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Electronic Engine Management Pt.12: Fueltronics’ Turbo Control Centre by Julian Edgar Australian company Fueltronics has recently released an automotive turbo­ c harger boost control unit. Called the “Turbo Control Centre”, the device under­cuts its competitors in price while also providing more features. It uses a Signetics 80C575 micro­con­troller and sophisticated software to control the turbo boost in modified cars. The unit consists of two solenoid-operated valves, an elec­ tronic control unit (ECU) housed in a diecast aluminium box, and a dashboard mount­ed liquid crystal display with control knobs. Conventional control Conventional turbocharger boost pressure control is by a wastegate – a valve which diverts exhaust gas around the tur­ bocharger’s exhaust turbine when the appropriate manifold super­charging pressure has been reached. The wastegate is operated by a rod connected to a diaphragm, one The Turbo Control Centre uses a microcontroller & sophisticated software to modulate turbocharger boost pressure. Note that the board in this photo is not totally populated, with the output transistors & some ICs still to be added. 16  Silicon Chip side of which sees manifold pressure. As boost pressure rises, the diaphragm deflects and the connecting rod opens the exhaust turbine wastegate valve. Electronic control The Turbo Control Centre makes use of this conventional mechanical control but changes the pressure seen by the wastegate actuator by bleeding air from the pressure line connecting the diaphragm to the manifold. A pulsed solenoid valve is used to do this and the greater the pulse-width used, the higher the boost pressure experienced. In order to allow relatively small valves and plumbing to be used, a restricting orifice is placed upstream of the bleed valve T-piece. There are several advantages in using electronic control of the wastegate. First, “wastegate creep” – where the rate of boost increase starts to taper off before the required level is reached –is avoided. With electronic control, the boost level can rapid­ly rise to the specified level before wastegate opening occurs. This means that quicker acceleration is enjoyed. Second, electronic management allows greater driver control over boost levels. The Turbo Control Centre uses two preset levels of boost, which are initially set with internal potentiom­ eters. These preset levels are selected using a pushbutton switch. Alternatively, the user can select the manual Turbocharger control is achieved using two solenoid-operated valves. The smaller valve on the left is used as a pulsed bleed valve & reduces the pressure seen by the wastegate actuator. The larger valve at right is used as a throttleclosed pressure-relief valve & is designed to give better driving response. Electronics engineer Jiri Bruderhans was responsible for the hardware development of the boost controller. The unit has been designed so that it can be interfaced with engine management software. boost setting, which allows control over the supercharging level via a dash-mounted 10-turn potentiometer. The other valve used by the Turbo Control Centre works as a throttle-closed blow-off valve. Conventionally, when the throttle is quickly closed following acceleration (eg, during gear chang­es), the rapid pressure build-up upstream of the turbo’s compres­sor slows the turbo. A slight lag in boost build-up then occurs when the throttle is opened again. The Turbo Control Centre blow- off system senses air pres­ sure both upstream and downstream of the throttle valve using electronic pressure sensors. When the pressure being experienced before the throttle blade is greater than after the butterfly, the large (25mm plumbing) valve is opened and this air is bled back to the turbo air intake. This allows the turbo to keep spinning at a high speed so that the boost quickly rises when the throttle is next opened. The difference required in the pressures either side of the throttle butterfly before the blow- off valve opens is adjustable with an internal pot. Shown on the dash-mounted LCD screen is the boost level in either numerical or bargraph form, and the selected boost control (Preset 1, 2 or Manual). In addition, the word “Blow!” is indi­cated when the blowoff valve is open. The display is also used during the setting-up procedure. The two signal inputs come from the pressure sensors, both of which are mounted within the ECU and are fed by small bore rubber hoses. The microprocessor and output drive circuitry is also within this box, with just the display electronics and input switches located in the dash-mounted unit. Hardware & software Running at a clock speed of 12MHz, the Signetics 8051-family 80C575 microcontroller has a tough job to do, especially in preventing boost oscillation around the selected boost level. The 8.5Kb program – which is written in 805X assembler language with ‘C’ 805X cross-compiler language used –required the great­ est design effort in allowing boost to rapidly rise without over-shooting the preset value. Inadvertent excessive boost can cause engine-destroying detonation. The program uses a differential equation for convergence, which allows the rising boost level to approach the preset level in a manner which allows system damping. The boost control valve remains closed until the boost level reaches a “window” set at 75% of the preset value. When this occurs, the control valve starts closing, with the software setting its pulse width on the basis of the rate of approach or the distance to the preset value. When the boost level reaches 95% of the preset value, the valve operation changes again, with a different feed­back loop being employed. Engine management Finally, since Fueltronics also rewrite software chips for factory engine management systems, the Turbo Control Centre has been designed to interface with new engine management software. This means, for example, that when the high boost preset is selected, an engine management program with the appropriate fuel and ignition maps can be automatically SC brought on-line. August 1994  17