Key points to be aware of when using a dynamometer

The vehicle chassis dynamometer is one of the important bench testing equipment for automobiles. It is used in automotive engineering for the overall inspection of technical conditions. Previously, it was mostly used in new car manufacturing and testing. In the 1980s, the application of dynamometers began to be popularized in China. Currently, thousands of in-use vehicle performance testing stations across the country are mostly equipped with this low-cost chassis dynamometer.

The requirements for vehicle testing chassis dynamometers are to ensure certain accuracy and compatibility with multiple functions while keeping the cost low. However, there are still many problems with such chassis dynamometers: such as low accuracy of measurement data; due to different mechanical structures, there are differences in internal resistance, resulting in poor reliability of the measurement data; it is difficult to make a correct evaluation of the vehicle's power performance and to be compatible with the dynamometer used for emission testing; it is difficult to accurately simulate road resistance, etc. Coupled with the low level of automation and intelligence, and cumbersome operation, many testing institutions cannot fully utilize and exert the role of the dynamometer. Based on the above situation, solving key technologies such as accurate detection data, compatibility of functions, stable and reliable performance, intelligence, and other aspects is the task for the promotion and application of chassis dynamometers.

After several years of efforts, by addressing some key issues in the use of in-service chassis dynamometers, and based on similar principles through mathematical analysis, optimizing the mechanical structure, improving the control methods, and establishing correction models, through a large number of experiments, it has been achieved that the function of the in-service vehicle testing dynamometer can be significantly improved without increasing the manufacturing cost.

The key points of the chassis dynamometer test bench structure are as follows: The vehicle-mounted chassis dynamometer is generally of a double drum structure, with the drums separated on the left and right to support the left and right wheels, resulting in a four-drum structure; for the vehicle, the left and right wheels are supported on the same shaft, resulting in a two-drum structure. The rolling resistance of the vehicle on the road and on the test bench is different. To simulate the road operation conditions of the vehicle on the test bench, the first step is to accurately simulate the motion inertia force and driving resistance of the entire vehicle. The diameter, spacing, and surface material of the drums are the main structural parameters of the chassis dynamometer. Its influence on the rolling resistance mainly manifests in the size of the tire deformation curvature. After many years of practice, most factories use a four-drum layout for chassis dynamometer testing, which consists of four short drums and has eight support points and at least two coupling devices. Due to the changes in assembly quality and stress caused by the test bench effect, the coaxiality of the internal motion resistance keeps changing, resulting in an unstable state. This is an important factor causing unstable detection data for this type of chassis dynamometer. The double-drum dynamometer is composed of two 2.5 drums, with half the number of supporting bearings and fewer coupling devices. The mechanical loss of the test bench is small, and its performance in practical applications has significantly improved compared to the four-drum dynamometer. The actual measured values of the internal resistance of the two-drum dynamometer and the four-drum dynamometer show that the internal resistance of the two-drum dynamometer is less than that of the four-drum dynamometer.

Through long-term observation, the long-term stability of the two-drum system is significantly better than that of the four-drum system, and it is worthy of promotion. The slip correction and acceleration resistance loading control used in the chassis dynamometer for in-use vehicle tests generally adopt mechanical inertia simulation. Due to the limited number of flywheels and the difference between the rolling resistance on the test bench and the road rolling resistance, variable condition measurements require the correction of the acceleration, slip distance and time measured by the test bench.

Through multiple experiments to explore simple, convenient detection methods that can meet certain accuracy requirements, the relationship between the sliding distance of the road and the sliding distance of the test bench is as follows: These parameters have been agreed upon. For example, the diameter of the drum is 218 mm for small car measurements; for heavy vehicle measurements, it is mostly 370 mm to 420 mm. The spacing of the drums is treated differently for small and large vehicles. Measures such as spraying hard alloy on the drum surface to increase roughness have been widely implemented. It is particularly important to note that the size and relative stability of the internal resistance of the dynamometer test bench have a significant impact on the measurement results and require attention.