How Fuel Injection Works
Petrol burns most effectively when it is mixed with air at just the right ratio. Depending on what your engine has to do, the optimum is about 14.5 parts air to 1 part fuel. This is called the stoichiometric ratio.The injection system exists to ensure that the correct quantities of air and fuel, at the stoichiometric ratio, are delivered to each cylinder so that it burns perfectly every time the spark plug in that cylinder fires.In say 5 seconds an engine could go from not running, to starting up, to idling to accelerating away. To you this may not sound like a long time but in 5 seconds your engine could easily have spun round a couple of hundred times. In a four cylinder engine that means that your spark plugs have fired fifty times.Back to the insides of the engine, because engines spin round, some measurements are measured in degrees, or a percentage of rotation and are known as dwell measurements; where dwell is the time that an action has to take place. In this case putting fuel and air into the engine.Before I go any further you need to know what bits and pieces are involved in an injection system.
The components of a fuel injection system are as follows:
1. Air flow meter - measures how much air is being sucked into the engine.2. Throttle potentiometer - smart name for a device that measures how hard your foot is pressing on the throttle.3. Engine speed sensor - calculates how many times a minute the engine is rotating.4. Coolant temperature sensor - measures the temperature of the air or liquid that cools the engine.5. Oil temperature sensor - measures the temperature of the oil that lubricates the engine.6. Air temperature sensor - measures the air temperature outside the engine.7. Air pressure sensor - determines the air pressure outside the engine, this is used in particular or high altitude driving.8. Manifold Absolute Pressure (MAP) sensor - determines the pressure of the air entering the engine.9. Oxygen sensor - measures how much oxygen is left over after being burned.10. Fuel pump - pumps fuel to the injector/s.11. Fuel pressure regulator - makes sure that fuel goes into the injectors at the right pressure.12. Fuel injector - a spring-loaded valve that allows an exact amount of fuel into the engine.13. Engine Management System (EMS) or Engine Control Unit (ECU) the 'brain' that makes that all-important calculation.
All these components exist to do one thing and that is to determine how long the fuel injector/s stay open. This is called the injector dwell. Because engines rotate, the injector dwell is calculated in degrees or percentage of a rotation. At idle an injector dwell will be around 4% of one rotation rising to 12% of one rotation at 3000 revolutions per minute. When you blip the throttle the dwell can go over 20%.So why is it necessary to make this calculation? For several reasons that go beyond this article but briefly because emissions laws determine that the gases coming out of your exhaust pipe have to meet ever more stringent requirements.
How do all those components work then?I think it easiest to think of each component making the injector dwell longer or shorter, so I have briefly explained what effect each component has on the dwell. So then, taking the components as I listed them above:
1. The airflow meter will increase injector dwell if the amount of air going into the engine increases, as you accelerate. Conversely if the amount of air going into the engine is reducing, while you are coasting down a hill, then the dwell is reduced.2. The throttle potentiometer increases the dwell as you press down on the throttle and reduces the dwell when you lift your foot off.3. As the engine speeds up so more fuel needs to be delivered, and the slower it goes the less required. This is of course dependant on where your foot is on the throttle. You could be flat out in first gear with you foot making a dent in the floor but the injection system knows that it has reached optimum load because even though your toes are going as white as your knuckles, the engine is spinning as fast as is reasonably sensible.4. A cold engine needs a longer dwell to keep it going, whereas a hot engine needs a shorter dwell.5. Cold oil moves round your engine reluctantly, and oil is the blood of your engine. Would you want to wake up on a winter's morning and go from bed to full sprint in under a second? Though t not; therefore if the oil is cold then less load is placed on the engine, ensuring less wear and tear.6. The colder the air the less likely it is to burn so the dwell is increased slightly.7. As you climb higher and higher, oxygen becomes thinner and thinner, seems obvious doesn't it but if this device was not in place then the same amount of fuel would be delivered for a decreasing amount of air therefore distorting the stoichiometric ratio. This means that too much fuel is sent into the engine. The result of this is that you see clouds of horrid black smoke coming from your exhaust and eventually your catalytic converter would get choked up.8. Again the MAP sensor is there principally to make sure that the measurement from the air pressure sensor can be matched against the air pressure of the air going into the engine.9. The oxygen or Lambda sensor is very important as it measures just how much oxygen is left in the exhaust gases. Why is this important? Too much oxygen means an inefficient burn.10. Fuel sprays or atomizes best when it is kept under a high pressure, typically 80 150 pounds per square inch. The smaller and the greater in number the droplets of fuel, the better they will burn.11. The fuel pressure regulator makes sure that fuel at just the right pressure is delivered to the injector/s.12. Where the real business finally happens. This valve is held shut by a spring and is opened by an electric current. The electric current only works for as long as it has to, a time determined by all the components listed above.
Well here ends the brief guided tour to the fuel injection system. I hope you have left more informed than when you arrived.
