*3.4. Effect of n-Butanol on DX HCCI Combustion and Emission Characteristics* 3.4.1. Effect of n-Butanol Blending Ratio on the Ignition Delay Time

At a constant engine speed of 85 rpm, equivalence ratio of 0.5, the total mole fraction of the mixed fuel is fixed. Figure 9a shows the effect of n-butanol blending ratio on the ignition delay time of the high-temperature part. It can be seen that when the temperature is higher than 1100 K, the ignition delay time of the mixed fuel rises with the increase of n-butanol blending ratio. The same trend also appears in the low-temperature stage (Figure 9b). The ignition delay time increases significantly with the increase of n-butanol blending ratio when the temperature is lower than 830 K. This is because the cetane number of n-butanol is lower than that of DX (Table 2). With the increase of n-butanol mixing ratio, the cetane number of the mixed fuel decreases. As a result, the ignition delay time of the mixed fuel becomes longer. The fuel and air have more time to mix fully and uniformly before the combustion starts, which can improve the fuel combustion efficiency effectively. The cetane number of DX is higher than that of diesel, and the cetane number of n-butanol is lower than that of diesel (Table 2). Therefore, adding a certain proportion of n-butanol to DX can make the cetane number of the mixed fuel closer to real diesel. It can be seen that when the blending ratio of n-butanol is 40%, the cetane number of the mixed fuel is closest to real diesel (Table 3).

**Figure 9.** (**a**) Effect of DX and n-butanol blending ratio on high-temperature ignition delay; (**b**) Effect of DX and n-butanol blending ratio on low-temperature ignition delay.

## 3.4.2. Effect of n-Butanol Blending Ratio on Combustion Temperature and Pressure

Figure 10 shows that the combustion temperature decreases with an increase of nbutanol percentage at a constant equivalence ratio of 0.5. This is because the lower heating value of n-butanol is lower than that of DX, the total heating value of fuel reduces with the increase of n-butanol percentage at the constant total mole fraction. The difference between the heating value of DX and n-butanol is not very large, it will not cause a significant decrease in the temperature of the main combustion process. However, the lower heating value will shorten the combustion process time. When the fuel is completely burned, the lower the heating value, the faster the combustion temperature decreases. The reduction of combustion temperature helps to reduce the production of thermal NOx. Therefore, the combustion temperature can be adjusted in an appropriate range by regulating the mixing ratios of DX and n-butanol to decrease NOx emissions

**Figure 10.** Effect of DX and n-butanol mixing ratio on combustion temperature in cylinder.

Figure 11 shows the effect of n-butanol mixing ratio on in-cylinder combustion pressure. It can be seen that the combustion pressure has not obvious change with the increase of n-butanol mixing ratio in a two-stroke diesel engine. Therefore, adding n-butanol to DX HCCI combustion will not cause a major change to the engine combustion pressure, which ensures the working efficiency of the engine.

**Figure 11.** Effect of DX and n-butanol mixing ratio on combustion pressure in cylinder.

3.4.3. Effect of n-Butanol Blending Ratio on NOx Emissions

Figure 12a shows the effect of DX and n-butanol blending ratio on NO emissions. It can be seen that NO emissions decrease significantly with the increase of n-butanol mixing ratio. When the blending ratio of n-butanol increases from 0% to 40%, the final NO emissions decreased by 21.9% (Figure 12b). Therefore, NO emissions can be reduced effectively by adding the n-butanol mixing ratio.

**Figure 12.** (**a**) Effect of DX and n-butanol blending ratio on NO emissions; (**b**) Effect of n-butanol percentage on final NO emissions.

The same trend also appears in NO2 emissions (Figure 13a). It can be seen that the final emission of NO2 shows a downward trend with the increase of n-butanol mixing ratio. It can be clearly seen that with the blending ratio of n-butanol increases from 0% to 40%, the final NO2 emissions decrease by 7.8% from Figure 13b. Therefore, NO2 emissions can be reduced effectively by adding n-butanol in DX HCCI combustion.

**Figure 13.** (**a**) Effect of DX and n-butanol mixing ratio blending ratio on NO2 emissions; (**b**) Effect of n-butanol percentage on final NO2 emissions.

NOx emissions show an obvious downward trend with the increase of n-butanol blending ratio. The main reason is that the lower heating value of DX is higher than that of n-butanol. The total heating value of fuel decreases with the increase of n-butanol blending ratio, causing the combustion temperature in the cylinder to decrease, which leads to a decrease in NOx emissions. At the same time, the increase of n-butanol blending ratio leads to a lower cetane number of the mixed fuel and increases the ignition delay time. The fuel has sufficient time for uniform mixing and shortens the combustion process. Since long-time combustion is also an important factor that causes the increase of NOx emissions. Therefore, the NOx emissions can be reduced by shortening the combustion process.
