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Chapter 2 All turbo cars of the last 15 years have electronic boost control. Some are closed loop (the boost pressure is monitored by a sensor which has an input into the ECU’s control strategy), while others are open loop (ie, there is no monitoring of boost). Advanced Engine Management Going beyond spark and fuel – other ECU functions. T HE FIRST CARS FITTED with engine management had systems that controlled only the spark timing, fuel injection and idle speed control. More recent cars use systems with many more outputs. Variable intake manifolds, electronic throttle, auto transmission and variable camshaft timing are all likely to be controlled by the main Electronic Control Unit or by additional control units. Variable Intake Manifolds Variable intake systems change the 14 PERFORMANCE ELECTRONICS FOR CARS length of the intake manifold runner or the volume of the plenum chamber. This allows the intake to have more than one tuned RPM – giving better cylinder filling at both peak torque and peak power, for example. The changeover is normally performed as a single step – the intake system is either in one configuration or the other. The intake system can be variably tuned in a number of ways, including (especially on 6-cylinder engines) connecting twin plenums at high RPM but having them remain separate smaller tuned volumes at lower revs. The introduction of a second plenum into the system at a particular RPM is another approach. However, the most common method is to have the induction air pass through long runners at low revs and then swap to short runners at high RPM. This doesn’t mean that the long runners need to be positively closed – opening parallel short runners is sufficient to change the effective tuned length of the intake system. The change-over is normally persiliconchip.com.au Fig.1: variable manifolds usually use a series of butterfly valves within the intake to change from long to short runners or to add another plenum volume. The valve actuator is operated by manifold pressure. [Mazda] The Ford Falcon 6-cylinder engine has a variable length intake manifold. The butterfly valves within the manifold open or close, depending on engine RPM, to provide long or short length intake runners. formed by a solenoid valve which directs engine vacuum to a mechanical actuator that opens or closes the internal manifold change-over valves. The change-over point can be based on engine RPM (this is most common), engine load or a combination of both. Variable Valve Timing Variable valve timing systems alter the timing and/or lift of the valves. Until recently, most variable camshaft timing has been on only one of the two camshafts and the camshaft timing has varied in a single step. That is, when the engine reaches a certain RPM and/or load, the ECU moves the camshaft timing – so one cam is either in the advanced or retarded position. Depending on the engine and manufacturer, that variable cam can be either the intake or exhaust cam. Continuously variable cam timing is now being used by many manufacturers. This allows lots of “in between” camshaft timing positions to be used, giving a far better result than singlestep cam timing variation. Continuously variable cam timing is most commonly used on just one camshaft but an increasing number of manufacturers are now using continuously variable cam timing on both the siliconchip.com.au Fig.2: variable camshaft timing uses oil pressure to operate an oil control valve or cam phaser. The oil pressure is varied by a solenoid (either switched or pulsed) that is controlled by the ECU. Both camshaft and crankshaft position sensors are used in variable cam timing systems. [Lexus] intake and exhaust camshafts. Systems that vary the valve lift as well as cam timing are also employed. Honda’s VTEC system is probably the best known of this type of single-step system. BMW has a design where the intake valve lift, as well as the exhaust and intake valve timing, are all able to be varied continuously. The techniques used to alter the camshaft timing and/or lift also vary. Where the camshaft timing alters in PERFORMANCE ELECTRONICS FOR CARS 15 Fig.3: the automatic transmission control system in this Calibra uses a separate control unit that communicates with a Motronic engine management unit. The inputs to the transmission control unit include PRNDL position, driver-selectable mode, kickdown switch, fluid temperature, brake light switch, transmission input and output speeds, and throttle opening. The main outputs control the transmission gear change solenoids, torque converter lock-up solenoid and the pressure regulator valve. [Holden] one step, an off/off signal from the ECU is used to activate a solenoid that feeds oil pressure to the mechanism, causing the change to take place. Where camshaft timing varies continuously, a pulsed solenoid is used to allow the cam phasers to vary in their position. The camshaft timing can be varied according to various input signals, such as engine RPM, 16 PERFORMANCE ELECTRONICS FOR CARS throttle position, coolant temperature and intake air flow. Automatic Transmissions On many cars, automatic transmission control is integrated into the engine management system. This allows the same input sensors (eg, throttle position, intake air-flow, engine temperature, etc) to be used for transmis- sion control and eliminates the need for duplicate sensors. It also allows the engine’s operating conditions to be varied as required; eg, the ignition timing can be retarded during gear changes to momentarily drop engine power and give smoother shifts. Automatic transmission control is achieved by actuating valves within the transmission. These hydraulic valves apply and release internal clutches and bands, causing the gearshifts to take place. Two main inputs – throttle position and road speed – are used to determine when gearshifts occur and the internal clamping pressures. There may be a throttle position sensor or the ECU may internally model the torque output of the engine (eg, by looking at throttle position, air flow, etc) and then use this information to control the transmission. However, some transmissions that are otherwise electronic still use a cable that mechanically connects the throttle to the transmission. Line pressure is also varied within auto transmissions. This controls the clamping forces and has a major influence on when gear changes occur; as engine power output increases, line pressure is increased. The torque converter also has a lock-up clutch, which stops any slip when it is engaged. This is controlled on the basis of road speed and load, and may also be automatically disengaged when braking. siliconchip.com.au BMW’s double “VANOS” system can continuously alter the timing of both the exhaust and intake camshafts. Fig.5: some cars calculate the engine torque output and the torque