Emission Characteristics of Nitrous Oxide (N₂O) from Conventional Gasoline and Hybrid Vehicles

Abstract

Considering the potential warming potential and long lifetime of nitrous oxide (N₂O) as a greenhouse gas, exploring its emission characteristics is of great significance for its control and the achievement of sustainable development goals. As vehicles are a significant source of N₂O emissions, in this study we conducted a detailed investigation of N₂O in the exhaust of light-duty vehicles using a chassis dynamometer. We selected one conventional gasoline vehicle and two hybrid electric vehicles. We found that the N₂O emissions from all the tested vehicles complied with the China 6 emission regulation, with emission factors of 7.7 mg/km, 6.8 mg/km, and 17.1 mg/km, respectively, for the three vehicles. Driving conditions played a crucial role in N₂O emissions, with emissions generated primarily during extra-high-speed conditions, possibly due to the higher driving speed and greater number of acceleration/deceleration events. Furthermore, while hybrid electric vehicles emitted less NOx compared to conventional gasoline vehicles, their N₂O emissions were closely tied to their engine operating conditions. Surprisingly, we discovered that hybrid electric vehicles emitted more N₂O during frequent engine start–stop cycles, which could be related to the mechanisms of N₂O generation. These findings contribute to a better understanding of the N₂O emission characteristics of vehicles and will inform the development of emission control strategies to better promote global sustainable development.

1. Introduction

In recent years, with the rapid development of vehicles, the pollutants emitted by vehicles have threatened human health and promoted the generation of secondary pollution [1,2,3,4]. This indicates that there is an urgent need for vehicle pollution control. As vehicle emission standards are upgraded, the amount of pollutants emitted shows a downward trend [5,6]. However, against the background of peaking carbon emissions in 2023 and the aim of achieving carbon neutrality by 2060 in China, in addition to controlling vehicle pollutants, attention should also be paid to greenhouse gas (GHG) emissions, and primarily carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O).

Regarding GHG emissions, countries worldwide are setting targets for controlling greenhouse gas emissions. In 2023, the United States Environmental Protection Agency (EPA) proposed the strictest-ever vehicle and truck emission standards, aiming to accelerate the transition to clean transportation. The plan aims to comprehensively reduce carbon emissions from newly sold passenger cars and light trucks by 2032 to reduce average greenhouse gas emissions by 56%. Specifically, regarding CO₂ emission limits, the EPA has set an upper limit of 85 g per mile for CO₂ emissions from new light-duty vehicles in the United States by 2032. Unlike the United States, European vehicle emission standards do not directly set limits on CO₂ emissions, but they indirectly promote improvements in vehicle fuel efficiency and emission performance by limiting the emissions of other pollutants, thereby contributing to the reduction of greenhouse gas emissions. In China, the “Limits and measurement methods for emissions from light-duty vehicles (China 6)” emission regulation [7] has introduced new control measures for N₂O, with a limit of 20 mg/km for category I vehicles. However, CO₂ and CH₄ are not regulated in the China 6 emission regulations. Therefore, the emission characteristics and control of N₂O are crucial for the development of vehicle emission control strategies.

N₂O may result from the reaction between ammonia (NH₃) and nitric oxide (NO) or nitrogen dioxide (NO₂) or the decomposition of ammonium nitrate (NH₄NO₃) generated in the catalyst [8,9]. Additionally, carbon monoxide (CO), hydrocarbons (THCs), and hydrogen (H₂) are also considered important precursors of N₂O [10,11]. In addition to precursor concentrations, N₂O emissions from vehicles might be highly correlated with the temperature inside catalytic converters, and low temperatures may promote the conversion of NO to N₂O [10]. The N₂O emissions for China 1~4 vehicles have been reported, with emission factors of 45 mg/km, 39 mg/km, 26 mg/km, and 21 mg/km, respectively [12]. Furthermore, researchers have revealed that China 6 vehicles have achieved a significant reduction in N₂O emissions compared to China 4 vehicles [8,12]. For China 6 vehicles with a driving mileage of less than 20,000 km and a displacement of 2.0 L, their N₂O emission factors were observed to be only 0.77 mg/km [8]. This is likely due to the fact that the China 6 emission regulation requires vehicles to adopt more efficient combustion technologies, which effectively reduces greenhouse gas emissions by improving exhaust gas treatment systems.

