Understanding fuel systems - Basics

What is the main function of a petrol or LPG fuel system?


When you control the accelerator pedal of a vehicle with a petrol or LPG engine you are not directly controlling how much fuel goes into the engine, you are actually limiting the amount of air that enters the engine - the further you put your foot down the more air can enter because the accelerator pedal opens an air valve (called the throttle valve). It is the job of any petrol or LPG fuel system to deliver to the engine the correct amount of fuel to mix with this controlled amount of air to give the correct air/fuel mixture.


Turning fuel into power to drive the wheels


Engines burn fuel to make heat, which expands air in cylinders, which raises the pressure in the cylinders, which pushes the pistons inside the cylinders, which turn a shaft called the crankshaft, which connects to the gearbox, which connects to the wheels. Burning is a chemical reaction where oxygen is combined with a fuel. If there isn't enough oxygen to combine with the fuel then some fuel will be left over. If there isn’t enough fuel to combine with the oxygen then some oxygen will be left over. Even if there are correct quantities of both oxygen and fuel, if they cannot get together to react chemically there will be both oxygen and fuel left over. This has implications for engines running on petrol because petrol is usually a liquid and oxygen can only get to the outside edges of a droplet of liquid. Engines don’t burn fuel continuously, they burn it in a very fast series of explosions. For this reason, an engine needs all the chemical reaction for each burn to take place within a fraction of a second, so the oxygen needs to be thoroughly mixed with the fuel. Petrol fuel systems must deliver petrol in tiny droplet form to ensure it has chance to evaporate so it can mix with the air properly. With LPG, fuel droplet size isn’t an issue because the LPG is in the form of a gas well before it gets near the engine and the two gasses, LPG and air, mix easily.


The importance of the correct air/fuel mixture


We often think of air and oxygen as the same but air is really a mixture of many gases - mostly nitrogen, then oxygen, then carbon dioxide. Petrol and LPG are chemically similar and both ‘hydrocarbon’ fuels - they both consist of complex combinations of hydrogen and carbon. When petrol or LPG is completely burned in an engine the oxygen from the air combines with the carbon to make carbon dioxide and with the hydrogen to make an oxide of hydrogen, also known as water, so steam always comes out of your exhaust but you usually can’t see it! If there is too much fuel for the amount of air some fuel is unused or at best partially used, so the unburned fuel is wasted as it leaves the engine with the rest of the exhaust gases. Not only is this fuel itself wasted, but beyond a certain point it can lower the temperature of the burn (much more with petrol than with LPG) and slow down the speed of the burn, so effectively waste some of the fuel that is actually burned. Fuel that doesn’t burn properly raises harmful emissions such as hydrocarbons (unburned fuel) and carbon monoxide (partially burned fuel). Another problem with a mixture that is too rich is that it can damage catalytic converters, because the fuel that hasn’t burned in the engine will partially burn in the catalytic converter and may overheat it. On the other hand, if there is not enough fuel for the amount of air then it will not make as much heat overall but will burn hotter and too quickly. The lower overall heat in the cylinder means less power so the driver has to apply more throttle to get the required power from the engine. This means the engine has to work harder to make the same power because it has to compress more air during the ‘compression stroke’ of the ‘4 stroke cycle’ to make the same amount of power. With the correct air/fuel mixture the nitrogen in the air is usually unaffected by being heated to the temperatures during the burn but the hotter temperatures reached when there is too much air can cause some of the nitrogen to bond with the oxygen creating nitrous oxides, another harmful emission. The higher temperatures could also damage the engine valves, spark plugs, or cause the entire engine to overheat.


