## 2 Planning Diagnostic Experiments
### Selection of the Location of Measurement Places
The reliability investigations carried out are aimed at estimating the values of reliability indicators of systems and components of tested engines in the operation phase. The operation phase, from the hand-over of the object to the user to the withdrawal from use, includes: awaiting the start of use, use, on-demand duty, stoppage between consecutive usage states, maintenance and awaiting maintenance. Figure 2: Fragment of the block diagram of the hierarchical decomposition of the marine combustion engine intended fuelled with distillate fuels: 0—engine level, 0.1—engine block, 0.2—piston-crank system, 0.3—exchanges of the working medium system, 0.4—fuel feed system, 0.4.1—settling tank, 0.4.2—transport pump, 0.4.3—filter of the transport pump, 0.4.4—fuel service tank, 0.4.5—centrifuge filter, 0.4.6—fuel centrifuge, 0.4.7—strainer fuel filter, 0.4.8—fine filter, 0.4.9—booster fuel pump, 0.4.10—injection pump, 0.4.11—injection pipe, 0.4.12—injector,... 0.4.16—check valve, 0.5—lubrication system, 0.6—cooling system, 0.7—starting system, 0.8—automatic steering and control system. The damage to the piston combustion engine was considered as random events because its occurrence is influenced by many factors: complex energy transformation processes, the impact of the environment and various operators. For reliability tests, information on the operation of 18 medium-speed engines by two ship-owners was collected according to the selected test plan. Reliability analysis was conducted on the basis of a passive experiment: data was obtained from machine documentation, computers' databases, observation of conditions and work parameters during ship stops in ports and shipyards. The reliability assessment was carried out according to a specified plan, and in relation to the tests of the systems and engine components, it was carried out according to the plan (\(N_{oe}\), \(R_{a}\), \(\theta_{k}\)) [2]. This means that the observations were subject to the \(N_{oe}=18\) engines. Combustion engines damaged in the analysed period were repaired (\(R_{a}\)) and tests of a particular engine were conducted from the beginning of operation and ended after three periodic maintenance (\(\theta_{k}\)) sessions for classification of the ship. The observation period for one engine was on average about 34,000 h of work. The article presents the results of auxiliary tests of marine engines of the same rotational speed and fuelled by distillate fuels due to restrictions on the use of residual fuels in many regions. Various reliability indicators can be used for reliability investigations [2; 8; 13; 15; 16; 17] and this work used the indicator of the individual damages of individual functional subsystems and components of \(K_{si}^{d}\):  
\[K_{si}^{d}=\frac{N_{si}^{d}}{N_{es}^{d}}\text{ for }i\ =\ 1,\ 2,\ 3,\ \ldots,\ n; \tag{9}\]  
where: \(N_{si}^{d}\)--number of damages of the \(i\)-th functional system, \(N_{es}^{d}\)--number of damages in all functional systems of the investigated sample of the internal combustion engines. Figure 3a shows that the most unreliable functional system in the quantitative terms is the fuel feed system of engines fuelled by distillate fuel; the second most unreliable is the working medium exchange system. The reliability analysis carried out for the functional systems of the engines showed that the most failing system of engines fuelled by residual fuel is the lubrication system [18]. Analogous estimates were made for the third level of decomposition of the investigated sample of combustion engines of one type and the indicator of the share of damages of particular elements was used \(K_{ei}^{d}\):  
\[K_{ei}^{d}=\frac{N_{ei}^{d}}{N_{ee}^{d}},\text{ for }i\ =\ 1,\ 2,\ 3,\ \ldots,\ n; \tag{10}\]  
where: \(N_{ei}^{d}\)--number of damages of the \(i\)-th functional system, \(N_{ee}^{d}\)--number of damages in all functional systems of the investigated internal combustion engines.