Automotive Electronics: Electronic Control of Transmissions

The transmission converts the engine torque and speed into the traction and speed required by the car, and can meet the change of rotation direction required to make the car move forward or backward. The electronically controlled automatic transmission can select the best gear by itself, making the car more fuel-efficient, and helping the driver to reduce operation in complex traffic conditions, such as in unfamiliar environments or bad weather conditions. It can significantly reduce the driver's fatigue in stop-and-go intermittent traffic.

Automotive transmission

Internal combustion engines do not operate with equal torque and equal power throughout their entire speed range . Their optimal "elastic" speed range is between the rated speed and the speed corresponding to the maximum torque. The transmission is essentially a speed-torque converter. For uphill driving and acceleration, the transmission converts the engine torque into the corresponding torque required by the vehicle. The transmission makes optimal use of the engine power and optimizes the engine's fuel consumption. There is a vehicle starting component on the transmission. It is necessary for the vehicle to go from a stationary state to a moving state.

Transmission diagram of three-axis five-speed transmission

1- input shaft 2- bearing 3- engagement ring 4- synchronizing ring 5- output shaft 6- intermediate shaft 7- engagement sleeve 8- intermediate shaft constant meshing gear

The transmission is the main influencer of the transmission system efficiency. The transmission power loss of the automobile transmission at the optimal working point is about 1.5%. In the daily driving conditions of the car, due to the various driving conditions of the car, its power loss is about 8%. The electronic control of the transmission can reduce the power loss by selecting the best gear position by itself.

Requirements for the transmission

Each car has some specific requirements for the transmission, so the various types of transmissions are different in structure and performance. The goals and focus of developing transmissions are concentrated in the following aspects:

Transmission diagram of two-axis five-speed transmission

1- input shaft 2- coupling sleeve 3- odometer gear 4- synchro ring 5- half shaft 6- driven gear of main reducer 7- differential case 8- half shaft gear 9- planetary gear 10, 11- output shaft 12- driving gear of main reducer 13- spline hub

1. Comfort:

Some key requirements for comfort are: 1. No shock or speed jump during shifting. 2. Easy shifting, not affected by engine load and working conditions. 3. Low noise. 4. No sacrifice of comfort throughout the service life.

Transmission remote control mechanism

1-shift lever 2-longitudinal cable 3-lateral cable

2. Fuel economy:

The conditions for effectively reducing fuel consumption from the transmission are: 1. Large transmission ratio. 2. High mechanical efficiency. 3. Intelligent shifting strategy. 4. Low control power. 5. Light weight. 6. Alternative control. 7. Hydraulic coupling. 8. Small oil stirring power loss (the resistance inside the transmission when the gears rotate).

3. Controllability:

The following functions of the transmission ensure good controllability of the transmission: 1. The shift point is coordinated with the driving conditions of the car. 2. It can identify the driving conditions of the car and the type of driver. 3. Good enzyme capacity. 4. Engine braking effect when going downhill in mountainous areas. 5. Preventing gear shifting when driving fast on curves. 6. Ability to identify winter road conditions.

4.Structural dimensions:

There are different regulations for the size of the transmission according to the drive mode. For rear-wheel drive transmissions, the diameter of the transmission should be as small as possible, while for front-wheel drive transmissions, the length of the transmission should be as short as possible. There are also detailed regulations to meet these requirements during impact tests.

5. Manufacturing cost:

The conditions for local manufacturing are: mass production, simple control structure and automated assembly.

In the first batch of power cars ready for driving in 1886, the driving force was transmitted directly to the wheels without a starting clutch. Therefore, the vehicle had to be pushed or the flywheel turned by hand to start. In order to transmit the driving force to the road, Mercedes-Benz used the following mechanical components. A flywheel was installed on the end of the engine crankshaft. Because the engine was installed horizontally on the rear axle, the vertically installed bevel gear transmission transmitted the engine's driving force to the belt drive mechanism, which reduced the speed of the intermediate shaft and further transmitted it to the drive shaft through the chain drive.

Basically, the types of transmissions are divided into: manual shift transmission, automatic shift transmission (AST), dual clutch transmission (DKG), automatic transmission (AT), continuously variable transmission (CVT), and toroidal transmission. We will introduce them one by one.

Manual shift transmission

The manual transmission is the simplest and cheapest transmission and is still widely used around the world. Due to the continuous increase in engine power, the increase in vehicle weight and the continuous decrease in vehicle drag coefficient, the 5-speed manual transmission has replaced the overwhelming 4-speed transmission since the 1980s. This measure allows the vehicle to start reliably and accelerate well, and at higher speeds, the engine can operate in a lower speed range, reducing fuel consumption.

Manual shift working principle diagram

The structure of a manual transmission can be divided into the following parts: 1. Gear sets mounted on two shafts. 2. Single-plate dry clutch as the starting part of the car, and cuts off the driving force when shifting gears. 3. Clutch components (as shifting components) synchronously operated by inertia force. The biggest advantages of a manual transmission are: 1. High efficiency. 2. Compact and simple structure. 3. Low manufacturing cost. Disadvantages: Relatively difficult to operate (when shifting gears, you must first step on the clutch and then manually shift gears). 2. The timing of shifting gears depends on the driver. 3. Traction is interrupted when shifting gears.

