Experimental study on the effects of blending PODEn on performance, combustion and emission characteristics of heavy-duty diesel engines meeting China VI emission standard

To study the influence of diesel fuel blended with polyoxymethylene dimethyl ethers (PODEn), a new alternative fuel with a high oxygen content and large cetane number, on the combustion characteristics, fuel economies, and emission characteristics of heavy-duty diesel engines that meet China VI emission standards, relevant tests were conducted on a supercharged intercooled high-pressure common-rail diesel engine. The PODEn were blended with diesel fuel at three different ratios (volume fractions of 10%, 20%, and 30%). The test results showed that the PODEn could optimize the combustion process of diesel engines that met the China VI emission standards, effectively improve the thermal efficiencies of diesel engines, and reduce the emissions of hydrocarbon (HC), carbon monoxide (CO), and soot. With an increase in the PODEn blending ratio, the peak values of the in-cylinder pressure, average in-cylinder temperature, and instantaneous heat release rate gradually decreased, and each peak progressively moved forward. As the start of combustion gradually moved forward, the combustion duration was shortened by 0.7–2.8°CA, the heat release process became more concentrated, and the effective thermal efficiency was increased by up to 2.57%. The effective fuel consumption gradually increased, yet the equivalent effective fuel consumption gradually decreased, with the largest drop being as high as 4.55%. The nitrogen oxides (NOx) emission increased slightly, and the emissions of HC, CO, and soot gradually decreased. The emissions of CO and soot declined significantly under high-speed and high-load conditions, with the highest reductions reaching 66.2% and 76.3%, respectively.

www.nature.com/scientificreports/ combustion analyzer. The heat release rate and the other combustion characteristics data were calculated by the KiBox combustion analyzer. The Horiba MEXA-584L exhaust gas analyzer was used to measure the emissions of NO x , HC, and CO, and the AVL 415SE smoke meter was used to measure the exhaust soot. The Siemens FC3000 mass flow meter was used to measure the fuel consumption.
Testing fuels. The PODE n used in the study were industrial PODE n with a purity of 99.9% that were produced by Shandong Yuhuang Chemical (Group) Co., Ltd., and they were primarily composed of PODE 2 , PODE 3 , and PODE 4 with mass fractions of 2.553%, 88.9%, and 8.48%, respectively. The test diesel was 0 # commercial diesel. In this test, three kinds of PODE n /diesel blended fuels were prepared, with PODE n volume percentages of 10%, 20%, and 30%, which were denoted as P10D90, P20D80, and P30D70, respectively, and the pure diesel was denoted as P0D100. The physical and chemical properties of the fuel are shown in Table 2. The physical and chemical properties of P10D90, P20D80, and P30D70 were calculated based on Eqs. (1) and (2) from a previous study 42 . Table 1. Technical specifications of the diesel engine.

Parameter Specifications
Engine type In-line 6-cylinder, 4-stroke, supercharged, intercooled, high-pressure common-rail Bore × stroke (mm × mm) 126 × 155 Compression ratio 17 Displacement (L) 11.596 Rated power (kW) 338 Rated speed (r·min −1 ) 1900 Fuel injection system Bosch CRSN2-16  Prior to the tests, all the instruments and the equipment were calibrated to ensure the reliability of the test and the accuracy of the test results. During the test, each operating point was repeatedly measured to eliminate uncertainty. After the engine was operating stably for 5 min, the fuel consumption was measured three times, and the average value was taken as the test result. The combustion analyzer collected the combustion characteristics data for 200 cycles and the average value of the data was taken for analysis. The exhaust gas analyzer and the smoke meter continued to be measured for 1 min, and with the invalid values eliminated, the average values were taken as the test results.

