Operational properties of forestry mulchers for cleaning field protection forest belts after sanitary cuttings

Abstract

Experimental studies were conducted with forestry mulchers in field protection forest belts and clearings in the North-Eastern region of Bulgaria . Regression models were elaborated to determine the operating performance and fuel consumption of forestry mulchers with a rated power of 70 kW to 245 kW for site preparation of clearings with the amount of wood residues, shoots and bushes from 15 t/ha to 48 t/ha. The mode of influence of the mulcher rotor speed , the concentration of comminuted biomass and the mulching unit power on the performance and fuel consumption were established. When treating clearings with small biomass concentration of 15 t/ha, the productivity of the mulching units with a greater power of 245 kW is 0.392 ha/h, and of those with a smaller power of 70 kW it is 0.086 ha/h. This difference is even greater in clearings with a biomass concentration of 48 t/ha, where the operating productivity of 245 kW mulchers is 0.304 ha/h and that of 70 kW mulchers is 0.021 ha/h. Mulching units with greater power also have a lower relative fuel consumption per unit area. This fact is more pronounced in clearings with a larger amount of wood residues of 48 t/ha, where the fuel consumption of bigger units with a power of 245 kW is 155 l/ha, of units with a medium power of 160 kW it is 217 l /ha, and of units with a smaller power of 70 kW it is 335 l/ha. For treating clearings with relatively less wood residues and shrubs of 15 t/ha, depending on the rated power of mulching units, these fuel costs are respectively 86 l/ha, 122 l/ha and 214 l/ha. The optimal power and rotor revolutions of forestry mulchers when processing cuttings with different concentrations of woody biomass were determined.

Full text

Silva Balcanica 24(2): 59-81 (2023)
doi: 10.3897/silvabalcanica.24.e109161
Copyright Georgi Georgiev. This is an open access article distributed under the terms of the Creative Commons
Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided
the original author and source are credited.
RESEARCH ARTICLE
Operational properties of forestry mulchers
for cleaning field protection forest belts after sanitary
cuttings
Konstantin Marinov, Konstantin Kostov, Dimitar Peev
University of Forestry, 10, “St. Kliment Ohridski” Blvd., Soa, Bulgaria
Corresponding author: Konstantin Marinov (ggeorgiev.fri@gmail.com)
Academic editor: Georgi Georgiev | Received 7 July 2023 | Accepted 26 September 2023 | Published 10 November 2023
Citation: Marinov K., Ko stov K., Peev D. 2023. Oper ational proper ties of forestr y mulchers for clea ning eld protec tion
forest belts a er sanitar y cuttings. Silva Balcanica 24(2): 59-81. https://doi.org/10.3897/silvabalcanica.24.e109161
Abstract
Experimental studies were conducted with forestry mulchers in eld protection forest belts and clear-
ings in the North-Eastern region of Bulgaria . Regression models were elaborated to determine the
operating performance and fuel consumption of forestry mulchers with a rated power of 70 kW to 245
kW for site preparation of clearings with the amount of wood residues, shoots and bushes from 15 t/ha
to 48 t/ha. e mode of inuence of the mulcher rotor speed , the concentration of comminuted biomass
and the mulching unit power on the performance and fuel consumption were established. When treat-
ing clearings with small biomass concentration of 15 t/ha, the productivity of the mulching units with
a greater power of 245 kW is 0.392 ha/h, and of those with a smaller power of 70 kW it is 0.086 ha/h.
is dierence is even greater in clearings with a biomass concentration of 48 t/ha, where the operat-
ing productivity of 245 kW mulchers is 0.304 ha/h and that of 70 kW mulchers is 0.021 ha/h. Mulching
units with greater power also have a lower relative fuel consumption per unit area. is fact is more
pronounced in clearings with a larger amount of wood residues of 48 t/ha, where the fuel consumption
of bigger units with a power of 245 kW is 155 l/ha, of units with a medium power of 160 kW it is 217 l
/ha, and of units with a smaller power of 70 kW it is 335 l/ha. For treating clearings with relatively less
wood residues and shrubs of 15 t/ha, depending on the rated power of mulching units, these fuel costs
are respectively 86 l/ha, 122 l/ha and 214 l/ha. e optimal power and rotor revolutions of forestry
mulchers when processing cuttings with dierent concentrations of woody biomass were determined.
Keywords
comminuting, woody biomass, performance, fuel consumption, power, regression models

60Konstantin Marinov, Konstantin Kostov, Dimitar Peev / Silva Balcanica 24(2): 59-81 (2023)
Introduction
Mulching technologies have indisputable ecological and technological advantages
compared to the bulldozer technologies for cleaning forest areas and clearings ap-
plied so far in Bulgaria (Marinov 2014; Glushkov et al., 2021). Forestry mulchers
are universal brush cutters for cleaning forest areas from undesirable tree and shrub
vegetation, for comminuting wood residues from logging, for cleaning re lines, elec-
trical overhead and underground cables, etc. ey have a milling working body that
chop the wood into chips that spread evenly over the surface of the land. Depending
on the rotor diameter, they can cut and shred trees, shrubs and stumps with a diam-
eter of up to 25-50 cm. Compared to other brush cutters, mulchers have wider tech-
nological capabilities (Marinov, 2021a). When operating with mulchers, there is no
need to collect and transport the processed wood, which reduces the overall produc-
tion costs (Jordanova and Marinov, 2015; Löf et al., 2012; Marinov and Jordanova,
2017a;). In addition, the risk of wildres and development of diseases and pests is
reduced (Hallbrook et al., 2006; Fornwalt et al., 2017). e mulchers are suitable for
clearing forest areas from wild res, windstorms, oods and other abiotic damages
to the forests (Von der Gönna, 1992; Löf et al., 2016; Smidt et al., 2019). e main
disadvantages of mulchers are their limited application into very stony terrain, as well
as the higher consumption of energy and fuel, compared to other rotary brush cutters
with disc and chain working bodies (Popikov et al., 2011).
From the research carried out with rotary brush cutters and mulchers, it was found
that higher rotor revolutions lead to lower energy costs and fuel consumption (Bukhto-
yarov et al., 2021; Furmanov et al., 2020; Marinov, 2019a). At working speed of the
mulching unit from 0.05 km/h to 1.0 km/h and rotor revolutions of 1000-2000 rpm, the
chips thickness is between 1.5 mm and 30 mm (Kostov, 2019a; Marinov and Kostov,
2019). Smaller chips decompose and assimilate faster in the upper soil layer, improv-
ing the organic composition and fertility of the soil. To reduce fuel load in high-re-
risk forests, comminuted wood fragments are recommended to be thicker than 3” or
7.62 cm (Fight and Barbour, 2004; Reiner et al., 2009; Fornwalt et al., 2017). erefore,
higher operating speeds of mulching units are recommended in such forests – 2.5-5
km/h. Depending on the rotor diameter and the power of the mulching unit, mulchers
can shred stumps and tree stems with a diameter of 15 to 60 cm (FAE, AHWI).
Modern technologies for site preparation for reforestation with forestry mulchers
and tillers, were introduced in Bulgaria relatively recently – from 2014. e research
carried out so far in Bulgaria for poplar clearings preparation with a self-propelled for-
estry unit PT-400 and a multitask milling machine FAE 300/S, showed the relatively
high technological and operational properties of this type of machine (Jordanova and
Marinov, 2015; Marinov et al., 2017; Marinov, 2019b; Marinov and Stefanov, 2019; Ma-
rinov, 2021b). e results obtained from the research of Glushkov et al. (2021) and Ma-
rinov and Jordanova (2017) establish that compared to the traditional technology with
bulldozer units, the new mulching technology is distinguished by higher economic and
environmental indicators. Research was also carried out with milling machines for soil
preparation of a non-renewed forest area in the lower at-hilly forest vegetation belt