With the vigorous promotion of clean transportation, the number of hybrid vehicles is gradually increasing [13]. Compared to traditional internal combustion engine vehicles, the start–stop characteristics of engine operation in hybrid vehicles may significantly impact their N₂O emissions, which requires further research and attention to be paid to the field of automotive emissions control. When comparing the N₂O emissions of hybrid electric vehicles (HEVs) and gasoline vehicles, HEVs exhibit significant carbon reduction benefits [14,15]. For vehicles with the same engine displacement, although the vehicle weight of HEVs is 20% higher than that of gasoline vehicles, their N₂O emissions showed a marked reduction. Specifically, turbocharged direct injection vehicles can achieve a reduction in N₂O emissions of over 10%, while naturally aspirated port injection vehicles can achieve a reduction of approximately 50% in their N₂O emissions [8].

Overall, to the best of our knowledge, while there are currently numerous reports on N₂O emission factors, there are relatively few comparative studies on transient emission characteristics, especially those focusing on gasoline vehicles and hybrid vehicles with different engine start–stop conditions. Therefore, in this study, we selected two gasoline vehicles and two hybrid vehicles, all of which comply with the China 6 emission regulation, in order to conduct a comprehensive analysis of N₂O emission characteristics.

2. Methodology

2.1. Test Vehicles and Fuels

In this study, we selected a total of three light-duty vehicles that all comply with the China 6 emission regulation, including one gasoline vehicle (vehicle #1) and two hybrid electric vehicles (vehicle #2 and vehicle #3). To minimize the deviation caused by the engine techniques used, all selected vehicles were equipped with Gasoline Direct Injection (GDI) engines, which are capable of achieving more precise fuel control. The gasoline vehicle is a five-seat passenger car, with a displacement of 1496 mL, a power of 138 kW, and a curb weight of around 1600 kg. The two HEVs are both plug-in hybrid electric vehicles. They were both equipped with a plug-in hybrid system consisting of a gasoline engine and a permanent magnet synchronous motor. Although both HEVs employ the same fuel injection method for their engines, namely GDI, they differ in their intake systems. Vehicle #2 has a turbocharged engine with a displacement of 1.5 L, while Vehicle #3 has a naturally aspirated engine with a displacement of 1499 mL. Furthermore, the curb weights of these two hybrid vehicles are 1840 kg and 2220 kg, respectively, which are heavier than those of gasoline vehicles, particularly vehicle #1.

During all driving tests, Research Octane Number (RON) 92 conventional gasoline generated by the automobile testing center was used, ensuring the consistency of the fuel’s composition during our experiment. RON 92 gasoline represents standard gasoline and meets national standards.

2.2. Experimental Protocol and Driving Cycles

The experiment was conducted on the chassis dynamometer at the vehicle testing center. All vehicle tests were performed in cold start mode, with a soaking period of at least 6 h at around 23 °C. The Worldwide Harmonized Light Vehicles Test Cycle (WLTC), newly introduced in the China 6 emission regulation, was adopted in these driving tests. The WLTC protocol lasts for 1800 s and around 23.25 km. It can be further divided into four sub-cycles: low-speed (589 s), medium-speed (433 s), high-speed (455 s), and extra-high-speed (323 s) [16].

