Injection Strategies
Injection Strategies
The piston recesses are important for the injection strategies of single, dual or triple injection. They allow late injection of fuel in order to create an ignitable air/fuel mixture around the spark plug for late ignition.
Engine start
Different injection strategies are used for the engine start, depending on the start temperature.
Triple injection
Generally, triple injection is used in the start phase for start temperatures with a coolant temperature of less than 140° F. (60° C.). Triple injection in the start phase offers a very great potential for reduction of the fuel injection quantity. The first two injection pulses occur early in the compression stroke with a small interval between injections, while the third injection pulse takes place at the end of the compression strokes. Triple injection made it possible to reduce the total injection quantity by up to 40 % in the temperature range from -22° F. (-30° C.) to 32° F. (0° C.) and also to shorten the starting times compared with the previous V8 DFI engines. The advantages of multiple injection are achieved primarily due to the fact that the evaporation surface area is increased by dividing the injection quantity between several injection pulses, thereby allowing more fuel to evaporate in the same time. This reduces the wall deposits of fuel for a cold start.
Single injection
Starting takes place by means of single injection as from a coolant temperature of 140° F. (60° C.). With this so-called high-pressure stratified-charge injection, injection takes place very late and just before the end of the compression stroke. Here, fuel is injected once into the specially molded piston recess in a targeted manner. This creates stratification around the spark plug, which in turn produces an ignitable mixture. The piston recess ensures that the injected fuel is channelled directly to the spark plug. This reduces both the amount of fuel required and the emissions compared with intake manifold injection.
Engine start with single and triple injection
The top part of the figure shows an engine start with single injection and the bottom part shows catalytic converter heating with dual injection.
Catalytic converter heating with duel injection
After engine starting by high-pressure stratified-charge ignition, the engine management system switches to the catalytic converter heating phase. In this operating state, dual injection helps to bring the catalytic converter to the temperature required for optimal conversion as quickly as possible by increasing the exhaust-gas temperature. For this purpose, the first injection of fuel takes place during the intake stroke and the second injection occurs into the piston recess when the intake valves are closed, just before the end of the compression stroke. The fuel-air mixture is ignited very late, thereby increasing the exhaust-gas temperature. This reduces the emissions during the starting phase.
Engine at operating temperature (homogeneous operation)
Single injection at idle speed and partial load up to 3,500 rpm
When the engine is at operating temperature, injection into the cylinder takes place only during the intake stroke. In this so-called homogeneous operation, there is uniform mixture formation for the entire cylinder charging process.
At speeds close to idle speed and in the lower partial-load range, the engine is operated with a single injection in the intake stroke since this relatively short injection time cannot be divided up.
Dual injection
Under conditions with high load (e.g. large valve lift), a dual injection takes place up to an engine speed of 3,500 rpm. The quantity of fuel required for combustion is shared between two successive injections. In the upper load range, both injections take place during the intake stroke (synchronous intake injection) with open intake valves, thereby ensuring better homogenization (spatial distribution in the combustion chamber) in order to save fuel and increase power output.
Dual injection, full load up to 3,500 rpm
Single injection, full load above 3,500 rpm
At high load and with an engine speed of over 3,500 rpm, injection takes place only once in the intake stroke, since there is not enough time to divide up the injection time at high engine speeds.