Emission Characteristics of Concrete Mixer Trucks Under Real-world Operating Conditions

Objective With the increasing emphasis on urban air pollution prevention and control efforts, studying the emission characteristics of concrete mixer trucks has become a critical factor in raising the synergetic control of environmental pollution and carbon emissions from mobile sources. This is particularly important given their high activity levels in urban central areas as heavy diesel vehicles. Most existing studies have primarily focused on heavy-duty freight vehicles, such as tractors, transport trucks, and dump trucks, while comparatively less attention has been given to concrete mixer trucks, which frequently operate in urban areas and pose potential threats to human health. In addition, most studies have concentrated on emissions under standard or controlled driving conditions. However, the actual operating conditions of engineering vehicles, including concrete mixer trucks, differ from those of typical heavy-duty vehicles. Therefore, conducting emission studies based on real-world conditions is essential for analyzing emission variations under different operating scenarios and for effectively controlling emissions from mobile sources.Methods This study selected concrete mixer trucks with high activity levels and representative operating areas in Chengdu to investigate the real emission characteristics of these vehicles during actual task processes. Real-world emission measurements of the entire driving process were conducted using a portable emissions measurement system (PEMS). The equipment continuously collected and recorded exhaust and pollutant emission data during the vehicle’s operation, along with per-second data on engine performance. In addition, it captured relevant environmental parameters, geographical location data, and other onboard diagnostics (OBD) information in real time. The three test vehicles selected were newly purchased National VI concrete mixer trucks, each having been in operation for less than three months. Specifically, Vehicle 1 accumulated a mileage of 4 712 km, Vehicle 2 accumulated 3 927 km, and Vehicle 3 accumulated 4 453 km. The study was conducted based on the actual task routes and processes of concrete mixer trucks, with one complete task cycle considered as a single test cycle. The test starting point, endpoint, and equipment installation site were located at a concrete mixing station in Chengdu. A total of 38 105 data points were collected, including 14 380 from Vehicle 1, 8 502 from Vehicle 2, and 15 223 from Vehicle 3. An analysis of the activity levels and operating conditions of the concrete mixer trucks was performed based on the test data, and the emission characteristics of CO₂, CO, and NO<italic>_x</italic> under different operating conditions and task periods were examined separately.Results and Discussions The research results indicated that: 1) The three selected concrete mixer trucks exhibited noticeable variations in emission factors for CO₂, CO, and NO<italic>_x</italic> under different typical driving conditions. Throughout the entire process, the average emission factors for CO₂, CO, and NO<italic>_x</italic> were 843.65, 1.21, and 2.29 g/(kW·h), respectively. During transportation periods, the average emission factors were 713.47, 1.06, and 0.51 g/(kW·h), while during waiting periods, they were 1 330.45, 2.35, and 6.64 g/(kW·h), respectively. 2) The distribution of task time for mixer trucks was significantly influenced by daily project scheduling and queue waiting times. The transportation period was the primary stage affecting CO₂ and CO emissions, with emission rates remaining relatively low during waiting periods and showing minimal change with increasing waiting time. However, during waiting and unloading periods, NO<italic>_x</italic> emissions accounted for the highest proportion, exceeding 70% of the total emissions throughout the entire process. The emission intensity of NO<italic>_x</italic> increased significantly with prolonged waiting time. 3) The actual working conditions of mixer trucks primarily involved low-load operations, with overall operating speeds typically remaining within 50 km/h. The vehicle specific power (VSP) was predominantly distributed in the [0, 2] kW/t range, with limited instances of acceleration. Even under different driving routes, significant differences were not observed, contrasting with typical heavy-duty vehicle driving conditions. In practical tasks, waiting times for mixer trucks were substantial, and prolonged idling contributed to the accumulation of pollutant emissions. Based on driving characteristics, it was determined that the emission intensity of mixer trucks exhibited a significant positive correlation with VSP, with acceleration conditions within the low-speed range (&lt; 20 km/h) resulting in higher emissions. 4) Compared to reference vehicles, mixer trucks exhibited emissions under different conditions that demonstrated some similarities to those of reference vehicles. This indicated that under similar conditions, the emission characteristics of mixer trucks and typical heavy-duty vehicles shared certain similarities. Therefore, emission control strategies for typical heavy-duty vehicles provided some guidance for mixer trucks operating under similar conditions. However, for mixer trucks, where NO<italic>_x</italic> emissions were most influenced during the waiting and unloading stages, specific considerations needed to be addressed.Conclusions A comparative analysis of engine power parameters, fuel combustion parameters, and exhaust emissions parameters recorded in PEMS indicated that CO₂, CO, and NO<italic>_x</italic> emissions are mainly influenced by vehicle power output and fuel combustion. In addition, CO and NO<italic>_x</italic> emissions are also affected by exhaust gas temperature and the operating temperature of the selective catalytic reduction (SCR) system. Based on an analysis of the entire task process of mixer trucks under actual road conditions, environmental management strategies for concrete mixer trucks are proposed, including optimizing driving routes and schedules to avoid road congestion, reducing vehicle waiting time, strictly controlling the operating temperature of the SCR system during vehicle operation, ensuring a consistent supply of urea for the SCR system, and standardizing driver behavior. These measures aim to provide reliable references for the synergetic control of environmental pollution and carbon emissions from urban mobile sources.