In the driving tests, the measurement of N₂O is achieved through the MEXA-ONE IRLAM Laser Spectroscopic Motor Exhaust Gas Analyzer, a cutting-edge commercial system from HORIBA, Ltd. (Kyoto, Japan), offering unparalleled precision and performance in the analysis of vehicle exhaust gasses. The measurement principle is Quantum Cascade Laser Infrared Spectroscopy (QCL-IR) [17], with NOx being simultaneously detected as an auxiliary parameter. The QCL instrument employs Tunable Diode Laser Absorption Spectroscopy (TDLAS) technology, combined with Wavelength Modulation Spectroscopy (WMS), to achieve the high-sensitivity detection of N₂O gas. The QCL laser emits a laser beam at a specific wavelength, which is absorbed by the N₂O gas. By detecting the changes in its absorption spectrum, the concentration of N₂O can be calculated. The system is designed with sufficient flexibility and precision to accommodate a wide range of N₂O concentrations, spanning from a low range (0–200 ppm) to high range (0–6000 ppm). Notably, it achieves a remarkable zero-noise (2σ) threshold of 0.4 ppm for measurements within the low range, and 12 ppm for those within the high range. Furthermore, the system boasts a rapid rise time (calibration gas line t_10–90) of 2.5 s or less for N₂O at 200 ppm, ensuring a rapid and accurate response during switching operations. The gas sampled in the QCL is directly sourced from the exhaust without any dilution. Therefore, the measured N₂O concentration, combined with the simultaneously measured exhaust flow rate, can be used to derive the mass of N₂O emissions. Furthermore, by incorporating the driving mileage, the mileage-based emission factors of N₂O can be obtained.

2.3. Vehicle Specific Power

To more comprehensively represent the impact of driving conditions on N₂O emissions, we calculated vehicle-specific power (VSP) for a comprehensive analysis. VSP is used to describe the power output per unit mass of a vehicle per unit time and is commonly employed to evaluate the energy consumption and emission characteristics of vehicles. The calculation of VSP is typically based on the vehicle’s instantaneous speed, acceleration, and mass. Compared to driving speed, VSP can better reflect overall vehicle emissions because it considers multiple factors that affect the instantaneous power of the motor vehicle’s engine, including the road gradient, frictional resistance, aerodynamic drag, and vehicle acceleration [18]. Through VSP, we can more accurately assess the emission characteristics of vehicles under different driving conditions. The calculation of VSP is illustrated in Equation (1) [14,19,20].

$$\mathrm{VSP}=\nu \left[1.1\mathrm{a}+98.1\left(\mathrm{atan}\left(\sin \mathsf{\theta }\right)\right)+0.132\right]+0.000302{\nu }^3$$

(1)

where the unit of VSP is kW/t; $\nu$ denotes the instantaneous vehicle speed, with the unit m/s; a denotes the instantaneous acceleration, with the unit m/s²; and θ denotes the road gradient.

Furthermore, researchers may categorize vehicle driving states into different intervals based on VSP values and calculate the emission factors or conduct other relevant analyses for each interval separately. In doing so, they can gain a more precise understanding of a vehicle’s emission performance under different power outputs, thereby providing a scientific basis for developing more effective emission reduction measures. In this study, we have divided VSP into 14 modes by referencing previous studies [2,3]. The corresponding VSP values, with the unit kW/t, for modes 1~14 are, respectively, (−∞, 2), [−2, 0), [0, 1), [1, 4), [4, 7), [7, 10), [10, 13), [13, 16), [16, 19), [19, 23), [23, 28), [28, 33), [33, 39), [39, ∞).

3. Results and Discussion

3.1. HONO Emission Factors of the Tested Vehicles

The N₂O and NOx emission factors of the gasoline vehicles and hybrid vehicles tested in this study are presented in Figure 1. Regarding N₂O emissions, the emission factors of the conventional gasoline vehicle were generally higher than those of the hybrid vehicles. The N₂O emission factors of gasoline vehicle #1 were 7.7 mg/km, and those of hybrid vehicle #2 and hybrid vehicle #3 were 6.8 mg/km and 17.1 mg/km, respectively. Since the conversion of NOx in the exhaust gas in the catalytic converter is an important source of N₂O in the exhaust gas, we have also presented the NOx emission factors here. However, the change trend of N₂O and NOx emissions was not as consistent as expected.

