Analysis of Skidder Fuel Consumption by Work Operations During Timber Extraction in Thinning of Even-Aged Forest on Mountainous Terrain: A Case Study

Abstract

The paper deals with the analysis of the fuel consumption of skidders during timber extraction from thinning of even-aged beech forest on mountain terrain. Fuel consumption research was conducted on the Ecotrac 140V cable skidder over 8 working days at the same worksite during real timber extraction work. The worksite was organized so that the empty skidder traveled uphill, and when loaded, it moved downhill. The skidder was equipped with measuring devices for collecting data from sensors, the motor, and data transfer. The key parameters measured include total fuel consumption (mL) and skidder GPS position, while slopes of skid trails and load volumes were measured directly on terrain. Fuel consumption (L, L/m³) was determined per work cycle and work cycle elements. The highest fuel consumption occurred while driving the unloaded skidder, accounting for 38% of the total. This is primarily because fuel usage during skidder movement is significantly affected by factors such as skidding distance, slope, and skid trail conditions, especially since the unloaded skidder was moving uphill. Guidelines for better and more efficient organization of work and reduction in fuel costs are presented, and the suitability of the skidder and harvesting system are considered based on the results of fuel consumption.

1. Introduction

Effective planning of forest operations can help lower costs and increase work efficiency, yet this requires in-depth data on the productivity and costs associated with different timber harvesting methods as part of the planning process [1]. Such data are also crucial for assessing the performance of forestry machinery and harvesting systems under diverse forest stand and terrain conditions [2]. Due to variations in stand, working conditions, and terrain factors, accurately measuring the productivity and cost of each harvesting method is essential to choosing the best option tailored to local conditions, which ultimately optimizes the profitability of logging operations [3].

The choice of timber harvesting techniques is typically driven by a balance between environmental compatibility and economic benefits, promoting sustainable forest resource management [4]. In mountainous regions, forest operations often involve semi-mechanized equipment, combining motor-manual felling with ground-based skidding using rubber-tired cable skidders [5].

In the recent scientific literature, skidding technology is receiving relatively little attention, especially when compared to CTL (cut-to-length) harvesting systems [6]. In Central and Southeastern Europe, skidders are the predominant method for timber extraction [7]. Lundbäck et al. [8] noted that, within the EU, skidders are used to extract at least 40% of the annual wood harvest, totaling over 100 million cubic meters.

In Croatia’s hilly and mountainous forests, skidders equipped with winches weighing up to 10 tons are primarily used to extract timber from regular broadleaf fellings and selective coniferous harvests. Approximately 55% of all timber assortments are extracted using winch-equipped skidders [9]. The operating environment significantly impacts the efficiency of forest harvesting activities [10]. In Croatia, forest harvesting technology is particularly influenced by the unique attributes of forest management practices, with a significant share of natural forests that require intensive management and complex regeneration and tending practices.

Estimating skidder production costs is challenging due to the constant variability of influencing factors, which could lead to inaccurate decision-making by forest managers [11]. The productivity and costs associated with skidder operations vary based on factors such as machine model, operator experience, terrain slope, tree diameter and volume, environmental conditions, and skidding distance [12,13].

Fuel consumption represents a major expense in timber harvesting. Baker [14] found that fuel expenses constitute 22.8% of total logging costs, while Grisso [15] observed fuel costs ranging from 16% to over 45% of total machine costs. Temba et al. [3] reported that fuel expenses make up 68.8% of the total variable costs for grapple skidders in tree-length harvesting systems. As timber harvesting and mechanization levels are expected to increase, the total fuel usage in future harvesting operations is also expected to rise.

Being able to predict fuel consumption is essential for budgeting and operational management [16]. Analyzing production efficiency enables decision-makers to identify performance variations among forest production units and find opportunities for improvement. For management, efficiency studies provide valuable insights into conditions that may be contributing to inefficiencies and factors that influence productivity trends [10].

