Effect of PODE on Emission Characteristics of a China VI Heavy-Duty Diesel Engine

Abstract

With its high cetane number and oxygen content, polyoxymethylene dimethyl ether (PODE) can promote engine combustion and reduce particulate emissions, which has become a key research object of diesel surrogate fuel. This study further explores the effects of blending PODE on emission characteristics of a China VI diesel engine. Diesel/PODE blends with the PODE volume blending ratios of 10%, 20% and 30% have been experimentally investigated in a China VI heavy-duty diesel engine at 1900 rpm and four different loads. Furthermore, the effects of EGR rates (Exhaust Gas Recirculation) rates (0–20%) on combustion and emission characteristics have been also discussed at 1700 r/min engine speed and 50% engine load condition. An exhaust gas analyzer and a particle counter were used to collect NOx, CO and THC emissions and particulate number (PN) emissions. The results show that the CO and THC emissions can be significantly reduced with the increase in the mixing ratio of PODE. Additionally, the particle number concentration can be also reduced, especially at low and high loads. The NOx emissions can be improved by increasing EGR rates. Interestingly, there is a trade-off relationship between PN and NOx emissions. In general, blending PODE can effectively reduce NOx and PN emissions simultaneously.

1. Introduction

As is well known, diesel engines have been applied widely due to their high thermal efficiency and power performance. However, an investigation [1] showed that 70.4% of nitrogen oxide and 98.8% of particulate matter account for 50% of diesel engines’ emissions. It is a big challenge for conventional diesel engines to reduce NOx and PM emissions simultaneously to meet the increasingly severe exhaust regulations. The most significant aspect is that the addition of oxygenate can significantly reduce engine soot emissions as well [2,3,4]. Among these, polyoxydimethyl ether (PODE) is becoming the most promising fuel due to its high cetane number, high oxygen content and low cost [5,6].

PODE has been widely concerned with improving PM, CO and HC emissions. Xie Meng et al. [7] conducted experiments of various PODE3-4/diesel blends with PODE3-4 in proportions of 30%, 50% and 100% on a China II diesel engine and proved that blending PODE could significantly improve the emissions of PM, CO and HC, except for NOx emissions. Liu et al. [8] carried out experiments with a small proportion blending diesel with PODE on a light-duty diesel engine, which indicated that soot emissions decreased by 90% under high load conditions, and the indicated thermal efficiency could be increased by 2%. Chen et al. [9] tested the effects of the blends of gasoline, diesel and PODE on the combustion and emissions of a Yuchai YC6G270-30 diesel engine in China III. The results showed that diesel/PODE (D80P20) and gasoline/PODE (D80G20) could increase the NOx emissions and decrease the soot emissions in most cases compared to diesel. Additionally, NOx and soot emissions can be balanced when adding PODE and gasoline to diesel by 10% volume simultaneously. By blending PODE and ethanol with diesel, the combustion and emission characteristics were studied by Liu et al. [10] on a turbocharged heavy-duty diesel engine (Euro IV), and they found that alcohols can be used as cosolvent of PODE/diesel mixture, which prolongs the ignition delay, reduces brake thermal efficiency, accelerates the combustion rate at the later stage of combustion under different loads and significantly reduces HC and soot emissions with a slight increase in NOx emissions. Pellegrini et al. [11] measured and evaluated the emission performance of neat PODE and a blend of 10% PODE and 90% commercial diesel fuel in an old Euro-2 diesel car over the NEDC driving cycle. Their results showed that PM emissions of neat PODE could be reduced by 77%, which is under the value of Euro 4, but NOx and CO emissions increase, compared to diesel. The preceding studies demonstrated that PODE, alone or in combination with diesel, can significantly improve engine combustion while also lowering soot emissions, but the addition of PODE increased NOx emissions that also pointed out by other works [8,12,13]. However, some researchers have found positive conclusions about NOx emissions [5,14,15].

The EGR strategy is an effective method to control diesel engine NOx emissions [16]. By using diesel/PODE blended fuel, with an even higher EGR rate, diesel engines can change the trade-off relationship between NOx and soot emissions, making it easier to meet emission regulations. Li et al. [17,18] carried out experiments with a single-cylinder engine to examine the effects of PODE and EGR rate on the engine combustion and emission characteristics. Jialin Liu et al. [14] studied the influence of blending PODE coupled with EGR strategy on emissions of a six-cylinder heavy-duty diesel engine, too. Their above experimental results showed that PODE could improve BTE even at a high load with a higher EGR rate on the one hand, and on the other hand, it can reduce soot and NOx emissions significantly. Additionally, Huang et al. [19] investigated the influence of EGR rates (0–40%) on the combustion and emission characteristics of n-butanol/diesel/PODE3-4 blends at low-temperature combustion mode in a diesel engine. The results show that as the EGR rate grows, the emission of NOx is sharply reduced; when the EGR rate is above 30%, as it grows, the emissions of soot, CO and HC drastically rise. As the EGR rate grows, the total particulate matter (PM) number concentrations first decline and then rise.

