Research on the effect of different test cycles on low temperature emissions and fuel consumption of Light plug-in hybrid cars

Based on a plug-in hybrid car equipped with a 1.5L turbocharged direct injection gasoline engine, this paper studies the emission and fuel consumption characteristics of the prototype vehicle under three different cycle conditions, WLTC city, WLTC and CLTC-P. The results show that for the four pollutants: CO, THC, NMHC and NOx, WLTC city cycle emissions are the largest, WLTC cycle emissions are the smallest, CLTC-P cycle emissions are the middle. For N2O, WLTC cycle has the largest combined emissions, CLTC-P cycle combined emissions are the smallest, and WLTC city cycle combined emissions are the center. The combined fuel consumption under WLTC city conditions is approximately 1.3 times the combined fuel consumption under the complete WLTC cycle. The combined fuel consumption under CLTC-P conditions is approximately 1.25 times that of the complete WLTC cycle. The first phase of each cycle is the phase with the highest emissions and fuel consumption.


WLTC test cycle and WITC city test cycle
WLTC test cycle is a four-part variable operating conditions test cycle, which is divided into low-speed section, medium-speed section, high-speed section and extra-high-speed section, in which low-speed section duration and time 589 s, medium-speed segment duration 433s,high-speed segment duration 455s, extra-high-speed segment duration 323s, total operating time 1800s, total distance 23.27 km [12]. The WLTC city test cycle is the first two parts of the WLTC: the low speed section and the middle speed section, which are used to measure the standard test cycle of the exhaust pollutants in the low temperature and cold start of the Chinese Six phase vehicles.

CLTC-P test cycle
Chinese passenger car driving condition (CLTC-P) include low speed (1 part), medium speed (2 parts) and high speed (3 parts) three speed intervals, the whole CLTC-P test cycle conditions total 1800 seconds [13], the operating conditions curve is shown in the diagram:

Comparison of characteristic parameters of two test cycles
WLTC test cycle and CLTC test cycle are variable condition test cycle,their total duration is the same as 1800 seconds,but there are differences in total cycle mileage and maximum speed.The characteristic parameters of the two working conditions are shown in the table1:

Test vehicle and test equipment
The main technical parameters of the vehicle under test are shown in Table2. The fuel used is China national six standard 95# benchmark gasoline. The environmental warehouse used for low temperature test is the SD 520'/30 type environmental warehouse produced by German WEISS Company, the Chassis dynamometer is the RPL1220 type single axis chassis dynamometer produced by Austrian AVL Company, and the emission analysis system is the MEXA-ONE type light vehicle emission analysis system produced by Japanese HORIBA Company. The measurement of THC is hydrogen flame ion method, CO by non-dividing infrared method, NOx by chemiluminescence method, and N2O by infrared absorption method.

WLTC
The principle of hydrogen flame ion method (FID) is that hydrocarbons can be ionized into electrons and free ions when they burn at about 2000℃ of high temperature of hydrogen flame, and the number of ions is basically proportional to the number of carbon atoms. The ions decomposed in the hydrogen flame form electron flow under the voltage between the plates of the ion absorption electrode, the current size represents the concentration of carbon atoms in the sample gas, so the result of FID detection is the ppm value of carbon atoms in the sample gas. Motor rating power 110kW Drive mode Front-wheel driven

Transmission form DCT
The non-dispersive infrared analysis (NDIR) is the best method for the determination of CO at present. Its working principle is based on measuring the energy absorption of gas to infrared rays at a specific wavelength. CO can absorb the wavelength of 4.5~5.0μm infrared ray, with absorption peak, the concentration of CO in the sample gas can be obtained by infrared ray through a certain length of the transmission energy of the gas. In order to reduce the interference of other gases, a filter chamber is set up in front of the sample chamber to filter out the wavelength corresponding to the other interference gases.
The chemiluminescence method (CLD) used to analyze the NOx,CLD can only determine NO directly. Sample gas and excess ozone are mixed and chemically reacted in the reaction chamber to form NO2, about 10% of which are in the electronically excited state. When the NO2 of the excited state attenuates to the ground state, the emission wavelength is 0.6~3μm photons. The chemiluminescence intensity is proportional to the product of NO and ozone concentration and is also related to the measurement conditions. However, when the measurement conditions are constant and the ozone concentration is constant and much higher than the NO concentration. The measurement NOx is actually the sum of the measured NO and NO2, so in the measurement The NO2 in the exhaust must be converted into NO. before the quantity is measured.

Vehicle pretreatment and test preparation
Prior to pretreatment, check whether the tire pressure of the vehicle meets the technical requirements, whether the engine oil level is normal, check whether the fuel in the vehicle tank meets the requirements, and preheat the chassis dynamometer at a speed of 100 km/h for 20 min. Drive the test vehicle or push it to the dynamometer for fixing and pretest the vehicle according to the WLTC cycle. When the pretest is finished, according to the method of determining the driving resistance CC the GB18352.6-2016 attachment, the vehicle coastdown is carried out according to the test quality and the road load coefficient F0,F1,F2, and the setting parameters of the chassis dynamometer are obtained,and then follow WLTC two cycles to preprocess [14]. the WLTC cycle pretreatment here is carried out twice to ensure that the vehicle is in the power holding mode during the vehicle test (that is, the battery SOC is in equilibrium). the vehicle will be pretreated according to the two cycles before each low temperature test later to ensure that the prototype battery SOC is at the same level before each test. When pre-treatment is completed, the vehicle shall be placed in an environmental warehouse for cryogenic immersion, and the prototype shall be placed not less than 12 h but not more than 36 h.. the average ambient temperature (dry ball) shall be maintained during this period: within -7℃±3℃ per hour.