In summary, previous studies highlight that fuel consumption in timber harvesting is influenced by various factors, including operator education, experience, driving skill, environmental factors (climate, field, stand, and terrain), technological factors (harvesting methods, work operations, load volumes), and machine-specific factors (engine size, design, and maintenance) [17,18,19].

The primary aim of this research is to analyze the fuel consumption of skidders during timber extraction from thinning of beech forest on mountain terrain. Furthermore, the suitability of the skidder and harvesting system will be considered based on the results of fuel consumption.

2. Materials and Methods

The research was conducted in a mountainous area in the Lika-Senj County. Data collection took place at a single worksite, where a skidder was used for the thinning of an even-aged beech forest on mountainous terrain. The forest was 55 years old, covering an area of 30.50 ha with a growing stock of 247 m³/ha. According to the Forest Management Plan, a total of 1548 m³ of wood was felled with logging intensity of 17.4%. The average diameter at breast height (DBH) of the felled trees was 24.8 cm, with an average timber volume of 0.53 m³ per tree. The tree felling was conducted using a chainsaw, hauling was performed with a skidder, and the skidder driver also served as the chokerman. The driver was 45 years old with 10 years of experience operating a skidder. The hauling took place along the tractor path in such a way that the half-tree-length wood assortments were first hauled from shorter distances, i.e., from parts of the forest that were closer to the landing site. The effective work time of the skidder was 6 h per day, with a 30 min break and a final preparatory time (arrival at the worksite, departure from the worksite, checking and maintaining the machine) of 1.5 h.

The volumes of skidder loads per cycle were measured regularly after unloading at the landing site near the forest road. The worksite was organized so that the empty skidder traveled uphill, and when loaded, it moved downhill. The skidder operated on three skid trails, with lengths of 900 m, 440 m, and 300 m, each having a constant longitudinal slope of 8%. The slopes of the skid trails were measured using a Stonex S900A GNSS (Stonex S.r.l., Paderno Dugnano (MI), Italy) device equipped with a high-performance GNSS board with 800 channels.

The research took place during the spring, on days without precipitation within a rainy period, in a mountainous region. The forest road surface was very wet and muddy, with wheel ruts up to 20 cm along the entire route.

Fuel consumption research was conducted on the Ecotrac 140V (Hittner d.o.o., Bjelovar, Croatia) cable skidder during real timber extraction work. The skidder was in use for two years with 2200 operating hours before research. The Ecotrac 140V skidder is powered by a water-cooled Cummins diesel engine (QSB5.5, 104 kW/140 HP) (Cummins Inc., Wellingborough, UK) and equipped with a two-drum hydraulically driven winch with a nominal tractive force of 100 kN. The weight of the empty skidder is 8060 kg. The size of the tires used on the skidder was 16.9-R30.

For collecting telemetry data, the skidder was equipped with a Fleet Management System (FMS) and the WIGO-E mobile unit. The WIGO-E is a professional industrial IoT Gateway that collects and stores sensor data using different communication protocols (WLAN, LAN, or GSM). During the research, the data were sent via GSM protocol to a web platform and stored in the cloud. In the absence of a GSM signal, a common issue in forest conditions, the mobile unit stores data in its internal memory, which is sent to the server retroactively when the vehicle reaches an area with signal reception. An integral part of the FMS is a GPS device used to record the vehicle’s position, while the GPS/GSM antenna enables data transmission and satellite signal reception for GPS operation. For precise fuel consumption measurements, a differential fuel consumption meter, the DFM 100CD (Figure 1), was installed on the skidder. This differential fuel meter allows for the measurement of both current and total fuel consumption with an accuracy of ±3% and a precision of 0.001 L.

The data are accessed through a dedicated web platform, which enables real-time monitoring of the skidder’s operations. The system provides the flexibility to view live data or to generate detailed reports in the form of Excel tables. These tables can be further processed and analyzed, making it easier to track trends and performance metrics. The key parameters measured include total fuel consumption (mL), skidder GPS position (latitude and longitude), winch use duration, engine speed (RPM), engine torque as a percentage of the maximum, throttle position (%), and engine temperature.

