Mixture Formation by Means of Gasoline Injection

Intake Manifold Injection Systems. The demands for low vehicle emissions and low fuel consumption are the primary influences on the design of modem intake manifold injection systems where fuel is injected through electronically controlled fuel injectors for individual cylinders into the intake arms of Otto engines. A typical configuration for fulfilling standards for minimum emissions is shown in Fig. 12-8.

The measures to reduce emissions beyond the basic functions of the engine control systems—injection and ignition—are mainly based on the required emissions standards, the untreated emissions of the combustion engine, and the vehicle weight class in the smog test. For example, in the aftertreatment of exhaust, measures such as secondary air injection in combination with retarded ignition are required to quickly heat the catalytic converter. These measures, as well as their diagnosis, increase the complexity of the engine control system with sensors, actuators, cables, and computer programming.

Fig. 12-8. Intake manifold injection system

Typical functional features of modem engine control systems are
- Torque-based load control with an electronically controlled throttle valve [electronic throttle control (ETC)]
- Model-based functions such as model-based intake manifold filling with load detection via a hot film air mass meter or an intake manifold pressure sensor
- Control of the position of a continuously adjustable camshaft on the inlet and/or outlet side

- Control of various relays to turn on or shut off components (main relay, fuel pump relay, fan relay, starter relay, air-conditioner compressor relay, etc.)
- Active camshaft position sensor for quickly detecting the camshaft position and, hence, quickly synchronizing the engine control while starting the engine
- Cylinder-selective knock control based on a crankcase vibration sensor to provide optimum output and consumption control of the moment of ignition
- Control of the tank ventilation valves to regenerate the carbon canister while the engine is running

- Special catalytic converter heating function with secondary air system, retarded ignition, and transmission shifting point control
- Precise control of the mixture composition via an oxygen sensor (“lambda sensor”) upstream from the catalytic converter, and a trim control via a second oxygen sensor downstream from the catalytic converter
- Onboard diagnosis (OBD) of all exhaust-relevant components and functions.

In the design of the fuel system, care is taken to ensure that the fuel in the fuel rail is only slightly heated. If the fuel is heated in the phase after shutting off the engine (hot soak), vapor bubbles can arise in the fuel rail that can lead to problems in a subsequent hot start.

A distinction is drawn between two basic fuel system designs:
1. Fuel system with a return system (Fig. 12-9): A characteristic of this fuel system is that the pressure regulator is directly on the fuel rail. The pressure diaphragm receives the intake manifold pressure on one side so that a constant differential pressure arises between the fuel in the fuel rail and the intake manifold. Given a constant fuel injector control time, this makes the injected fuel quantity independent of the intake manifold pressure.

Fig. 12-9. Design of a fuel distribution system with return flow

The advantages of the fuel system with a return system are
- Favorable fuel pressure control dynamics
- Good hot start behavior from raising the fuel rail with cool fuel from the tank
- The injected fuel quantity is independent of the intake manifold pressure

A substantial disadvantage is that the fuel is heated in the tank (up to 10 К in contrast to systems without a return). This increases the fuel evaporation in the tank and the load on the carbon canister.

2. For this reason as well as to reduce system costs, return-free fuel systems were developed (Fig. 12-10). They are characterized by the integration of the fuel pump and pressure control valve in the tank or close to the tank.

Fig. 12-10. Layout of a return-free fuel distribution system

The advantage of this design is that the excess fuel does not have to be first pumped into the engine compartment and then flowed via the pressure regulator back into the tank. The injection times are correspondingly corrected by the engine control system as a function of the constant fuel pressure of approximately 350 kPa (3.5 bar ±0.5 bar).

To avoid large pressure fluctuations in the fuel rail that can lead to fluctuations in the injected fuel quantity, pressure pulsation dampers are used in return-free fuel systems.