Air-Supported Direct Injection

In addition to liquid high-pressure direct injection systems, there are air-supported direct injection systems such as ОСР_TM. Air-supported direct injection systems enable stable combustion with good stratification that is compatible with large amounts of recycled exhaust gas because of the high quality of the mixture preparation. The main feature of air-supported direct injection is an arrangement of an electromagnetic fuel injector and an electromagnetically actuated air injection valve (Fig. 12-12) that injects finely atomized fuel into the combustion chamber.

Fig. 12-12. Air and fuel rail with a fuel injector and air injection valve swirling method with duct closure

Figure 12-13 provides an overview of the air-supported direct injection system. The injection system is divided into two subsystems, the compressed air path and the fuel path. A compressor driven by gears or a belt generates the required compressed air. The pressure level is set by a mechanical pressure regulator to a set point, and the fuel is conveyed by an electrical fuel pump. The fuel pressure is regulated to be at a constant differential pressure from the compressed air (approximately 0.7-1.5 bar).

Fig. 12-13. System overview of an air-supported direct injection system

The fuel is metered with a conventional fuel injector for intake manifold injection. The fuel is injected into a venturi in the air injection valve. By means of the air injector, a finely atomized mixture cloud is introduced into the combustion chamber that can be directly ignited. In stratified-charge operation, jet-directed combustion is, therefore, possible with low untreated emissions. By synchronizing the phase angle of injection (the actuation of the air injector) and ignition, an optimum air-fuel ratio in the mixture can be maintained at the spark plug for stable combustion under all operating conditions.

As Fig. 12-13 shows, the system consists of an air filter, an air mass meter with an integrated air temperature sensor, an electrical throttle valve actuator, and the intake manifold.

The externally recycled exhaust can be introduced into the individual intake tubes via the manifold or via a line in the cylinder head. In any case, a position-controlled EGR valve is necessary to provide precise metering. Optionally, internal exhaust recycling can be provided by adjusting the phase angle of the intake and exhaust camshaft, allowing the torque characteristic and output of the engine to be improved.

The exhaust aftertreatment subsystem consists of a three-way catalytic converter close to the engine and an underfloor NO_x storage reduction catalytic converter. A broadband oxygen sensor allows the air-fuel ratio to be controlled in lean operation, including the regeneration phase and the stoichiometric operation. The exhaust temperature sensor optimally controls the NO_x storage catalytic converter and trigger measures to protect it. A binary lambda sensor downstream from the NO_x storage catalytic converter is required for the control of the regeneration phase to work properly. Alternately, a NO_x sensor can be used.

The compressor in the system offers a new and interesting solution for scavenging the carbon canister. The air inducted by the compressor is guided through the carbon canister. This allows stratified-charge operation to also be used to attain a sufficient scavenging rate without the disadvantages of untreated emissions.

The process of fuel preparation in the OCP_TM injector differs from that in a high-pressure direct fuel injector: With a high-pressure injector, the jet decomposes primarily from turbulence and inertia in the liquid jet itself. Up to the end of the decomposition process, a distance of approximately 10-50 times that of the orifice diameter is necessary. In the case of an air-supported injector, the jet dissipates when the aerodynamic forces exceed the surface tension in the liquid. The pressure level in the air injector is such that the critical pressure ratio at the valve orifice is exceeded during injection. The resulting sound velocity of the airflow causes strong aerodynamic forces to be exerted on the fuel jet.

The essential part of the atomization process is over directly at the valve outlet. Other thermodynamic effects such as evaporation play a special role particularly when injecting fuel into a high- temperature medium. This interaction of the fuel spray with the cylinder charge represents the interface between the fuel system and the combustion system. The atomization quality can be seen in Fig. 12-14. The average Sauter mean diameter (SMD) is 10.3 µm; only an insignificant number of droplets have a diameter over 40 µm.

Fig. 12-14. Drop size distribution in relation to volume and time in air-supported direct injection

The required air mass flow for the OCP_TM air injector in reference to the entire amount of air inducted by the engine varies from 15% in homogeneous idling to 1.5% in full-load operation. As an absolute quantity, this results in approximately 5-9 mg air per injected fuel per pulse the air rail is preferably set at 6.5 bar. Compressed air is generated by means of a water-cooled piston compressor that is driven by the engine via gears or a belt.

Because of the high turbulence and stratification in the air-supported OCP_TM direct-injection system, normally no additional measures are required to move the charge such as swirl valves. Because of the low sensitivity to internal cylinder flow, this injection system is particularly suitable for engines with different valve configurations without active or passive measures to move the charge.