The skidder’s work cycle was broken down into specific work elements to better analyze its performance. These elements include: unloaded travel from the roadside landing to the felling site, winch operations (rope extension and load winching), loaded travel back to the landing area, and unloading at the roadside.

From the collected data, each work cycle was segmented to identify the start and end times of each phase within a cycle, as well as the specific durations of each work element. Fuel consumption data, measured in milliliters through a differential flowmeter, was allocated to the various work cycles and their corresponding elements based on recorded times, allowing for a detailed understanding of fuel use across different phases of the skidder’s operation.

3. Results and Discussion

A total of 8 working days were recorded at that worksite, and during that time, the skidder extracted 126.90 m³ of wood mass in 56 skidding cycles.

Table 1. Load volumes characteristics.

	Minimum	Total/Average	Maximum
Load volume, m3	0.82	2.27	3.91
Number of pieces in load	8	10.3	15
Mean piece length, m	2	6.4	10
Mean piece diameter, cm	10	20.4	52
Mean piece volume, m3	0.02	0.44	1.06

shows the summary of the dimensions of the timbers extracted in this study. The presented numbers indicate a small piece size, a crucial factor that influences fuel consumption per volume of extracted wood. The number of pieces in load was in the range from 8 to 15 pieces of timber assortments.

Corresponding to Table 1,

Table 2. Load volumes per cycle.

	Load Volumes (m3)
Cycle	Day 1	Day 2	Day 3	Day 4	Day 5	Day 6	Day 7	Day 8	Total
1	2.19	2.47	1.76	2.42	0.82	3.04	1.82	2.43	16.94
2	2.22	1.83	1.70	2.58	1.73	3.03	3.91	2.87	19.88
3	3.06	2.04	0.89	2.15	2.13	1.10	2.28	2.52	16.18
4	2.67	2.12	1.47	3.11	1.41	2.75	2.46	2.42	18.41
5	1.83	1.17	2.04	2.80	2.66	2.15	1.98	1.50	16.12
6	1.63	1.86	1.54	3.16	2.76	3.61	2.57	1.93	19.06
7	3.09	2.35	2.77	2.52	2.89	-	-	-	13.62
8	2.42	1.99	2.29	-	-	-	-	-	6.70
Total	19.10	15.82	14.47	18.73	14.40	15.69	15.02	13.67	126.90
Average	2.39	1.98	1.74	2.68	2.06	2.62	2.50	2.28	17.76

shows load volumes, which are two to three times smaller than the nominal capabilities of the observed skidder. The average load volumes per working cycle was 2.27 m³ in the range between 0.82 m³ and 3.91 m³. The average daily output of extracted timber assortments was 17.76 m³/day.

In total, 242.28 L of Diesel fuel was consumed during the research. The average fuel consumption per cycle was 4.33 L, with the lowest value of 2.82 L per cycle and the highest value of 7.80 L cycle (

Table 3. Fuel consumption per cycle.

Fuel Consumption Per Cycles (L)
Cycle	Day 1	Day 2	Day 3	Day 4	Day 5	Day 6	Day 7	Day 8	Average
1	3.93	3.16	3.38	3.94	4.72	4.37	5.94	4.96	4.30
2	2.83	3.65	3.08	3.09	3.68	4.54	5.85	4.97	3.96
3	2.99	4.11	3.41	4.41	6.68	4.87	5.85	5.48	4.72
4	3.68	3.59	3.47	3.50	4.79	7.80	6.32	5.30	4.81
5	3.98	3.61	3.26	3.76	5.05	7.19	5.11	4.89	4.60
6	2.82	3.35	3.09	3.73	6.44	5.77	4.19	5.68	4.38
7	3.04	3.76	3.50	3.50	5.00	-	-	-	3.76
8	3.72	2.83	3.19	-	-	-	-	-	3.25
Average	3.37	3.51	3.30	3.70	5.19	5.76	5.55	5.21	4.33

).

According to

Table 4. Fuel consumption per work operations.

