Engine Management System
Direct injection
Direct injection of petrol gives advantages such as low fuel consumption, low emissions and high power.
Injection takes place directly in the cylinder at high pressure, between 3-20 MPa (30-200 bar). To achieve these high pressures, a mechanical fuel pump driven by the exhaust camshaft is used. The fuel pump supplies the fuel rail with fuel under high pressure. The fuel is distributed on to the fuel injectors, which are controlled by the engine control module (ECM).
Excess fuel from the high pressure pump is returned to the inlet of the high pressure pump. The fuel system has thus no separate return line to the tank. Injection into the cylinder is either by:
- An injection during the compression stroke, so-called stratified combustion.
- Two injections, one during the induction stroke and one during the compression stroke, so-called semi-stratified combustion.
- An injection during the compression stroke, so-called homogeneous combustion.
- Stratified combustion
- Semi stratified combustion
- Homogeneous combustion
Stratified combustion
Injection takes place during the compression stroke's second half. When all of the drawn in air is not involved in combustion, a portion of the air is used to isolate and concentrate the optimum stoichiometric air-fuel mixture (14.7:1) to the spark plug and the piston combustion chamber, which creates a stable combustion.
(coolant temperature/catalyst temperature). In colder conditions semi stratified combustion is also used during the starter motor sequence.
Stratified combustion is used during the period when the starter motor is activated and the fuel pressure has reached at least approx. 3 MPa (30 bar). When the engine has started, the engine's operating mode shifts to semi stratified combustion in cases where catalyst heating is required. Stratified combustion is used in all conditions at temperatures above -20 °C.
The concentration of the fuel air mixture to the spark plug and piston occurs by the airflow created through the inlet ports' and combustion chamber's shape, as well as the fuel injector's position relative to the piston. When combustion has been initiated, the fuel air mixture is said to be 30:1 or even leaner for the entire cylinder.
Semi-stratified combustion
Injection takes place through two injections. The first takes place during the first half of the induction stroke and the other during the second half of the compression stroke. The fuel quantity is distributed so that the largest amount of fuel is injected during the induction stroke and the remainder during the compression stroke. This provides a lateral and stable combustion with low peak pressure and low temperature, resulting objects in low NOx emissions. With the low walls impingement and efficient combustion with lean fuel-air mixture (lambda >1), HC emissions are also low.
Semi-stratified combustion is used during catalytic converter heating when the engine has started. The time in this operating mode can be anything from 0 seconds when the engine coolant is at operating temperature and the catalytic converter is warm to approximately 30 seconds when both the coolant and catalytic converter are cold. Important parameters for the calculation of time by the engine control module (ECM) are the signals from the coolant temperature sensor and a calculation model for catalytic converter temperature. When the catalytic converter is considered sufficiently hot, the engine operating mode changes to homogeneous combustion.
Homogeneous combustion
Injection occurs in the first half of the induction stroke compared to conventional port injection. The incoming air is cooled by the fuel evaporation. This gives high knock tolerance.
Direct injection also gives slight wall impingement which results in low HC emissions. Homogeneous combustion is used during the driving cycle except in the above conditions.
MISCELLANEOUS
- If the fuel pressure in the fuel rail at start has fallen to 0 kPa (0 bar) it takes approx. 2-3 pump strokes before the pressure reaches 3 MPa (30 bar). 3 pump strokes corresponds to 1.5 crankshaft revolutions.
- During the period of stratified combustion, 4-20 injections occur.
- During the period of semi-stratified combustion, the idle speed is raised to approx. 1200 (varies depending on market)
- During the period of semi-stratified combustion the exhaust camshaft is controlled to a later position. This means that the hot exhaust gases reach the exhaust manifold which gives reduced HC emissions. Together with late ignition the catalytic converter is rapidly heated.
- During the period of stratified and semi-stratified combustion, fuel injection occurs at approx. 5-10° crankshaft degrees before ignition occurs.
- For all strategies the injection time is adapted depending on the different parameters such as load, engine speed, operating mode and injected fuel volume.