Results and discussion
Effects of blending PODE n on combustion characteristics of diesel engines. Figure 2 shows the variation of the in-cylinder pressure and instantaneous heat release rate of the diesel engine with different PODE n blending ratios when n = 2000 r·min −1 , BMEP = 0.38 MPa, and BMEP = 1.14 MPa. Since the fuel injection consisted of two stages, the exothermic process was divided into two phases: pre-injection and maininjection heat release. Since the pre-injection lasted for a relatively short time, the amounts of fuel injection and heat released were small, and the blending of the PODE n exerted no significant effect on the peak value of the pre-injection heat release rate. However, since the duration of the main-injection phase was longer, the peak value of the main-injection heat release rate decreased with the increase in the PODE n blending ratio, with the peak phase gradually moving forward. Because the PODE n had a lower heat value than diesel, under the injection conditions controlled by the electronic control unit (ECU) of the original engine, the blended fuel emitted less heat than the diesel with a declining peak value of the heat release rate. Moreover, the PODE n had a large cetane number, which shortened the ignition delay period of the blended fuel and advanced the ignition timing. Figure 2 also shows that as the PODE n blending ratio increased, the maximum in-cylinder pressure decreased, with the corresponding phase of the peak in-cylinder pressure gradually moving forward. Due to the large cetane number, great self-ignitability, and good volatility of the PODE n , the ignition delay period of the blended fuel became shorter, with the corresponding phase of the peak in-cylinder pressure moving forward. The peak incylinder pressure decreased due to the following two factors. First, the lower heat value of the PODE n reduced the heat value of the blended fuel and the heat release during the main-injection phase. Second, the shortening of the ignition delay period of the blended fuel caused the start of combustion (SOC) to occur earlier, shortened the mixing time of the fuel and the air, and reduced the premixed combustion ratio. Figure 3 shows the variation of the average in-cylinder temperature with different PODE n blending ratios when n = 2,000 r·min −1 , BMEP = 0.38 MPa, and BMEP = 1.14 MPa. As the PODE n blending ratio increased, the peak in-cylinder temperature dropped and the curve shifted forward. Since the latent heat of vaporization of the blended fuel increased after the blending of the PODE n , after the fuel was injected into the cylinder, part of the heat was absorbed, resulting in a relatively significant temperature drop, which led to a decrease in the peak in-cylinder temperature. However, due to the large cetane number of the PODE n , the combustion heat release of the blended fuel was advanced, thereby shifting the peak in-cylinder temperature forward. Figure 4 shows the variation of the combustion duration of the engine with different PODE n blending ratios when n = 2,000 r·min −1 , BMEP = 0.38 MPa, and BMEP = 1.14 MPa. In this study, the crank angle corresponding to a cumulative heat release rate of 10% (CA10) was defined as the SOC, the crank angle corresponding to a cumulative heat release rate of 90% (CA90) was defined as the end of combustion (EOC), and the crank angle between the SOC and the EOC was defined as the combustion duration. With BMEP = 1.14 MPa, compared with www.nature.com/scientificreports/ P0D100, when P10D90, P20D80, and P30D70 were used, the CA10 was advanced by 0.5°CA, 0.8°CA, and 1.0°CA, respectively, and the CA90 was advanced by 1.2°CA, 2.6°CA, and 3.8°CA, respectively, and the combustion duration was shortened by 0.7°CA, 1.8°CA, and 2.8°CA, respectively. This indicated that as the PODE n blending ratio increased, the combustion duration was gradually shortened, and the heat release was more concentrated, which was conducive to improving the thermal efficiency of the engine. Figure 5 shows the variation of the cumulative heat release of the engine with different PODE n blending ratios when n = 2,000 r·min −1 , BMEP = 0.38 MPa, and BMEP = 1.14 MPa. The cumulative heat release decreased after the PODE n were blended, and with the increase in the blending ratio, the cumulative heat dropped more significantly. At a BMEP of 1.14 MPa, when P0D100, P10D90, P20D80, and P30D70 were used, the maximum cumulative amounts of heat released were 4,959.9 J, 4,863.6 J, 4,789.1 J, and 4,733.6 J, respectively. Compared with P0D100, the cumulative amounts of heat released were reduced by 1.94%, 3.44%, and 4.56% when P10D90, P20D80, and P30D70 were burned, respectively. At the same speed and load, the effective power values of the   www.nature.com/scientificreports/ engine were equal for all the blends. The lower the cumulative heat release was, the higher the thermal efficiency of the engine was. This indicated that blending PODE n helped to improve the effective thermal efficiency of the engine. This conclusion is further discussed in "Effects of blending PODE n on fuel economy of diesel engines".