Prototype test
The chassis dynamometer and emission analyzer are preheated before the start of the test, and the zero and span points of the emission analyzer are calibrated after preheating. push the test vehicle onto the chassis dynamometer for fixing, check whether the oil temperature and coolant temperature of the test vehicle are in the range of -7±3℃, check the tire pressure of the vehicle and the SOC of the battery to confirm that the status of the prototype is consistent before each low temperature test [15][16]. The final,start the test and run test cycle, during the test, the heating and defrosting device should be closed, etc., the driver must not see the speed tolerance. Maximum tolerance ,2.0 km/h, time ±1.0 seconds inner; tolerance lower limit ,-2.0 km/h, time within ±1.0 seconds. The allowable speed tolerance is greater than the specified requirement, but the excess time can not exceed 1 second. During the test, there should be no more than 10 times of excess speed. During the test, use the tracking fan to cool the engine, the fan should be within 300 mm in front of the car. After the test, the sampling results were analyzed by reading bag, and the final low temperature emission and fuel consumption results were obtained.

Test results and analysis
The test results obtained by conducting low temperature tests under three different cycles of WLTC city, WLTC and CLTC-P are shown in the table.   It can be seen from figure 5 that the emission of gaseous pollutants is mainly produced in the first phase, and the emission level of pollutants in the first phase under the two cycles is approximately the same. Not difficult to understand, because the WLTC city cycle is originally part of the WLTC cycle, the first phase of the two cycles is exactly the same working conditions, so the emission level of pollutants in the first phase is roughly the same.  The emissions of four gaseous pollutants at each phase of the CLTC-P and WLTC cycles were compared separately, as shown in figures 6 and 7. It can be seen from the diagram that the emission of gaseous pollutants is mainly produced in the first phase, and the emission level of pollutants in the first phase of the two cycles is very different. In the first phase nitrous oxide (N2O) and nitrogen oxide (NOx) emission levels are basically the same. The emission levels of the three pollutants THC, CO and NMHC under the CLTC-P cycle are significantly higher than those in the WLTC cycle, which is about twice the combined emissions under the WLTC cycle.  Then, the comparison of low temperature comprehensive fuel consumption under three cycles is carried out. The WLTC city cycle fuel consumption is the highest for the cycle comprehensive fuel consumption, as shown in figure 8, is 11.73 L/100km; The WLTC cycle fuel consumption is the lowest ,9.06 L/100km;The CLTC-P cycle comprehensive fuel consumption is in the middle ,11.15 L/100km.. We can see that under the low temperature state, under the WLTC city working condition the comprehensive fuel consumption is about 1. It can also be found from figure8 that the integrated fuel consumption in the first phase of each cycle condition is the highest, and the integrated fuel consumption in the first phase of CLTC-P cycle is 21.271 L/100km, which is about 1.32 times that of the integrated fuel consumption (16.101 L/100km) in the first phase of WLTC cycle. The combined fuel consumption level of the second phase under the three cycles is approximately the same, and the combined fuel consumption level of the fourth phase of the WLTC cycle (extra-high speed section) and the third phase of the CLTC-P cycle (high speed section) is approximately the same.
The instantaneous emission curves of four gaseous pollutants CO, THC, NOx and N2O under WLTC and CLTC-P cycles are shown in Fig.9 and Fig.10, respectively, where the units of CO, THC and NOx are the units of g/s,N2O is the units of mg/s. It can be seen from the image that the emission peaks of gaseous pollutants in 4 appear within 50 seconds after the cold start of the first phase.  Draw the 1st minute pollutant transient emissions is shown in figure 11-14.We can see that the peak of CO in the WLTC cycle appears at 25.4 seconds, the peak of CO emission is 0.52 g/s, the peak of concentration is 2597.49ppm, the peak of CO in the CLTC-P cycle appears at 27.0 seconds, and the peak of CO emission is 0.53 g/s, the peak of concentration is 2618.36 ppm;THC peaks in in the WLTC cycle appears at 28.8 seconds, the peak of THC emission is 0.103 g/s, the peak of concentration is 1043.05 ppm;the peak of THC in the CLTC-P cycle appears at 30.9 seconds, and the peak of THC emission is 0.103 g/s, concentration peak is 1029.14 ppm;The peak of NOx in the WLTC cycle appeared at 28.7 seconds, NOx emission peak was 0.029 g/s, concentration peak was 113.98 ppm;the peak in the CLTC-P cycle appeared at 38.0 seconds, NOx emission peak was 0.021 g/s, concentration peak was 82.86 ppm;The peak of N2O in the WLTC cycle appears at 38.9 seconds, the peak of N2O emission is 0.275 mg/s, the peak of concentration is 0.875 ppm; the peak of N2O in the CLTC-P cycle appears at 38.5 seconds, and the peak of N2O emission is 0.298 mg/s, the peak of concentration is 0.938 ppm.      CLTC-P CO ppm CLTC-P THC ppm CLTC-P NOX ppm CLTC-P N2O ppm