High-Pressure Injection

Another possibility for directly injecting fuel into the cylinder is liquid fuel injection using the common rail principle (fuel injection from a common pressure line).

Figure 12-15 provides a system overview of high- pressure gasoline injection divided into an engine control system and a fuel system.

A spark-ignition engine with direct injection requires the use of an electronically controlled throttle valve for the various modes of operation with homogeneous charging and stratified charging. To control the mixture to produce lean mixtures with stratified charging requires a linear Л sensor that can also ensure this function for homogeneous operation with λ = 1.

Fig. 12-15. Gasoline direct injection—system overview

The high-pressure pump is fed from the low-pressure system that has a pressure of 1-4 bar. In the mechanically driven high-pressure pump, the fuel pressure is increased to 120 bar. The high-pressure is controlled via an electrically controlled pressure control valve. The return from the high-pressure line ends directly from the pressure control valve in the feed for the high-pressure pump. A pressure sensor serves to detect the pressure. For reasons of safety, an overpressure valve is integrated in the high-pressure circuit that limits the maximum fuel pressure. If the fuel pressure in the fuel rail is kept constant over the entire speed and load ranges, the electrical pressure regulator can, in principle, be replaced by a mechanical pressure regulator.

The fuel injector is located directly in the cylinder head. Because of the high fuel pressure, the magnetic force required to open the valve needle must be much higher than with low-pressure fuel injectors. In addition, the valve needle must be opened and closed extremely quickly for charge stratification and metering.

The fuel atomization quality strongly depends on the fuel pressure, the counterpressure, the flow calibration, and the spray dispersal angle.

Figure 12-16 shows the atomization quality of a high- pressure fuel injector in comparison to a low-pressure fuel injector and an air-supported fuel injector. Different types of combustion and combustion chambers require different flow calibrations and jet shapes.

Fig. 12-16. Comparison of the atomization quality of high-pressure injection valves and intake manifold fuel injectors

The high-pressure pump provides the pressure (50 to 120 bar) for the fuel rail. The injection pressure has recently been approaching 200 bar, and no limit is yet in sight for jet-directed procedures. Given a multihole nozzle, this yields a stable spray dispersal angle, good evaporation, and mixture preparation. The high-pressure pump is driven directly by the engine camshaft and is, therefore, mounted on the engine. A distinction is drawn between radial and axial high-pressure pumps.

Figure 12-17 shows an axial plunger pump. A swash plate is rotated by a mechanically driven shaft that is responsible for the alternating stroke movement of the three pistons. Fuel passes into the cylinder through a groove in the swash plate and is ejected via a nonreturn valve in the outlet. Each piston is mounted to the swash plate by a ball-and-socket joint. The bearing and pump chambers are separated by a shaft-sealing ring. The combined materials and coatings are adapted to the wear and lubrication requirements of gasoline operation. The extraordinarily narrow tolerances require the fuel to be finely filtered.

Fig. 12-17. High-pressure fuel pump

For high-pressure direct injection, there are numerous new requirements on the engine control system:
- The pressure in the high-pressure fuel system must be regulated.
- Lean operation in engines with direct injection requires a linear lambda sensor that covers the lean range and operating range where λ = 1.
- The high-pressure fuel injectors require a control system that is adapted to the special requirements of this technology. The high fuel pressure and greater demand for linearity and reproducibility from injection to injection make it necessary to alter the fuel injector control system. For the fuel injectors to open quickly, an increase in voltage and current to 80 V/ 10 A is required. In integrating the driver stages in the electronic control unit, the greater power loss of the driver stages must be taken into consideration.

- For lean engines, the use of an electrical throttle valve is essential. The control of this engine-operated throttle valve ensures that the pedal and throttle valve positions are completely independent.
- In unthrottled engine operation, there is no pressure differential for scavenging the carbon canister. To attain the necessary scavenging rates, a pump is required for scavenging the carbon canister in engine designs with a high-stratified charge component.

In contrast to conventional engines with intake manifold injection systems that work under nearly all operating conditions and a homogeneous stoichiometric mixture (i.e., the mixture is enriched only in special engine states such as cold start, warm-up, and full load), the Otto engine with direct injection is operated using different injection and combustion strategies.

