a gas, mixes easily with air and cannot condense). Full group and sequential petrol injection systems work almost exactly the same in most other respects - The injectors are fed with petrol that is usually maintained at a constant pressure above the air pressure in the engine inlet manifold. The injectors contain tiny valves which are opened and closed very quickly by electronic solenoids, hence the electrical wires. Because the fuel pressure is constant above engine inlet manifold pressure, the amount of fuel injected is directly proportional to the amount of time the injector is held open by the electrical control system which is controlled by a computer. The computer uses various inputs to determine the amount of air that is entering the engine, as well as other inputs such as accelerator position and engine revs, to determine how much fuel should be injected. It knows to a fair degree of accuracy how much fuel should be injected for any operating situation by means of a fuel ‘map’. The map is data stored in a computer in the ECU (electronic control unit) of the fuel injection system by the vehicle manufacturer. It contains information on how much fuel to inject for any operating condition of the engine. The map is best imagined as being similar to a computer spreadsheet or a piece of paper divided into cells (boxes) with reference points along the top for engine revs and down the side for airflow. By looking up information from the map where the row and columns cross for the engine’s current rpm and airflow the computer can retrieve information from the cell that best matches the operating conditions at the time. For engine revs and airflows that fall between reference points (cells / boxes) the computer can interpolate, that is use a figure somewhere in-between cells but proportionally closer to the cells that are nearest the reference points that the operating conditions are closest to. These types of fuel systems usually have an advanced closed loop system, the computer stores information regarding how far on average it changes the air/fuel mixture for different parts of the fuel map. This information is stored  in ‘fuel trims’ in another part of the computer’s memory. There are slow and fast fuel trims, these are not intuitive so we will explain them. Fast fuel trims are spur of the moment adjustments made by the fuel injection computer. So while you are driving if the computer detects a lean air/fuel mixture from the lambda sensor it will immediately start applying a positive fast fuel trim to correct the air/fuel mixture. The fast fuel trim always starts at zero and quickly climbs or lowers until the air/fuel mixture is seen to be correct. The fast fuel trim is not stored in the computer’s memory but it effects the slow fuel trim which is stored in memory. If we imagine the map again being like rows and columns full of numbers containing information on how much fuel to supply the engine with, then there is a similar but smaller number of rows and columns in the computer to store the slow fuel trims in. Each slow fuel trim covers a certain wider area of the map. Let’s suppose you are driving at 60mph with your foot a quarter of the way down on the accelerator. The engine might be doing 2500rpm and the map might contain information to inject 6ms (milliseconds) of fuel per injector opening. So let’s suppose the closed loop system finds this air/fuel mixture to be lean, in which case it will immediately start applying a positive fast fuel trim (if it was rich the computer would apply a negative fuel trim). The fast fuel trim will quite quickly rise until the air/fuel mixture is correct and then fluctuate around that level. The fast fuel trim slowly effects the slow fuel trim, so the slow fuel trim will gradually change from it’s current value to it’s current value plus the fast fuel trim. The actual amount of fuel that the computer tells the injectors to inject is based on the information from the map, added to the slow fuel trim which covers that part of the map, added to the fast fuel trim. Since the slow fuel trim is stored in memory, in future when the computer has to run the engine under the same conditions it will likely provide the engine with an air/fuel mixture that is closer to correct straight away by looking up the information from the map and the slow fuel trim, before even applying a fast fuel trim. There are situations where the computer will go into an open loop mode. This usually occurs when you put your foot down past a certain point on the accelerator. To get most power out of an engine requires a slightly rich air/fuel. A slightly rich mixture doesn’t make the best use of the fuel in terms of economy or emissions, but allows more power to be made at the expense of wasting some fuel. So to get the most power it isn’t helpful to have the computer try to keep the mixture correct when what we want is a rich mixture. When in open loop mode the computer will send just the amount of fuel that the map tells it to, and usually this will provide for a slightly rich mixture to make the most power. The best type of LPG system to fit on these types of petrol fuel system engines is usually a fully sequential multipoint LPG fuel system. Many competitors only fit this type of LPG system but there are now many manufacturers producing them, each with their own selling talk which can sound very convincing to most potential customers, but which in many cases we know to be complete rubbish! There are pros and cons with any LPG manufacturer’s system - Some are generally better than others for most vehicles but there are exceptions where a system we wouldn’t usually rate might be the best option for a particular vehicle. We fit most manufacturers LPG systems, are aware of the differences between them and can help you decide which is the best system for your vehicle and/or budget. On this website we

 have devoted a full page to the subject of comparing different manufacturers sequential LPG systems but you may still find it more helpful to discuss your requirements with us. On older models or even certain more recent models of vehicle we may even advise a single point closed loop system as a money

saving option that will still give almost identical results.