	Fuel Consumption Per Work Operation (L)
Cycle	Day 1	Day 2	Day 3	Day 4	Day 5	Day 6	Day 7	Day 8	Average
Unloaded travel uphill	1.39	1.24	1.17	1.10	1.70	2.12	2.74	2.25	1.65
Loading (winching)	1.01	1.16	0.90	1.20	1.24	1.31	0.97	1.29	1.12
Loaded travel downhill	0.33	0.47	0.60	0.59	1.10	1.28	1.13	0.81	0.76
Unloading	0.64	0.64	0.63	0.81	1.15	1.05	0.71	0.86	0.80
Total	3.37	3.51	3.30	3.70	5.19	5.76	5.55	5.21	4.33

, it was found that the skidder’s fuel consumption is higher when moving an unloaded skidder uphill than when skidding wood downhill. When skidding small and insufficient loads regarding the size of the skidder, fuel is consumed primarily to obtain energy for the movement of the skidder.

The average fuel consumption for each work operation was as follows: driving the unloaded skidder used 1.65 L; timber winching consumed 1.12 L; driving the loaded skidder used 0.761 L; and operations at the landing consumed 0.80 L.

The highest fuel consumption occurred while driving the unloaded skidder, accounting for 38% of the total. This is primarily because fuel usage during skidder movement is significantly affected by factors such as skidding distance, slope, and skid trail conditions, especially since the unloaded skidder was moving uphill. Timber winching was the second most fuel-intensive operation, using 26% of the total fuel. Driving the loaded skidder consumed 17% of the total fuel, which is less than half of what was used for driving the unloaded skidder.

These findings indicate that load size does not have an impact on fuel consumption. The results also align with FP Innovations’ guidelines for reducing consumption rates, which recommend downhill skidding to improve fuel efficiency [20]. Also, the results are completely different from the findings of Proto et al. [21], where skidder fuel consumption had greater value during extraction (21.3 kg/h) than bunching (18.7 kg/h) due to load size and the number of pieces in the load.

In previous research on skidder fuel consumption in the different terrain conditions, different units were used [21,22,23,24,25], includingliters per unit of extracted timber (L/m³), liters per productive machine hours (L/PMH) or scheduled machine hours (L/SMH), liters per unit of power (L/kWh), and liters per unit weight of the machine (L/ton). In this research, fuel consumption per cycle and working cycle elements are presented in liters per unit of extracted timber (L/m³), which is common in forestry practice.

Fuel consumption (

Table 5. Fuel consumption per unit volume of timber.

Fuel Consumption Per Cycles (L/m3)
Cycle	Day 1	Day 2	Day 3	Day 4	Day 5	Day 6	Day 7	Day 8	Average
1	1.79	1.28	1.92	1.63	5.76	1.44	3.26	2.04	2.39
2	1.27	1.99	1.81	1.20	2.12	1.50	1.50	1.73	1.64
3	0.98	2.01	3.84	2.05	3.13	4.43	2.57	2.17	2.65
4	1.38	1.69	2.36	1.13	3.40	2.83	2.57	2.19	2.19
5	2.17	3.08	1.60	1.34	1.90	3.34	2.58	3.26	2.41
6	1.73	1.80	2.01	1.18	2.33	1.60	1.63	2.94	1.90
7	0.98	1.60	1.26	1.39	1.73	-	-	-	1.39
8	1.54	1.42	1.39	-	-	-	-	-	1.45
Average	1.48	1.86	2.02	1.42	2.91	2.52	2.35	2.39	2.09

) per extracted volume ranges from 0.98 L/m³ to 5.76 L/m³. Average fuel consumption (Table 4) per extracted volume of 2.09 L/m3 is well over the ones reported in previous studies. Borz et al. [26] determined that the fuel consumption of skidders was significantly higher in even-aged forests rather than in selective forests. The average fuel consumption during timber skidding in even-aged forests ranges from 0.62 to 1.00 L/m³ [26], while Kopseak et al. [9] reported higher rates in similar field conditions (from 1.38 to 1.65 L/m³). Janeček and Adamovský [27] concluded that fuel consumption of clambunk skidder ranged from 0.52 to 1.30 L/m³ over a skidding distance 150–730 m.