Operational properties of forestry mulchers for cleaning field protection forest belts...61
in Bulgaria. In one of the rst studies with such machines, it was found that mulchers
have about twice the shi productivity and 30% lower fuel consumption compared to
universal bulldozers (Marinov, 2014). e research by Kostov (2019a and 2019b) and
Marinov and Kostov (2019) show that, compared to traditional technologies for soil
preparation with dozers and plows, milling machines provide higher quality and lower
time and labor costs. In the study by Kostov (2019c) of a forestry tiller FAE SSM/HP for
soil preparation in clearings with wood residues 36 t/ha, and stumps with diameter 18
cm and density 31 pcs/da, it was established that the cost time of the new technology is
33.8 h/ha, and according to the traditional one – 98.5 h/ha. e average price according
to the new technology is 2180 €/ha, and according to the traditional one – 2955 €/h. In
a recent research by Marinov and Kostov (2022) on forestry tillers for deep tillage up
to 40 cm and shredding of stumps and roots of hard broad-leaved species of dierent
diameter and density, it was established that of all the signicant factors, the diameter
has the strongest inuence on the fuel consumption and performance of the milling
machines. In the study of Berude et al. (2021) on the production costs from applying
a mulching technology to growing young Scots pine plantations, it was found that, in
addition to better economic indicators, higher stand growth was also achieved. Bukhto-
yarov et al. (2021) studied the eectiveness of a rotary brush cutter for cleaning shoots
and brush up to 3 m in height, and up to 2 cm in diameter, in young forest plantations,
establishing that higher rotor speeds resulted in lower energy costs. In the previous
research published by Bulgarian and foreign authors on the cleaning of eld protection
forest belts and clearings in the lower forest vegetation belt of oak forests, the inuence
of the amount of comminuted wood, the power of the drive unit, and the rotor rotation
frequency of the forestry mulchers, have not yet been suciently well studied.
e aim of this study is to establish the way the rotor rotation speed, the concen-
tration of woody biomass, and the rated power of the mulching unit, all inuence the
main operational properties of forestry mulchers for cleaning eld protection belts
and clearings.
Materials and Methods
e methods of mathematical modeling, regression analysis and planning of the ex-
periment were used. To dene the operating conditions, the experimental trial areas
method wa s used.
Location and objects of study
e experimental areas with testing polygons are located in the cuttings of eld pro-
tection forest belts on the territory of the North-East State Forest Enterprise of Shu-
men. e main tree species are sessile oak, turkey oak, black locust and ash. Accord-

62Konstantin Marinov, Konstantin Kostov, Dimitar Peev / Silva Balcanica 24(2): 59-81 (2023)
ing to the classication scheme of the habitats in Bulgaria, the experimental areas fall
into the Myzian forest vegetation area, in the lower plain-hilly belt, sub-belt of the oak
forests – M-I-2, D2,3 (Raikov et al. 2011) e terrains are at to sloping with a slope of
4-10 degrees. e altitude is 220-350 m. e boundary coordinates of the experimen-
tal areas are 43°06.42 – 43°99.31 N and 27°21.78 – 28°58.28 E.
e trial areas are located on the territory of the following forestry units and divi-
sions:
•State Forestry of Dobrich: divisions: 145-д1; 190-1; 224-м; 1072-а,-б1;
•State Forestry of Tervel: divisions: 19-б,з; 67-г; 114-д,и,р; 142-б; 318-а;
•State Forestry of Balchik: divisions: 1714-б; 2004-в; 2026-а; 2077-а,б; 2078-а; 2079а;
2104-б; 2189-в; 2365-а,б,в,г; 2799-а; 43-14;
•State Forestry of General Toshevo: divisions: 115-п; 154-и; 1681-а,б; 1790-а;
•State Forestry of Varna: divisions: 246-ж; 1853-а.
e objects of the research are dierent mulching units for comminuting wood
waste, shoots and brushes in clearings and eld protection forest belts. e subject of
research are the operational properties of the machines, such as operating productiv-
ity and relative fuel consumption. e following mulching aggregates were studied:
*Forestry mulcher AHWI 550m with working width Bр = 2.4 m, driving by Fendt 936
Vario tractor with rated power Nnom = 243 kW/330 h.p. (Fig. 1A);
*Forestry mulcher FAE SFM 225 with working width Bp = 2.25 m, driving by Fendt 933
Vario tractor with rated power Nnom = 221 kW/300 h.p.;
*Forestry mulcher SEPPI M/225 with working width Bp = 2.25 m, driving by Valtra
T214 tractor with rated power Nnom = 158 kW/215 h.p.;
*Forestry mulcher FAE UML/S/DT 225 with working width Bp = 2.25 m, driving by John
Deere 6155R tractor with rated power Nnom = 116 kW/155 h.p;
*Forestry mulcher FAE UML/VT 175 with working width Bp = 1.82 m, driving by Bob-
cat Т770 mini track loader with rated power Nnom = 69 kW/92 h.p. (Fig. 1B).
Parameters and conditions of the study
Input parameters. Rotor speed is a major factor that can be managed and maintained
at dierent levels when conducting a mixed active-passive experiment with forestry
mulchers. e input factors, which during the study cannot be managed, but can be
controlled and maintained at certain levels, form the group of unmanaged controlled
factors. ese factors include the technical indicators of the machines and the work-
ing conditions – slope of the terrain, woody species, concentration of the wood bio-
mass, working width, rated power of the unit, technical condition of the machines,
etc. Factors that can be held constant at dierent levels are suitable for conducting
passive and active-passive experiments. e controlled input factors are established
at levels corresponding to the working conditions in the lower forest vegetation zone
of the oak forests in Bulgaria.

