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Chapter 3 Other Electronic Systems A quick rundown on other electronically-controlled systems in a car. I N ADDITION TO engine management, there’s a host of other car systems which are electronic. And if they’re electronic, they’re potentially cheap and easy to modify! In the past, few people even thought of modifying these systems but the on-road gains can be very worthwhile. 4-Wheel Drive While there are many different all-wheel drive performance systems available, many use electronic control. This usually takes the form of a wet 20 PERFORMANCE ELECTRONICS FOR CARS multi-plate clutch that is controlled electro-hydraulically. When the electronic control system directs that 4-wheel drive is needed, the clutch (a little like a clutch pack in an automatic transmission) progressively clamps up, passing power to the wheels that are not normally driven. The benefit of this approach over a mechanical 4-wheel drive system that requires a variation in front/ rear wheel speeds before it activates is that the electronic system can be pro-active. In other words, it can put the car into 4-wheel drive before it is actually needed. (Note: whether this approach is better than traditional viscous-coupled constant 4-wheel drive is open to debate; it very much depends on how the electronic system is programmed). The most famous car to use this electro-hydraulic approach to 4-wheel drive is the R32 Nissan Skyline GT-R. It is primarily a rear-wheel drive vehicle but the front wheels are powered when certain conditions are met. Fig.1 shows a flow diagram of its electronic siliconchip.com.au Fig.1: the R32 Skyline GT-R’s 4-wheel drive electronic control system initially looks complex. However, when the inputs (wheel speeds and longitudinal and lateral acceleration) and the outputs (a warning lamp for malfunctions and a pulse width modulated solenoid valve to engage 4-wheel drive) are looked at in isolation, it becomes a lot easier to understand. Modifying the accelerometer input dramatically changes the on-road attitudes of the car. [US Patents Office] control, with the diagram taken from the original US patent for the system. All four wheel speeds are sensed and in addition, two lateral acceleration sensors and one longitudinal acceleration sensor have inputs to the ECU. From the wheel speeds, a front/rear speed differential is calculated – this is the primary input for deciding when 4-wheel drive is needed. However, the outcome of this calculation is heavily influenced by the lateral and longitudinal acceleration. To provide traditional power oversteer, the progression into 4-wheel drive is slowed when the car is cornering. A simpler version of this system is used on the current Nissan X-Trail, with this car’s approach shown in Fig.2. Another brilliant car that uses a complex electronically-controlled 4-wheel drive system is the Mitsubishi Lancer Evo VII. Fig.3 shows the layout of its control system. In addition to siliconchip.com.au Electronic stability control uses an ABS hydraulic actuator to brake individual wheels to pull the car back onto the cornering line. This photo shows the four wheel speed sensors, a steering angle sensor, a yaw-rate sensor, the ECU and the hydraulic control unit. PERFORMANCE ELECTRONICS FOR CARS 21 Nissan X-Trail 4-Wheel Drive Fig.2: the current Nissan X-Trail uses a 4-wheel drive system based on the GT-R Skyline, although it is normally in front-wheel drive mode rather than the Skyline’s rear-wheel drive mode. As with the Skyline, it uses a wet multi-plate clutch to transfer torque to the normally undriven wheels, however its electronic control system uses only one accelerometer sensor. [Nissan] front/rear torque split, the electronic control system can alter the rear differential’s left/right split. The inputs to the system comprise information on steering angle, throttle opening, individual wheel speeds, longitudinal acceleration and lateral acceleration. In addition, a driver-select mode switch, the parking brake and ABS system have inputs. Two multi-plate clutches control the torque splits. Power Steering Electronically variable power steer- ing alters the weight of the steering on the basis of road speed. This is in contrast with previous variable weight systems that usually altered steering rate hydraulically with engine speed – at high RPM the assistance was reduced. Road speed systems use a variable solenoid valve to control the steering effort. The hydraulic flow through this valve usually resists the steering input in some way – ie, it works against the normal assistance. In other cars, the amount of fluid available to do the assisting is changed. The primary input to a variableweight electronic control system is normally road speed but some cars also use an additional steering angle sensor input. The output solenoid is controlled by varying its duty cycle. ABS ABS (Anti-skid Braking System) prevents wheels locking under heavy braking, to shorten stopping distances and also allow steering control to be maintained. In operation, the wheel speeds are individually monitored and individually varied in braking effort (this is a 4-channel system). Alternatively, the rear wheels may be treated as a pair in terms of speed monitoring and control (this is a 3-channel system). Inputs to the system comprise the Mitsubishi Lancer Evo VII 4-Wheel Drive Fig.3: the Mitsubishi Lancer Evo VII has arguably the best 4-wheel drive high-performance chassis in the business. The main inputs into its electronic control system are steering angle, throttle opening, and lateral and longitudinal acceleration. The outputs are the solenoids that control the front/rear and rear lateral torque splits. [Mitsubishi] 22 PERFORMANCE ELECTRONICS FOR CARS siliconchip.com.au The Skyline GT-R uses an electronically controlled 4-wheel drive system which can be easily returned to rear-wheel drive. Various interceptors can be used to vary the system’s behaviour, allowing driver adjustment of the car’s on-power handling characteristics. wheel speeds and often an accelerometer that monitors actual deceleration under braking. An hydraulic unit controls the wheel braking, while an ECU provides overall system control. In many cars, this now also includes stability control