Figure 2 shows fuel consumption as a function of load volume. The results of this case study imply a very wide range of data that cannot correlate the dependent and independent variables. This confirms, in this case, that fuel consumption does not depend on the volume of the load.

Trend lines of 1 L/m³ and 1.5 L/m³ have been inserted into the data, based on the cited literature sources on skidder fuel consumption during timber extraction in even-aged forests. The data on fuel consumption per unit volume of timber for 19 cycles out of a total of 56 cycles are in the range between 1 L/m³ and 1.5 L/m³, which we can consider acceptable consumption under the tested conditions. In most cycles (39 cycles or 70% of cycles), the fuel consumption per unit volume of timber is too high and can be considered unsatisfactory for the cost analysis of the skidder operation.

Considering the research results, it can be concluded that a large number of small pieces of timber in the load leads to greater time consumption (more time is spent on hanging the load and operating the winch) as well as higher fuel consumption when working at the felling site. Also, an optimal load for that size of the skidder was not provided. The Ecotrac 140 V skidder is not the most suitable solution for extracting wood in the tested conditions of working in thinnings, and the possibility of using a lighter and smaller skidder should be taken into account, which would certainly achieve lower fuel consumption per unit volume of wood.

On the other hand, there is a big organizational problem of Croatian forestry practice in the area of mountain forests. The Forest Management Plan is the basic framework for regulating the management of forests and forest land based on the principle of sustainable production, natural regeneration and permanence of income, while preserving and improving the diversity and other generally useful functions of forests [28]. The forests within forest management areas are managed according to the provisions of the Forest Management Plan, which is drawn up for a period of ten years. In doing so, the total amount of felling is prescribed for each forest department by tree species and economic classes.

Beech and fir selective forests predominate in the mentioned area, and the Forest Service (Forestry Administration Office) primarily procures and uses skidders weighing more than 8 tons due to difficult terrain conditions. Even-aged forests are rarely represented in lower areas, and when implementing the regulations of the Forest Management Plan, especially when carrying out thinning, forestry practice does not have satisfactory forest machinery. It should also be noted that the importance of private forestry contractors, who could address the organizational problems, is significant. Unfortunately, no private enterprises have been developed in the forest sector for wood extraction works in the mentioned sub-region.

Šporčić et al. [29] have presented the state of forestry entrepreneurship in European countries and have established differences in the position of private contractors in Croatia and surrounding countries in relation to Western European forestry practice. Forest entrepreneurs have become irreplaceable actors in carrying out timber harvesting operations, but they are primarily small companies with occasional organizational problems [30]. The territorial distribution of private contractors licensed for forest harvesting operations varies by region, and the minority of entrepreneurs are located in the mountainous region of Croatia [31], where the research was conducted.

4. Conclusions

This case study is part of a complex research of the skidder’s energy consumption under different work tasks and field conditions, which was carried out during 272 working days of the skidder. The case study was chosen because the results of the research clearly indicate certain advantages and disadvantages of skidder operation during timber extraction.

It should be emphasized that the uniqueness and originality of the detailed analysis of skidder fuel consumption according to the working elements of the skidder cycle is such that it provides guidelines for better and more efficient organization of work and reduction in fuel costs.

Analysis of fuel consumption by work components of the shift can provide guidelines for optimal work organization with the aim of reducing work costs. It can be concluded that it is more energy- and cost-efficient to organize the work of the skidder to skid downhill.

In the organization of the skidder’s work, field conditions have a great influence, primarily the processing method and the type of felling. The tested skidder (weighing over 8 tons) is not an optimal solution for pulling wood from early thinning. In the tested conditions, when choosing the means of operation for the wood harvesting system, the use of another forest machine—a skidder of smaller mass and dimensions and/or a smaller forwarder—should have been considered in terms of energy and cost-efficiency.

The problem of the lack of different types of forest machines within state forest enterprise in mountainous region is evident, as well as the importance of private forest harvesting contractors.