EFFECTIVENESS ASSESSMENT OF DIESEL LOCOMOTIVES OPERATION WITH THE USE OF MOBILE MAINTENANCE POINTS

Based on the example of 6Dg type diesel locomotives, the paper presents a new maintenance strategy concerning periodical checks at the P1 maintenance level. Currently, such locomotives are sent off to service points every 102 hours of operation or every 14 days for a P1 level check. Studies demonstrate that the average distance to be covered by a locomotive to arrive at a service point is in excess of 60.0 km, and the quantity of fuel consumed is more than 88 litres. It is costly and time consuming to have locomotives out of service and considerable resources of the railway carrier are engaged which could be made use of in the transport process. The aim of the newly developed strategy of P1 checks is to eliminate the need for locomotives to exit their routes to reach rolling stock maintenance points. The control/diagnostic and maintenance activities specified in the Maintenance System Documentation will be performed by so-called mobile maintenance points. The development of the new strategy required: identification of the current condition of the maintenance system, development of the concept of a new strategy of P1 maintenance checks, conduct of studies and operational analyses for SM42 series locomotives, performance of a durability, reliability analysis, assessment of safety together with an analysis of the risks involved in the proposed changes. In order to review and assess the efficiency of the new strategy, an observed operation of selected locomotives was conducted together with a railway carrier. During the study, the maintenance activities and processes were monitored and the costs of P1 checks were recorded. The analysis of efficiency of the new strategy of performing P1 checks without the need for a locomotive to exit its route to reach a rolling stock maintenance point demonstrated that depending on the distance covered by the locomotive to reach a maintenance point, the unit costs of a P1 level check are lower by up to 67.1% compared with the currently applied method.


Introduction
The subject of the paper is an assessment of the introduction of a new strategy for periodical checks at the P1 maintenance level for SM42 6Dg diesel locomotives. Currently, such locomotives are sent off to service points every 102 hours of operation or every 14 days for a P1 level check. SM42 6Dg locomotives are modern rolling stock manufactured by NEWAG S.A., where modern on-board diagnostics solutions are applied. Excluding the modernised locomotives from the traffic in order to carry out the P1 inspection is costly and involves significant resources of the carrier, which could be used in the transport process. Projects to improve the efficiency of the process of operation railway vehicles by changing the preventive maintenance, are amongst the core areas of the strategies pursued by rail transport companies. This is due to the fact that the costs of preventive maintenance are up to 30.5% of the Life Cycle Costs (LCC) of rail vehicles and are the second cost category after the costs of power or fuel (Babeł and Szkoda 2016, CUT 2016, CUT 2017, Szkoda and Tułecki 2017, Szkoda and Satora 2019). Construction or optimisation of rail vehicle maintenance strategies are also described in professional writings. For example paper Soh, et al. (2012) presents a method of setting the optimum maintenance plan intended to minimise the total maintenance costs and the scope of maintenance activities. The solution can be applied to vehicles, infrastructure, rail traffic control systems. Matusevych, Kuznetsov and Syvchenko (2018) point that increase of maintenance system effectiveness can be generally reached in three main interdependent directions (technical, economic, organizational) that demands to work towards the following main objectives: improvement of maintenance strategy of equipment, improvement of operational and repair documentation or improvement of the organization of maintenance. Wu and Lin (2016) demonstrate that changes in the maintenance plan of a railway vehicle may consist in reducing the frequency of individual inspections and periodic repairs, i.e. extending the duration of intervals between them. Gill (2017) presents the problem of optimising the maintenance system of selected tram elements account being taken of the risk involved. The costs of dealing with the risk and the values of risk reduction achieved as a result of avoiding damage to the ele-ments of the object have been assumed as components of the objective function. Pietrzyk and Uhl (2005) indicate that reliability analysis facilitates analysis and risk management in operation. One of the methods which can be used for this purpose is the RCM (Reliability Centered Maintenance) method, which combines analyzes related to safety, reliability and costs. The authors presented its application for the optimization of railway equipment maintenance. Cheng and Tsao (2010), Cheng et al. (2006) emphasize that safety should be the most important factor taken into account when choosing a maintenance strategy for railway vehicles. In turn, Ten and Ghobbar (2013), using three types of performance indicators related to reliability, availability and maintenance costs, the authors describe a method for changing the times to repair of rail vehicles. The method has been implemented in order to reduce the maintenance costs of rolling stock for the Dutch carrier. The authors point to the proper identification of the sub-assemblies affecting the maintenance strategy. In turn, Magiera (1982) notes that already at the time of implementation of a maintenance strategy, there is a trend towards an increase in the mileage between particular maintenance activities. Park, et al. (2011), Yun, et al. (2012, as part of the criteria for selecting the appropriate maintenance strategy, the Life Cycle Cost (LCC) and the technical availability index are taken into account when determining the maintenance intervals for assemblies and sub-assemblies of a railway vehicle. The aim of this paper is to assess the effectiveness of introducing a new P1 maintenance strategy for SM42 series locomotives without the need to go to repair and maintenance points. It is envisaged that the control and diagnostic and maintenance activities specified in the Maintenance System Documentation (DSU) for P1 level checks would be carried out in the range of 200 moto hours instead of 102 hours, or 21 calendar days instead of 14 days, by traction teams, examiners and so-called mobile maintenance points without the need to go down to the rolling stock maintenance points.

