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Engine braking from an automatic! Mitsubishi’s int automatic tran Mitsubishi’s smart new automatic transmission adjusts its shift points to suit the driver’s style. What's more, it shifts gear in a much more intelligent manner than previous automatic transmissions. Here's how it works. By JULIAN EDGAR 20  Silicon Chip Many automatic transmissions are now at least partially electronically controlled. Some use a hybrid system of electronic and hydraulic control, while others are fully electronic. The latest innovation is the adaptive “self-learning” au­ tomatic transmission, as fitted to the new TE Magna from Mitsu­ bishi. In this system, a transmission control unit is used to constantly monitor the driver’s style. Depending on what it “learns”, it then adjusts its control behaviour accordingly. With this type of system, the economy/power switch fitted to some transmissions is made redundant. Drive the car hard and the gear changes will occur at higher engine speeds; gently toddle along and the changes will slur through early. However, there’s more to adaptive shift control technology than changing gear shift points, as we shall see. Common problems In most modern automatic transmissions, gear selection is based mainly on throttle position and vehicle speed. However, there are many situations where the gear selected is not appro­ priate for the driving conditions – the control system literally selects the “wrong” gear. A good example of this occurs when driving an automatic car uphill along a winding road. In this situation, a slight easing of the throttle prior to each corner can result in an up-shift, the transmission then down-shifting again after the corner has been negotiated. Obviously, if driving a manual car, the driver would not change into a higher gear prior to entering a corner. Because the automatic transmission does, it losses engine braking and so some degree of control is lost. Fig.1: Mitsubishi’s earlier “Fuzzy Shift Scheduling” system allowed the transmission control unit to use engine brak­ing and hill-climbing modes. Self-learning was not incorporated into the system, however. Downhill driving in a conventional automatic also results in a lack of engine braking, unless the driver manually selects a lower gear. (Incidentally, it is good driving practice to manual­ly lock a conventional auto into a single appropriate gear in both of the above scenarios – lazy drivers take note!) Getting back to the TE Magna, Mitsubishi’s research indi­ cated that a conventional automatic transmission could be in the “wrong” gear for a given situation up to 60% of the time telligent nsmission – an extraordinarily high figure and perhaps only possible if the car were being driven on a racetrack! However, there are certainly times when the control system needs more brains. For Mitsubishi, the first step in overcoming these problems involved the development of “Fuzzy Shift Scheduling” – see Fig.1. In this system, additional inputs are used to allow the system to select from three shift modes: engine braking, standard and uphill. The current Mitsubishi system is an extension of this design. Fig.2 shows the basis of the new system. The engine Elec­tronic Control Unit (ECU) and the Transmission Control Unit (TCU) are linked, and exchange data on engine speed and airflow rate (ie, engine load). In addition, the TCU receives additional inputs on the throttle position, brake operation, steering angle and the transmission shaft speeds. The throttle opening is de­rived from a throttle position sensor, the frequency and/or duration of brake operation by monitoring the brake Fig.2: the new Mitsubishi adaptive system accepts additional inputs, including steering angle and the frequency and/ or dura­tion of brake application. This allows the system to better calculate appropriate shift behaviour during downhill coasting and to match the style of the driver. December 1996  21 Fig.3 (above): if the TCU selects a gear which is too low and thus provides excessive engine braking, the action of the driver applying throttle will cause a correction to the downshift. Conversely, applying the brakes excessively when coasting down a hill – as in Fig.4 (right) – will cause the TCU to shift to a lower gear, thus increasing engine braking. light switch, and the steering angle by a dedicated sensor. The speed of both the input and output shafts of the auto transmission is also measured. This allows the TCU to monitor road speed and to calculate the amount slippage occurring through the transmission. It can then use these inputs as a feedback mechanism to reduce “shift shock”. In some versions of the sys­tem, longitudinal and lateral accelerometers are also employed. Engine braking Engine braking is achieved by calculating an index called “engine brake applicability”. This is carried out by a so-called “neural network” which links together the road gradient, vehicle speed, braking frequency and steering angle with varying degrees of importance. The influence of these various factors depends on empirical data originally gathered by monitoring the gear-shift­ing behaviour of experienced drivers. The aim here is to approximate the