Effects of blending PODE n on fuel economy of diesel engines. The effective fuel consumption b e and the effective thermal efficiency η et are two important parameters that characterize the fuel economy of the engine, and they are critical indicators for measuring the engine performance. Because the low heat value of PODE n differs greatly from that of diesel, directly comparing the fuel consumption of the fuels could not accurately reflect the total heat consumed by the fuels. Therefore, the concept of equivalent effective fuel consumption b eq was introduced; i.e., the total heat value consumed by the PODE n /diesel blended fuel was converted to the diesel consumption corresponding to an equivalent heat value. The variations of the effective fuel consumption b e with different PODE n blending ratios are shown in Fig. 6. Given the same speed and load, the effective fuel consumption gradually increased with the increase in the PODE n blending ratio. Due to the low heat value of the PODE n , the heat values of the blended fuels P10D90, P20D80, and P30D70 were 93.6%, 87.3%, and 81.3% of that of diesel, respectively, and the amount of heat released by combustion within the unit mass of the blended fuel was reduced. Without changing the fuel injection strategy, the output torques corresponding to the same BMEP operating condition were the same. Hence, it was important to increase the injection amount of the blended fuel, resulting in an increase in the effective fuel consumption. Figures 7 and 8 show the variations of the equivalent effective fuel consumption b eq and the effective thermal efficiency η et of the engine with different PODE n blending ratios. Given the same load, with the increase in the PODE n blending ratio, the equivalent effective fuel consumption gradually decreased, and the effective thermal efficiency increased significantly. With n = 2000 r·min −1 and BMEP = 1.52 MPa, compared with P0D100, the equivalent effective fuel consumptions of P10D90, P20D80, and P30D70 decreased by 2.20%, 3.26%, and 4.55%, respectively, with the effective thermal efficiencies increasing by 0.59%, 1.48%, and 2.57%, respectively. Due to the large cetane number of the PODE n , once the PODE n were blended, the ignition delay period was shortened, and the combustion advanced with the heat release and the peak in-cylinder pressure in the main-injection phase being closer to the top dead center (Fig. 2), which was conducive to the improvement of the thermal efficiency. Furthermore, once the PODE n were blended, the combustion duration shortened (Fig. 4), the heat released  www.nature.com/scientificreports/ became more concentrated, and the low in-cylinder temperature (Fig. 3) reduced the heat transfer loss. Furthermore, with the increase in the PODE n blending ratio, the oxygen content in the blended fuel increased, which reduced the area of oxygen-deficient combustion and improved the combustion reaction speed. In summary, blending PODE n effectively improved the effective thermal efficiency of the engine and reduced the equivalent effective fuel consumption.

Effects of blending PODE n on emission characteristics of diesel engines. High-temperature and
oxygen-rich conditions with a long high-temperature duration are required for the formation of NO x 43 . Figure 9 shows the effect of the PODE n blending on the engine emission of NO x under different operating conditions. Under low-speed conditions (n = 1300 r·min −1 ), the in-cylinder temperature was low. As the PODE n blending ratio increased, the in-cylinder temperature further declined (Fig. 3). Moreover, the shortened combustion duration shortened the high-temperature duration and suppressed the generation of NO x . However, the increased oxygen content in the high-temperature region increased the emission of NO x . Combining the two effects, blending PODE n into the diesel fuel at different ratios made little difference under low-speed conditions. Under high-speed conditions (n = 2000 r·min −1 ), the in-cylinder temperature increased rapidly. Although blending PODE n decreased the in-cylinder temperature, the in-cylinder temperature remained high (the peak temperature shown in Fig. 3 was about 2000 K). At that time, the NO x was quite sensitive to the oxygen concentration in the cylinder during combustion, so the high oxygen content of the PODE n rapidly increased the emission of NO x . In addition, the NO x emission at a speed of 2,000 r·min −1 was generally lower than that at 1,300 r·min −1 , which was likely because after the speed increased, the fuel reaction time was reduced, the high-temperature duration decreased, and the emission of NO x decreased. At the same speed, as the load increased, the emission of NO x increased due to the increase in the in-cylinder temperature.
