# Thermodynamic Analysis of In-Cylinder Steam Assist Technology within an Internal Combustion Engine
## 3 Results and Discussion  
### Effects of Injection Temperature and Mass on Cycle Performance  
Before focusing on the in-cylinder steam-assisted cycle, the influence of steam injection parameters should be analyzed. Figure 4a presents the relationship between in-cylinder temperature at TDC and steam injection mass under different steam injection temperatures. As presented, the in-cylinder temperature decreases as the steam injection mass increases from 0 to 0.4 g/cycle. The highest temperature in the dry cycle is up to 2867 K, but the temperature decreases to 1680 K at a steam injection temperature of 300 K. The in-cylinder temperature after high-temperature steam injection increases to 1931 K, which is also much less than that in dry cycles. As a consequence, both room- and high-temperature steam can decrease the in-cylinder temperature at TDC. The reason for this is that the steam injected into a cylinder absorbs some combustion heat and evaporates immediately, which is beneficial in controlling abnormal combustion and optimizing low-temperature combustion. Furthermore, high-temperature steam injection also adds energy to the system. Figure 3: Flowchart of boundary verification. Figure 4a also presents the exhaust temperature at the expansion end under various steam injection temperatures. As displayed in the figure, the exhaust temperature of the dry cycle is about 1793 K, which is higher than the intake temperature. The exhaust temperature rises from 1224 K to 1343 K at steam injection mass = 0.2 g/cycle, while the steam injection temperature increases from 300 K to 600 K. Hence, the increasing injected steam temperature tends to raise the exhaust temperature. However, the results suggest that the exhaust temperature drops as the steam injection mass increases from 0 to 0.4 g/cycle. At a steam injection temperature of 450 K, the exhaust temperature decreases from 1793 K to 1021 K as the steam injection mass increases from 0 to 0.4 g/cycle. Since injected steam increases the working fluid mass, combustion heat can be efficiently transformed into an increase in in-cylinder pressure, thereby producing more work. Therefore, the exhaust temperature decreases with increasing steam injection mass. Meanwhile, although the model does not consider heat transfer loss, it is plausible that the thermal efficiency will increase dramatically if the exhaust energy is utilised sufficiently. The results are shown in Figure 4b, which illustrates the relationships between thermal efficiency and steam injection mass at various steam injection temperatures. The calculations of steam injection mass range from steam injection mass = 0 (no steam injected, dry cycle) to steam injection mass = 0.4 g/cycle. Under an injection pressure of 20 MPa, the thermal efficiency is 46.34% without steam injection. It gradually increases to 51.11% with increases in steam injection mass at a steam injection temperature of 300 K and increases more markedly to 61.25% at 600 K. According to the thermodynamic model in Section 2, the increment in thermal efficiency is derived from the addition of water steam enthalpy. Increasing the steam injection mass means more working fluid is used in the expansion stroke, enhancing thermal efficiency and work output. The rise in injected steam temperature results in an increase in the specific enthalpy of water steam. It is obvious from the slopes of the lines in Figure 4b that increasing the steam injection temperature increases the thermal efficiency at a certain steam injection mass. Therefore, the thermal efficiency of the developed cycle increases with the injected water steam enthalpy. The work output at different steam injection masses is shown in Figure 5 for a steam injection temperature of 450 K. The gas work and steam work are 178 J and 934 J, respectively, at a steam injection mass of 0.12 g/cycle. The gas work drops to 814 J, but the steam work increases dramatically to 407 J at steam injection mass = 0.32 g/cycle. It is apparent that the work generated by steam rises as the steam injection mass increases.