The current China 6 emission regulation has set the emission limit for N₂O at 20 mg/km. It can be seen that the N₂O emissions of all the tested vehicles in this study were below the limit and that the N₂O emissions were only as high as one-third of the limit. This may be mainly due to the relatively short driving mileage of the vehicles in this study. The driving mileage of vehicle #1 was 11,404 km, whereas the driving mileage of the other two vehicles was less than 10,000 km. Additionally, displacement would affect N₂O emissions, as reported in previous research. Based on a statistical analysis of N₂O emissions from a large number of vehicles reported by Yin et al. [8], it was found that among vehicles with a displacement of 1.5 L, the majority of vehicles have N₂O emission factors within the range of 0.5~1.0 mg/km, accounting for approximately 40% of the total. As for vehicles with a displacement of 2.0 L, the majority of vehicles have N₂O emission factors within the range of 0.6~0.8 mg/km, accounting for approximately 25% of the total [8].

When comparing gasoline vehicles and hybrid vehicles in terms of pollutants such as CO, THC, and NOx, the use of electric motors would lead to a reduction in emissions from the engine of hybrid vehicles. Unexpectedly, in Figure 1, the N₂O emissions of hybrid vehicles did not decrease as significantly as those of NOx emissions. This indicates that despite the lower pollutant emissions of hybrid vehicles, they may not be able to achieve a reduction in N₂O emissions, which might be attributed to their N₂O generation mechanisms.

Regarding emission standards, N₂O emissions of the tested China 6 vehicles in this study were significantly lower than those reported by He et al. [12]. This reveals that with the tightening of emission standards, N₂O emissions have been better controlled. As the National-VI emission standard includes N₂O emissions, N₂O has received significant attention in motor vehicle control. However, CO₂ and CH₄ are not currently included in China’s emission regulations. In future, while controlling pollutants and N₂O, attention should also be paid to the control of other GHGs such as CO₂ and CH₄.

3.2. Effects of Driving Conditions on N₂O Emissions

To investigate the transient characteristics of N₂O emissions, we used gasoline vehicle #1 as an example and presented its emission concentrations throughout the WLTC protocol in Figure 2. It can be observed that there was an emission pulse of N₂O during the start-up phase which coincided with a pulse in NOx emissions. This synchronicity primarily stems from NOx being one of the most significant precursors of N₂O. Nevertheless, it is evident that N₂O emissions can be more sensitive to changes in the exhaust flow compared to NOx emissions. Specifically, N₂O emissions increased rapidly during acceleration phases when the exhaust flow suddenly increased and generally decreased when the exhaust flow diminished. Considering the influence of driving speed on N₂O emissions, it is clear that, apart from the significant N₂O emission pulses caused by start-up and the two main rapid acceleration periods, N₂O emissions at low speeds were relatively low. It is worth noting that two steep peaks appeared in the low-speed phase, which may be related to the formation mechanism of N₂O, suggesting that NO can selectively form N₂O at lower temperatures [3]. Both peaks are preceded by extended periods of idling. Prolonged idling may lead to a decrease in engine and aftertreatment temperatures, thereby promoting the substantial formation of N₂O. As the temperature rises, NOx is more favorably converted into NH₃ rather than N₂O. Furthermore, as depicted in the pie chart in Figure 2, N₂O emissions are primarily concentrated in the extra-high-speed phase. The mass percentages of N₂O emissions in the low-speed, medium-speed, high-speed, and extra-high-speed phases were 10.5%, 22.7%, 20.5%, and 46.3%, respectively. This underscores once again the crucial impact of speed on N₂O emissions.

As a comprehensive parameter considering speed, acceleration, road gradient, tire rolling resistance, and aerodynamic resistance, VSP can represent different vehicle driving conditions. Figure 3 represents the N₂O emission rate under different vehicle-specific power modes. As the VSP modes increase, the overall emission of N₂O shows an upward trend. When the VSP was less than 0 kW/t, for example, in model 1 and model 2, the average emission of N₂O was less than 0.05 mg/s. This driving condition denotes low-load conditions such as deceleration or downhill driving. Conversely, when the VSP was higher than 0 kW/t, the vehicle was characterized as being under high-load conditions, such as acceleration or uphill driving. The average emission rate of N₂O was 1.0 mg/s or more when the VSP mode reached 12. Additionally, the lowest N₂O emission rate was observed in VSP mode 3, which is the idling stage. It is noteworthy that the lowest emission of N₂O occurred during the idling stage, rather than the deceleration phase. This indicates that acceleration, and not only driving speed, plays a crucial role in influencing N₂O emissions.

