Part 1: Importance, Classification and Technical Details of Automotive Transmissions
1. Importance of Automotive Transmissions
The automotive transmission is a core component of the vehicle powertrain. Its fundamental existence is attributed to the extremely complex driving conditions of vehicles: the speed range can vary from less than 5 km/h to over 160 km/h, a span of more than 30 times; the driving force demand can range from approximately 0.15 times the Gross Vehicle Weight (GVW) during constant-speed driving on flat roads to more than 2.5 times the GVW during rapid acceleration, a span of more than 16 times. Neither internal combustion engines nor drive motors can meet such a broad and variable range of requirements with a single reduction ratio alone. Therefore, a transmission with multiple gear ratios is crucial—it enables the optimal conversion of engine power into vehicle driving force over a wide speed range, thereby achieving the best balance among power performance, fuel economy and drivability. This paper specifically points out that automatic transmissions are the culmination of electromechanical-hydraulic control integration, and are generally regarded as the most technologically advanced and complex products in automobiles.
2. Detailed Classification of Automotive Transmissions
This paper comprehensively classifies passenger car transmissions and analyzes their respective technical characteristics:
Manual Transmission (MT)
Definition: A transmission that requires the driver to manually operate the clutch and gear shift lever. The Automated Mechanical Transmission (AMT), an upgraded version based on MT, is also classified in this category in this paper.
Principle: Changes the transmission ratio by selecting gear sets with different gear ratios via synchronizers.
Advantages: Simple structure, low maintenance cost, high transmission efficiency, good fuel economy, and engaging driving experience.
Disadvantages: Cumbersome operation, high requirements for drivers, and easy fatigue in congested road conditions.
Sequential Manual Gearbox (SMG)
Definition: A manual transmission that only allows gear shifting up or down one gear at a time, with no skip shifting available.
Characteristics: Simple structure, small volume, light weight, fast gear shifting and high transmission efficiency, thus widely used in racing cars and motorcycles.
Automated Mechanical Transmission (AMT)
Definition: Based on a manual transmission, it adds an electronically controlled hydraulic (or electronically controlled electric) clutch operating system and gear selection/shifting system to realize automatic gear shifting.
Advantages: High cost performance, simple operation, and good fuel economy.
Disadvantages: Severe gear shifting jitter, poor ride comfort, and relatively slow gear shifting speed.
Automatic Transmission: Refers to transmissions that shift gears automatically according to vehicle speed and throttle depth, mainly including three types:
Automatic Transmission (AT)
Composition: Consists of a torque converter, planetary gear system, hydraulic system and electronic control system.
Advantages: Most mature technology, high reliability, strong load-bearing capacity, and automatic gear shifting to reduce driver burden.
Disadvantages: Complex structure, high manufacturing and maintenance costs, and relatively low transmission efficiency.
Dual Clutch Transmission (DCT)
Principle: Adopts two clutches connected to the gear sets of odd and even gears respectively; when one clutch is in operation, the other can pre-select the next gear, thus achieving fast gear shifting without power interruption.
Derived Models: Including Volkswagen’s DSG, Porsche’s PDK, Audi’s S-Tronic, BMW’s M DCT, Ford’s PowerShift, etc.
Advantages: Fast gear shifting, high transmission efficiency, no power interruption, and compatibility with the mature gear technology of MT.
Disadvantages: Relatively complex structure, high requirements for control software, and possible jitter during gear shifting.
Continuously Variable Transmission (CVT)
Principle: Achieves continuous stepless variation of the transmission ratio by changing the working radii of two cone pulley sets and the transmission belt (or chain).
Advantages: Extremely smooth gear shifting without jitter, and continuous and smooth power output.
Disadvantages: Slightly slow power response, lack of driving fun, and small maximum torque that can be withstood.
3. Current Status and Trends of Transmissions for New Energy Vehicles
Current Status: At present, the vast majority of electric vehicles adopt fixed-ratio reducers (single-speed transmissions), which feature a simple structure, low cost and low development difficulty, and usually adopt two-stage gear reduction with an integrated differential.
Inevitability of Development towards Multi-speed Transmissions: This paper clearly points out that it is an inevitable trend for electric vehicles to adopt two-speed or even multi-speed transmissions for three reasons:
- Improve power performance: A larger reduction ratio of the low gear can provide stronger acceleration and climbing capabilities; a smaller reduction ratio of the high gear can achieve a higher top speed.
- Improve system efficiency: Enable the drive motor to operate in the high-efficiency range for more time.
- Reduce system cost: Allow the use of smaller, low-speed motors, and standard bearings and gears can be adopted, thereby reducing the cost of motors, inverters and transmission components.