Operational properties of forestry mulchers for cleaning field protection forest belts...63
e concentration of wood biomass per unit area is a major factor in the study
of forestry mulchers. It is dened as the sum of the remaining wood residue mass –
branches, tree tops and defective wood, and the mass of standing vegetation – trees
and shrubs. is mass can be measured and established in any experimental area. To
establish the amount of mulched woody mass per unit area, the method of the trial
sites, in which control samples are composed, is used. For this purpose, on an area of
1 decare, 25 trial sites with dimensions of 2x2 m were set evenly. e number of trial
sites is such that their total area should cover 10% of the overall experimental area. At
each site, control samples of mulched wood chips are drawn up and weighed on site.
e mean wood biomass in each plot was determined as an average weighted value.
e rotor rotation speed as a managed factor is studied in the range of 500 min-
1 to 1000 min-1. It is good that this factor has a larger variation interval, which will
reduce the probability of obtaining inadequate models, and confounding factors will
have a less signicant inuence on the accuracy of the obtained results (Vuchkov and
Stoyanov, 1986). From preliminary trials, it was found that at lower rotor speeds,
relative fuel consumption was greater and performance was lower. For this reason,
the lower level of this factor is assumed to be 500 min-1 and the upper level to be at the
maximum rotor revolutions of 1000 min-1.
Output parameters. e initial parameters Yj were chosen in accordance with
the purpose of the study and express the main operating properties of the mulch-
ers. e following indicators were studied as such parameters: Y1 – operating hourly
productivity, Wh [da/h]; Y2 – relative fuel consumption, Go [l/da]. e operating pro-
ductivity Y1 is determined by the duration of the process and mulching area by the
formula,
Wh = Y1 = F, da/h (1)
t
Where F is the mulching area, da; t is the duration of the process, h.
Figure 1. Forestry mulchers for comminuting wood waste, stem and shrubs (Photo by K.
Kostov)

64Konstantin Marinov, Konstantin Kostov, Dimitar Peev / Silva Balcanica 24(2): 59-81 (2023)
e relative fuel consumption Y2 is an essential indicator of the operating prop-
erties of forestry mulchers and has a major weight in determining the cost of site
preparation. It is used as the main operating criteria for nding optimal technological
solutions (Marinov, 2021). e relative fuel consumption is expressed by the amount
of fuel that is consumed per unit of mulched area, Q
Go = Y2 = Q, l/dka, (2)
F
Where Q is the amount of the fuel consumed, l.
e mass of the mulched materials is measured with an electronic “Kern” scale,
with an accuracy of 1 g. e fuel consumed is determined by the on-board computer
of the tractors. An electronic stopwatch with an accuracy of 0.1 sec is used to measure
the time of the operations. Garmin-550 GPS is used to measure areas. To measure
linear dimensions, a Bosch GLM 80 laser tape measure is used, with a range of up to
80 m and an accuracy of ±1 mm, and a Yucon Extend LRS-1000 laser rangender,
with a range of up to 1000 m and an accuracy of ±1 m.
Design of experiments (DOE)
From the studies carried out so far with forestry milling brush cutters in poplar cut-
tings, it was established that the obtained dependencies for determining the relative
fuel consumption, as functions of the response of the input factors, are described by
second-order polynomials
(Marinov 2019b). Active-passive and passive experiments with two input factors
are designed to determine the inuence of the rated power of the mulching unit, the
concentration of the mulching wood biomass and the rotor speed.
Design of a two-factor active-passive experiment. It is used to establish the joint
impact of the rotor speed and wood biomass concentration on the operating produc-
tivity and relative fuel consumption of forestry mulchers. e rst factor rotor rota-
tion frequency is actively managed, while the second factor wood biomass concentra-
tion is maintained at several levels. e study was conducted with an AHWI 550m
forestry mulcher driven by a Fendt 936 Vario tractor.
e input factors and variation levels have the following notations and levels:
Х1 is the rotor rotation frequency: 5 levels of variation: 500, 625, 750, 875 and 1000 min-1.
Х2 is the concentration of mulched woody mass: 4 levels of variation: 15, 25, 36 and 48 t/ha.
e unmanaged input factors that are kept at one level are: rated power of the
mulching unit Nnом = 243 kW; working width В = 2.4 m; wood species – turkey oak,
sessile oak, black locust and ash; terrain slope 4-8 degrees.
e design of a complete two-factor active-passive experiment to determine the
inuence of rotor speed and woody biomass concentration on the operating produc-
tivity and fuel consumption of an AHWI 550m forestry mulcher driven by a Fendt
936 tractor is given in Table 1. To carry out the calculation procedures, the natural
values are replaced by coded ones.

Operational properties of forestry mulchers for cleaning field protection forest belts...65
Design of a two-factor passive experiment. is experiment aims to establish
the joint inuence of the rated power of the mulching units and the concentration
of the woody biomass on the operating properties of the mulchers. e study is con-
ducted with mulching units mentioned above. e experimental areas are located in
clearings with a woody biomass concentration of 15 t/ha, 25 t/ha, 36 t/ha and 48 t/ha.
e other input factors, such as wood species of the mulched wood, working width,
terrain slope, etc., are kept at constant levels.
e controlled input factors and their levels of variation are:
Х1 – rated power of the mulching unit: 5 levels of variation: 69, 116, 158, 221 and
243 kW.
Х2 – concentration of wood mass per hectare: 4 levels of variation: 15, 25, 36 and
48 t/ha.
e design of a complete two-factor passive experiment with forestry mulching
units of dierent rated power is given in Table 1.
Table 1. Design of experiments
№ of
exp.
Active-passive experiment Passive experiment Output parameters
Input factors
Natural form Coded form Natural form Coded form Operating
productivity
Relative fuel
consumption
X1
min-1
X2
t/ha X1 X2 X1
kW
X2
t/ha X1 X2 Y1
da/h
Y2
l/da
1 500 15 -1 -1 69 15 -1 -1 Y1,i,j Y2,i,j
2 625 15 -0.5 -1 116 15 -0.460 -1 …… ……
3 750 15 0 -1 158 15 0.023 -1 …… ……
4 875 15 0.5 -1 221 15 0.747 -1 …… ……
5 1000 15 1 -1 243 15 1 -1 …… ……
6 500 25 -1 -0.394 69 25 -1 -0.394 …… ……
7 625 25 -0.5 -0.394 116 25 -0.460 -0.394 …… ……
8 750 25 0 -0.394 158 25 0.023 -0.394 …… ……
9 875 25 0.5 -0.394 221 25 0.747 -0.394 …… ……
10 1000 25 1 -0.394 243 25 1 -0.394 Y1,i,j Y2,i,j
11 500 36 -1 0.273 69 36 -1 0.273 …… ……
12 625 36 -0.5 0.273 116 36 -0.460 0.273 …… ……
13 750 36 0 0.273 158 36 0.023 0.273 …… ……
14 875 36 0.5 0.273 221 36 0.747 0.273 …… ……
15 1000 36 1 0.273 243 36 1 0.273 …… ……
16 500 48 -1 1 69 48 -1 1 …… ……
17 625 48 -0.5 1 116 48 -0.460 1 …… ……
18 750 48 0 1 158 48 0.023 1 …… ……
19 875 48 0.5 1 221 48 0.747 1 …… ……
20 1000 48 1 1 243 48 1 1 …… ……