N₂O is produced from the reaction between ammonia (NH₃) and NOx or the decomposition of ammonium nitrate (NH₄NO₃) generated in catalytic converters, suggesting a greater dependence on NOx and NH3 concentrations [8,10]. The maximum formation of N₂O was reported to occur between 200 °C and 350 °C, with the principal precursors being CO and NOx [10]. Besides CO and NOx, H₂ and HCs are also considered precursors of N₂O [11]. Taking into account the aforementioned generation mechanisms of N₂O, the concentrations of precursor substances such as CO, NO, and HCs all play a role in influencing the production of N₂O. Regarding these precursors, previous studies have already demonstrated a close correlation between their emissions and VSP.

3.3. N₂O Emission Characteristics of Hybrid Electric Vehicles

Compared to the extensive research on the emission characteristics of traditional gasoline vehicle exhausts, research into hybrid vehicle exhaust emissions is relatively scarce. However, due to the presence of electric motors in hybrid vehicles [21], they may exhibit different emission characteristics from traditional gasoline vehicles. This further underscores the necessity of researching the pollutant characteristics of hybrid vehicle exhaust emissions. The HEVs studied here are plug-in hybrid electric vehicles. Figure 4 presents their transient N₂O emission characteristics and related parameters investigated in this study. Unlike traditional gasoline-powered vehicles, the pollutant emissions of hybrid vehicles are not related to speed. Instead, the pollutant emissions of hybrid vehicles are correlated with their exhaust gas flow, primarily because when there is no exhaust gas flow, the vehicle is driven by an electric motor.

The two hybrid vehicles depicted in Figure 4 exhibit significantly different engine operating characteristics and pollutant emission profiles. For the hybrid vehicle #2 in Figure 4A, during the medium-speed phase, the engine commences operation and, as expected, its NOx emissions show a sharp increase upon the emergence of an exhaust gas flow, while N₂O only exhibit a minor pulse. In the high-speed phase, with an increasing exhaust gas flow, its NOx emissions display numerous small pulses, whereas its N₂O emissions undergo a sharp increase. However, the occurrence of N₂O emission pulses lagged behind that of the NOx emission pulses. From the operating characteristics observed in the extra-high-speed segment, it is evident that the vehicle’s speed in this phase was primarily driven by the engine. Concurrent with engine operation, high concentrations of N₂O emissions were observed.

Unlike the operating characteristics of hybrid vehicle #2, depicted in Figure 4A, hybrid vehicle #3, shown in Figure 4B, was driven by its engine during the low-speed phase, and there was a sharp increase in NOx emissions during its start-up phase. The emission of N₂O lagged behind the emission of NOx, resulting in high N₂O emissions during the low-speed phase. In the next three speed phases, the engine remained operational, leading to significant N₂O emission pulses. Nevertheless, its N₂O emissions in the high-speed and ultra-high-speed phases were lower than those in the low-speed and medium-speed phases. This is primarily because the engine flow in the high-speed and ultra-high-speed phases was relatively stable compared to the low-speed and medium-speed phases, with fewer acceleration and deceleration events. This indicates that acceleration may have a greater impact on N₂O emissions than driving speed. In addition to the transient emission characteristics, there is also a significant difference in the N₂O emission concentrations between the two hybrid vehicles. Vehicle #2 exhibited N₂O peak concentrations ranging between 40 and 60 ppm, whereas the N₂O peak concentrations from Vehicle #3 exceeded 150 ppm. This disparity can be primarily attributed to the distinct operating states of their engines. The engine of vehicle #2 operated for shorter durations, whereas that of vehicle #3 operated more frequently. This difference in engine operating patterns directly influenced the generation and accumulation of N₂O emissions.

