*2.1. Background*

The described engine has a high overall expansion ratio (εdcp) of the cylinder charge equal to 21. This value results from the fact that the compression and expansion ratios of the fired cylinders of the engine are equal to 10.5. The volume of the additional expansion cylinders at Bottom Dead Center (BDC), at the end of the second expansion, is twice as high as the volume of the gas at the end of the first expansion. The mentioned value of <sup>ε</sup>dcp allows us to obtain a relatively high degree of energy recovery of the exhaust gas in the additional expansion cylinders at wide open throttle (WOT). In one study [5], the results of the theoretical calculations of the increase of the thermal efficiency of an engine cycle with the additional expansion were presented. To standardize the analysis of the engine cycle, processes are described as though the additional expansion was carried out in the same working volume. In the Otto-cycle, in which the compression and expansion ratios are the same, the thermal efficiency of the cycle is independent of the amount of heat added to the cycle. In the cycle in which the expansion ratio is higher than the compression ratio (e.g., five-stroke engine cycle), the increase in thermal efficiency is higher with more heat is added to the cycle. Under some simplification, the heat added to the

theoretical cycle may be identified as the load of a real engine, so the higher the load, the higher the increase in the efficiency can be expected. On the other hand, in the real engine at partial load, when the cylinder pressure does not reach high values, the additional expansion process may give a charge pressure lower than the ambient pressure, which causes negative work and energy losses. In this situation, you should take into account that the cylinders of additional expansion, instead of giving additional power to the engine shaft, will be required to bring the power from the fired cylinders, which is a highly undesirable phenomenon for obvious reasons. The idea of an additional expansion process carried out in a separate cylinder together with the undesirable phenomenon of expansion below the ambient pressure in some cylinders is shown in Figure 3. As seen in the figure above, when the maximum pressure of the cycle is too low, a vacuum occurs in the additional expansion cylinders. This will produce an area of negative work of the cycle. When it becomes dominant over the rest of the cycle of the second expansion, it turns out that the overall balance of the additional expansion cylinder is negative—instead of giving power to the engine shaft, it will require its delivery and will act as an additional load of the engine, decreasing its efficiency.

**Figure 3.** The concept of realizing the additional expansion process in a separate cylinder; Vcyl—displacement of one cylinder of the engine; Vch—volume of the combustion chamber.

In addition, in the five-stroke engine, the volume of the transfer port connecting the fired cylinder with the cylinder of an additional expansion is a parasitic volume, because expansion of exhaust gases occurring there causes the loss of part of the energy. In an ideal situation, this volume should be equal to 0. In practice, this is impossible due to the constraints of the conventional engine arrangement. Similarly, it would be preferable that the volume of the chamber above the piston in the cylinder of the additional expansion would be as small as possible. Unfortunately, in the tested engine, improvement of these parameters was not feasible, because during the construction of the test engine, a modified cylinder head of the original mass-produced engine was used.
