SI Engine Air Charge Control

The cylinder air charge in a gasoline engine with a turbocharger is controlled through the position of the throttle and the waste gate (WG) [18]. The throttle position predominantly affects the intake manifold pressure and is used to restrict the air mass flow into the engine. The waste gate position affects the exhaust back pressure and the boost pressure, which allows an increase in air mass flow into the engine. The task of air charge control can be divided into the coordination between throttle and WG position and the actual control of the desired actuator positions.

a) Actuator Coordination

The coordination of throttle and WG position is a trade-off between transient response and fuel efficiency as shown by Eriksson et al [18]. The highest fuel efficiency is achieved if the WG is kept as open as possible to meet a specific air charge demand. Any increase in exhaust gas pressure increases the pumping work during the exhaust stroke and consequently reduces the fuel efficiency. However, as demonstrated by Gorzelic et al [19] the response of air charge to a change in WG position is much slower compared to a change in throttle position, which makes a smooth transition from throttled into boosted operating mode extremely difficult. The slow response to the WG actuator can be explained by the inertia of the turbocharger as well as by the filling and emptying of the intake and exhaust system, which further delay the build-up of boost pressure. The best response can be achieved if the WG is kept as closed as possible to meet a specific air charge demand. In this case, the WG is only used to limit the maximum boost pressure and the throttle is used to control the intake manifold pressure. However, the higher the boost pressure during throttled engine operation, the higher the penalty in fuel efficiency due to the increased pumping

work. A good compromise between fast response and high fuel efficiency can be achieved with the use of a boost buffer as shown by Beckman et al [20]. The aim of such a strategy is to build up a specific boost pressure when the intake manifold pressure is below ambient pressure and to maintain a specific pressure difference across the throttle once the intake manifold pressure exceeds ambient pressure. This allows a smooth transition from the throttled into the boosted operating range as well as a fast air charge response at high load.

Since most modern SI engines are equipped with a Mass Air Flow (MAF) sensor, it is theoretically possible to control throttle and WG position with a simple Proportional-Integral-Derivative (PID) controller. However, this would result in a very slow and poor response of the desired air charge [13]. To achieve a highly responsive but robust system, a more advanced control structure is required. In literature, a wider range of approaches with different complexity is available. The simplest methods combine feedback with static feedforward control as shown in Thomasson et al [21], Wakeman and Wright [22] or Iserman [13]. Replacing the static feed forward model with a linearized dynamic feed forward model, as shown by Colin et al [23], Kranik et al [24], Kalabic et al [25], Moulin et al [26] and Leroy et al [27], can further improve the transient response. The most advanced solutions, as presented by Cieslar [28] and Colin et al [29] make use of model predictive control which theoretically allows a close to optimal operation.

b) Actuator Position Control

All advanced control structures mentioned above require a static or dynamic model of the air-path. The most common solution for air charge control is the combination of feedforward and feedback control as shown in Isermann [13]. The engine air charge of a SI engine at a specific engine speed is mainly dependent on the intake manifold pressure. Therefore, a two-dimensional map, which describes how the air charge depends on intake manifold pressure and engine speed, forms the core of such a control strategy as shown by Colin [29]. The air charge model is multiplied by a one-dimensional map, which describes how the air charge is affected by the intake manifold temperature as illustrated in Figure 1.9. The inversion of this model allows converting the desired air charge into a desired intake manifold pressure as shown in Figure 1.10.

Figure 1.9: ECU air charge model Adopted from [13]

Figure 1.10: ECU air charge model inversion Adopted from [13]

Combining the desired intake manifold pressure with the desired delta throttle pressure allows an estimate of desired boost pressure. The throttle is then used to control the intake manifold pressure, and the waste gate is used to control the boost pressure. Using the interconnected control structure, throttle and WG can be controlled as demonstrated in Figure 1.11 and Figure 1.12.

Figure 1.11: ECU throttle controller Adopted from [13]

Figure 1.12: ECU waste gate controller Adopted from [13]

The feedforward controller can be either a simple inverted stationary model or a linearised inverted dynamic model. The complete air charge control strategy is summarised in Figure 1.13.

Feedforward torque based engine control strategies rely on a number of invertible stationary models. Generating these models is called the engine calibration process and requires a large amount of test data, which cover the entire engine operating range [14]. It is crucial that the calibration is performed with steady- state test data. In case the models do not describe the engine at steady-state conditions, accurate and fast responsive torque control is not possible, and thus the car will have very poor drivability as well as a poor fuel consumption and high emissions.