Characteristics of research object -6Dg diesel locomotive
In 2007 NEWAG S.A. company performed prototype modernisation of the 6D diesel engine which has been used in Poland for over forty years. 6D is the most common series of locomotives in Poland (in December 2019 there were 599 such vehicles). The main job of the locomotive is shunting manoeuvres at hump yards. In 2009, after two-year testing of the prototype vehicle, the first modernised locomotive was delivered to PKP Cargo S.A., the biggest Polish rail carrier. After the modernisation the locomotive was given the symbol 6Dg (Figure 1a). The modernisation scope included the replacement of the a8C22 diesel engine used till then by a new 12-cylinder C27 Caterpillar diesel engine, of 653 kW power (since 2010 of 708 kW power), meeting the exhaust emission standard according to 2004/26/WE Directive. Selected technical parameters of 6Dg locomotive are shown in Fig. 1b, and its detailed description is given in Szkoda (2017  The maintenance plan for the SM42 6Dg diesel locomotive series was specified in the Maintenance System Documentation (DSU) approved by the President of the Railway Transport Office (Table 1).

Concept of the new strategy for P1 checks
The new strategy for P1 maintenance level checks assumes that some of the activities specified in the DSU would be carried out by drivers, examiners and a mobile maintenance point without having to go to the rolling stock maintenance facility. The analysis also takes into account the possibility to change the values of the P1 interval measures according to the assumptions presented in Table 2. The scope of the P1 maintenance activity covers a number of assemblies and subassemblies relevant for railway safety: − body, − underframe, − bogies, − castors with bearings, axle boxes and suspension for traction motors, − drag and collision sets, − braking and compressed air systems, − internal combustion engine and accessories, − propulsion systems, 10 Szkoda, M., Satora, M., Konieczek, Z., Archives of Transport, 54(2), [7][8][9][10][11][12][13][14][15][16][17][18][19]2020 − electrical machinery including: main generator, auxiliary generator, traction motor, auxiliary machinery, − electrical circuits, apparatus and equipment, − ventilation, air conditioning and heating, − firefighting equipment, − lubrication system. The development of a new strategy for P1 checks required, inter alia: − identification of the current state of play with regard to the locomotive maintenance system, − development of a concept of a new strategy for P1 checks, − performance of operation tests for SM42 locomotives, − performance of a durability and reliability analysis, − assessment of the risk involved in the proposed changes, − assessment of the efficiency of the new maintenance strategy. Table 3 presents an example of the activities according to the new strategy, within the P1 maintenance level for a locomotive body. Table 4 presents selected statistical data concerning failures of a 6Dg locomotive detected within P1 checks done between July 2016 and June 2017 by rail carrier. The data comprise a total of 4078 checks for 119 6Dg locomotives. The analysis carried out showed that, from the point of view of reliability and safety of railway traffic, the significant risks detected in maintenance level P1 checks concern: 1) Electrical apparatus and equipment:

