All gasoline (petrol) fuel systems need ways or method that will be calculating amount of the air that is entering the engine.This is true as regards mechanical fuel injection, carburetors and electronic fuel injection.Speed density system fall under one of this method. "Speed" in this context means the speed of the engine while "density" means density of the air. The Speed is been measured using the distributor, crankshaft position sensor could be used here. The Density emerge from from measuring of the air pressure which is inside the induction system, using a typical "manifold absolute pressure" (MAP) sensor as well as air temperature sensor. Using pieces of information above, we can calculate
mass flow rate of air entering the engine as well as the correct amount of fuel can be supplied. With this system,
the mass of air can be calculated. One of the drawback if this system is that
Any modification will result to incorrect calculations. Speed-Density can be regarded as method use in estimation of airflow into an engine so that appropriate amount of fuel can be supplied as well as. adequate spark timing. The logic behind Speed-Density is to give prediction of the amount of air ingested by an engine accurately during the induction stroke. Then this information could now be used in calculating how much fuel is required to be provided, This information as well can be used in determining appropriate amount of ignition advance. It should be noted The speed density fuel injection system uses the Map sensor as the primary sensor to determine base pulse width
