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Electronic Engine Management Pt.13: Electronic Transmission Control by Julian Edgar Anyone who has driven a car equipped with an automatic transmission has, in a sense, driven a computer-equipped car. Even if it’s a 1965 Valiant, the gear changes of the automatic transmission were computer-controlled – in this case, by an analog computer. Traditionally, an automatic transmission uses pressurised hydraulic fluid to control the movement of valves. However, in more recent times, the analog computer has been replaced by a digital version. The automatic transmission The automatic transmission is the most sophisticated me­chanical component in a car. A simple 3-speed transmission (ie, a unit with three forward ratios) uses a gear set comprising an annulus gear, forward and The Holden Jackaroo V6 uses full electronic control for its transmission. This view shows the Transmission Control Unit (TCU). 14  Silicon Chip reverse sun gears, and short and long pinions mounted in a planetary pinion carrier. Controlling these gears are two multi-plate clutches, front and rear braking bands, and a one-way clutch. The action of these friction elements is in turn controlled by four shift valves, two hydraulic pressure regulator valves, a governor valve, and five other valves per­forming one-way or other functions. Central to automatic transmissions are planetary gear sets. Fig.1 shows a planetary gear set with three pinions. All the gears remain enmeshed at all times, with different ratios gained by driving a particular gear member while the others are held stationary. This makes the system amenable to automatic operation, with hydraulically-operated clutches or bands being used to control the rotation of parts of the planetary gear set. The single gear set shown in Fig.1 would not provide suffi­cient forward ratios for a car. Either two simple gear sets connected together or a compound gear set (which shares some gears between two planetary sets) is required to provide the correct ratios and direction of rotation for a car transmission. Four (and now five!) speed auto transmissions require even more internal gears but all use various combinations of planetary gear sets. A torque converter connects the automatic transmission to the engine. This internal speed sensor is from a Magna electronically-con­trolled automatic transmission. This acts as a sophisticated fluid coupling. Hydrau­lic oil is driven around inside the housing by the action of spinning blades and torque is transferred from the impeller (which rotates at engine speed) to the turbine (which is attached to the transmission input shaft). When the turbine is stationary (ie, the car is stopped) but the impeller is spinning quickly (ie, the engine is being revved), the engine’s output torque is multiplied by the convert­er by a ratio which may be as high as 2:1. This torque multipli­cation is reduced as the rotational speeds of the two spinning elements become closer. However, while the action of the torque converter is advan­tageous during acceleration, some slippage will always occur when the car is being driven at a constant speed. Conventional control Conventional transmission control is by means of an hydrau­lic computer (in reality, the transmission’s valve body) and this uses oil pressure to perform its function. Oil is pressurised by an internal pump and this pressure is modulated by two main variables: (1) road speed, and (2) throttle position. In the simplest two-speed transmission, these two variables oppose each other by bearing on opposite ends of the same spool valve. Fig.2 shows a schematic of this type of control system. A spring holds the shift valve in the “first gear” position, to allow the car to start off in low gear. If throttle pressure is high (ie, the accelerator is hard down), then the shift valve will resist the rising governor pressure (which is proportional to road speed). However, when the throttle pressure Fig.1: automatic transmissions use several sets of these plane­tary gears. This allows the ratios to be changed while the gears are in constant mesh. FIRST GEAR SECOND GEAR SPRING GOVERNOR (ROAD SPEED) INPUT THROTTLE PRESSURE SHIFT VALVE PRESSURE REGULATOR INPUT Fig.2: the basis of hydraulic transmission control is the spool valve, which is subject to varying hydraulic pressures. This diagram shows a simple 2-speed system. Fig.3: this diagram shows the pressure flow for a simple 2-speed hydraulicallycontrolled transmission. THROTTLE VALVE PUMP PRESSURE REGULATOR X FIRST GEAR SHIFT SECOND GEAR GOVERNOR VALVE X X = DRAIN October 1994  15 Fig.4: an automatic transmission is the most complicated mechani­cal component in a car. This cutaway drawing is of the Jatco L4N71B 4-speed transmission which uses hybrid electro-hydraulic control. drops (ie, the accel­erator has been lifted) or the road speed rises sufficiently, then the governor pressure will cause the valve to move to the right. A 1-2 gear change will then occur as the valve directs fluid to the correct planetary gear control clutch and/or band. Obviously, if gear changes are to be completed quickly and aspects such as kickdown are required, then some additions to Fig.2’s simple system are required. A manual control valve (so that P-R-N-D-1 can be selected) is also needed. However, all hydraulic transmissions are essentially controlled using this type of valve-pressure approach. Fig.3 shows a flow diagram of this simplified version. The transmission’s hydraulic control valves are located in the valve body at the base of the transmission. Machined to very fine tolerances, these valves generally work for very long serv­ice lives with little maintenance, as long as regular transmis­ sion fluid changes are carried out and overheating of the oil does not occur. Electro-hydraulic control With the hydraulic control of automotive transmissions very well entrenched, full electronic control was not immediately introduced when the technology became available. The high cost of transmission development meant that hybrid transmissions appeared next, using some elements of electronic control matched to FUSE CONVERTER CLUTCH SOLENNOID CONTROL UNIT 17 OD CANCEL SOLENOID DOWNSHIFT SOLENOID 2 1 POWER SHIFT SWITCH INHIBITOR SWITCH 18 6 7 10 5 8 THROTTLE VALVE SWITCH 3 22 11 VEHICLE SPEED SENSOR 16  Silicon Chip 4 CONTROL UNIT 9 21 KICKDOWN SWITCH 12 1-2 2-3 16 3-4 15 TEMP SENSOR 19 THROTTLE VALVE SENSOR Fig.5: hybrid control transmissions generally have a limited range of electronic control capabilities. This unit is able to override the hydraulics only in the selection of overdrive (fourth gear), kickdown and torque converter lockup (Holden). Fig.6: this Bosch system integrates engine & transmission manage­ment