Aftermarket April 2022

BY Damien Coleman, Product Manager/ EBI Specialist at Snap-on T here are many systems and functions in the modern motor vehicle which require a method of controlling and reversing the electrical polarity of a circuit. The most common process is controlling the direction of rotation of a motor, e.g. electric window motor, electric power steering motor, electronic throttle control motor, etc. Electronic throttle control has now become standard fitment on all spark ignition engines. This has allowed for advancements in technology such as cruise control, traction control, limited operating strategy (limp home mode) and engine speed limiter. Although these functions previously existed, electronic throttle control has further refined the operation of the vehicle, complementing these functions. Another advantage of electronic throttle control has been seen on vehicles with direct fuel injection which can function in two modes: Homogeneous and stratified. In homogeneous mode, the engine runs on a stoichiometric air to fuel ratio of 14.7:1, and fuel is injected late on the exhaust stroke/early on the intake stroke. The fuel and air mix to give a homogenous fuel mixture. In stratified mode, fuel is injected late on the compression stroke and the mixture formation is heterogeneous. Please refer to the diagram as seen in Fig.1. On the diagram the blue circles represent air and the red circles represent fuel. As can be seen, the mixture is exceptionally rich in the centre of the heterogeneous mixture, but overall as a whole the mixture is lean. Under stratified running mode the throttle valve is held fully open and the driver demand (accelerator pedal input) is varied by adjusting the quantity and pressure of the fuel being delivered. Without electronic throttle control this functionality would be at best extremely complex or more realistically, would be impossible as there has to be a 32 AFTERMARKET APRIL 2022 TECHNICAL/SNAP-ON www.aftermarketonline.net MOTOR CONTROL SYSTEMS This month, Snap-on’s Damien delves deeply into the functioning of motor control systems complete disconnect between the accelerator pedal input and the control of the throttle valve. Vehicles fitted with electronic control can precisely control idle speed and also use additional processes such as closing the throttle valve slightly under cranking to improve engine start-up. This is controlled by changing the electrical polarity of the throttle motor. Please refer to Fig.2, which shows a basic electronic throttle control system. As can be seen from the image, accurate monitoring of the accelerator pedal position (APP) and throttle motor position (TMPS) are required. These sensors have two/three outputs for redundancy and fault detection. In the case of a single track/circuit failure then the system can still function, also electrical short and open circuits can easily be detected. Fig.3 and Fig.4 offer examples of both APP and TMPS position sensor waveforms respectively. As had previously been outlined, the polarity of the motor is reversible which allows accurate control of the entire system. A way to control the polarity is by designing a circuit referred to as an H-drive bridge circuit. The bridge circuit can be created using insulated gate bipolar junction transistors (IGBT) or metal oxide field effect transistors (MOSFETS), the latter being the most common way of controlling such a circuit. A control circuit using mechanical switches is shown as Fig.5 for demonstration purposes. A) Mechanical switches B) IGBTs C) MOSFETs Next, Fig.6 and Fig.7 demonstrate the fundamental operation of the circuit, basic mechanical switches are used to simplify the circuit. With switches S1 and S4 closed, the current flow through the motor results in the motor