66Konstantin Marinov, Konstantin Kostov, Dimitar Peev / Silva Balcanica 24(2): 59-81 (2023)
From the research carried out so far in Bulgaria with a self-propelled unit РТ-400
with a multifunctional forestry milling machine FAE 300/S for soil preparation of
poplar clearings (Marinov, 2019b), it was established that the models of the studied
parameters, are described by second-order polynomials. For this reason, the same hy-
pothesis is assumed here as well, that the regression models for operating productiv-
ity and fuel consumption, as a functions of response, are described by second-order
polynomials, according to the following relationship:
 
𝑦𝑏
0 + ∑ 𝑏𝑖𝑥𝑖+ ∑ 𝑏𝑖𝑗𝑥𝑖𝑥𝑗+ ∑ 𝑏𝑖𝑖𝑥𝑖
2(3)
𝑖 𝑖<𝑗 𝑖
Where -1 ≤ xi ≤ 1 are the coded values input factors, i =1,2,…,m; m = 2 are the
number of input factors; b0 is the regression coecient of the free term; bi are the
regression coecients of the linear terms; bij are the regression coecients of the in-
teraction between the linear terms; bii are the regression coecients of squared terms.
Regression coecients are estimated at a signicance level of α = 0.05. e ad-
equacy of the obtained regression models is performed by evaluating the signicance
of the multiple correlation coecient R (Vuchkov and Stoyanov, 1986). For these
models to be usable, the multiple correlation coecient R should be as close as pos-
sible to 1.0. In addition to the R coecient, the quality of the regression models is
also assessed by the F-criteria. e condition for signicance of the coecient R is
checked according to the following criteria:
➢ If the condition F > Ftable (α,ν1,ν2) is met , then the multiple correlation coecient
R is signicant, the model is adequate and can be used to predict the studied parameters;
➢ If the inequality F ≤ Ftable (α,ν1,ν2) is met, then the multiple correlation coef-
cient R is insignicant and the model is inadequate. is may be due to incorrect
selection of a lower order model or the presence of other strongly inuencing factors
not included in the model sought.
Optimization
Optimization can be carried out with an operational criterion (Marinov, 2021). For
this purpose, single-objective optimization is used to nd the minimum value of the
relative fuel consumption. From the previous studies with multitask forestry milling
machines in poplar clearings it was found that the single-objective optimization with
the gradient algorithm method gave optimal solutions, which did not dier signi-
cantly from those with multi-objective optimization, where operating productivity is
used as the second objective function (Marinov, 2019b; Marinov and Kostov, 2022).
Optimization is performed aer establishing adequate regression models for the rela-
tive fuel consumption function. It is carried out using the method of the gradient
algorithm, in which the minimum of the function y2 is sought,
ŷ2 → min. (4)

Operational properties of forestry mulchers for cleaning field protection forest belts...67
Results and discussion
e research was carried out in the period 2019-2021. Active-passive and passive ex-
periments were conducted in accordance with the research methodology.
Results and analysis of the two-factor active-passive experiment
is experiment was conducted with an AHWI 550m forestry mulcher driven by a
Fendt 936 Vario tractor with rated power of 220 kW. e rst managed input fac-
tor Х1 – rotor rotation frequency is regulated in the interval from 500 min-1 to 1000
min-1, in 125 min-1. e second input factor X2 – concentration of woody mass in the
clearings was established and maintained at levels of 15 t/ha, 25 t/ha, 36 t/ha and 48
t/ha. e obtained results were used to elaborate regression models and to conduct
single-objective optimization to establish the optimal rotor speed. e experimental
results obtained for the output parameters Y1 – operating productivity and Y2 – rela-
tive fuel consumption are given in Table 2.
Table. 2. Results of the active-passive experiment with an AHWI 550m forestry mulcher
№ of
exp.
Input factors Output parameters from
experiments
Output parameters from
regression models
Natural form Coded form Operating
productivity
Relative fuel
consumption
Operating
productivity
Relative fuel
consumption
X1 min-1 X2
t/ha X1 X2 Y1
da/h
Y2
l/da y1 da/h y2 l/da
1 500 15 -1 -1 2.93 16.65 2.91853921 16.53485046
2 625 15 -0.5 -1 3.19 13.58 3.183936555 13.57718643
3 750 15 0 -1 3.42 11.22 3.41790533 11.2227367
4 875 15 0.5 -1 3.61 9.37 3.620445535 9.47150127
5 1000 15 1 -1 3.79 8.25 3.79155717 8.32348014
6 500 25 -1 -0.394 2.84 17.59 2.858796039 17.48084187
7 625 25 -0.5 -0.394 3.09 14.52 3.105912155 14.62384513
8 750 25 0 -0.394 3.33 12.35 3.321599701 12.37006268
9 875 25 0.5 -0.394 3.51 10.69 3.505858677 10.71949454
10 1000 25 1 -0.394 3.66 9.88 3.658689082 9.67214069
11 500 36 -1 0.273 2.71 19.18 2.717060451 19.43583415
12 625 36 -0.5 0.273 2.95 16.62 2.944055147 16.68963786
13 750 36 0 0.273 3.16 14.56 3.139621274 14.54665587
14 875 36 0.5 0.273 3.31 13.11 3.303758831 13.00688819
15 1000 36 1 0.273 3.43 12.13 3.436467818 12.0703348
16 500 48 -1 1 2.49 22.72 2.47189021 22.65733254
17 625 48 -0.5 1 2.67 20.13 2.676953465 20.03190377
18 750 48 0 1 2.83 18.05 2.85058815 18.0096893
19 875 48 0.5 1 2.99 16.65 2.992794265 16.59068913
20 1000 48 1 1 3.11 15.56 3.10357181 15.77490326

