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A boost controller is a device to control the boost level produced in the intake manifold of a turbocharged or supercharged engine by affecting the air pressure delivered to the pneumatic and mechanical wastegate actuator.
A boost controller can be a simple manual control which can be easily fabricated, or it may be included as part of the engine management computer in a factory turbocharged car, or an aftermarket electronic boost controller.
Operation
The purpose of a boost controller is to reduce the boost pressure seen by the wastegate's reference port, in order to trick the wastegate into allowing higher boost pressures than it was designed for.
Many boost controllers use a needle valve that is opened and closed by an electric solenoid. By varying the pulse width to the solenoid, the solenoid valve can be commanded to be open a certain percentage of the time. This effectively alters the flow rate of air pressure through the valve, changing the amount of air that is bled out instead of going to the wastegate's reference port. Solenoids may require small diameter restrictors be installed in the air control lines to limit airflow and even out the on/off nature of their operation. Two-port solenoid bleed systems with a PID controller tend to be common on factory turbocharged cars.
An alternative design is to use a stepper motor. These designs allow fine control of airflow based on position and speed of the motor, but may have low total airflow capability. Some systems use a solenoid in conjunction with a stepper motor, with the stepper motor allowing fine control and the solenoid coarse control.
At the component level, boost pressure can either be bled out of the control lines or blocked outright.[citation needed] Either can achieve the goal of reducing pressure pushing against the wastegate. In a bleed-type system air is allowed to pass out of the control lines, reducing the load on the wastegate actuator. On a blocking configuration, air traveling from the charge air supply to the wastegate actuator is blocked while simultaneously bleeding any pressure that has previously built up at the wastegate actuator.
Control system
Most modern designs are electronic boost controllers that use an electronic control unit to control the boost via a solenoid or stepper motor. The operating principle is the same as older manual boost controllers, which is to control the air pressure presented to the wastegate actuator. Electronic controllers add greater flexibility in management of boost pressures, compared with the manual controllers.
The actuation of an electronic boost controller can be managed by one of two control systems:
- Open-loop is the simpler option, where control output is merely based on other inputs such as throttle angle and/or engine speed (RPM). Since an open-loop system does not include any monitoring of boost pressure, the boost pressure may vary based on outside variables (such as weather conditions or engine coolant temperature). For this reason, open-loop systems are less common.[citation needed]
- Closed-loop systems rely on feedback from a manifold pressure sensor to maintain a target boost pressure as closely as possible. These systems are more sophisticated and can more accurately control boost pressure under varying circumstances.
Advantages
Since less positive pressure can be present at the wastegate actuator as desired boost is approached the wastegate remains closer to a completely closed state. This keeps exhaust gas routed through the turbine and increases energy transferred to the wheels of the turbocharger. Once desired boost is reached, closed loop based systems react by allowing more air pressure to reach the wastegate actuator to stop the further increase in air pressure so desired boost levels are maintained. This reduces turbocharger lag and lowers boost threshold. Boost pressure builds faster when the throttle is depressed quickly and allows boost pressure to build at lower engine RPM than without such a system.
This also allows the use of a much softer spring in the actuator. For instance, a 7 psi (0.48 bar) spring together with a boost controller may still be able to achieve a maximum boost level of well over 15 psi (1.0 bar). The electronic control unit can be programmed to control 7 psi (0.48 bar) psi at half throttle, 12 psi (0.83 bar) at 3/4 throttle, and 15 psi (1.0 bar) at full throttle, or whatever levels the programmer or designer of the control unit intends. This partial throttle control greatly increases driver control over the engine and vehicle.
Limitations and disadvantages
Even with an electronic controller, actuator springs that are too soft can cause the wastegate to open before desired. Exhaust gas backpressure is still pushing against the wastegate valve itself. This backpressure can overcome the spring pressure without the aid of the actuator at all. Electronic control may still enable control of boost to over double gauge pressure of the spring's rated pressure.
The solenoid and stepper motors also need to be installed in such a way to maximize the advantages of failure modes. For instance, if a solenoid is installed to control boost electronically, it should be installed such that if the solenoid fails in the most common failure mode (probably non-energized position) the boost control falls back to simple wastegate actuator boost levels. It is possible a solenoid or stepper motor could get stuck in a position that lets no boost pressure reach the wastegate, causing boost to quickly rise out of control.
The electronic systems, extra hoses, solenoids and soforth add complexity to the turbocharger system. This runs counter to the "keep it simple" principle as there are more things that can go wrong. It is worth noting that virtually all modern factory turbocharged cars, the same cars with long warranty periods, implement electronic boost control. Manufacturers such as Subaru, Mitsubishi and Saab integrate electronic boost control in all turbo model cars.
Past and future
There are other outdated methods of boost control, such as intake restriction or bleed off. For instance, it is possible to install a large butterfly valve in the intake to restrict airflow as desired boost is approached. It is also possible to actually release large amounts of already compressed air similar to a blowoff valve but on a constant basis to maintain desired boost at the intake manifold. The currently popular exhaust gas bypass via wastegate is quite superior if compared to creating intake restriction or wasting energy by releasing air that has already been compressed. These methods are rarely used in modern system due to the large sacrifices in efficiency, heat, and reliability.
Other methods may come into widespread use in the future, such as variable geometry turbochargers. With a sufficiently large turbine, no wastegate is necessary. Low speed response and faster spool up are then obtained using variable turbine technologies rather than a smaller turbine. These systems may replace or supplement typical wastegates as they develop. Control methods for the variable mechanical controls, such as the principles of closed loop will still apply even if they no longer involve pneumatics.
See also
References
External links
- MBC Concept Overview, Installation, and Setup
- How To Guide - Manual Boost Controller Installation at addictiveperformance.com.