From the above works, the engines with PODE/diesel blends are all under the China VI level. Therefore, it is important to re-evaluate the effects of diesel/PODE blends on emissions for China VI diesel engines. In particular, whether or not mixing in PODE has a negative impact on NOx emissions still remains inconclusive. Thus, the effects of diesel blended with 10%, 20% and 30% PODE (in mass) on the emissions of a China VI diesel engine are discussed at various engine loads and EGR rates.

2. Experimental Setup

2.1. Test Engine and Measurement Equipment

The test engine is a six-cylinder inline Weichai WP12.460 heavy-duty diesel engine in which the inlet air is turbocharged and intercooled. The main specifications of the engine are listed in

Table 1. Engine specifications.

Parameters	Values
Number of cylinders	6
Bore × stroke (mm)	126 × 155
Compression ratio	17
Displacement (L)	11.596
Rated power (kW)	340.7
Rated speed (rpm)	1900
Max torque (N·m)	2200
Emission regulation	China VI
Fuel injection system	CRSN2-16

. The engine meets China VI emission regulations. The engine is equipped with a common rail fuel injection system.

The schematic of the experimental setup is presented in Figure 1. The output end of the engine crankshaft is connected to a GW400 eddy current dynamometer produced by Xiangyi Power Test Instrument Co., Ltd. The Horiba MEXA 7100DEGR gas analyzer is used to measure HC, CO and NOx emissions. Among them, CO is measured by a non-dispersive infrared absorption analyzer (NDIR), HC is measured by hydrogen flame ion analyzer (FID) and NOx is measured by a chemiluminescence analyzer (CLD). The engine exhaust is directly sampled for detection without dilution treatment. In order to reduce the error caused by the fluctuation of the engine operating conditions, the average value of the gas emission concentration of the three measurements is used as the research object. The measurement error and measurement accuracy are shown in

Table 2. Measurement error of emissions.

Emissions	Measurement Error (%)	Measurement Accuracy (ppm)
CO	2	1
CO2	2	1
HC	2	0.1
NOx	2	0.1

.

In the experiments, an AVL 489 Particle Counter is used to measure PN emissions. AVL489 uses n-butanol to wrap a single particulate and detects the number of particles by interfering with the set light path, which can achieve high-precision measurement of the number in the size range of 23 nm~2.5 um emitted by the engine. In order to avoid condensation of volatile hydrocarbon components on the pipe wall, the connecting pipe between the dilution channel and the exhaust pipe is controlled at 150 °C by electric heating. During the test, in order to avoid data fluctuations caused by changes in the dilution ratio, the total dilution ratio was controlled at 20,000 and remained unchanged.

In the test, EGR rate can be calculated by Equation (1).

$$\mathrm{EGR}=\frac{{({\mathrm{CO}}_2)}_{\mathrm{intake}}-{({\mathrm{CO}}_2)}_{\mathrm{ambient}}}{{({\mathrm{CO}}_2)}_{\mathrm{exhaust}}-{({\mathrm{CO}}_2)}_{\mathrm{ambient}}}\times 100\%$$

(1)

where the parameters represent the EGR ratio and the concentration of CO₂ in the ambient, exhaust and intake, respectively. Additionally, the exhaust flow rate can be flexibly adjusted by a throttle valve installed prior to the EGR cooler. As shown in Figure 1, EGR is air-cooled by a plate-fin fan. Additionally, the EGR rate is measured by the HORIBA 7100DEGR gas analyzer.

2.2. Test Fuels

The diesel fuel used in the tests was purchased from gas stations, which is 0^# China VI standard diesel produced by CNPC. The PODE was bought from Jinchang Co., Zibo, China. The main PODE components were measured from PODE3~6, and their mass fraction is 37.5:23.2:19.5:12.6, accounting for 92.8%. Some physical properties of diesel and PODE are shown in

Table 3. Fuel physical parameters.

Properties	Diesel	PODE
Molecular formula	C10~C21	CH3O(CH2O)nCH3
Density (kg/L)	0.81~0.845	1.046
Cetane number	≥51	88
Lower heating value (MJ/kg)	42.83	13.6
Oxygen content (wt%)	0.06	54.86
Kinematic viscosity (mm2/s, 20 °C)	3.96	0.86

.