multiplication (for the torque converter) before deciding on the optimal transmission line pressure. In this case, engine torque is calculated by the engine CPU on the basis of inputs from various engine sensors, including throttle opening, intake air flow, coolant temperature and engine RPM. This information is then fed to the transmission CPU which also accepts sensor signals based on transmission input and output speeds, the transmission fluid temperature and the gear-lever position. The resulting output from the transmission CPU is a variable duty cycle pulse signal which controls the line pressure solenoid valve. [Lexus] Automatic transmission control, either by the engine management system or a dedicated controller, is now universal. In addition to allowing “Tiptronic” style up-shifts and down-shifts, it allows the transmission to electronically adapt to different engine loads. Fig.4: in the Lexus V8, long runners are used at less than 60° throttle opening at all engine speeds. At throttle openings over 60°, the long runners are also used at engine speeds between 2500 RPM and 4900 RPM. For smaller throttle settings, the short runners are used. [Lexus] Transmission fluid control solenoids use two approaches – they’re either turned on or off or they are a variable flow design controlled by the ECU. The solenoids that control the gear-change process are generally either on or off, whereas fluid pressure control and torque converter clutch engagement are achieved by continuously varying the amount of fluid that flows through their respective solenoids. These variations in flow are achieved by varying the duty cycle of the solenoids. Turbo Boost Control Nearly all turbocharged cars use siliconchip.com.au electronic boost control. The is based on the old approach of using a wastegate which is controlled by a springloaded diaphragm – see Fig.9a. When the boost pushing against the diaphragm overcomes the spring tension, the diaphragm is deflected (Fig.9b), in turn moving a lever that opens the waste-gate to allow exhaust gases to bypass the turbo. This prevents the turbo from rotating any faster and so limits the peak boost that can be developed. Electronic control adds a variablePERFORMANCE ELECTRONICS FOR CARS 17 Fig.8: an automatic transmission pressure control solenoid varies line pressure on the basis of engine load – at high loads, the pressures are higher resulting in firmer shifts and better friction surface clamping. This solenoid valve is varied in duty cycle to continuously control the valve position. Similar valves are used to gently engage the torque converter lock-up clutch. [Holden} Fig.6: the boost control solenoid is placed close to the turbocharger and its duty cycle varied to alter its flow. The nearby air-bypass valve (commonly known as a blow-off valve) can also have an input into boost control – it may not close until a relatively high manifold pressure is reached (altering the way boost rises) and it may open at very high boost levels to prevent over-boosting. [Mazda] Note that electronic turbo boost control systems can be open or closed loop. In open loop systems, the signal sent to the solenoid valve has been completely pre-mapped – ie, the system doesn’t have any way of directly monitoring the resulting boost level. Note, however, that many cars have an over-boost fuel cutout to shut the engine down if something goes catastrophically wrong. Other cars use a closed loop boost control system, where the boost level is monitored by a manifold pressure sensor. This adjusts the duty cycle of the solenoid valve described above to give the desired boost level, even at different altitudes and temperatures. Electronic Throttle Control Fig.7: most cars with electronic throttle control use a DC motor to control the opening and closing of the throttle butterfly. This allows functions such as cruise control, traction control and stability control to be easily and effectively integrated. In this Lexus system, the “Limp Mode Lever” allows the throttle to still be controlled even if the electronic throttle system completely fails. [Lexus] duty cycle solenoid that bleeds air from the waste-gate hose, thus altering the pressure that the waste-gate actuator sees. Waste-gate actuators in 18 PERFORMANCE ELECTRONICS FOR CARS electronically controlled boost systems have quite weak springs – that is, if no boost is bled from the line by the solenoid, peak boost levels will be low. Electronic throttle control replaces the throttle cable connection from the accelerator pedal to the throttle blade. Instead, pushing on the accelerator moves a position sensor (one or two potentiometers) which sends this “torque request” information to the ECU. The ECU then controls an electric motor which opens the throttle blade. The actual opening of the throttle is siliconchip.com.au Fig.9(a): electronic boost controls are still very closely based on this older, all-pneumatic design. Here, all the exhaust gases are being channelled through the turbine because the waste-gate (or swing valve) is closed. It will only open when boost pressure starts to overcome the spring tension in the controller. Cars equipped with an electronic throttle have no mechanical connection between the driver and the throttle blade. Instead, the driver’s “torque request” is processed by the ECU which then directs a DC electric motor or a stepper motor to open or close the throttle. monitored by a throttle position sensor similar to those fitted to conventional engine management systems. Elaborate safeguards prevent the throttle operation from going awry if any faults develop in the system. Electronic throttle control is now being widely adopted – expect to see it in all new cars in the next few years. It has significant advantages in the integration of traction control, stability control and cruise control, and can also be programmed to reduce emissions. Note than in systems with electronic siliconchip.com.au Fig.9(b): here boost pressure has risen to the extent that the waste-gate actuator diaphragm is compressing the spring, in turn opening the waste-gate. A proportion of the exhaust gas is then bypassed around the turbine, preventing the turbo from rotating faster and so limiting boost to this value. Electronic boost control simply adds a solenoid that is “tee’d” into the waste-gate line to bleed boost pressure from it, so controlling the pressure seen by the waste-gate actuator. [Nissan] throttle, the terms “accelerator position” and “throttle position” are no longer synonymous – all electronic throttle systems at times use throttle blade openings that don’t directly match the driver’s request! In systems where a DC motor is fitted, it is driven in either direction by a variable duty cycle, variable-polarity current. Other systems use stepper motors, which are controlled by sequentially pulsing their windings.  PERFORMANCE ELECTRONICS FOR CARS 19