Investigation of methods for measuring fuel economy and emissions of heavy-duty hybrid-electric vehicles (HEVs)

. Hybrid-electric vehicles can achieve low fuel consumption and emission by optimal combination of electrical energy and internal combustion engine power. There are four main test method of energy consumption and emission for heavy-duty HEVs, including traditional engine bench test, powertrain test, HILS and chassis dynamometer test. The tradition engine operating conditions are distinguished from that in HEVs, which can’t reflect the performance of HEVs. Powertrain test can achieve braking energy regeneration, but need bulky installation on testbed. HILS test solves the problem that the engine test cycle is different from the actual operating condition on road and is cost-effective. However, it requires a lot of effort to monitor the HILS simulation input parameters and carry out vehicle validation. Chassis dynamometer test method can better reflect the real-road driving condition. But it is nearly impossible to test all different HEVs types on chassis dynamometer and the test cost is very expensive. By comparing different heavy-duty HEVs energy consumption and emission test methods, the advantages and disadvantages are identified, which provides guidance for the formulation of the next-stage standards of heavy-duty HEV, and promotes the healthy and orderly development of the HEV industry.


Introduction
At the General Debate of the 75th Session of the United Nations General Assembly, China made it clear that it will adopt more forceful policies and measures, strive to peak its carbon dioxide emissions by 2030, and strive to achieve carbon neutrality by 2060.According to statistics, the carbon emissions of the transportation industry account for 15% of the national terminal carbon emissions, and the growth rate is 5% year by year.Strengthening the coordinated control of carbon emissions and emission pollution, enhancing the application of new energy and clean energy, and intensifying the implementation of vehicle emission inspection and maintenance regime are the only way for the transportation industry to reach the carbon peak.Through a highly efficient Rechargeable Energy Storage System (RESS) and an electric drive system, the hybrid-electric vehicles (HEVs) achieve optimized combination of electric power and internal combustion power to realize reduce the power demand of the internal combustion engine and the optimization of the working state of the engine.Therefore, HEVs can achieve energy saving and emission reduction.
With the national government vigorously promoting the relevant policies of new energy vehicles, the market of hybrid-electric vehicles will gradually expand.The production and sales of hybrid-electric vehicles shows a trend of increasing year by year, and the growth rate increases year by year.Due to the significant differences between the working principle of HEVs and the traditional internal combustion engine vehicle, the test method of the energy consumption and emission for the traditional vehicle will not be able to accurately measure the real energy consumption and emission level of HEVs, nor can it make a correct evaluation of the aftertreatment device of HEVs.
At present, there are two standards for light hybrid-electric vehicles in China: GB/T 19753-2021 "Test methods for energy consumption of light-duty hybrid electric vehicles" and GB 19755-2016 "Technical requirements and measurement methods for emissions from light-duty hybrid electric vehicles ", which indicate test methods of energy consumption and emission for light hybrid electric vehicles are required respectively [1,2] .However, for heavy-duty hybrid-electric vehicles, there is only GB/T 19754-2020 "Test methods for energy consumption of heavy-duty hybrid-electric vehicles standard", which makes specific requirements for energy consumption test methods [3] .The emission certification of heavy-duty hybrid-electric vehicles is conducted on engine test bench in the same way as that of traditional diesel vehicles according to GB 17691-2018 "Limits and measurement methods for emissions from diesel fuelled heavy-duty vehicles (CHINA VI)s" [4] .However, for heavy-duty hybrid-electric vehicles, there is a great gap between the actual engine operating condition and the test condition, so it is obviously unreasonable to use the traditional engine test method to evaluate the performance of heavy-duty hybrid-electric vehicles.The test standards of hybrid-electric vehicles in China are summarized in Table .1.In the present work, the development situation of energy consumption and pollutant emission test standards for heavy-duty hybrid electric vehicles at home and abroad is firstly reviewed.Then different test and evaluation methods of heavy-duty hybrid-electric vehicles are summarized and compared.Finally, the prospect of energy consumption and emission standards for heavy-duty hybrid electric vehicles in China in the next stage is put forward.By comparing different test standards and methods for heavy-duty HEVs, the advantages and disadvantages are analyzed, which may provide guidance for the formulation of heavy-duty HEVs standards in the next stage and promote the healthy and orderly development of heavy-duty HEVs industry.