In the experiments, diesel fuel was blended with 10%, 20% and 30% PODE in weight. Hereinafter, diesel and the above blends were marked as D100, DP10, DP20 and DP30, respectively.

2.3. Operating Condition Setup

The toxic emissions are huge (CO, NOx, PN) at low speeds and at high loads, which is why we did not conduct research under such conditions. So, the experiments were conducted at the rated speed of 1900 r/min, and the loads were set to be 25%, 50%, 75% and 100%, respectively.

Further, in order to access overall engine performance, the operating condition of 1700 r/min and 50% load was chosen to study the influence of EGR rate on engine emission characteristics. EGR rates were set to be 5%, 10%, 15%, and 20%, respectively.

During the engine test, the lubricant temperature was kept around 90 °C, the inlet air after the intercooler was kept at 60 °C and the cooling water was kept between 85 and 90 °C. When changing the fuel, compressed air was first used to discharge residual fuel in the low-pressure fuel system, such as the pipeline, filter and return fuel cooler, and then the engine was driven for more than 15 minutes under 1000 Nm@1000 r/min by the new fuel to remove the influence of the previous fuel.

3. Results and Discussions

3.1. The Influence of Blending PODE to Emission Characters

Figure 2 shows that as the load increases, the CO emission trends of the four fuels are similar; that is, they decreased and then increased. The reason is that the excess air coefficient in the cylinder is relatively large and the oxygen content is greater under the small load, and further loading increases the in-cylinder combustion temperature and promotes CO oxidation. At 75% to 100% load, CO emissions increased slightly due to the decrease in the excess air coefficient.

Meanwhile, as the blending ratio of PODE grew, CO emissions decreased, especially at medium and large loads at the same EGR rate. The reason for this is the high oxygen content of PODE reduces the areas of the over-concentrated mixture and promotes CO oxidation.

At low loads, the injection pressure and combustion temperature are low, resulting in poor fuel atomization. The small increase in oxygen content of DP10 and DP20 has a limited effect on combustion improvement. At the same time, CO may be produced in the C–O bond of PODE in the region of the over-concentrated mixture, making the CO emission level of DP10 and DP20 equal to that of D100.

Figure 3 shows as the load increases, the HC emissions decrease due to a rise in in-cylinder temperature, which aids HC oxidation. The relatively large increase from D100 and DP10 at 100% load is due to a large amount of fuel injection being inconducive to fuel atomization, resulting in more areas of over-concentrated mixture beyond the ignition limit, aggravating incomplete combustion.

The areas of the over-lean mixture produce the majority of the HC emissions. The HC emissions of blended fuels were significantly lower than diesel at the same load. HC emissions decreased as the proportion of added PODE increased. HC emissions from DP30, in particular, were reduced by up to 50%. On the one hand, the mixing with PODE shortens the ignition delay and promotes the ignition characteristic due to the high cetane number, which reduces the areas of the over-lean mixture. On the other hand, the higher reactivity of PODE can promote the oxidation of HC.

As is well known, the thermal mechanism of the NOx formation is mainly affected by the oxygen content of fuels, combustion temperature and staying in the higher temperature zone [20]. Figure 4 shows that NOx emissions first increase and then decrease from all four fuels with further loading. It is due to the fact that the excess air coefficient was high enough at a low load, and the low maximum combustion temperature was the dominant factor in the formation of NOx. When the combustion duration and the in-cylinder combustion temperature were increased with further loading, the generation of NOx emissions was promoted. At 75% load, the oxygen content played a more important role in NOx generation under high in-cylinder temperature conditions, so the reduction in the excess air coefficient inhibited the formation of NOx.

As the oxygenated fuels, PODE had high oxygen content, which was supposed to result in high NOx emissions. However, the lower heating value of PODE was only half of that of diesel, and the latent heat of vaporization was higher, which led to a decrease in combustion temperature in the NOx formation areas, making the NOx emission slightly lower than that of diesel [21].

Figure 5 shows the PN emissions first drop and then rise slightly with the increment of loading. Soot forms when there is a high temperature and a lack of oxygen. The PN emissions are relatively high at low load because the in-cylinder combustion temperature is lower, which is inconducive to the oxidation of soot. At 75% to 100% load, PN emissions increase gradually due to the fact that the excess air coefficient decreases, and there are more areas of the over-concentrated mixture, causing incomplete combustion.