68Konstantin Marinov, Konstantin Kostov, Dimitar Peev / Silva Balcanica 24(2): 59-81 (2023)
e obtained experimental results were subjected to statistical processing with
the computational program for regression analysis QstatLab. Aer stepwise analysis
and exclusion of non-signicant regression coecients, adequate models were ob-
tained. To determine the operating productivity ŷ1 and relative fuel consumption ŷ2,
as functions of the response to the input factors X1 and X2, the following regression
models were obtained:
ŷ1 = 3.2238 + 0.3763X1 – 0.2837X2 – 0.0629X1X1 – 0.0895X2X2 – 0.06039X1X2, (5)
ŷ2 = 13.54 – 3.7735X1 + 3.3935X2 + 1.2064X1X1 + 1.0762X2X2 + 0.3323X1X2, (6)
To assess the adequacy of the obtained models in equations (5) and (6) and their
suitability for predicting the output parameters, according to the accepted methodol-
ogy, the coecient for multiple correlation R and F-criterion of the Fisher distribu-
tion were used. e obtained results of the variance analysis for evaluating the quality
of the models ŷi are given in Table 3.
Table 3. ANOVA analysis of ŷi functions of response from active-passive experiments
ANOVA
Source Sum of squares Degrees of
freedom
Mean squared error F Р
ŷ1 – operating productivity, da/h
Model 2.39677 5 0.47935 2784.00027 1.838E-20
Residual 0.00241 14 0.00017
Tot al 2.39918 19
T(0.025,14)= 2.14479; F(0.050,5,14)=2.95825; Residual СКО= 0.013122; R-sq=0.999; Radj-sq=0.99864; PRESS =
0.00576; R-sq(pred)=0.9976
ŷ2 – relative fuel consumption, l/da
Model 282.56099 5 56.51220 3220.05895 6.6446E-21
Residual 0.24570 14 0.01755
Tot al 282.80669 19
T(0.025,14)= 2.14479; F(0.050,5,14)=2.95825; Residual СКО= 0.13248; R-sq=0.99913; Radj-sq=0.99882; PRESS =
0.67850; R-sq(pred)=0.9985
e results of the analysis of variance show that the multiple correlation coef-
cients R of the two models are signicant. From these results, it can be seen that the
F-criterion values are greater than the tabular ones determined at the corresponding
degrees of freedom, with the condition F > Ftable (α,ν1,ν2). e squares of the coef-
cients of multiple correlation R2 are close to being equal (for y1 → R-sq = 0.999 and
for y2 → R-sq = 0.99913), which means that according to the principles of variance
analysis, all signicant inuences in the studied process were taken into account. e
squares of the coecients of determination of the predicted R-sq (pred) values also
have high values. e estimate for operating productivity is R-sq (pred) = 0.9976 and
for fuel consumption is R-sq (pred) = 0.9985. is means that in the condence inter-

Operational properties of forestry mulchers for cleaning field protection forest belts...69
val 0.95, the obtained regression models predict the mulching process with sucient-
ly high accuracy. From the conducted statistical analysis, it can be concluded that the
proposed regression models in equations (5) and (6) are adequate and describe the
studied output parameters with suciently high accuracy. ey can be used to deter-
mine the operating productivity and relative fuel consumption of forestry mulchers
with rated power of 220 kW in the studied interval of variation of the input factors Х1
и Х2. e calculated values of the output parameters y1 and y2 obtained from regres-
sion models are given in Table 2. From these models, graphs were plotted in gure 2
and gure 3 depicting the variation of operating productivity and fuel consumption
as a function of rotor speed revolutions and woody biomass concentration.
e obtained results show that both studied factors have a strong inuence on
the operational properties of forestry mulchers. Regardless of woody mass concen-
tration, an increase in rotor speed leads to an increase in productivity and a decrease
in relative fuel consumption. As the amount of mulched wood mass increases, pro-
ductivity naturally decreases, and fuel consumption per unit area increases, but the
dynamics of this change are dierent. e obtained results show that the inuence of
woody mass is weaker in clearings with a concentration of up to 20-25 t/ha, while in
those with a biomass concentration of more than 30-35 t/ha, this inuence is more
pronounced. For example, at a rotor speed of 500 rpm, the relative fuel consumption
in cuttings with biomass from 15 t/ha to 25 t/ha increases from 16.5 l/da to 17.5 l/
da, which is about 1.0 l/ da more, while in cuttings with larger amounts of woody
biomass from 36 t/h to 48 t/ha, this consumption increases from 19.4 l/da to 22.7 l/
Figure 2. Variation of the operational performance of an AHWI 550 forestry mulcher de-
pending on the rotor revolutions and the concentration of woody biomass

70Konstantin Marinov, Konstantin Kostov, Dimitar Peev / Silva Balcanica 24(2): 59-81 (2023)
da, which is about 3.3 l/da more, or almost 3.3 times. At the same levels of woody
biomass concentrations, but at a maximum rotor speed of 1000 rpm, the relative fuel
consumption increases from 8.3 l/da to 9.7 l/da, which is 1.4 l/da more and from 12.1
to 15.8 l/da, which is 3.7 l/da more, or almost 2.7 times. From the graph in Figure 4,
it can be seen that as the rotor speed increases from 500 rpm to 1000 rpm, the slope
of the relative fuel consumption curve decreases and towards the end the inuence of
this factor begins to weaken, but nevertheless it is seen that it has not yet reached its
minimum. e evolution of the curve outside the studied factor space indicates that
the fuel consumption will continue to decrease until about 1500-2000 rpm. Regard-
ing the operating productivity, depicted in the graph in gure 3, it can be seen that its
curve develops relatively more smoothly and with a smaller drop until reaching the
maximum rotor speed of 1000 rpm. Here it is seen that the performance will contin-
ue to increase at a speed above 1500-2000 rpm. Such an increase in speed is likely to
lead to an increase in fuel consumption and to greater dynamic loads in the working
bodies and transmission of the mulchers. However, further studies with higher rotor
revolutions above 1000 rpm are needed to conrm these hypotheses.
According to the optimization methodology, i n order to determine the optimal
rotor revolutions in the studied range from 500 to 1000 rpm, it is necessary to nd the
minimum of the relative fuel consumption of the mulching unit for cleaning clear-
ings with dierent concentrations of woody biomass. For this purpose, the area is
sought in which the response surface of the objective function y2 will acquire a mini-
mum value. e optimization is carried out according to the method of the gradient
Figure 3. Variation of the relative fuel consumption of an AHWI 550 forestry mulcher de-
pending on the rotor revolutions and the concentration of woody biomass

