*3.2. Effect of Initial Pressure on DX HCCI Combustion and NO Emissions*

In order to keep the combustion pressure consistent with the actual two-stroke diesel engine in the cylinder, at an equivalence ratio of 0.5 and an initial temperature of 400 K,

four different pressures of 0.8, 1.0, 1.2, and 1.4 atm are selected as the initial pressure of the two-stroke diesel engine HCCI combustion. Figure 5 shows the effect of different initial pressures on the in-cylinder pressure in the DX HCCI combustion. It can be seen that the peak combustion pressure increases with an increase of initial pressure. When the initial pressure is 1.4 atm, the DX HCCI combustion pressure is consistent with the experimental pressure [38]. The actual combustion pressure in the cylinder rises to the highest point after the top dead center (TDC) compared with HCCI combustion. The reason is that HCCI is an ideal combustion mode, multiple ignition points are distributed in the cylinder. When the fuel reaches ignition conditions, all the fuel is combusted at the same time in the cylinder, and the pressure rises to the highest point instantly in the cylinder. The fuel combustion reaction needs some time to progress gradually in the actual diesel engine, the fuel is completely burned after the TDC, and the cylinder pressure reaches the maximum. Therefore, the study selects 1.4 atm as the initial combustion pressure of the two-stroke engine without affecting the engine efficiency.

**Figure 5.** Comparison of different initial pressures and experimental pressure.

Figure 6 shows the effect of different initial combustion pressures on NO emissions. NO increases very slightly with the increase of initial intake pressure at a 0 crank angle degree. This is because the increase of pressure leads to an increase in the reaction rate of NO formation at the TDC, which increases the production of NO. It can be seen that the NO emission decreases slightly with the increase of initial pressure at a 50–119 crank angle degree. The reason is that the increase in combustion pressure accelerates the disturbance of the gas in the cylinder, speeds up the destruction of NO, and reduces the final production of NO. Therefore, the increase of initial combustion pressure can help to reduce NO emissions.

**Figure 6.** Effect of initial combustion pressures on NO emissions.

## *3.3. Effect of Initial Combustion Temperature on DX HCCI Combustion and NO Emissions*

At a constant initial combustion pressure of 1.4 atm, equivalence ratio of 0.5. The study selects 380, 400, 420, and 440 K as the initial combustion temperatures for DX HCCI combustion. Figure 7 shows that when the initial temperatures are 400 K and 420 K, the combustion peak pressures in the cylinder are close to the actual combustion pressure [38]. At the same time, the fuel ignition delay time at 420 K is shorter than that at 400 K, so DX HCCI combustion can be synchronized with the actual combustion pressure curve earlier at the initial temperature of 420 K.

**Figure 7.** Effect of initial combustion temperatures on in-cylinder pressure.

Figure 8 shows the effect of different initial combustion temperatures on NO emissions. It can be seen that the NO emissions at the TDC increase significantly with an increase of initial temperature, which is because the maximum in-cylinder combustion temperatures rise with the increase of the initial temperature. The final emissions of NO at the exhaust port increase slightly at different initial temperatures.

**Figure 8.** Effect of initial combustion temperatures on NO emissions.

Therefore, the DX HCCI combustion will not increase significantly at the initial combustion temperature of 420 K. The simulated combustion pressure can maintain a good consistency with the actual diesel engine combustion pressure in the cylinder.