The effect of PODE n blending on the HC emission of the engine under different operating conditions is shown in Fig. 10. With n = 1,300 r·min −1 , with the increase in the PODE n blending ratio, the emission of HC gradually decreased. This was because the emissions of HC were mainly generated in the lean gas mixture and wall quenching layer around the injection fuel spray. The large cetane number of the PODE n improved the ignition performance of the blended fuel and shortened the ignition delay period, thereby reducing the HC emission in the lean gas mixture and wall quenching layer. Moreover, due to the low boiling point and high oxygen content of the PODE n , the speed of mixing and combustion of oil and gas improved, which was beneficial for the oxidation  www.nature.com/scientificreports/ of HC. With n = 2,000 r·min −1 , the in-cylinder temperature was relatively high, and the emission of HC was low and decreased slightly with the increase in the PODE n blending ratio. As the load increased, the emission of HC significantly decreased. This was because as the load increased, the in-cylinder temperature gradually increased, more HC was oxidized, and the emission of HC gradually decreased. The emission of CO was mainly generated in rich mixed gas areas or low-temperature areas where the excess air coefficient was less than 1. Figure 11 shows the effect of PODE n blending on the CO emission of the engine under different operating conditions. At a speed of 1,300 r·min −1 , the emission of CO was quite low when different fuels were used, and the emission changed slightly as the load increased. Since the excess air coefficient was large at low speeds, the emission of CO was comparatively low. At a speed of 2,000 r·min −1 , the emission of CO declined slightly with the increase in the PODE n blending ratio at low and medium loads, whereas at high loads, the emission of CO decreased significantly as the PODE n blending ratio increased. With BMEP = 1.52 MPa, the CO emissions of P10D90, P20D80, and P30D70 were reduced by 35.0%, 53.4%, and 66.2%, respectively, compared with that of P0D100. At low and medium loads, the diesel engine basically worked with lean mixtures with an excess air coefficient greater than 1. There were fewer low-temperature and hypoxic areas in the cylinder, and the emission of CO was relatively low. Thus, the PODE n had a comparatively small impact on the emission of CO. At high loads, there was a relatively rich mixture of local hypoxia in the cylinder, resulting in a rapid increase in the emission of CO. The high oxygen content of PODE n increased the air-fuel ratio and excess air coefficient, improved the combustion conditions in the local hypoxic areas in the combustion chamber, and accelerated the oxidation rate of CO, which significantly decreased the emission of CO. Figure 12 shows the effect of PODE n blending on the soot emission under different operating conditions. Blending PODE n could effectively reduce the soot emission of diesel engines, and the reduction was more evident under large loads. With n = 2,000 r·min −1 and BMEP = 1.52 MPa, the filter smoke number (FSN) values of P10D90, P20D80, and P30D70 decreased by 35.1%, 62.8%, and 76.3% compared to that of P0D100. Unlike diesel, the PODE n exhibited great volatility and a low viscosity and boiling point. Therefore, blending PODE n contributed to the evaporation and atomization of the fuel, improved the homogeneity of the mixed gas, and effectively avoided fuel cracking caused by an uneven gas mixture under high-temperature and hypoxia conditions 44 . Furthermore, blending PODE n increased the oxygen content in the blended fuel, which supplied the oxygen during the combustion process, thereby improving the combustion conditions in some areas during  www.nature.com/scientificreports/ the diffusion combustion process of the diesel engine and promoting the post-oxidation process of the soot. In particular, the PODE n molecules did not contain C-C bonds, which could significantly reduce the amounts of olefins and PAHs generated during the combustion reaction, thereby lowering the amount of soot. The above factors greatly improved the soot emission of the diesel engine after the blending of PODE n . Furthermore, under high speeds and loads, the soot emission decreased more substantially with the increase in the PODE n blending ratio. With increases in the speed and the load, the combustion temperature in the cylinder increased, and the high-temperature hypoxic areas increased. Hence, due to the high oxygen content and excellent volatility, the PODE n had a more evident effect on improving the combustion conditions when the gas mixture had an excessively high concentration.

Conclusions
PODE n , with a high oxygen content and a large cetane number, are a promising additive for diesel. In this study, a series of tests were performed on a supercharged intercooled high-pressure common-rail diesel engine powered by diesel fuel blended with PODE n at different ratios, i.e., 10%, 20%, and 30%, to explore the influences of PODE n blending on the combustion and emission characteristics as well as the fuel economies of a heavy-duty diesel engine that met the China VI emission standards. The following conclusions were obtained: (1) Blending PODE n significantly affected the combustion characteristics of the diesel engine. As the PODE n blending ratio increased, the peak in-cylinder pressure and the average in-cylinder temperature gradually decreased, and the phase corresponding to the peak gradually shifted forward. The peak heat release rate in the pre-injection stage varied slightly, and the peak heat release rate in the main-injection stage gradually decreased, with the phase corresponding to the peak gradually moving forward. (2) As the PODE n blending ratio increased, the start of the combustion gradually shifted forward, the duration of combustion was shortened by 0.7-2.8°CA, the heat release became more concentrated, the combustion heat release of the fuel was closer to the top dead center, and the maximum effective thermal efficiency could be improved by 2.57%. After blending with PODE n , the effective fuel consumption increased, but the equivalent effective fuel consumption decreased significantly, with the maximum reduction reaching 4.55%. (3) Blending PODE n into diesel could effectively improve the emission characteristics of the diesel engines.
Particularly under high-speed and high-load conditions, the PODE n , due to its high oxygen content and great volatility, improved the oxygen concentration in the cylinder and the uniformity of the combustible mixture, which effectively inhibited the generation of soot and reduced the emission of soot by as much as 76.3%. With the increase in the PODE n blending ratio, the emission of NO x increased slightly, but the emissions of HC and CO gradually decreased, with the highest reduction of the CO emissions reaching 66.2%.