The Fundamental Structure and Operating Principle of Automotive Chassis Dynamometers

　　A vehicle chassis dynamometer is an experimental setup used to test various performance characteristics of automobiles, such as power output and emission performance. When combined with other test benches and instrumentation, it can simulate a wide range of real-world driving conditions, accurately measuring the output power delivered by the vehicle's drive wheels. Additionally, it enables the measurement of multi-condition emission levels and fuel consumption, providing valuable guidance for environmental agencies monitoring vehicle exhaust emissions. Compared to outdoor road tests, conducting vehicle experiments on a chassis dynamometer offers significant advantages, including enhanced safety, higher efficiency, and superior repeatability. Below, we’ll introduce the basic structure and operating principles of several common types of vehicle chassis dynamometers.

　　1 The Fundamental Structure of a Chassis Dynamometer

　　According to the JT/T 445-2008 "Chassis Dynamometer for Vehicles" standard, chassis dynamometers are classified by their rated load capacity into 3-ton, 10-ton, and 13-ton models. The 3-ton and 10-ton models are two-axle types, while the 13-ton model is a three-axle type. Two-axle chassis dynamometers are primarily used for testing light- and medium-duty single-axle drive vehicles, whereas three-axle chassis dynamometers are mainly designed for heavy-duty single-axle drive vehicles as well as vehicles with dual drive axles.

　　The standard dual-axis chassis dynamometer typically consists of a main dynamometer frame, a power absorption unit (electro-magnetic eddy current brake), an inertia simulation unit (flywheel assembly), a lifting mechanism, safety devices (such as limit guide rollers), and various associated sensors. It can be used for speedometer testing, odometer testing, chassis dynamometer performance tests (including constant-speed control and multi-point continuous power measurements), acceleration time detection, coasting distance measurement, and steady-speed fuel consumption testing. This versatile equipment effectively meets the needs of automotive testing and fault diagnosis, making it widely applicable in automotive repair shops, automobile manufacturers, and comprehensive vehicle performance testing facilities.

　　As the atmospheric environment continues to deteriorate and the number of motor vehicles rapidly increases, environmental protection authorities are stepping up efforts to monitor vehicle exhaust emissions. Consequently, chassis dynamometers have become essential equipment for environmental testing stations and Class M repair businesses. Compared to standard chassis dynamometers, condition-based chassis dynamometers equipped with exhaust pollutant testing capabilities (as shown in Figure 2) require, in addition to the basic components mentioned above, a backdragging mechanism. Moreover, both the backdragging mechanism and the inertia simulation system must comply with the relevant regulations outlined in HJ/T 291, "Technical Requirements for Equipment Used to Measure Exhaust Pollutants from Gasoline-Powered Vehicles under Steady-State Conditions," as well as HJ/T 292, "Technical Requirements for Equipment Used to Measure Exhaust Smoke from Diesel-Powered Vehicles under Loaded Deceleration Conditions."

　　The three-axis chassis dynamometer is equipped with a dual-test bench system, a power-absorption unit (consisting of two eddy-current dynos), a 151-type simulation unit (featuring a flywheel assembly that can be configured with multiple sets as needed), a lifting mechanism, safety devices (including limit-position guide rollers), and various associated sensors. During measurement, the drive axle (the dual front wheels) rests on the rollers of the first two axles, while the dual rear wheels are positioned on the third axle's roller. All three axle rollers rotate synchronously via a timing belt.

　　2 How a Chassis Dynamometer Works

　　When a vehicle is in motion, it possesses kinetic energy and experiences driving resistance. The so-called simulation of vehicle operating conditions on various road surfaces is achieved by replicating the vehicle's dynamic inertia—specifically its kinetic energy—through an inertia-mimicking setup connected to a chassis dynamometer. Meanwhile, the driving resistance is simulated using a power-absorption system, while frictional rollers are employed to mimic the road surface that the vehicle encounters during operation.

　　Before testing, all preparatory work must be thoroughly completed, including setting up the testing area and ensuring proper power supply. When the equipment is operating normally, the lift should remain in the raised position. The vehicle under test should then be driven onto the dynamometer test bench, with its drive axle resting securely on the lift. Ensure that the wheels are centered precisely over the rollers. Next, lower the lift so that the wheels make full contact with the rollers. Start the vehicle moving slowly, and carefully observe whether the vehicle exhibits any tendency to drift off course. If drift is detected, raise the lift again, adjust the vehicle’s position until it no longer drifts, and then lower the lift once more. As the vehicle rolls smoothly on the rollers, the linear surface speed of the rollers will match the vehicle’s actual driving speed, simultaneously engaging the inertia-mimicking flywheel assembly to simulate rotational inertia. This setup allows the system to accurately measure the vehicle’s speed, as the roller’s rotational velocity—detected by a speed sensor mounted on the rolling shaft—is converted into the vehicle’s speed data. Finally, the main roller assembly is connected to an eddy-current power absorber. By applying an excitation current to the eddy-current machine, a resistance force is generated that is equal in magnitude but opposite in direction to the vehicle’s driving force. This opposing force is dynamically balanced by a force sensor installed on the eddy-current machine’s arm, which converts the measured force into an electrical signal. The signal is then sent to a computer for processing and control, enabling precise monitoring and adjustment of the entire testing process.