Electronic fuel injection / EFI  - Multipoint electronic / Full Group / Sequential / Port Injection

Direct injection systems for petrol engines have been around a long time in one form or another, there are two main types. The older systems that were around before the now common computer controlled EFI systems (described above) were completely mechanical and worked in the same kind of way as the fuel system on many diesel engines, using a mechanical pump to inject fuel straight into the engine cylinders. This type of system was used on certain 70’s Mercedes vehicles and works only open loop. These days direct injection systems have electronic and computer controlled direct injectors and use many of the same parts as the above EFI systems, except the injectors pump fuel straight into the engine’s cylinders instead of into the air at the other side of the inlet valve to the cylinder. The main advantage to doing this is that unlike a normal EFI system that mixes fuel with air before it enters the cylinder, a direct injection system can aim fuel at a certain area inside the cylinder and can inject fuel at any time even if the inlet valve is closed. The benefit of this is that if the combustion chamber area of the cylinder is also designed in a certain way, for example with a pocket to contain the fuel that is directly injected, it is possible to create a localised area in the cylinder where the fuel and air mixture is correct even though there has not been enough fuel injected overall to provide a correct mixture to fill the whole cylinder. So, direct injection engines might in theory be able to offer better economy by being able to run with a mixture that would be too lean for a normal EFI system. A performance engine might be able to inject fuel at a certain point during the compression or even during the power stroke to avoid detonation, or there may be a compromise between these two advantages. In reality the economy and performance gains are small because, in the terms of economy, they use a small localised explosion rather than the full area of the combustion chamber to push the big piston when only minimal power is needed, and performance engines might generate as much power by having a combustion chamber designed in a way that does not have to consider the direct injection system. The fuel injectors on these systems are exposed to the full heat and pressures within the engine cylinders, so live a hard life and are more expensive than normal petrol injectors. Direct injection injectors are usually protected somewhat from the heat by the cooling effect of the petrol as it flows through them into the cylinders - This can be a problem for LPG installers because, although these engines will run fine on LPG, if we stop the flow of fuel through the direct petrol injector it can lead to that petrol injector being damaged, hence direct injection engines are not the best to convert. It is possible to convert some of them because certain LPG systems have the ability to also control the petrol injectors to inject a small amount of petrol whilst using mainly LPG, but this means a compromise between the maximum economy of running wholly on LPG and protecting the petrol injectors by injecting more petrol to keep them cool. The majority of petrol injected engines don’t use direct injection, only a few models do, but any convertor should inform you about the pros and cons of converting a direct petrol injected engine before undertaking the work. To convert a direct petrol injection engine, if possible, we would either advise a fully sequential LPG system or a single point closed loop system, both types may need modification by us to enable them to be used on a direct injection petrol engine.


When converting vehicles with this type of fuel system to LPG, it is best to tune the LPG system to keep all the fuel trims at the same levels while running on LPG as would be on petrol. This gives the smoothest changeovers between LPG and petrol and can prevent issues such as drivability problems and slightly higher than ideal LPG consumption, because the slow fuel trim will remain the same between petrol and LPG so the fuelling will be more correct when the engine changes rpm or you move your foot on the accelerator. There are only one set of fuel trims, if the LPG system calibration is wrong these have to be re-learned every time you switch between petrol and LPG or vice/versa. More on this when we explain sequential LPG systems.


Most categories use a computer to control how much LPG is delivered to the engine.  The category will tell you if the computer works independently of the petrol system, or if it works by interpreting signals from the computer that controls the petrol system.

Direct injection