decision-making process adopted by a driver in a manual car. However, while downshift timing is primarily controlled by the “neural network” from empirically-collected data, the calculat­ ed timing is not appropriate for all drivers because of their individual preferences and driving styles. A feedback mechanism dubbed “Learning Control” has therefore been added. This judges the driver’s dis­satisfaction with the amount of engine braking being provided by the TCU and corrects the downshift condition until the driver’s preference is reached. This judgement is carried out by monitoring the frequency and/or duration of brake use and by monitoring throttle varia­tions when the vehicle is coasting downhill. If throttle needs to be applied (Fig.3) then the transmission is in too low a gear. Conversely, if the brake is applied (Fig.4) the gear selected is too high. Fig.5 shows how the learning system changes the ease with which up-shifts and down-shifts occur in response to brake and accelerator movement during downhill coasting. Variable shift patterns Fig.5: the self-learning behaviour of the system can be seen here, where the ease of selection of either a downshift or upshift varies with the driver’s preference – as sensed by the TCU through brake or accelerator application while driving down­hill. 22  Silicon Chip A conventionally-controlled transmission has a shift pat­tern similar to that shown in Fig.6. An upshift from second to third, for example, occurs at a certain combination of throttle position and vehicle speed. Similarly, at another precise mix of speed and throttle, the downshift from third to second will occur. It’s this fixed approach which causes the problem of upshifts before corners when climbing a hill. To avoid this, it is necessary to move the upshift lines to a higher speed range. Just how much the upshift points are moved depends on the road gradient, as derived from the TCU sensors. Variations in individual driving styles also require changes to the shift points. For example, a “sporty” (or aggres­sive) style means that the lower gears need to be held to higher engine speeds and also selected more readily. The driving style is evaluated by a variable that Mitsubishi’s engineers call the “Sporty Driving Index”. The “Sporty Driving Index” is calculated by selecting the larger of two input factors – either the engine load index or the tyre load index. On some Mitsubishis (but not on the Australian Mag­na), the tyre load index is calculated by comparing the actual lateral and longitudinal accelerations with the maxima of which the tyre is capable. Similarly, the engine load index is calculated by measuring the actual acceleration and comparing this with the maximum possible acceleration. How hard the car is being cornered or accelerated varies the “Sporty Driving Index”, with the shift point maps then moved as a result. Fig.7 shows a schematic summary of the complete TCU system. Does it work? According to Mitsubishi, the new system selects the correct gear for 80% of the time. This represents a considerable improve­ment on the 40% of a conventional auto transmission and 55% for their second-generation fuzzy system. In Australian Government AS2877 fuel economy tests, the V6 automatic transmission Magna has equal econ­ omy to its manual equivalent on the highway cycle and is only 5% worse Fig.6: in a conventional shift control system the up and down changes always occur at a precise combination of speed and throt­tle position. in the city cycle. By comparison, the current model automatic Commodore is 6% worse than the manual version on the highway and 9.5% worse on the city cycle. It would certainly appear that the more sophisti­ cated transmission control system of the Magna yields economy benefits! So what’s it like to drive? We took an automatic TE Magna sedan for a run to find out. On the road we found that the electronic control system had some noticeable advantages over more traditional transmission control systems. Most obvious was the transmission down-changing to provide engine braking when slowing for a red traffic light, for example. And on country roads, the downhill engine braking was also noticeable. However, many of the traditional disadvantages of an au­tomatic transmission appeared to remain. Any demand for instant power (eg, when overtaking on a country road) still results in a relatively slow response, there being approximately a 1-second time lag for the transmission to “think” and then change down a gear. A quicker response was possible by manually changing down. Jerky changes also occurred in some situations – for example, when accelerating hard away from a standstill and then suddenly lifting the throttle. That said, Mitsubishi’s new adaptive control system repre­sents a real improvement in automatic transmission technology. It matches the shift points to suit the driver and it provides superior shift patterns in certain driving situations. And it does this unobtrusively. Certainly it never occurred to me that the transmission was pre-empting my decisions or matching its change SC patterns to my driving style! Fig.7: block diagram of the TCU. The shift pattern is calculated from a range of input data. December 1996  23