What is the correct air/fuel mixture


The correct mixture for burning petrol in air to allow for complete combustion as described above is 14.7 parts air to 1 part petrol, a ratio of 14.7:1. This ratio is the ‘stoichiometric’ (chemically correct) ratio for burning petrol in air and is based on the masses (weights) of the petrol and air, not the volumes. A gas is not as dense as a liquid, so for the same volume will weigh much less - imagine comparing the weights of a bucket of air and a bucket of petrol.  We need 14.7 times as much weight in air as we do weight in petrol, so you can see that for a little petrol we need a lot of air. A mixture with more fuel than this ration is said to be ’richer’ and a mixture with less fuel is said to be ’leaner’. A richer mixture up to around 13.9:1 will give more engine power for a given amount of air but at the expense of fuel economy and emissions, a leaner mixture down to around 15.4:1 can give more fuel economy at the expense of engine power and emissions but this is dependant on the design of aspects of the engine such as the shape of the ‘combustion chamber’ which is the area within an engine cylinder where most of the burn takes place. The stoichiometric ratio for burning LPG in air is between 15.6:1 and 15.8:1. There are two figures for LPG because LPG is a mixture of propane and butane gas which have slightly different stoichiometric ratios. In the UK LPG is usually wholly propane, on the continent it is more of a mixture of propane and butane, but this has little significance in how the engine runs - a vehicle running on LPG in the UK will run exactly the same as a vehicle running on LPG on the continent.  For most engines the ideal fuel system should be able to keep the mixture close to the stoichiometric ratio for most driving conditions, but when maximum power is required it should be able to make the mixture richer. When the air/fuel mixture gets much leaner than the ideal ratio then very poor engine power, very poor fuel economy, rough running, overheating, backfiring and risk of engine damage are symptoms. When the air/fuel mixture gets much richer than the ideal ratio then reduced engine power and poor fuel economy (though not as bad as a lean mixture), misfiring, engine oil contamination (with petrol), backfiring and potential catalytic converter damage can be symptoms.


Lean burn engines


Many engines have combustion chambers and further aspects of the overall design that are designed to minimise the negative effects of running slightly lean (such as the nitrous oxide emissions, fuel detonation and overly quick burn times), to take advantage of the marginally improved fuel economy of a slightly lean mixture. The actual improvements in economy when cruising are small and, as with every aspect of engine design, the ability to run with a lean mixture without ill effect will compromise in some other area such as engine power at certain engine speeds. We are not saying that lean burn engines make any more or less power than other engine designs by other manufacturers, but that they might have been able to make more power if they were not designed to be lean burn. When the driver applies more throttle these engines also make better power from a correct or slightly rich mixture for full power anyway.


Direct injection engines


We describe the way in which direct injection systems work in more detail on the Types Of Petrol Fuel System page, but they are worth mentioning here because one of the ideas behind direct injection is to allow a localised area of the combustion chamber to contain an air/fuel mixture that is correct, even though the amount of fuel entering compared to the amount of air would usually provide only an extremely lean mixture. The operative word here is ‘localised’ - a small area in the combustion chamber has the correct air/fuel ratio because fuel is injected straight into that area of the combustion chamber and timed so that it is injected just before it is ignited so it does not disperse into the surrounding air in the combustion chamber before ignition. Although these engines can burn a very lean air/fuel mixture they are not the same as the more common lean burn engines described above.


Closed loop / Open loop


When describing fuel systems there are terms which are used frequently - ’Open Loop’ and ’Closed Loop’ are some of the most frequent terms. It is best to explain what these terms mean before we move onto the different types of fuel system, both so that you will have prior knowledge and to avoid repetition later. Closed loop systems are the more advanced type but we will explain these first, because Open loop simply means the lack of a Closed loop system.  A Closed loop system is one that has the ability to continuously measure the air/fuel ratio that the engine is actually burning, by means of a sensor in the exhaust that measures the oxygen content of the exhaust gas (called a Lambda sensor). The amount of oxygen in exhaust gas is directly related to how rich or lean the air/fuel mixture was that has just been burned in the engine was. The Lambda sensor gives either a variable voltage or variable electrical resistance which corresponds to the air/fuel mixture that was burned. The rest of the closed loop system uses this information from the sensor to continuously adjust the air/fuel mixture in order to keep the air/fuel ratio closer to the ideal ratio, usually around the stoichiometric 14.7:1 ratio. What actually happens with closed loop systems is that the mixture constantly fluctuates very closely around the 14.7:1 ratio because the closed loop system always very slightly over adjusts - it takes a short time for the exhaust gas to reach the lambda sensor after the fuel has burned, then for the sensor to change it’s output information to accurately give information about the exhaust gas, then a short time for the control system to react. In practice the reaction time can be so fast that the mixture may fluctuate around the correct ratio many times a second under most conditions and never moves far from the chemically correct air/fuel ratio under most vehicle cruising conditions.  Now, remembering from the section above that a slightly rich mixture gives more power at the expense of fuel efficiency, then for optimum power when the driver requires it we don’t want the closed loop system to keep the mixture at the correct ratio but to instead give a slightly rich mixture. So most closed loop systems go into an open loop mode when you put your foot far enough down on the accelerator. If you drive at anything approaching full throttle even the most advanced fuel injection system is designed to go into an open loop mode and start sending a set amount of fuel usually regardless of the emissions to give a slightly rich mixture.