Operational properties of forestry mulchers for cleaning field protection forest belts...71
algorithm, by setting dierent values of the controllable factor X1 – rotor rotation
frequency, with established constant levels of the factor X2 – concentration of woody
biomass from 15 t/ha to 48 t/ha. e calculations continue until the minimum value
of the objective function 𝑦2 is found. QstatLab soware is used for the calculation
procedures. Aer processing the results and nding the minimum of the function
y2, optimal solutions for the variable X1 were obtained at the basic, upper and lower
levels of the factor X2. e obtained optimal solutions are given in Table 4.
e results show that the relative fuel consumption of the mulching unit is the
smallest at the maximum speed of the rotor 1000 min-1 and the upper level of the fac-
tor X1 = 1. is speed is optimal for all levels of wood mass concentration. At the min-
imum concentration 15 t/ha and upper level of the input factor Х2 = -1, the function y2
for the relative fuel consumption is Go = 10.97 l/da. At wood mass concentration 31.5
t/ha and basic level of factor X2 = 0, fuel consumption is Go = 8.32 l/da, at 48 t/ha and
upper level of factor X2 = 1, this consumption reaches to Go = 15.77 l/da. At this speed,
the maximum performance of the mulcher has been established. At a lower level of
X2 and a minimum concentration of wood mass, the operating productivity naturally
is the highest Wh = 3.792 da/h, at a basic level X2 = 0 it is Wh = 3.537 da/h and at an
upper level X2 = 1 the productivity is the smallest Wh = 3.103 da/h.
With a rotor diameter 550 mm of an AWHI 550m mulcher and an optimal rotor
revolutions 1000 rpm, the peripheral speed of the working body and cutting speed is
equal to vо= 28.8 m/s. At the operating speed of the mulching unit vр = 1.5 km/h, the
kinematic index of milling is λ = 68.5.
e graphical results for the objective function surface y2 and the diagram of lines
with constant values of the relative fuel consumption of the AHWI 550m forestry
mulcher with a Fendt 936 Vario tractor are depicted in Figure 4.
Table 4. Results of the single-objective optimization to nd the optimal rotor speed

72Konstantin Marinov, Konstantin Kostov, Dimitar Peev / Silva Balcanica 24(2): 59-81 (2023)
Results and analysis of the two-factor passive experiment
e rst input factor X1 rated the power of the mulching unit, and is established and
maintained at 5 levels in the interval from 69 kW to 243 kW. e second factor X2 –
concentration of woody biomass, like the active-passive experiment, is maintained
at 4 levels – 15 t/ha, 25 t/ha, 36 t/ha and 48 t/ha. From the optimization carried out
in the previous active-passive experiment, an optimal rotor revolution of 1000 rpm
was established. For this reason, in this experiment the rotor speed is kept constant at
an optimal value of 1000 rpm. e obtained experimental results for operating pro-
ductivity Y1 , and relative fuel consumption Y2 of mulching units of dierent power
when working in clearings with dierent concentrations of woody biomass, are given
in Table 5.
e obtained results of the experimental observations were subjected to statistical
processing with the QstatLab regression analysis program. Aer a stepwise analysis
to determine the signicance of the regression coecients bi, bij and bii at a signi-
cance level of α = 0.05, the following regression models were obtained to determine
the operating productivity and relative fuel consumption as functions of the response
of the input factors Х1 and Х2:
𝑦1 = 2.409 + 1.474Х1 – 0.383Х2 – 0.334Х1Х1 – 0.065Х2Х2 – 0.057Х1Х2, (7)
𝑦2 = 15.956 – 7.710X1 + 4.772X2 + 2.631X1X1 + 1.169Х2Х2 – 1.315Х1Х2 (8)
e adequacy of the obtained models and their suitability for predicting the out-
put quantities 𝑦1 и 𝑦2 were evaluated by means of the multiple correlation coecient
R and the F test. e obtained results of variance analysis of the models ŷi are given
in Table 6.
Figure 4. Surface of the objective function y2 and diagram of lines with constant values of
relative fuel consumption of AHWI 550 forestry mulcher with Fendt 936 Vario tractor

Operational properties of forestry mulchers for cleaning field protection forest belts...73
Table 6. ANOVA analysis of 𝑦i functions of response from passive experiments
ANOVA
Source Sum of squares Degrees of
freedom
Mean squared error F Р
𝑦1 – operating productivity, da/h
Model 25.44640 5 5.08928 4776.22892 4.2164E-22
Residual 0.01492 14 0.00107
Tot al 25.46132 19
T(0.025,14)= 2.14479; F(0.050,5,14)=2.95825; Residual СКО = 0.032643; R-sq = 0.99941; Radj-sq= 0.9992; PRESS
= 0.03618; R-sq(pred) = 0.99858
𝑦2 – relative fuel consumption, l/da
Model 908.46414 5 181.69283 1972.26427 2.0454E-19
Residual 1.28974 14 0.09212
Tot al 909.75388 19
T(0.025,14)= 2.14479; F(0.050,5,14)=2.95825; Residual СКО = 0.30352; R-sq = 0.99858; Radj-sq = 99808; PRESS
= 3.80057; R-sq(pred) = 0.99582
Table 5. Results of the 2-factor passive experiment
№
of
exp
Input factors Output parameters from
experiments
Output parameters from
regression models
Natural form Coded form Operating
productivity
Relative fuel
consumption
Operating
productivity
Relative fuel
consumption
X1 kW X2
t/ha X1 X2 Y1
da/h
Y2
l/da
y1
da/h
y2
l/da
1 69 15 -1 -1 0.862 21.21 0.86176255 21.3794047
2 116 15 -0.46 -1 1.946 15.95 1.952215745 15.85230676
3 158 15 0.023 -1 2.808 12.53 2.762271362 12.20858162
4 221 15 0.747 -1 3.687 8.84 3.684181333 9.045834986
5 243 15 1 -1 3.914 8.32 3.92365107 8.5909323
6 69 25 -1 -0.394 0.685 24.05 0.719059813 24.08020386
7 116 25 -0.46 -0.394 1.768 18.38 1.790757162 18.12265151
8 158 25 0.023 -0.394 2.603 14.34 2.584036717 14.09390881
9 221 25 0.747 -0.394 3.464 10.42 3.480799962 10.35403442
10 243 25 1 -0.394 3.676 9.78 3.711482238 9.69745587
11 69 36 -1 0.273 0.525 27.75 0.506776942 28.04583298
12 116 36 -0.46 0.273 1.551 21.12 1.557830481 21.61449666
13 158 36 0.023 0.273 2.365 17.21 2.332645295 17.16198051
14 221 36 0.747 0.273 3.232 12.74 3.201730542 12.78688464
15 243 36 1 0.273 3.431 12.13 3.42274081 11.90832952
16 69 48 -1 1 0.258 34.15 0.20949515 33.5533529
17 116 48 -0.46 1 1.184 26.35 1.238047864 26.60561335
18 158 48 0.023 1 1.957 21.56 1.99273694 21.69120321
19 221 48 0.747 1 2.837 16.42 2.831654415 16.62374449
20 243 48 1 1 3.053 15.67 3.04212263 15.5032449

