Boost controller: Difference between revisions

Revision as of 22:37, 9 July 2017

Revision as of 22:40, 9 July 2017

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Control for the solenoids and stepper motors can be either [[PID controller|closed loop]] or [[open loop]]. Closed loop systems rely on [[Feedback loop|feedback]] from a [[Manifold Absolute Pressure sensor|manifold pressure sensor]] to meet a predetermined boost pressure. Open loop systems have a predetermined control output where control output is merely based on other inputs such as throttle angle and/or engine [[Revolutions per minute|RPM]]. Open loop specifically leaves out a desired boost level, while closed loop attempts to target a specific level of boost pressure. Since open loop systems do not modify control levels based on MAP sensor, differing boost pressure levels may be reached based on outside variables such as weather conditions or engine coolant temperature. For this reason, systems that do not feature closed loop operation are not as widespread.

Boost controllers often use pulse width modulation (PWM) techniques to bleed off boost pressure on its way to the reference port on the wastegate actuator diaphragm in order to (on occasion ) under report boost pressure in such a way that the wastegate permits a turbocharger to build more boost pressure in the intake than it normally could. In effect, a boost-control solenoid valve lies to the wastegate under the [[engine control unit]]´s (ECU) control. The boost control solenoid contains a needle valve that can open and close very fastquickly. 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 rate at which air bleeds out of the T in the manifold pressure reference line to the wastegate. This effectively changes the air pressure as seen by the wastegate actuator diaphragm. 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.

The wastegate control solenoid can be commanded to run in a variety of frequencies in various gears, engine speeds, or according to various other factors in a deterministic open-loop mode. Or, by monitoring manifold pressure in a feedback loop-, the engine management system can monitor the efficacy of PWM changes in the boost control solenoid bleed rate at altering boost pressure in the intake manifold, increasing or decreasing the bleed rate to target a particular maximum boost.

The basic algorithm sometimes involves the EMS (engine management system) "learning" how fastquickly the turbocharger can spool and how fastquickly the boost pressure increases. Armed with this knowledge, as long as boost pressure is below a predetermined allowable ceiling, the EMS will open the boost control solenoid to allow the turbocharger to create overboost beyond what the wastegate would normally allow. As overboost reaches the programmable maximum, the EMS begins to decrease the bleed rate through the control solenoid to raise boost pressure as seen at the wastegate actuator diaphragm so the wastegate opens enough to limit boost to the maximum configured level of over-boost.

Stepper motors 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.

Many configurations are possible with 2-, 3-, and 4-port solenoids and stepper motors in series or parallel. Two -port solenoid bleed systems with a [[PID controller]] tend to be common on factory turbocharged cars.

=== Advantages ===