Automotive Engine Performance

Piston Designs

21-02 Explore the components of a GDI engine.

A comparison of the piston design of a conventional port fuel-injected engine and that of a GDI engine reveals that the pistons in a GDI engine are designed to direct the fuel that is injected into the cylinder toward the spark plug. By directing the fuel up and toward the spark plug, the air-fuel ratio can get as high as 30:1. This lean condition allows for a highly efficient engine. The downside of running an engine this lean is that it runs very hot, which can lead to the production of oxides of nitrogen (NOx). This is remedied by introducing exhaust gas recirculation (EGR) into the cylinder to cool the combustion chamber temperatures.

The design of a GDI piston is contingent on the location of the injector in the combustion chamber. The purpose of the design is to push the fuel injected into the cylinder toward the spark plug (FIGURE 21-6). The interchangeability of pistons within this type of engine is not possible, because depending on the location of the piston in the engine, it may affect the fuel direction when it hits the piston (FIGURE 21-7). The atomization of fuel within the cylinder is one of the features of the pistons; another is the higher compression ratio. By increasing the compression ratio within the engine, the efficiency and the power output increase. Because of the increased pressures within the cylinder, the pistons in a GDI engine must be much stronger than those in a conventional engine. As these pressures increase, the use of forged pistons in GDI applications is becoming the acceptable practice by manufactures of these types of engines.

FIGURE 21-6 The GDI piston is designed differently from a conventional piston: It plays a role in directing fuel toward the spark plug for more efficient combustion.

FIGURE 21-7 The GDI piston head helps to direct fuel toward the spark plug so that it is able to be efficiently combusted to create the most possible power.

Injectors

A GDI engine, like a conventional combustion engine, uses a fuel injector to introduce fuel into the combustion chamber (FIGURE 21-8). The location of the injector is different, though, because the GDI injector is located within the combustion chamber. The ability of the injector to directly inject fuel into the combustion chamber allows the PCM to precisely control how much fuel enters the cylinder and at what point. The port fuel-injection system sprayed fuel at the backside of the intake valve, hoping that all of it will be delivered into the cylinder. To make the injection event happen at the correct time, the PCM uses camshaft position (CMP) and crankshaft position (CKP) sensors to coordinate the opening of the injector. This allows for having multiple injection events to better fill the cylinder before the ignition event happens.

FIGURE 21-8 The port fuel injector is smaller than the GDI injector because it only has to reach the intake port to fill up the backside of the valve with fuel. The GDI injector has to reach all the way into combustion chamber.

Due to the extreme pressures that the GDI injectors are subjected to, vehicle battery voltage is not sufficient enough to open the injector’s pintle. Because of this increased pressure, the injector must first be opened with a higher voltage, around 65 volts. This is used to overcome the increased pressure; once the injector is open, the voltage returns to 12 volts to maintain the position of the injector pintle. The injection control voltages above the normal battery voltage are created within the PCM, using step-up transformers so that it can induce the needed voltage to overcome the increased pressures (FIGURE 21-9). In order to introduce the fuel into the combustion chamber in the correct location their tip of the injector has precision machined holes in specific locations to direct the fuel toward the machined surface of the piston head (FIGURE 21-10). The injector has a slender tip so that it can protrude through the cylinder head, and it allows the cooling jacket around the injector to help cool the head of the injector in the combustion chamber (FIGURE 21-11). To help control emissions and carbon inside the engine, manufactures are trying different styles of injectors to increase longevity and decrease maintenance.

FIGURE 21-9 The GDI injectors use step-up transformers to induce a higher voltage to overcome the increased pressures that the injector is operating in. The injector is trying to overcome both the combustion pressures and the increased fuel pressures.

FIGURE 21-10 The machined holes in the tip of the injector help to atomize and direct the fuel flow toward the correct location on the piston head. The pressure and direction allow for the piston head to control the splash and direct it toward the spark plug for a more complete combustion event.

FIGURE 21-11 The GDI engine must be cooled to help with keeping the fuel in the injector in a liquid form so that it can be injected properly. If the injector is not cooled, the fuel would vaporize, and the injector may overheat and not operate correctly.

Regulator

The regulator is integrated into the low-pressure side of the GDI fuel system so that when the fuel in the line expands, it has a place to exhaust (FIGURE 21-12). When the vehicle sits for a period of time and hot soaks, the regulator allows for the expansion of the fuel in the fuel line. When the fuel expands, the regulator allows the increased pressure to be returned to the fuel tank from the line to prevent component damage. For normal operation, the regular should not be needed, because the PCM is constantly getting a fuel pressure reading from the high-pressure fuel sensor on the fuel rail or injection pump. This sensor allows the PCM to use pulse-width modulation (PWM) on the high-pressure fuel pump so that excess pressure will not be built up in the high-pressure fuel system. Knowing the pressure on the high-pressure side of the fuel system will indicate the amount of pressure needed from the low side of the system to fulfill the needs of the engine. Fuel pump modulation increases the life of the fuel pump and prevents the need for an active fuel pressure regulator.

High-Pressure Fuel Sensor

To help the PCM regulate the on time of the low-pressure fuel pump and the high pressure, the GDI fuel system employs a high-pressure fuel senor that monitors the high-pressure injection pump’s output (FIGURE 21-13). This sensor is basically an electrical transducer that takes a mechanical pressure and turns it into an electrical signal that can be used to determine how the fuel system is operating. The electrical transducer has a very simple layout for operation: a one-wire 5 Vref signal and ground (FIGURE 21-14). This sensor is essential to the operation of the fuel system on a GDI engine. Without the input from the high-pressure fuel sensor, the PCM would not know what the fuel system was doing or the needs of the engine.

FIGURE 21-14 The high-pressure fuel sensor is a very simple transducer that gives a feedback to the PCM so that the PCM understands the amount of pressure that is present in the high-pressure side of the fuel system.

High-Pressure Injection Pump

The heart of the high-pressure fuel system is the high-pressure injection pump (FIGURE 21-15). The high-pressure pump operates of a three- or four-lobe design located on the camshaft that allows it to create the pressure needed to overcome the combustion chamber pressures. By using the camshaft to actuate the fuel pump, the pump will operate as long as the camshaft is turning, when the camshaft stops, pressure development stops. The camshaft lobe pushes a single-piston barrel pump that the PCM controls by monitoring the fuel pressure sensor located on or near the pump. Using a control valve in the fuel pump, the PCM changes the amount of pressure that the injection pump is creating. The fuel pumps on a GDI-equipped vehicle have an infinite number of combinations for controlling each fuel pump so that the injectors receive the correct amount of fuel pressure.