74Konstantin Marinov, Konstantin Kostov, Dimitar Peev / Silva Balcanica 24(2): 59-81 (2023)
e results of the analysis indicate that the multiple correlation coecients R of both
models are signicant. From the obtained estimates in Table 6, it can be seen that
the calculated values of the F-distribution are greater than the tabular Ftable (α,ν1,ν2)
determined at the respective degrees of freedom ν1 and ν2, and the condition F > Ftable
(α,ν1,ν2). e squares of the multiple correlation coecients (R-sq) are close to unity,
indicating that all signicant workow inuences are accounted for. e values of the
squares of the coecients of determination Rsq (pred) of the predicted values also
have high values, for the performance it is R-sq (pred) = 0.9986 and for the fuel con-
sumption R-sq (pred) = 0.9958. erefore, the obtained models in equations (7) and
(8) are adequate and with suciently high accuracy describe the studied mulching
process. From the variance analysis, it can be concluded that the proposed regression
models are suitable for determining the operating productivity and fuel consump-
tion of forestry mulching units with a power of 69 kW to 243 kW for cleaning clear-
ings and eld protection forest belts with a concentration of wood waste, shoots and
bushes from 15 t/ha to 48 t/ha
Output parameters calculated values from the regression models as functions of
the response y1 and y2 are given in Table 5. From the obtained results, graphical de-
pendencies were drawn in gures 5 and 6, depicting the way the productivity and fuel
consumption of forestry mulchers changes depending on the rated power of the unit
and the concentration of comminuted wood biomass in clearings and eld protective
forest belts.
Figure 5. Variation of the operating productivity of forestry mulchers depending on the rated
power of the unit and the concentration of comminuted woody biomass

Operational properties of forestry mulchers for cleaning field protection forest belts...75
e results indicate that at optimal rotor speed, both factors have a signicant
impact on the productivity and fuel consumption, but in the studied factor space, the
rated power of the mulching units has a greater weight. is inuence is particularly
pronounced when the rated power of the mulching unit increases to 160 kW, aer
which its inuence begins to weaken. From the graphs in gure 7, it can be seen that
this inuence is more pronounced on the curve depicting fuel consumption in clear-
ings with a concentration of 15-25 t/ha, and aer reaching a value of about 220 kW,
the inuence of this factor signicantly decreases. From the same graphs, it can be
seen that when cleaning cuttings with a concentration of wood biomass of 15 t/ha, the
minimum relative fuel consumption will be reached at a unit power of about 240-245
kW. When clearing cuttings with 25 t/ha residue mass, the minimum fuel consump-
tion would probably be realized at a power of around 250260 kW. From the graphs
in Figure 6, it can be seen that the productivity of the mulching units continues to
increase at a high rate even aer reaching the upper level of the studied power factor
space of 245 kW. From these results it is evident that forestry mulching units with
bigger power naturally have higher productivity.
e results of this experiment are used to determine the optimal power of the
mulching units for cleaning clearings with dierent concentrations of woody bio-
mass. e optimization is carried out by the method of the gradient algorithm in
accordance with the accepted methodology, where the minimum of the relative fuel
consumption is sought – ŷ2 = Go → min. For this purpose, at pre-xed levels of the
Figure 6. Variation of the relative fuel consumption of forestry mulchers depending on the
rated power of the unit and the concentration of comminuted woody biomass

76Konstantin Marinov, Konstantin Kostov, Dimitar Peev / Silva Balcanica 24(2): 59-81 (2023)
concentration of woody mass X2, dierent values of the rated power X1 were set in the
interval from 69 kW to 243 kW. e calculations continue until the minimum value
of the objective function 𝑦2 is found. Aer processing the results, optimal solutions
were obtained for determining the rated power of the mulching unit, with xed basic,
upper and lower levels of the concentration of woody mass. e resulting optimal
solutions are given in Table 7.
e obtained optimal solutions for cleaning cuttings and eld protection forest
belts show that the relative fuel consumption is lower when using mulching units
with higher power. Forestry mulchers, driven by tractors with a rated power of 245
kW, are optimal for work in all clearings with a concentration of woody biomass
from 15 t/ha to 48 t/ha. e obtained optimal solutions correspond to those obtained
in the active-passive experiment at a rotor speed of 1000 rpm. When working with
a mulching unit with a power of 245 kW (upper level X1 = 1) and a minimum value
of the concentration of 15 t/ha (lower level X2 = -1), the function y2 for the fuel con-
sumption is equal to Go = 8.59 l/da. In cuttings with a concentration of woody mass
31.5 t/ha (basic level X2 = 0), the relative fuel consumption is equal to Go =10.88 l/da,
and in those with a concentration of 48 t/ha (upper level X2 = 1) this consumption
is Go = 15.5 l/da. When working with mulching units with a higher power, a higher
operating productivity is naturally established. e operating productivity in clear-
ings with a concentration of woody biomass of 15 t/ha (X2 = -1) is Wh = 3.924 da/h, in
those with a concentration of 31.5 t/ha (X2 = 0) it is Wh = 3.548 da/h, and in clearings
with concentration of 48 t/ha (X2 = 1) it is Wh = 3.042 da/h.
e surface of the objective functions y2 and y1 and a diagram of the lines with
constant values of the operating productivity, and relative fuel consumption of forestry
mulching units of dierent rated power when cleaning eld protection forest belt, and
clearings with dierent concentrations of wood biomass are given in gure 7 and 8.
Table 7. Results of the optimization to nd the optimal power of the mulching unit