Standards of energy consumption and emission testing for heavy-duty HEVs HEVs standards abroad
In order to deal with the energy and environmental problems brought by the transportation industry, the United States, Europe, Japan and other major countries and regions in the world have formulated automobile energy consumption and emission standards and regulations.However, there are relatively few test standards of energy consumption and emission for heavy-duty HEVs.

HEVs standards in the USA
For heavy-duty HEVs, there is SAE J2711 "Recommended Practice for Measuring Fuel Economy and Emissions of Hybrid-Electric and Conventional Heavy-Duty Vehicles", which was issued in 2002, stabilized in 2018 and revised in 2020.This SAE Recommended Practice was established to provide an accurate, uniform, and reproducible procedure for simulating use of middle-duty/heavy-duty conventional vehicles and hybrid-electric vehicles, as well as plug-in hybrid-electric vehicles and battery electric vehicles on chassis dynamometer for the purpose of measuring emissions and fuel economy.It is recommended that the vehicle be tested using three different driving cycles: the Manhattan Cycle, which is representative of transit bus operation in city service (illustrated in Fig. 1); Orange County Cycle, which represents mid-speed heavy-duty vehicle operation (illustrated in Fig. 2); and heavy-duty Urban Dynamometer Driving Schedule (UDDS), which mimics higher speed operation (illustrated in Fig. 3) [5].Chassis dynamometer test method solves the problem that the engine test condition breaks away from the vehicle real condition.At the same time, through the inertial simulation of the chassis dynamometer, the braking energy can be recovered during the test process, which can evaluate the performance of the heavy-duty HEVs comprehensively and reasonably.In addition, the repeatability and accuracy of the measurement results are good.
Currently, most major medium-duty and heavy-duty truck OEMs and Tier1 suppliers provide integrated powertrains that have been optimized as a system.Therefore, it makes sense to test powertrains as a system to capture the gains that they enable and that might be otherwise overlook when testing a stand-alone engine.The revised SAE J2711-2020 defines clear procedures for powertrain testing.

HEVs standards in Japan
Hybrid technology was developed earlier in Japan, thus the HEVs standards is relatively perfect.With respect to heavy-duty HEVs, there are test standards in Japan: 12-40966 "Test methods for emission of heavy-duty hybrid electric vehicles", 12-2-30 "Test procedure for fuel consumption grading of heavy-duty hybrid electric vehicles", and 12-2-35 "Interim procedure for heavy-duty hybrid electric vehicles certification test using HILS system" in "Automobile Type Approval Handbook for Japanese Certification".Test methods recommended by the above standards including system bench method and HILS (Hardware In the Loop Simulation) method.The system bench method in Japan is similar with the powertrain test method mentioned in the USA.
(1)System bench method Compared with traditional diesel vehicles, the HEVs add a RESS and an electric drive System.When using system bench method, the engine, electric motor-generator system, RESS and other components are installed on the engine bench according to the hybrid powertrain layout scheme.The dynamometer is employed to simulate the vehicle load, and the fuel consumption meter, emission analysis system, voltameter and related sensors are used to measure the energy consumption and emission of the hybrid powertrain.The JE05 Driving Cycle is employed and depicted in Fig. 4. It is noted that the JE05 Driving Cycle is a curve of vehicle speed versus time.Before the test, it is needed to be converted into the torque-engine speed-time curve of the engine by using vehicle parameters such as tire load radius and transmission ratio.(2) HILS method With the development of computer simulation technology, HILS technology develops and is widely used in engine ECU rapid development and test.The models of each unit (engine, RESS, generator, motor, transmission, etc.) and the real ECU are used to simulate the actual HEV operation and calculate the fuel consumption and emissions.HILS method has simple operation and high calculation precision, which can effectively shorten the test duration and save the test cost.However, due to the various types and different control strategies of HEVs, different HILS models are needed when measuring different HEVs.Therefore, the development of high-precision and widely applicable models is the premise for the wide application of the HILS method.