into one unit. This allows the easy use of sophisticated techniques like retarding the ignition timing during gear chang­es. Many of the input sensors for the engine & transmission control are the same. trans­ missions which are essentially hydraulically controlled. Generally, the electronic control exercised in these hybrid transmissions is for features such as kickdown and torque con­verter lock-up. One example is the Jatco L4N71B, as used in the Holden VL Commodore, Nissan Skyline and some Mazda models. This transmission is a real “Grandpa’s axe”, with the 1970s 3-speed 3N71B having had an overdrive unit added and then some electronic control juxtaposed on top! Fig.4 shows this transmission in cutaway form. Ten input signals to the Transmission Control Unit (TCU) are used and the system controls three transmission functions. Fig.5 shows the circuit for this system. The electronics is able to override the hydraulics only in the selection of overdrive (fourth gear), kickdown and torque converter lock-up. The TCU uses vehicle speed, throttle position, transmission fluid temperature, and the positions of the hydraulic shift valves as its main in- B A Jatco’s hybrid electro-hydraulic controlled transmission uses a solenoid (A) to control kickdown & a temperature sensor (B) to indicate transmission fluid temperature to the control unit. puts. By using a switch mounted on the gear lever, the driver can select between “power” and “economy” modes, with different shift behav- iour experienced in each mode. Fast acceleration will also automatically select the power mode. In this mode, the upshift and downshift points October 1994  17 The traditional hydraulic control system uses valves mounted inside an intricate valve body to determine when gear shifts occur. Hydraulically-operated wet multiplate clutches are used in all auto transmissions – whether they are controlled electronically or not. the transmission oil temperature is less than 45°C; (2) during acceleration; (3) during a gear change; (4) when the throttle is closed; and (5) when the transmission is in first and second gears. Actual control of the clutch operation is hydraulic, with the TCU operating a bleed-off solenoid. Full electronic control All automatic transmissions use planetary gear sets. They are compact & remain permanently engaged – even during ratio changes. generally occur at higher engine rpm than in economy mode. The electronic module controls the overdrive function, with change into overdrive inhibited when the accelerator is floored, when the transmission is in power mode, and when the transmission fluid is at a temperature of less than 45°C. The latter inhibition occurs because exhaust emissions would suffer if using low rpm and wide throttle angles when the engine was still relatively cold. (Note that with this system, the engine management 18  Silicon Chip and transmission control electronics are entirely separate – there is no engine coolant temperature input to the transmission control). Because of the intrinsic slippage experienced in torque converters during cruise conditions, manufacturers have started building in lock-up converter clutches. However, if the torque multiplication function is still to occur, then the clutch should not lock-up under certain conditions. For example, the L4N71B torque converter will not lock-up (1) when With the adoption of transmissions expressly designed for full electronic control, a different approach to the design could be taken. The fundamental sensor requirements for gear selection control could be re-evaluated and greater versatility and accura­cy built-in. Electronically-controlled transmissions still use hydraulics to apply the clutches and bands but all the valves are triggered by the electronic control unit. Up to 14 inputs are used in some transmissions, with sometimes six internal hydraulic solenoids controlled by the electronics. In many cars, the engine and transmission management computers are integrated, allowing the manufacturer to include refinements such as retarding the ignition during gear changes to give smoother progress. In one car (Subaru Liberty), the electronic transmission control is divided Fig.7: a “hot” chip can be used to reprogram a fully electroni­cally-controlled automatic transmission. This diagram shows the results obtained from a Holden Jackaroo V6 using a Fueltronics-modified transmission control unit. into six areas. These areas are: (1). Gear Shift Control: here, the TCU controls the gear change points using different internal maps, depending on whether the economy or power pattern has (automatically) been selected. It also holds fourth gear longer when the cruise control is being used (thereby stopping unnecessary down-shifts) and locks the transmission in third gear when the anti-lock braking system (ABS) is operating. If the transmission fluid temperature is too low, it prevents the use of fourth gear. It also holds each gear when the gear selector is being manually used. (2). Lock-Up Control: the TCU determines when torque converter lock-up will occur. This depends on the gear used, the throttle position and vehicle speed. (3). Over-Running Clutch Control: engine braking is performed by using the TCU to determine the operation Even the simplest automatic transmission is a complex mix of hydraulic valves, gear sets and friction elements. Electronic control is now taking over from full hydraulic control. of an over-run clutch. The operation of this clutch depends on the power/ economy range being used, vehicle speed, and cruise control operation. When the cruise control is in operation, full engine braking is imposed to prevent speed build-up when coasting down hills. (4). Line Pressure Control: during gear shifting, the hydraulic oil pressure is dropped to reduce “shift shock”. As vehicle speed increases, the line pressure is brought up to provide better hy­draulic clamping of the clutches and bands. During engine start­ ing, the hydraulic pressure is reduced to impose less cranking load on the starter motor. (5). Automatic Power/Economy Selection: when the speed of the throttle opening exceeds an internal value, the TCU switches over to its power shift map and then returns to the economy mode when the throttle opening falls below a preset amount. (6). Shift Timing Control: by using variations in hydraulic pressure at the appropriate times, the TCU is able to smooth up-shifts and down-shifts. Incidentally, with full microprocessor control, new programs can now be written which modify the change points. By using a “hot chip”, for example, the power mode can be made to kick down at higher speeds than for the standard chip, with the intermediate gears holding on for longer before SC changing up. October 1994  19