Technical Examples:
- GKN SynchroShift: Applied to the front axle of the BMW i8, it is the world’s first mass-produced two-speed electric drive transmission. It adopts an AMT structure, with gear shifting driven by a small shift motor via a synchronizer. Specific parameters: Transmission ratio 11.3 (1st gear)/5.85 (2nd gear), maximum input torque 250 Nm, maximum input speed 11,400 rpm, mass 27 kg (including oil).
- Schaeffler Two-speed Parallel Shaft Electric Drive Axle: Adopts a modular design, integrating a water-cooled permanent magnet synchronous motor, reduction gear set, differential and shift mechanism. Its parallel shaft structure results in a center distance of only 127.5 mm between the rotor shaft and the differential output shaft, which is extremely compact and suitable for the rear axle layout of SUVs.
- GKN eTwinsterX: Integrates coaxial arrangement, torque vector distribution (achieved through two wet clutches) and two-speed shifting functions. Its two-speed mechanism adopts a special planetary gear set composed of two sun gears and one planet carrier (without a ring gear).
- Porsche Taycan Two-speed Transmission: This paper emphasizes that its development is mainly aimed at “enhancing power performance”. With the large reduction ratio of the first gear, the Taycan achieves a 0-100 km/h acceleration in 2.61 seconds. This transmission does not focus on motor miniaturization (rear axle motor parameters: 335 kW, 550 Nm, 16,000 rpm) or efficiency improvement.
Part 2: Case Study on Design Optimization of High-efficiency Reducers for Electric Vehicles
This paper takes a specific design project as an example to systematically discuss the theoretical and practical methods for improving reducer efficiency.
1. Theoretical Measures to Improve Reducer Efficiency
Design Aspect: A detailed analysis of the influence of gear macro parameters and bearings on efficiency:
- Reference circle diameter (d): Under heavy load, gears with a larger diameter have lower load, are easier to form an oil film, result in less sliding friction loss and higher efficiency.
- Module (z): With the same reference circle diameter, gears with a larger module have a longer contact line, greater sliding loss and lower meshing efficiency.
- Transmission ratio (i): Requires appropriate distribution. Under heavy load, the large gear diameter corresponding to a large transmission ratio is conducive to forming an oil film and can improve efficiency; however, an excessively large transmission ratio will change the meshing area and have an adverse effect on efficiency.
- Face width (b): Under light load, an increase in face width will increase rolling friction loss and reduce efficiency; under heavy load, the face width has little effect. However, an increase in face width will significantly increase churning loss.
- Bearings: With the same diameter, ball bearings have less loss than roller bearings; at high speeds, bearing loss increases with the increase of diameter. Appropriate bearing types and series should be selected.
Manufacturing Aspect:
Heat treatment, surface processing (such as gear grinding) and special treatment (such as carburizing) of gears will affect the surface roughness and friction coefficient, thereby affecting efficiency.
Lubrication Aspect:
- Viscosity: On the premise of ensuring sufficient lubrication, low-viscosity lubricating oil should be selected as much as possible to reduce churning loss. Low-viscosity oil is used for high-speed and light load, and high-viscosity oil is used for heavy load to form an oil film.
- Oil supply: The larger the oil supply, the greater the churning loss; however, an insufficient amount will lead to poor lubrication. Precise control is required to minimize the oil supply on the premise of ensuring lubrication.
2. Specific Design Optimization Process and Verification
Design Objective:
Develop a two-stage reducer for electric vehicles with a maximum input torque of 350 Nm, a total transmission ratio of 9.1, and a maximum input speed of 12,000 rpm.
Design and Calculation Process:
- Transmission ratio distribution: According to the equal strength principle, the primary and secondary transmission ratios are initially determined: .
- Center distance determination:
- Primary center distance: ( takes 8.9~9.3)
- Secondary center distance:
- Gear parameter determination: Finally, the primary gear pair with 22/69 teeth and the secondary gear pair with 21/61 teeth are adopted.
Efficiency Optimization Implementation:
On the basis of the initial design, four optimization measures are taken:
- Improve the box structure: Add oil baffles inside the box and reduce the internal volume space, thereby reducing the required amount of lubricating oil and lowering churning loss.
- Upgrade bearings: Replace domestic bearings with higher-performance imported bearings.
- Optimize lubrication: Replace high-viscosity lubricating oil with low-viscosity oil of the same grade and reduce the total oil filling amount.
Experimental Verification and Conclusions:
Bench efficiency tests (based on the NEDC working condition) were carried out on the reducer prototypes before and after optimization.
Results: The NEDC comprehensive efficiency of the optimized reducer is as high as 95.8%, exceeding the advanced domestic level.
Core Conclusions:
- Optimizing the box structure to reduce internal volume and churning loss has an obvious effect on efficiency improvement.
- On the premise of ensuring lubrication, using low-viscosity lubricating oil and precisely controlling the oil amount can effectively improve efficiency.
- Through the comprehensive optimization of structure, bearings and lubrication, this case successfully achieved the design goal of a high-efficiency reducer.