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Automatic Shift Transmission (AST)

Automatic Step Transmission (AST ), also known as Automatic Manual Transmission (AMT), can simplify transmission operation and improve fuel economy. The shifting process of the previous manual transmission is now achieved by pneumatic, hydraulic or electric.

Electronic Clutch Management (EKM) is used in automatic transmissions (AST) and is supplemented with two servomotors for gear selection and shifting . The servomotors with AST enable automation and increased convenience without any additional costs. The required electronic control signals come directly from the gear lever operated by the driver or from a serially connected electronic control, depending on the system.

In the simplest AST system, the remote control replaces the mechanical lever. The shift lever (tap lever or H shift diagram shift lever) only sends a signal, and the start process and clutch engagement are operated as in a manual transmission. In a fully automatic AST system, the transmission and the vehicle start components are automated. The driver's operating component is the shift lever or key switch . In order to realize automatic control of common transmissions, a comprehensive shift strategy is required, including consideration of the current driving resistance of the vehicle, such as load and road conditions. When the traction force is interrupted during the shift, the electronically controlled engine must be terminated for a short time and automatically to ensure the synchronization of the transmission.

Its structural features are: 1. The basic structure is the same as the manual shift transmission. 2. The clutch and gear shift are controlled by (starting, hydraulic or motor) controller and electronic control.

Advantages: high transmission efficiency, compact structure, adaptable to existing manual shift transmissions, simple operation, effective shifting strategy to achieve optimal fuel economy, cheaper than stepped automatic transmissions or continuously variable automatic transmissions. The disadvantage is that traction mid-section occurs during the shift.

We have previously introduced manual transmissions and automatic transmissions (AST). Today we will continue to introduce two other types of transmissions: dual clutch transmissions (DKG) and automatic transmissions (AT). These two are relatively common transmissions, so let's briefly introduce them.

Dual Clutch Transmission

Dual Clutch Transmission (DKG)

The dual clutch transmission is a development of the automatic shift transmission (AST), which does not have the main disadvantage of the AST transmission, which is the interruption of traction during gear shifting. And the fuel saving over the AST transmission is a major advantage of the DKG. It is equivalent to the AST in terms of gear shifting comfort and functionality. The DKG transmission is mainly used in high-end cars.

DKG also meets the modular design requirements of automobile manufacturers. The modular design concept is that manual transmission and automatic transmission can be produced on the same production line. Its main structural features are: The basic structure is the same as that of the manual transmission. The gear set is supported on three shafts. Two clutches. The clutch and shift mechanism are operated by the transmission control and actuator.

Torque Converter

The working principle is to divide the gears that arrange the gears into two groups, the even-numbered gears and the odd-numbered gears. Although the basic arrangement is similar to that of a conventional countershaft shift transmission, the fundamental difference is that the main shafts of the DKG are separated, one is a solid shaft and the other is a hollow shaft sleeved outside the solid shaft. The solid shaft and the hollow shaft are connected together by a gear set. The solid shaft and the hollow shaft at the input end of the transmission are equipped with clutches. Because two gears are embedded during the shift (i.e., the active gear and the pre-selected adjacent gear), it is possible to shift quickly between the two gears, just like an automatic transmission without traction interruption.

In terms of performance, the advantages of DKG are: high working efficiency, similar comfort to AST, no traction interruption when shifting, and the ability to skip a gear. The disadvantages are that it is larger in size than AST, has greater support force, and has a rough structure.

Planetary gear

Automatic transmission (AT)

The automatic transmission (AT) plays the role of starting, selecting speed ratio and automatically shifting gears. With simple gear shifting, the automatic transmission can jump gears by connecting and disconnecting the shift mechanism. The starting component of the automatic transmission is the torque converter.

In terms of structure, many 4-speed automatic transmissions are based on the well-known Ravigneaux planetary gear set. Automatic transmissions with more than 4 forward gears cannot be shifted. A 5-speed automatic transmission requires another basic transmission or an additional planetary gear set before or after the 4-speed automatic transmission. Engineer Lepdil found a clever way to shift 5 or more gears. He added a front planetary gear set to the two-axis Ravigneaux planetary gear set so that it can be driven at a different drive speed. The number of starting parts remains unchanged. The additional gears are simply used multiple times. The advantages of this solution are small size, light weight and low cost.

Torque Converter

The torque converter was mentioned just now. The function of the torque converter is to help the car start. It plays the role of an additional gear in the process of starting the car. The hydraulic torque converter is used as a starting component in most automatic transmissions that require comfort. Since the hydraulic torque converter is a liquid mechanism, it is an ideal starting component and can absorb vibration. In order to improve the transmission efficiency of the hydraulic torque converter under high-speed ratio conditions, a clutch is connected across the pump wheel and turbine of the torque converter, thus forming a hydraulic torque converter with a locking clutch. Avoid power loss and increased fuel consumption due to slippage of the hydraulic torque converter.

The multi-plate clutch ensures that there is no traction mid-section phenomenon during gear shifting and the engine torque can still be transmitted. During the gear shifting process, the clutch lining and steel plate as well as the shift brake bear the dynamic torque and transmit the load torque after the gear shift.

The planetary transmission is the core component of the automatic transmission. Its function is to adjust the transmission ratio and ensure uninterrupted power transmission. The planetary transmission is composed of a gear arranged in the center, namely the sun gear, multiple planetary gears, and an internal meshing gear.