Understanding Power Loss in Torque Converters for Automatic Transmissions

No machine or device is perfect, and this principle applies to the torque converter in an automatic transmission. When torque is multiplied, some power is inevitably lost. This article aims to explore power loss in torque converters, explaining the reasons and how it impacts the performance of automatic transmissions.

Power Loss Factors in Torque Converters

There are several reasons for power loss in a torque converter, primarily due to slip, hydraulic resistance, and heat generation. These factors can significantly impact the overall efficiency of the system.

Slip

A torque converter allows for some amount of slip between the engine and the transmission, particularly during acceleration. Slip is a result of the converter's fluid dynamics, which means that not all of the engine's power reaches the wheels. This inefficiency can lead to a notable loss in power.

Hydraulic Resistance

The torque converter uses fluid dynamics to transmit power. As the fluid moves within the converter, it experiences resistance, which translates to energy loss. This energy is dissipated as heat, which further reduces the overall efficiency of the system.

Heat Generation

Friction and turbulence within the torque converter generate heat. This heat represents lost energy that could otherwise be used for propulsion. Even modern torque converters, which are more efficient than their predecessors, still typically operate at an efficiency of around 85-90%. This means that 10-15% of the engine's power may be lost in the conversion process.

Diagrammatic Representation and Energy Loss

Figure 1 below is a diagrammatic representation of a torque converter, showing the flow of fluid through its components. When the flywheel rotates the pump, also known as the impeller, the fluid is thrown outward by centrifugal force. This fluid then travels up to the top of the pump fins and exits into the turbine, making the turbine rotate. The turbine, in turn, rotates the transmission.

The stator inside the torque converter ensures that the fluid is reused efficiently. However, at the junction where the fluid enters and exits the turbine, energy is always lost due to friction and slippage. This slippage is a result of viscous and hydraulic forces.

Energy Loss Process

1. Impeller and Fluid Dynamics: As the impeller rotates, it throws the fluid outward due to centrifugal force. The fluid then travels up to the top of the pump fins and exits into the turbine directionally.

2. Turbine Rotation: The turbine, being rotated by the fluid, then imparts its rotation to the transmission, connected via a shaft. However, the turbine rotates in the opposite direction to the impeller due to the fluid's reaction torque.

3. Fluid Recirculation: The process is repeated as the fluid flows outward and then back inward to the turbine, further dissipating energy through friction and turbulence.

Manufacturing Defects and Slippage

No manufacturing process can achieve 100% precision, and even within tolerance limits, defects can still lead to slippage. For example, free-play is observed due to minimal slippage between the splines connecting the turbine and transmission. These small slip factors contribute to the overall power loss in the torque converter.

Estimates suggest that the total power loss due to these factors can range from 30-35%. However, the exact figure may vary based on various factors, including the quality of the components and the design of the torque converter.

Conclusion

Though torque converters are crucial for smooth operation and provide significant benefits such as increased torque during acceleration, they do incur power loss compared to direct drive systems like manual transmissions. The power loss is a result of slip, hydraulic resistance, heat generation, and manufacturing defects. Understanding these factors is essential for optimizing the performance of automatic transmissions.

Keywords: power loss, torque converter, automatic transmission