# Effect of PODE on Emission Characteristics of a China VI Heavy-Duty Diesel Engine
## 3 Results and Discussions  
### 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,  
Figure 2: CO emission of four fuels under different loads. 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  
Figure 4: NOx emission of four fuels under different loads. Figure 3: HC emission of four fuels under different loads. 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.