Observed operation of locomotives according to the new strategy
In order to collect the data necessary to assess the effectiveness of the new P1check strategy, the operation of six 6Dg locomotives within the rolling stock of rail carrier within the company's maintenance plants (Zakład Północny and Śląski) was observed: Initially, the observed operation included a period of two maintenance cycles between maintenance level P2/1, i.e. in practice about 3÷4 months (06.2017 ÷ 09.2017, stage I) for three locomotives. After collecting and analysing the collected data and drawing up the appropriate reports, it was decided to continue the research and carry out an extended observed operation (10.2017 ÷ 11.2017, stage II and 01.2018 ÷ 02.2018, stage III), which, apart from locomotives in the Company's Zakład Północny, also comprised three vehicles in Zakład Śląski. This allowed the observation of the course of the locomotives' operation under various conditions, and thus to obtain reliable and extensive data to assess their effectiveness. The extended observed operation included one cycle between successive periodic checks at the second level of maintenance, both for the locomotives of both Zakład Północny and Zakład Śląski. The data on the time of operation and kilometrages of the surveyed locomotives are presented in Table 5.  12 Szkoda, M., Satora, M., Konieczek, Z., Archives of Transport, 54(2), [7][8][9][10][11][12][13][14][15][16][17][18][19]2020 6. Assessment of the effectiveness of the new maintenance strategy In the course of the observed operation, in addition to the measurement and control of wear and tear of assemblies and subassemblies important from the point of view of traffic safety, there was also an ongoing recording of the costs of P1 checks of selected locomotives, which was the basis for assessing the effectiveness of the new maintenance strategy. − costs of environmental charges during an exit/arrival at P1 (KOS).

Direct costs of a P1 check at a rolling stock maintenance point
According to the current DSU, within a time of 102 hours of operation or 14 days, 6Dg locomotives are sent to repair and maintenance points for P1 level checks. The data gathered during the observed operation indicate that the costs of a P1 check of a SM42 locomotive at a rolling stock maintenance point (KP1P) are on average:

Costs of fuel consumption during an exit for P1
Based on the data gathered during the observed operation, the quantity and costs of fuel (KZP) consumed during exits/arrivals of selected locomotives for a P1 check were recorded. Table 6 presents an example of a record for one of the six tested SM42-1315 locomotives from Zakład Północny. Table 7 presents the total costs of fuel consumed during exits/arrivals of locomotives for P1.

Costs of access to the infrastructure involved in an exit for a P1 check
Based on the observed operation, the costs were recorded of access to infrastructure (KDI) during exits/arrivals of selected locomotives at P1 checks. Table 8 presents the costs of access to the infrastructure for one of the locomotives No. SM42-1315. Table 9 presents the total costs of access to the infrastructure during exits/arrivals of selected locomotives at P1 checks.

Labour costs of traction teams during an exit for a P1 check
During the observed operation, the costs were recorded of the work of traction teams (KDT) during the exits/arrivals for selected locomotives for P1 checks. Table 10 presents the labour costs of traction teams during exits for one of the locomotives No. SM42-1273. Table 11 presents the total labour costs of traction teams during the exits/arrivals of selected locomotives for P1 checks.

