Fuel injection for automotive engines

In internal combustion engines, the primary performance factor, once the operating cycle has been given, is the quality of the combustion reaction. This reaction only takes place in the gas phase and within certain concentration limits for the reactants. Compliance with these conditions - vapour phase and concentration - has the primary effect of optimizing heat release in the combustion chamber and limiting the formation of pollutants.

Given the brevity of the combustion reaction in reciprocating engines (a few milliseconds), as well as the temperature levels reached (2,000 K) in the reaction zone, it is not envisaged to directly control its progress, but to achieve initial conditions such that the characteristics of the reaction (speed, temperature, products) are under control. The formation of the mixture, which ensures the initial conditions of the reaction, thus conditions the quality of combustion and the evacuation of gases, in particular burnt gases. The mixture is made up of fuel and air, ideally in a gaseous state, in proportions that may or may not be stoichiometric, depending on the type of engine. They are animated by movements that promote molecular contact between oxygen and fuel. Various engine components are involved in the formation of the mixture: the intake ducts, the valves or ports, the piston and cylinder head which, by their shape, define the geometry of the combustion chamber, and finally the injection system.

The primary purpose of fuel injection in engines is therefore both to condition the fuel and to dose it. However, to ensure that the dosing specified by the designer is actually carried out at the time and place of the reaction, the supply of reagents and the removal of reaction products must be guaranteed, and the injection system can contribute to this by generating stirring movements whose characteristic scale can be very diverse.

Depending on the type of engine, fuel and operating conditions, the relative importance and criticality of these functions can vary. For example, in the conventional gasoline engine, where the reactive mixture is stoichiometric and homogeneous, the metering function only became critical with the integration of catalytic converters in the exhaust. Without the catalytic converter, air and fuel concentrations can vary over a wider range without affecting engine operation.

In the gasoline engine in particular, mixing precedes combustion in the operating cycle. Mixing can be carried out during the intake phase or during the compression phase, the essential being that the mixture is formed by the time ignition is triggered by the spark from the spark plug. The fuel may be introduced outside the combustion chamber (indirect injection) or directly into the combustion...