Energy consumption test standards for HEVs
The energy consumption test methods for heavy-duty HEVs are identified in GB/T 19754-2020 "Test methods for energy consumption of heavy-duty hybrid-electric vehicles standard".This standard was issued in 2005, and then revised in 2015 and 2020.Chassis dynamometer test method is recommended for energy consumption of HEVs in GB/T 19754, referring to SAE J2711.In GB/T 19754-2020, the China heavy-duty Test Cycle (CHTC) is recommended.CHTC-B Driving Cycle is recommended for city bus, shown in Fig. 5.

Emission test standards for HEVs
In China, there is no national standard for emission testing of heavy-duty HEVs, but only the industry standard QC/T 894-2011 "On-board measurement methods for emissions from heavy-duty hybrid electric vehicles" [7] .At present, the emission certification of heavy-duty HEVs is conducted on engine test bench in the same way as that of traditional diesel vehicles according to GB 17691-2018.As discussed above, it is obviously unreasonable to use the traditional engine test method to evaluate the performance of heavy-duty HEVs.Therefore, it is essential to establish a specific emission test standard for heavy HEVs.

Test and evaluation methods for heavy-duty HEVs
The test and evaluation methods for heavy-duty HEVs are mainly divided into the following four categories: engine bench test, powertrain test, HILS test and chassis dynamometer test [8][9][10] .

Engine bench test
Currently, the HEVs type test is conducted on engine bench using the WHTC/WHSC Driving Cycle consistent with the traditional diesel engine.On the one hand, the engine operation in HEVs is totally different with that in traditional diesel vehicle.On the other hand, the braking energy regeneration of HEVs can't be achieved on the traditional engine bench.Therefore, it is apparently unreasonable to use the traditional engine bench test method to evaluate the performance of heavy HEVs.

Powertrain test method
The powertrain test method is an upgrade based on the traditional engine bench test to reflect the unique energy utilization of the HEVs.The engine, electric motor-generator system, RESS and other components are installed on the engine bench according to the hybrid powertrain layout scheme.The dynamometer is employed to simulate the vehicle load, and the fuel consumption meter, emission analysis system, voltameter and related sensors are used to measure the energy consumption and emission of the hybrid powertrain.The schematic diagram of powertrain test method for parallel hybrid power system developed by Li et al. [11] is depicted in Fig. 6.
Compared with traditional engine bench, the powertrain test can achieve barking energy regeneration.Modular test can provide the required test environment for different types of engines, motors, batteries and vehicle controllers, shortening the research and development duration.However, engine operation is distinguished from the real-road condition.Moreover, it is difficult to install and arrange the bench, and the test cost is high.

HILS method
The HILS method was proposed by Kenji Morita, Kazuki Shimamura et al. from the Japan Automotive Research Institute [12] .Using the Digital Signal Processor (DSP), the ECU operation mode is consistent with that installed on the actual vehicle through vehicle modeling and component modeling.By using HILS, the vehicle speed-time curve can be transformed into the engine torque-speed-time cycle, which solves the problem that the engine test cycle is separated from the actual driving condition of the vehicle.
As shown in Fig. 7, the real ECU is first connected to the DSP-based vehicle model during the HILS test.DSP can simulate the running state of the vehicle powertrain, and then obtain the engine torque-speed-time sequence.The energy consumption is calculated using the converted engine torque-speed-time series combined with the fuel consumption chart.The emission is measured on the engine bench using the simulated engine torque-speed-time cycle sequence .
The HILS method solves the problem that the engine operation is distinguished from the real-road operation.In addition, the HILS test method can control the cost.However, the hybrid model in HILS is not standardized and has poor generality, and it requires a lot of effort to monitor the HILS simulation input parameters and carry out vehicle validation.

