## 3 Results
### Mechanical Efficiency of the Tested Engine  
For conventional internal combustion engines, calculations of the mechanical efficiency (\(\eta_{\text{m,env}}\)) based on the known indicated mean effective pressure do not pose any problems. This is described by Formula (3):  
\[\eta_{\text{m,env}}=\frac{\text{BMEP}}{\text{IMEP}}=\frac{\text{P}_{\text{e}}} {\text{P}_{\text{i}}}, \tag{3}\]  
Brake Mean Effective Pressure at a certain rotational speed is determined by measuring the torque of the engine relative to a displacement volume of the engine and to the engine type, i.e., whether it is a two-stroke or four-stroke engine. Indicated Mean Effective Pressure is determined based on the measured in-cylinder pressure as a function of CA. For a multi-cylinder engine, the pressure is typically measured for one cylinder and it is assumed that all the other cylinders operate in the same  
Figure 13: Effective power and BSFC as a function of engine rotational speed at WOT and with rel. AFR = 1.0; Brake Specific Fuel Consumption (BSFC); Wide Open Throttle (WOT); Relative Air-to-Fuel Ratio (rel. AFR). way. For obvious reasons, for the classic engine, the BMEP/IMPEP ratio is exactly the same as the ratio of effective power (\(\mathrm{P_{e}}\)) to the indicated power of the engine (\(\mathrm{P_{i}}\)). In the case of an engine with the additional expansion of exhaust gas in a separate cylinder, the situation becomes more complicated. Besides the cylinders, where a conventional four-stroke working cycle is carried out, the engine has a cylinder for the additional expansion of the exhaust, which should deliver work to the output of the engine. This cylinder is an integral part of the engine so its effect cannot be ignored in the analysis of the mechanical efficiency of the engine, because the calculated value of the mechanical efficiency would be artificially high and, in particular, the engine operating conditions could prove to be even higher than 1. Under the conditions of a sufficiently high load of the engine, where the cylinder of additional expansion provides additional power to the output, its indicated mean effective pressure is higher than zero. In the case of low engine load when the pressure of the exhaust gas going into the cylinder is low, the additional expansion cylinder requires additional propulsion from the engine, and the value of the indicated mean effective pressure becomes negative; of course, the same thing happens to the value of power indicated from the cylinder. Considering the case above, we conclude that the effect of the operation of the additional expansion cylinder has to be included in a certain way when determining the mechanical efficiency of the five-stroke engine. The values of IMEP for the fired and additional expansion cylinders are not additive. This happens because in a general case they relate the different displacement volumes, and it should be noted that the fired cylinders operate in four-stroke mode, while the cylinders of additional expansion operate in two-stroke mode. Avoiding this problem is possible by calculating the mechanical efficiency from the calculated values of the effective power and the indicated power of the engine. To determine the mechanical efficiency of the five-stroke engine, the author of this paper proposes a summation of the indicated power for the fired cylinders \(\mathrm{P_{i\_{f\_{f\_{f\_{f\_{f\_{f\_{f\_f\_f\_f\_f\_f\_f\_f\_f\_f\_  
An analysis of the diagram indicates that the obtained value of the mechanical efficiency of the engine increases with engine load, reaching a value slightly larger than 0.8. This took place at the rotational speed of 3000 rpm, when the boost pressure p\({}_{\text{bst}}\) was 0.5 bar. It is also seen that in the analyzed range of the rotational speed of the engine, the mechanical efficiency does not depend significantly on the rotational speed.