The fuel preparation strategies that produce different homogeneous operating conditions and stratified charging are discussed below (Fig. 12-18).

Fig. 12-18. Engine operation with homogeneous and stratified charging

The goal of stratified charging is to concentrate a well- prepared fuel-air mixture at the spark plug so that a locally limited, ignitable mixture arises (λ ≈ 1) that creates favorable conditions for combustion despite the overall lean mixture. Because of the local concentration of the mixture in the center of the combustion chamber, stratified charge operation also allows high exhaust gas recycling rates. The stratification of the mixture around the spark plug is attained by late injection during the compression cycle. The throttle valve is opened completely for maximum air induction into the cylinder.

The jet direction, jet shape, jet penetration depth, and air flow in the cylinder are the decisive parameters for a successful stratification of the injected fuel near the spark plug.

The jet direction cannot be changed during engine operation. The penetration depth depends on the difference between the fuel jet speed and the air flow speed in the cylinder. The jet speed can be influenced by the injection pressure. To attain a specific movement of air flow in the combustion chamber with turbulence near the spark plug (for good mixture preparation), the design of the intake duct and the combustion chamber (a key responsibility of the engine manufacturer) must be adapted and optimized. Present intake systems are constructed with a tumble or swirl design.

When the operating state of the engine is to be changed from stratified charging to homogeneous charging with the same engine output, the inducted air mass must be reduced by closing the throttle valve; simultaneously, the amount of injected fuel in the cylinder must be increased to compensate for the greater throttling loss. To produce a homogeneous mixture in the combustion chamber, the fuel is injected during the intake cycle at the point in time in which the air speed is at its maximum.

Problems with mixture preparation, drivability, and, above all, exhaust emissions prevent stratified charging from being used over the entire operating range of the engine.

The different operation states over the working range of the engine are shown in Fig. 12-19. The example also includes cooling water temperature to illustrate the influence of different environmental states.

Fig. 12-19. Operating strategies in the engine map

The following combustion states exist:
- Homogeneous rich
- Homogeneous λ = 1 with or without exhaust gas recycling
- Homogeneous lean with exhaust gas recycling
- Stratified charging with a high exhaust gas recycling rate

At low and medium loads and speeds, the engine is operated with stratified charging and a high exhaust gas recycling rate. This yields lower fuel consumption. The exhaust temperature determines the operating range at low loads in which the engine can be operated unthrottled. For the catalytic converter to convert the pollutants, the catalytic converter temperature may not fall below 250°C. Completely unthrottled operation is, therefore, impossible while the engine is idling. Even in a cold start and during warm-up, the engine runs with a homogeneous, slightly lean mixture to quickly start the catalytic converter. A hot engine can function with throttled stratified charging.

At a low partial load and high speeds, it is difficult to attain good mixture preparation with stratified charging because of the short mixture preparation time and the danger of soot formation. A homogeneous mixture with EGR is, therefore, preferable.

The NO_x emissions and the danger of soot formation pose limits to stratified charging in the upper partial-load range. In this range, operation with homogeneous charging and EGR has only a slightly negative effect on fuel consumption in comparison to stratified charging with EGR; however, the N0^ emissions are lower, and there is no danger of soot formation.

Homogeneous lean operation is limited by the exhaust temperature. At temperatures over 500°C, storage catalytic converters are no longer able to store nitrogen oxides so that the engine is operated with a stoichiometric mixture and high EGR rates to reduce the N0_x emissions and fuel consumption.

Exhaust gas recycling is not possible when operating at full load. The engine is controlled in the same manner as engines with intake manifold injection, i.e., with a mixture for maximum performance and optimal catalytic converter protection.

In addition to changing the operating states during the transitions between different load states (such as the transition from stratified charging at partial load to a homogeneous enriched mixture at full load during acceleration), a change between two operating conditions can also be necessary in an unchanging load for exhaust aftertreatment. The main requirement is a transition without a change in torque since this is perceived by the driver.

To regenerate the catalytic converter during lean operation, increased fuel consumption of up to 3% is to be expected.