and 4-wheel drive or traction control (if fitted). that use open (ie, non-locking) diffs, where the power distribution – both from side-to-side and front-to-rear – can be controlled by individual wheel braking. Some cars mix approaches, reducing engine torque and individually braking wheels when slippage occurs. Traction Control Stability Control System Traction control systems limit wheel spin. Inputs are from the wheel speed sensors (normally the same ones as for ABS), with the system reducing engine torque in a variety of ways when wheel spin is detected. The most common method on current cars is to close an electronicallycontrolled throttle. Alternatively, some cars use a second throttle in series (electronically controlled, even when the main throttle isn’t) or cut fuel and/or retard ignition timing to drop engine torque. Another form of traction control is to brake the wheel that is spinning. For example, in a front-wheel drive vehicle, a spinning lefthand wheel will be braked, which in turn sends power to the righthand wheel. This type of traction control can be taken to another level in 4-wheel drive vehicles siliconchip.com.au Stability control systems help correct car attitude when the car is understeering or oversteering. The system does this by braking individual wheels. If a car is understeering (ie, the front running wide), the inside rear wheel is braked, causing the car to pivot around it. This causes the nose to be pulled back onto the cornering line. Conversely, in an oversteering car, the outside front wheel is braked, Fig.4: variable-weight electronically-controlled power steering usually alters the duty cycle of an hydraulic solenoid to control the flow of oil resisting the steering movement. This system relies on just a single input – ie, the road speed. [Holden] PERFORMANCE ELECTRONICS FOR CARS 23 Fig.5: climate control systems can vary a lot in complexity – this system is a “mid level” one. Input sensors include temperatures and sunlight intensity, while the primary outputs are the air-conditioner compressor clutch, fan speed control and the positions of various duct flaps. [Nissan] As more and more cars are fitted with electronic stability control, modifying the systems to achieve required performance outcomes is going to become increasingly common. It’s just a matter of modifying the sensor outputs before they are fed to the ECU. 24 PERFORMANCE ELECTRONICS FOR CARS which again has the effect of reducing the slide. In addition, engine power is often varied – for example, if a rearwheel drive car is power oversteering, engine torque will be reduced at the same time as the braking corrections are being made. Stability control systems are normally integrated with the traction control system. In some ways, the two systems perform a similar task, although it should be noted that stability control is far more sophisticated and effective. For example, it can also control car cornering attitudes when no throttle at all is being used; eg, in a lift-off oversteer situation. In addition to wheel speed inputs used by the traction control and ABS systems, stability control has inputs from a steering angle sensor, yaw rate sensor and longitudinal acceleration sensor. Fig.7 shows one system – note that in this car, a single electronic control unit (ECU) looks after antilock braking (ABS), traction control siliconchip.com.au (TRC) and stability control (VSC). It is the difference in the yaw angle of the car compared with the predicted yaw based on steering input which is the main determinant of the braking and throttle outcomes of the system. Understanding Traction Control Systems Climate Control Climate control systems regulate the interior temperature and air flow. Depending on the car, their complexity varies immensely. Typically, a control unit has inputs from interior and exterior temperature sensors, the coolant and the airconditioning evaporator. In addition, a sunlight sensor (normally mounted on top of the dashboard) is used. Each of these sensors is usually a variable resistor (ie, a thermistor). Outputs include the ventilation fan speed control, air-conditioner compressor magnetic clutch, and actuators to control the position of the various flaps that direct air. The flap actuators can be stepper motors or vacuum actuators switched by solenoids. Many cars now also have an autorecirculation function that activates when the air outside is polluted; eg, when following a car or truck with a smoking exhaust. Headlight Height Adjustment Fig.6: traction control systems can take a number of forms. This design uses a second electronically-controlled throttle butterfly to reduce engine torque when wheel spin is detected. The main input signals are from the wheel speed sensors, which are shared with the ABS. [Lexus] Manual in-cabin headlight height adjustment is common on European cars, while automatic height adjustment (which takes into account any car attitude changes caused by load variations) is used on all cars with high-intensity gas discharge headlights. These systems use front and rear suspension height sensors as the main ECU inputs, with the outputs going to the headlight height control motors. Conclusion The reason that we’ve covered these car systems is that each can be easily modified by cheap interceptors. For example, if you regard the steering in a car as being too light, it can be altered by using an interceptor (provided, of course, that the steering weight is electronically-controlled). If you drive a car with electronicallycontrolled 4-wheel drive, it’s easy to change the system’s behaviour. The same goes for the climate control – perhaps you’d like the system to be more sensitive to sunlight changes, for example. Automatic headlight height siliconchip.com.au Fig.7: in this car, a single electronic control unit (ECU) looks after anti-lock braking (ABS), traction control (TRC) and stability control (VSC). The main inputs are from individual wheel speed sensors, a yaw rate sensor and a steering angle sensor. [Lexus] control? – it’s easy to add a knob that allows manual height changes as well. While many of the systems shown here are seldom modified, there’s absolutely no reason why you can’t personalise  them to suit your preferences. PERFORMANCE ELECTRONICS FOR CARS 25