The following are the reasons for this: blending PODE with diesel can significantly reduce soot emissions. Firstly, the high oxygen content of PODE can increase oxygen for combustion and promote high-temperature combustion, which is beneficial for the oxidization of soot. Secondly, there is no C–C bond in the PODE molecule. This molecule structure is effective in inhibiting the formation of soot precursor particles. Thus, PN emissions decrease.

The PN emissions of DP30 and DP20 are higher than those of DP10 at a low load. This is because the in-cylinder combustion temperature is relatively low. The PODE with high volatility and a low boiling point contacts the cylinder wall at a lower temperature, forming a certain thickness of the combustion-quenching layer, and the volatiles are easily squeezed into the slot between the piston and the cylinder wall. The resulting PODE is unburned or oxidized into particles with very small particle sizes, and a high concentration of nuclear mode particles will be emitted. Because the number of nuclear mode particles is dominant at low load, the PN emissions of DP30 and DP20 are higher than those of DP10 [8,22,23].

3.2. Effects of EGR Rates on Emission Characteristics

Figure 6 shows the thermal efficiency decreases slightly with an increase in the EGR rate. This is because as the EGR rate increases, the amount of exhaust gas in the cylinder increases, and the triatomic gases such as CO2 and H2O increase the specific heat capacity of the mixed gas, which means the temperature in the cylinder decreases. At the same time, due to the decrease in oxygen content, the combustion gradually deteriorates, resulting in a decrease in effective thermal efficiency. At the same EGR rate, as the PODE blending ratio increases, the effective fuel consumption rate also increases, which can be explained by the shortened combustion duration [7,9].

Figure 7 shows the average in-cylinder pressure and instantaneous heat release rate curves of the four blended fuels at different EGR rates. The average in-cylinder pressures of the four blended fuels essentially show a downward trend as the EGR rate increases. The peak instantaneous heat release rate drops slightly as the crankshaft angle corresponding to the peak shifts back, as does the heat release start point. This is due to the fact that as the EGR rate rises, the amount of exhaust gas entering the cylinder rises while the oxygen concentration in the cylinder falls, slowing the combustion rate. During the compression process, however, the content of triatomic gases, such as CO2 and H2O, in the cylinder increases, increasing the mixture heat capacity and lowering the temperature in the cylinder.

Figure 8 shows the CO emissions at various EGR rates. As the EGR rate increases from 0 to 20%, the CO emissions increase by 200%, 111.3%, 127.8% and 105.6%, respectively, corresponding to D100, DP10, DP20 and DP30. Because as the EGR rate grows, the intake oxygen concentration declines and the heat capacity of the in-cylinder mixture will increase, which leads to the peak in-cylinder temperature reducing and the CO emissions rapidly increasing [19]. A high oxygen content can facilitate the combustion and promote the oxidation of CO, so the emissions of CO when using DP10, BP20, and BP30 are higher than those of D100.

Figure 9 shows that the HC emissions increase continuously as the EGR rate grows. The reason is that the peak combustion temperature declines and the combustion rate slows down due to the decrease in fresh charge amount and the increase in inert gas content in the cylinder, which results in an increase in wall quenching layer and HC emissions. Besides, unlike the results of CO emissions, blending PODE has no significant effect on THC emissions.

Figure 10 shows NOx emissions with different EGR rates. From the chart, as the EGR rate grows, the emission of NOx significantly reduces. The reasons are as follows: the oxygen concentration of the in-cylinder mixture decreases and the higher heat capacity of the in-cylinder mixture causes the peak in-cylinder temperature to decline [24]. After blending with PODE, the NOx emissions further decrease. The PODE’s influence on NOx emissions can be attributed to the competition between the decrease in the adiabatic combustion temperature and the increase in oxygen content. Due to the low blending ratio, the combustion characteristics of DP10 are similar to diesel. At EGR rates of 5%, 10%, and 15%, DP10 facilitates the combustion process and provides a favorable condition for NOx formation under low and medium load conditions so that the NOx emission of DP10 is equivalent to or slightly higher than that of D100. However, when EGR rates or PODE blending ratios increase, the reduction in combustion temperature (as shown in Figure 7) plays a dominating role in the formation of NOx, thus reducing NOx emission.

Figure 11 demonstrates the PN emissions of four fuels at different EGR rates. When the EGR rate is above 10%, PN emissions of D100 increase sharply from 5.3 × 10⁶ to 3.24 × 10⁷ (at the EGR rate of 20%), with a 6-fold increase. At the same EGR rate, blending PODE can effectively reduce PN emissions. Because the molecular structure of PODE does not contain C–C bonds, it will not generate soot precursor particles during combustion. Moreover, the high oxygen content of PODE is conducive to the oxidation of accumulated particles into nucleated particles, or even complete oxidation, which ultimately reduces PN emissions [25,26].