Chassis dynamometer method
The chassis dynamometer test method comprehensively uses the inertial mass of the drum itself and the electric inertia of the motor to simulate the actual road driving resistance of the vehicle.The vehicle runs on the chassis dynamometer using prescribed cycle.The emission is measured by the emission test system.Fuel consumption meter or carbon balance method is used to measure the fuel consumption, and wattmeter is used to measure the electric energy consumption.
Chassis dynamometer test method is recommended in SAE J2711and GB/T 19754.According to GB/T 19754-2015, the test method of energy consumption for heavy-duty HEVs on chassis dynamometer is introduced in the following.
Net Energy Change (NEC) is the main judgment condition for OVC-HEV (off-vehicle charging hybrid electric vehicle) and NOVC-HEV (not off-vehicle charging hybrid electric vehicle) test.For NOVC-HEV, in order to truly compare the fuel consumption of HEVs and tradition vehicles, the HEVs results should be revised to ensure that the NEC of the RESS is zero, so that all the energy is provided by the engine.At least three sets of tests should be conducted to ensure that there is sufficient data to correct the SOC.

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(1)  8) is adopted under 65% load; or C-WTVC under 100% load.For other heavy-duty vehicles, C-WTVC is employed under 100% load.In GB/T 19754-2020, the test cycle is revised, and CHTC (China Heavy-duty Test Cycle) is adopted to measure the energy consumption.During the test, the ambient temperature should be between 20 ℃ and 30 ℃.The test procedure varies according to the type of HEVs, including NOVO-HEV, OVC-HEV with a pure electric operating mode, and OVC-HEV without a pure electric operating mode.
For NOVC-HEV, the test is conducted on the chassis dynamometer, and at least three sets of tests should be conducted to ensure that there is sufficient data to correct the SOC.The relative change of NEC is taken as the condition to judge whether the test is effective or not, and whether SOC correction is needed for the results.The relative change of NEC is calculated using Equ (5).If the absolute value of relative change of NEC is less than or equal to 1%, the test results do not need to be corrected.If the absolute value is greater than 1% but less than 5%, the test results need to be corrected.If the value is less than -5%, the vehicle continues to discharge, then directly list fuel consumption (L/100 km) and electric energy consumption (kW•h/100 km).If the value is greater than 5%, it is considered that the test is invalid and the vehicle control strategy is unreasonable and needs to be adjusted.
where: ηtransmission2: the transmission efficiency from the NEC node to the wheel edge At least three sets of tests should be conducted to ensure that there is sufficient data to correct the SOC.Moreover, at least one NEC result is required to be positive, and at least one NEC result to be negative, so as to ensure that SOC correction is calculated based on interpolation method.Linear interpolation method is used for SOC correction.When the linear regression coefficient R 2 is ≥0.8, the predicted results are considered acceptable.Example of fuel consumption correction after 6 tests is depicted in Fig. 8. achieve braking energy regeneration.However, the test operating conditions are still separated from the actual operating condition on road.HILS test method transforms the vehicle speed-time sequence to engine torque-speed-time sequence through DSP, solving the problem that the engine test cycle is different from the actual operating condition on road.In addition, the test cost using HILS method is relatively low.However, However, the hybrid model in HILS is not standardized and has poor generality, which requires a lot of effort to monitor the HILS simulation input parameters and carry out vehicle validation.Chassis dynamometer test method can better reflect the real-road driving condition.But it is nearly impossible to test all different HEV types on chassis dynamometer and the test cost is very expensive.
At present, in China, the stage III fuel consumption test standard of heavy-duty vehicles is conducted on the chassis dynamometer.Moreover, the stage VI emission standard also requires the actual road test of the vehicle.The whole-vehicle test evaluation method will also be the development direction of the next stage standard of heavy HEVs.And further optimizing the test method to decrease the test cost and reduce the burden for automobile enterprises will also be an important aspect to be considered in the next stage standard formulation.
Support by CATARC Automotive Test Center TJKY2021019.

Table 1 .
Standards for HEVs in China.