## 3 Results and Discussion  
### Density and Fuel Content in Oil  
The oil samples were collected from the existing running engine operated in real conditions so there was no operational opportunity/possibility to stop the engine from taking the oil samples. Moreover, the design of the engine and fuel injection pump determined the method of extracting oil from the pump crankcase. That is why measuring the volume of the lubricating fluid (oil-fuel mixture) inside of the pump was almost impossible to be made precisely. For this reason, it was decided to collect oil samples each time after a complete oil change in the crankcase, maintaining the specified operating time of each sample. It was also impossible to determine the amount of fuel in the oil directly. The amount of fuel in oil was determined using the mass and volume ratio of clean and used oil, which is presented in Equation (1).  
Based on the measurement of the oil density for each sample and the comparison with pure oil, mass balance (first line of system of Equation (1)) and volume balance (second line of system of Equation (1)) allowed to estimate the amount of fuel that leaked to the crankcase for a specific period of engine operation. As a result of the transformation of Equation (1)-the Equation (2) was obtained. Based on this transformation the amount of fuel for individual oil samples was calculated and the results are presented in Table 2.  
\[\left\{\begin{array}{l l l}q_{LO}\times\rho_{LO}+q_{FO}\times\rho_{FO}=q_{ ULO}\times\rho_{ULO}&\quad\text{[kg]}\\ q_{LO}+q_{FO}=q_{ULO}&\quad\text{[litres]}\end{array}\right. \tag{1}\]  
\[q_{FO}=1.8\frac{\rho_{LO}-\rho_{ULO}}{\rho_{ULO}-\rho_{FO}}\quad\quad\text{[ litres]} \tag{2}\]  
where:  
\(q_{LO}\)--quantity of Lubricating Oil (Fresh Oil) = 1.8 in liters,  
\(q_{FO}\)--quantity of Fuel Oil in liters,  
\(q_{ULO}\)--quantity of Used Oil in liters,  
\(\rho_{\text{LO}}\)--Lubricating Oil density = 0.8886 g/cm\({}^{3}\) (the density of fresh oil measured in the laboratory, which is consistent with the oil manufacturer's data),  
\(\rho_{\text{FO}}\)--Fuel Oil density = 0.8317 g/cm\({}^{3}\)[23],  
\(\rho_{\text{ULO}}\)--Used Oil density in g/cm\({}^{3}\).  
Since the fuel density was provided as a standard value at 15 \({}^{\circ}\)C, all calculations were performed assuming this temperature. Table 2 presents calculated ratio [%] how much of fuel was leaking into the lubricating oil.  
Based on the data from Table 2, it can be easily noticed that the amount of oil in the injection pump lubricating oil increases (as expected) over time to above 50% (Figure 4),  
\begin{table}
\begin{tabular}{l c c c} \hline \hline
**Oil Working Period** & \begin{tabular}{c} **Lubricating Oil** \\ **Density at 15 \({}^{\circ}\)C** \\ \end{tabular} & \
\begin{tabular}{c} **Fuel Oil** \\ **Quantity** \\ **(Volume)** \\ \end{tabular} &
\begin{tabular}{c} **Fuel Oil** \\ **Quantity** \\ **(Volume)** \\ \end{tabular} \\ \hline  & [g/cm\({}^{3}\)] & [L] & [\%] \\ \hline
**Fresh oil** & 0.8886 & 0 & 0.0\% \\ after 1 h & 0.8749 & 0.5708 & 13.7\% \\ after 2 h & 0.8744 & 0.5986 & 14.2\% \\ after 4 h & 0.8723 & 0.7227 & 15.3\% \\ after 6 h & 0.8708 & 0.8194 & 15.4\% \\ after 8 h & 0.8689 & 0.9532 & 18.1\% \\ after 10 h & 0.8645 & 1.3226 & 21.3\% \\ after 18 h & 0.8625 & 1.5253 & 35.5\% \\ after 24 h & 0.8614 & 1.6485 & 43.8\% \\which is a very large dilution of the oil causing a significant change in its chemical properties, as shown in work [5]. Additionally, it can be noticed that the dilution of oil with fuel changes significantly after the first hour and remains at a relatively stable level for several hours. Thereafter, the fuel content increases proportionally to time by approximately 1.5% per hour.