# Thermodynamic Analysis of In-Cylinder Steam Assist Technology within an Internal Combustion Engine
## 3 Results and Discussion
### Analysis of Thermal Efficiency Boundary  
It is an efficient method to enhance power output and thermal efficiency to increase the compression ratio. Due to limitations in the combustion chamber's material strength and thermal load, increasing the compression ratio of engines is a great challenge. Steam injection technology is considered to be a technical solution to achieve controllable combustion at high compression ratios. Adding steam to a cylinder can decrease the temperature of the charge in a flexible manner, especially the temperature of the end gas in the cylinder, and increase knock suppression. Hence, a more extensive range of compression ratios is applicable. The thermal efficiency limit of the in-cylinder steam-assisted cycle is analyzed in this section. Figure 8 shows plots of in-cylinder and exhaust temperatures at various compression ratios. The in-cylinder temperature increases slightly from 2323 K to 2385 K (steam injection mass = 0.2 g/cycle) as the compression ratio increases from 10 to 16, but the exhaust gas temperature decreases dramatically from 1368 K to 1250 K. This means that more energy is converted into work output, thereby improving the thermal efficiency. However, because of the increased compression ratio, the exhaust temperature decreases and the maximum steam injection temperature (as heated by the exchanger) is limited. This means that the gain in thermal efficiency brought by the in-cylinder steam-assisted cycle drops with decreases in steam injection temperature. Therefore, there is a trade-off between the compression ratio and the maximum temperature of injection steam heated by the exchanger. Hence, it is necessary to analyse the thermal boundary under different compression ratios. Figure 9 presents the thermal efficiency and its boundary for an in-cylinder steam-assisted cycle under a compression ratio range of 10-16 at a 20 MPa injection pressure and 0.1 MPa intake pressure. To allow the designed heat exchanger to make the most of the exhaust energy, the temperatures of steam injected at different masses are validated using the method introduced in Section 2.3. As shown in Figure 9, Case 1, thermal efficiency greater than the boundary cannot be achieved. At a compression ratio of 10, the highest efficiency (about 54.0%) appears between steam injection temperatures of 500-550 K. For the in-cylinder steam-assisted cycle, the steam injection mass and temperature are the most critical parameters in cycle efficiency optimization. As analyzed above, the increase in steam injection mass increases thermal efficiency, but the exhaust gas temperature drops with increases in injected mass. If the steam injection mass is too high or the exhaust gas temperature is too low, the thermal efficiency cannot attain the best cycle performance. Therefore, there is a trade-off between the steam injection mass and specific enthalpy. For a given value of injected steam enthalpy, the capacity to enhance thermal efficiency varies. Figure 7: Relationship between cycle performance and steam injection mass at various intake pressures (\(\text{T}_{\text{steam}}\) = 450 K; \(\text{P}_{\text{steam}}\) = 20 MPa; CR = 10). (**a**) In-cylinder/exhaust temperatures; (**b**) Thermal efficiency. As presented in Figure 9, higher thermal efficiency is achieved with the same steam injection mass at a higher compression ratio. Moreover, the thermal efficiency boundary rises from 54.0% at a compression ratio of 10 to 59.71% at CR = 16, and the maximum thermal efficiency reaches 59.71%. Overall, the increased compression ratio benefits the thermal efficiency of the in-cylinder steam-assisted cycle. A gain and loss analysis of thermal efficiency is presented in Figure 10. For each increment in compression ratio, the gains in WHR reduce by 0.25%, but the thermal efficiency gains because of the increased compression ratio increase by 1.1%, compensating for the efficiency loss in WHR.