Types of petrol fuel system

Put very simply, carburettors work by allowing petrol to be sucked into the flow of air as it enters the engine. They don’t suck petrol in the same way you might expect, they make use of a principal called the ‘venturi effect’, but ‘sucking’ is close enough for the purpose of this explanation. There is nothing to control the flow of the petrol except for mechanical metering by the ‘jets’ from which the petrol flows inside the carburettor. There is usually only one carburettor on an engine but there may be up to one for each engine cylinder.  Some carburettors have more than one ‘barrel’, where each barrel may act as an individual carburettor. To the designer of the carburettor it doesn’t usually matter too much how many engine cylinders it provides with fuel (as long as it is the appropriate size and has the correct characteristics for the application), because the fuel is added to the air in the carburettor itself, and downstream this flow of fuel and air may feed a single engine cylinder or if the flow is split it could serve the whole engine. An area in which one carburettor can be compared against another is in the size of the droplets of petrol that come out of the jets. As described above, the petrol must be delivered in tiny droplets. Because of the way in which carburettors work the internal petrol pressure is very low, so the jets through which the petrol flows must be quite large. Because of the low pressure and large jets, they deliver droplets that, while small, are large compared with a fuel injection system. With a warm engine most of the small droplets will evaporate and mix with the air before the burn, but with a cold engine some of the petrol will stay in droplets that are too large to turn into a vapour and properly mixed with the air to form a combustible mixture. This has the effect of ‘leaning’ the air/fuel mixture (not enough fuel for the amount of air).  A ’choke’ is therefore fitted to carburettor engines which facilitates more fuel to enter the engine for any given amount of air, compensating for the fuel that isn’t properly mixed with air and cannot be burned in an effort to correct the effective air/fuel mixture. Any fuel that cannot be burned exits the engine straight out of the exhaust, so is wasted. Carburettors also have a system called the ‘accelerator pump’, which as the name might suggest, pumps extra petrol straight into the airflow through another jet when you push further down on the accelerator. The reason for this is because at part throttle there will be a partial vacuum between the carburettor and the inlet valves compared to outside air pressure, but containing air and fuel that is already mixed by the carburettor and moving towards the inlet valves. If you suddenly were to open the throttle further by pressing the accelerator down quickly, the vacuum between the carburettor and the inlet valves will suddenly rise in pressure as the extra air rushes in but before the extra airflow through the carburettor has time to effect the amount of petrol being sucked in - remember a carburettor has to suck liquid petrol using the force of air in gas form. Under these conditions, without an accelerator pump, the extra air without extra petrol would cause the engine to momentarily run very lean and cause drivability problems such as ‘flat spots’ and ‘kangarooing’. The best type of LPG system to fit on carburettor vehicles is usually the single point open loop type.  We can sometimes improve on this system by fitting a single point closed loop type which would also require us to fit a Lambda probe in the vehicle exhaust. If the vehicle is run on petrol from a carburettor for extended periods the chances are that the incorrect mixture from the exhaust when running on petrol will eventually cause soot to be deposited on a Lambda probe and prevent it working, causing the closed loop LPG system to stop working correctly. It may also be possible to fit a multipoint non sequential LPG system or multipoint LPG system with individual injectors but again these both require us to also fit a Lambda probe.




This page is included on the website to help you identify the type of petrol fuel system that is on your vehicle, to help you better understand the basic way in which it works and to help you understand why we would usually advise a particular type of LPG fuel system for a type of petrol system. There are many exceptions to these general guidelines, so it is still best to discuss the options for your particular vehicle with us rather than just taking it upon yourself to find out which type of fuel system is on your vehicle and deciding you need a particular type of LPG system. We are not suggesting you need to look at the engine on your vehicle yourself and find anything out, because we will already know! But the information is here for people that like to better understand our advice.



These systems usually have one petrol injector per cylinder. On the pic you cannot see the injectors, which are below the metal box on the engine inlet manifold. The injectors are each fed by their own fuel line which leads to a single fuel measuring and distribution unit, shown at the extreme bottom right of the pic. This unit is in two adjoined halves, one half of which measures the amount of air flowing into the engine by means of a flap which is deflected or lifted by the air flowing upwards past it, the other half controls the amount of pressurised fuel which is sent to the injectors by opening a kind of valve which is connected to the air flap via a lever arm. Essentially, the more air that flows, the more fuel pressure is sent to the injectors which will allow more fuel to flow the higher the pressure, but the measuring and distribution unit also has to be designed to avoid the movements of the vehicle from affecting the operation of the air flap and has some quite advanced features to allow the relationship between changing air flow and fuel delivery to be more advanced than a simple ratio. For instance, if you are holding a constant steady position on the accelerator and suddenly put your foot down, the flap will be affected by the extra air entering the engine but will over-swing the usual position for the new quantity of air momentarily, to give ’acceleration enrichment’ for the same reason as the accelerator pump on a carburettor described above. Because of the high fuel pressures the injectors can be made with a smaller hole for the petrol to flow from than the jets in a carburettor, so these systems deliver much smaller droplets of fuel than a carburettor. The smaller droplets turn into a vapour more readily and don’t need as much time or heat to do so. This allows the injectors to be placed very close to the engine’s inlet valves, as opposed to carburettors which need to be placed some distance from the inlet valves to better allow the fuel to vaporise in a heated air inlet manifold. Placing the fuel injectors closer to the inlet valves better allows the measuring and distribution unit to more quickly supply the correct amount of fuel for any changing operating conditions of the engine, such as when you change accelerator position. The more advanced versions of these systems are closed loop (described above), they have a Lambda sensor in the exhaust to measure the emissions and partially electronically controlled adjustment of the air/fuel mixture by means of a system that can adjust internal valves within the fuel metering and distribution unit.  The best type of LPG system to fit on mechanical injection vehicles is usually the single point closed loop type. If the petrol system does not have a Lambda probe we might advise a single point open loop type. It may also be possible to fit a multipoint non sequential LPG system or multipoint LPG system with individual injectors.


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Mechanical / continuous petrol injection systems