Operational properties of forestry mulchers for cleaning field protection forest belts...77
e research that was conducted established that when mulching clearings with
wood residues from 15 t/ha to 25 t/ha with a mulching unit with the largest rated
power of 245 kW, the relative fuel consumption is the smallest – from 8.6 l/da to 9.7
l/da. When working with a medium power mulching unit of 160 kW in the same
clearings, the fuel consumption is from 12.2 l/da to 14.1 l/da, with the excess being
3.6-4.4 l/da. When mulching with a less powerful unit of 70 kW, this consumption
is from 21.4 l/da to 24.1 l/da, and the excess is greater – from 12.8 l/da to 14.4 l/
da. For cleaning clearings with a larger amount of residues from 36 t/ha to 48 t/
Figure 7. Surface and diagram of the isolines of the objective function y1 for operating pro-
ductivity of forestry mulchers with dierent power in clearings with dierent biomass con-
centration
Figure 8. Surface and diagram of the isolines of the objective function y2 for fuel consump-
tion when operating forestry mulchers with dierent power in clearings with dierent biomass
concentration

78Konstantin Marinov, Konstantin Kostov, Dimitar Peev / Silva Balcanica 24(2): 59-81 (2023)
ha, when working with the more powerful 245 kW mulching unit, the relative fuel
consumption is from 11.9 l/da to 15.5 l/da. In these clearings, the medium-powered
unit has a fuel consumption of 17.2 l/da to 21.7 l/da, with the excess already greater
from 5.3 l/da to 6.2 l/da, and the least powerful unit has a consumption of 28.0 l/
da to 33.5 l/da, with the excess already more signicant from 10.8 l/da to 11.8 l/da.
Based on these results, it can be summarized that when mulching clearings with
small remaining amounts of wood biomass up to 20 t/ha, the fuel consumption
of medium-powered units of 160 kW is higher by about 4 l/da compared to more
powerful units with a rated power of 245 kW. If we suppose that the relative costs
of materials, amortization and maintenance of a mulching unit (forestry mulcher
and tractor) with a power of 160 kW are about 6-7 €/da lower than those of a 245
kW unit, and at current price of diesel fuel 1.5-1.7 €/liter, then the total costs of
mulching per unit area will equalize. In cases where these costs turn out to be lower
than those mentioned above and the price of fuel is maintained or falls below the
above levels, this is likely to reduce the bottom line costs of using medium pow-
ered mulching units. However, these assumptions require additional research and
analysis of the regional market of this type of machinery in our country, which is
still in the process of development. Regardless of the aforementioned hypotheses,
mulching units with greater power have higher operating performance. For exam-
ple, when cleaning forest areas and cuttings with a small amount of wood residue
and bushes with a mass of 15 t/ha, the productivity of a mulching unit with a greater
power of 245 kW is 3.92 da/h, while for a medium powered unit of 160 kW, it is 2.76
da/h, which is 1.16 da/h or 30% less. When operating with a small power unit of
70 kW, this productivity is 0.86 da/h, which is 3.06 da/h or 78% less. For clearings
with more residue 48 t/ha, the productivity of a high power mulching unit of 245
kW is 3.04 da/h, for a medium powered unit of 160 kW it is 1.99 da/h, which is by
1.05 da/h or 35% less, and a unit with less powerful unit of 70 kW it is already 0.21
da/h, which is 2.83 da/h or 93% less. ese results show that regardless of the total
operation and maintenance costs per unit area, the use of more powerful forestry
mulchers will reduce site preparation time
Conclusions
e technological capabilities and operational properties of forestry mulchers de-
pend on the working conditions and technical parameters of the machines. As a re-
sult of the conducted research, functional models have been elaborated to determine
the operating productivity and relative fuel consumption of mulching units with a
rated power of 70 kW to 245 kW for cleaning cuttings from wood residues, shoots
and bushes with a concentration of biomass from 15 t/ha to 50 t/ha. e mode of
inuence of the amount of wood waste, the rotor rotation frequency and the rated
power of the mulching unit on the operating productivity and fuel consumption has
been established. e optimal rated power and rotor revolutions of mulchers for op-

Operational properties of forestry mulchers for cleaning field protection forest belts...79
erating in clearings with dierent concentrations of wood biomass were determined.
As the rotor speed increases from 500 rpm to 1000 rpm, fuel consumption decreases
and productivity increases. e relative fuel consumption reaches its minimum at an
optimal rotor revolutions of 1000 rpm. To establish how these parameters change be-
yond the studied speed range, additional research with mulchers having higher rotor
speed above 1000 rpm is needed.
Mulching units with higher rated power have lower fuel consumption per decare
and higher operating productivity. In clearings with wood mass concentration of 15
t/ha, the productivity of mulchers with a greater power of 245 kW is 3.92 da/h, for
those with a medium power of 160 kW it is 2.76 da/h, and for those with a small pow-
er of 70 kW – 0.86 da/h. e dierence becomes even greater in cuttings with a larger
amount of wood mass 48 t/ha, where depending on the rated power of the mulcher,
the productivity is respectively equal to 3.04 da/h, 1.99 da/h and 0.21 da/h. Mulching
units with a higher power are more economical, i.e. more powerful mulchers have a
lower fuel consumption per unit area, regardless of the amount of comminuted bio-
mass. is fact is more pronounced in clearings with a greater amount of wood waste
48 t/ha. In them, the fuel consumption of mulching units with a greater power of 245
kW is 15.5 l/da, for units with a medium power of 160 kW it is 21.7 l/da, and orf those
with a small power of 70 kW – 33.5 l/da. In clearings with a smaller amount of wood
waste 15 t/ha, depending on the power of the mulching unit, these costs are respec-
tively 8.6 l/da, 12.2 l/da and 21.4 l/da. When mulching such clearings, it is found that
the dierence in relative fuel consumption between the 245 kW and 160 kW units is
a relatively small 3.6 l/da. In order to determine the impact of other running costs on
the total costs per unit area, additional economic research and analysis of the regional
market for this type of machinery are needed. Regardless of the results of such future
research, the use of more powerful forestry mulchers will shorten cuttings prepara-
tion times and reforestation deadlines.
e obtained results can be used to develop norms for productivity, time cost
and fuel consumption of forestry mulchers for operating in the North-Eastern State
Forestry in Bulgaria.
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