Costs of unavailability of locomotives going for P1
The performance of P1 level checks at the rolling stock maintenance points involves time wasted for the exit and arrival of the locomotive, as well as the need to provide a higher number of locomotives for shunting operations, to replace the vehicles which are not in operation. Based on the data gathered during the observed operation, a database was developed of the times of exits/arrivals of the locomotives going to P1 checks.  In order to assess the availability, within a specific time interval, e.g. between periodic checks, the availability ratio can be estimated from the formula: where: TZtime of locomotive availability, TNtime of locomotive unavailability due to current repairs, TOtime of locomotive unavailability due to preventive maintenance.
Based on the 2016÷2017 operation data gathered by the carrier, the technical availability ration for a 6Dg locomotive is:

Direct costs of P1 check at a mobile maintenance point
The direct costs of a P1 level check for a SM42 locomotive at a mobile maintenance point (KP1M) are similar to those at a rolling stock maintenance point and the duration of the check should not be longer than 4 hours.

Labour costs of traction teams and examiners during P1 check activities
In accordance with the scope of activities involved in a P1 check, Table 13 presents the labour costs of traction teams and examiners (KDR) during a P1 check.

Costs of unavailability of locomotives exiting for a P1 check
Elimination of significant losses of time for exits/arrivals at rolling stock maintenance points with the new P1 check strategy, of an average of 652 hours/year for a locomotive operating in Zakład Północny and 453 hours/year for a locomotive operating in Zakład Śląski, will improve the locomotive availability ratio in comparison to the current condition. It is estimated that increased availability of the locomotive with the new strategy of P1 checks will be higher by:

Resultsa comparison of the current and the new strategy
Based on the analysis it was concluded that the efficiency of the new strategy of P1 checks depends on the distance covered by the locomotive to exit/arrive at the rolling stock maintenance point. Analysing the structure of unit costs of a P1 check (Fig. 2), it can be seen that the direct costs of P1 (material and labour costs) according to the current strategy constitute only 10.7%, and the indirect costs related to fuel consumption, costs of access to infrastructure, labour costs of traction teams and the unavailability during an exit for a check as much as 89. 2%

Conclusions
The paper presents the results of the feasibility study on the project to introduce a new strategy for periodic checks at the P1 maintenance level for SM42 6Dg diesel locomotives. Generally, the new strategy consists in the performance of a technological maintenance process at the P1 level without the need to go down to rolling stock repair and maintenance points at the interval of every 200 moto-hours, 2500 km or every 21 calendar days whichever occurs earlier. The contractors of the process (operations, procedures, activities) will be traction teams, examiners and so-called mobile maintenance points. The concept of the new strategy has been subjected to research, analysis and evaluation in the following aspects: operation, combined with research and analysis of durability and reliability and effectiveness of the project. The work done demonstrated that: − the new strategy ensures that the vehicle's availability ratio is improved by eliminating the significant loss of time for existing the route to go to maintenance points; − the operation undertaken, observed on selected locomotives, confirms the validity of the project which has been undertaken. The analysis of the effectiveness of the new P1 check strategy for the modernised SM42 6Dg locomotives without the need to go down to the maintenance points showed that, depending on the distance the locomotive travels to exist/go down to the maintenance point: − the unit costs of a P1 level check for one locomotive are lower by up to 67.1% compared to the current method; − the costs of P1 level checks, annually, for one locomotive, are lower by up to 78.5% compared to the current method; − the costs of P1 level checks in the full maintenance cycle (30 years of operation) for one locomotive are lower by between PLN 1.3 million and PLN 1.7 million compared to the current method; − with the full number of 6Dg locomotives in stock and assuming that all locomotives exit to rolling stock maintenance points, the annual savings are up to PLN 6.9 million. The conducted analyses showed that it is possible to verify the maintenance plan for SM42 6Dg locomotives in the Maintenance System Documentation and to introduce records in accordance with Table 2. The next stage of work should be a safety assessment together with an analysis and evaluation of risk in order to confirm that the changes to be introduced do not violate the railway traffic safety conditions.