With the increase in EGR rates, the addition of PODE reduces PN emissions more. At a high EGR rate, particles mainly exist in the form of accumulated particles [19]. As the proportion of added PODE grows, higher oxygen content can weaken the adverse effect of EGR on combustion [27]. Meanwhile, the increase in combustion temperature promotes the oxidation of particles and reduces the regions of the over-concentrated mixture. The two reasons co-promote the decrease in the peak value of accumulated particles concentration. EGR rate has little effect on PN emissions of DP20 and DP30, which only slightly increase at a 20% EGR rate. Compared with diesel operating conditions, PN emissions of DP20 and DP30 decrease by 89.7% and 85.7%, respectively.

3.3. Emission Characteristics of PN-NOx

For diesel engines, conventional control measures of NOx emissions usually lead to the increase in soot emissions due to the inherent trade-off relationship between these two pollutants [20,28,29]. Some previous studies have pointed out that the addition of PODE increased NOx emissions [8,12] due to the presence of PODE, higher temperatures and oxygen-richness in the combustion chamber. However, there are also other studies that have reported that lower NOx emissions were obtained for blended fuels, especially for low and medium loads [5,14,15], as a result of the higher cetane number and reduced lower heating values of the blends. Interestingly, Zhao et al. [30] found that the trade-off between the NOx and PM emissions was significantly improved by blending PODE at 0% EGR, 1900 r/min speed, low and high loads. However, this issue at different EGR rates has not been reported yet. As a result, Figure 12 plots the PNC-NOx trade-off relationship for different test fuels under various EGR rates (0%, 5%, 10%, 15% and 20%) with 1700 r/min speed and 50% load.

As illustrated in Figure 12, the PN-NOx emission characteristics of the engine fueled with four different fuels, in which the ordinate represents the PN emissions and the abscissa represents the NOx emissions. It can be seen that the use of DP10 can achieve a substantial reduction in PN emissions and a slight reduction in NOx emissions at the same time. The use of DP20 further reduces PN and NOx emissions. Even though the use of DP30 leads to higher PN emissions, NOx emissions are still lower than diesel. When adopting DP30 and EGR rate of 10%, NOx emission is 567 ppm that is 51.7% lower than that of diesel. The reduction in PN emissions due to the addition of PODE is in line with expectations. Firstly, the dilution effect of PODE decreased the concentration of aromatics, i.e., the precursor of soot formation [31]. Secondly, PODE has higher oxygen content, resulting in a reduction in soot formation [9]. Thirdly, due to the lack of a C–C bond, the soot-free nature of the PODE significantly reduces the formation of PAHs. For NOx reduction, the greater cetane number of PODE shortens the ignition delay, resulting in a lower premixed combustion ratio [30]. On the other hand, the reduction in combustion time and peak heat release rates also resulted in a fall in the mean in-cylinder gas temperature (as shown in Figure 7), which aided NOx generation reduction.

4. Conclusions

In this paper, the effects of blending PODE on emission characteristics of an inline, six-cylinder, turbocharged and intercooler Weichai WP12.460 heavy-duty diesel engine equipped with a common rail fuel injection system were studied. Furthermore, the effects of the blending ratio of PODE (10%, 20% and 30%), the engine loads (25%, 50%, 75% and 100%), and the EGR rate (5%, 10%, 15% and 20%) have been discussed. The experimental results of this study are as follows.

(1) Blending PODE can further reduce CO, HC and NOx emissions. In addition, adding PODE can also reduce PN emissions at low and high engine loads. Under 100% load, PN emissions of DP30 are 45.0% lower than that of D100, and NOx emissions are reduced by 10.8%.

(2) With an increase in the EGR rate, the CO and HC emissions of the engine increase. The addition of PODE reduces CO emissions, but the impact on HC emissions is not obvious.

(3) With an increase in the EGR rate, NOx emissions are significantly reduced, PN emissions gradually increase, and PN emissions increase rapidly at a high EGR rate. Additionally, PN emissions are always at a low level of 10⁶ ppm with DP20 and DP30 blends, which is more than 85% lower than that of diesel.

(4) In general, blending PODE can achieve a simultaneous reduction in PN and NOx at various EGR rates. Even if PN emissions sometimes rebound with DP30 blends, the quantity is still lower than that of pure diesel.

In the current work, all results are obtained from the engine bench test. However, the road test is also very important. In the future, we will conduct further road tests to explore the emission performance of PODE/diesel blends under complex conditions.