Based on the emission factors depicted in Figure 1, it is evident that the N₂O emission factor of hybrid vehicle #3 was higher than that of gasoline vehicle #1, whereas that of hybrid vehicle #2 was lower than that of gasoline vehicle #1, despite both hybrid vehicles having lower NOx emissions compared to the tested gasoline vehicle. The lower NOx emissions from the hybrid vehicles in general can be attributed to their more efficient engine designs and the utilization of electric power during certain driving conditions, which reduces the need for fossil fuel combustion and thus lowers NOx formation. Notably, among the three vehicles, hybrid vehicle #3 had the lowest NOx emission factor but paradoxically the highest N₂O emission factor. This may be due to the low engine temperature caused by the frequent start–stop of the engine; the lower temperature may facilitate the conversion of NOx to N₂O [10]. Both the exhaust temperature and catalyst temperature are considered to be important for N₂O emissions. Unfortunately, in this study, we did not directly measure the catalyst temperature and the exhaust temperature. However, qualitatively speaking, the exhaust temperature of HEVs is generally lower than that of traditional gasoline vehicles, primarily due to the following two reasons. First, the engine of HEVs is typically not continuously operational but starts only when the battery is low or high-power output is required. In contrast, the engine of traditional gasoline vehicles operates almost continuously during driving. Since the engine of HEVs operates for a shorter duration and predominantly within a high-efficiency range, their exhaust temperature tends to be relatively lower. Second, the energy management system of HEVs optimizes the operating modes of both the engine and the electric motor to reduce fuel consumption and emissions. When the electric motor can meet the power demand, the engine shuts down or reduces its output, which also contributes to lowering the exhaust temperature.

Although the above indicates that hybrid vehicles can significantly reduce primary pollutant emissions such as NOx, they may not necessarily have a positive effect on reducing the emissions of secondarily generated pollutants such as N₂O. Factors such as engine temperature, the air–fuel ratio, and the presence of catalysts can all influence the distribution of the nitrogen oxides emitted. In future, we should conduct synchronized measurements of the relevant parameters that affect emissions during vehicle emission tests to more thoroughly analyze the formation mechanisms of pollutants.

4. Conclusions

In this study, we conducted tailpipe emission tests on a chassis dynamometer based on the WLTC protocol. In order to compare the N₂O emission characteristics between conventional gasoline vehicles and hybrid vehicles, we selected one gasoline vehicle and two plug-in hybrid vehicles. Herein, we analyzed the N₂O emission factors of the tested vehicles and the influence of driving conditions on vehicular N₂O emissions and performed a comparison of the transient N₂O emission characteristics between hybrid vehicles and gasoline vehicles. The following are the main conclusions drawn:

(1) The N₂O emission factor of the conventional gasoline vehicle tested in this study was 7.7 mg/km, while the emission factors of the two hybrid vehicles were 6.8 mg/km and 17.1 mg/km, respectively. The N₂O emission factors of all tested vehicles met the newly introduced limit of 20 mg/km contained in the China 6 emission regulation. The N₂O emission factors measured for the China 6 vehicles used in this study were lower than those previously reported for China 4 and China 5 vehicles, highlighting the effectiveness of emission standard upgrades at controlling pollutants and greenhouse gasses.

(2) During the WLTC protocol of conventional gasoline vehicles, N₂O emissions were concentrated mainly in the extra-high-speed phase, with emissions in the low-speed phase accounting for only 10.5% of the total WLTC protocol. Additionally, VSP had a significant impact on N₂O emissions; the lowest emissions occurred during idle periods. Additionally, N₂O emissions exhibited a general upward trend as the VSP increased.

(3) Although hybrid vehicles emit lower NOx emissions compared to conventional gasoline vehicles, their N₂O emissions are highly dependent on engine operating conditions. Compared to gasoline vehicles, unexpectedly, the frequent start–stop cycles in hybrid vehicles can lead to lower engine temperatures, which can promote the conversion of NOx to N₂O. Therefore, while hybrid vehicles may reduce NOx emissions, their impact on greenhouse gasses compared to conventional gasoline vehicles still requires further study.

Our conclusions outline the profiles of the N₂O emission characteristics of hybrid vehicles and conventional gasoline vehicles, evaluate the level of vehicle N₂O emissions under the implementation of China’s most recent emission regulations, and provide a basis for setting N₂O emission limits in the next phase of emission regulations introduced to meet the Sustainable Development Goals and Carbon Neutrality Targets.