ARTICLE IN PRESS BIOSYSTEMS ENGINEERING
98 (2007) 286 – 296
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/issn/15375110
Research Paper: PM—Power and Machinery
Tractor energy requirements in disc harrow systems Joa˜o M. Serrano, Jose´ O. Pec- a, J. Marques da Silva, Anacleto Pinheiro, Ma´rio Carvalho Engineering Department, University of E´vora, ICAM, Nu´cleo da Mitra, Apartado 94, 7002-554 E´vora, Portugal
ar t ic l e i n f o
Soil-working operations in conventional farming systems involving the use of the tractor are some of the operations that incur the highest energy costs. The sustainability of such
Article history:
systems requires a strictly controlled management of resources leading to a significant
Received 6 August 2006
reduction in crop-production costs derived from savings in fuel consumption. The
Received in revised form
configuration of the tractor-harrow combination, based on the measurement of the
1 August 2007
draught required under operational conditions, provides the manufacturers with a reliable
Accepted 1 August 2007
indication of the recommended power required for each model of harrow produced. With
Available online 20 September 2007
this type of information farmers can take decisions regarding the selection of a suitable tractor–implement combination for their farms. As a consequence there is improved tractor-harrow productivity and field efficiency. This study centred on the validation of the mathematical models used to estimate the draught of disc harrows in medium-textured soils presented by ASABE. A 3-year research project was developed to study tractor–implement dynamics in tillage operations. The field tests were performed under real working conditions, using more than 20 four-wheel-drive tractors and trailed disc harrows combinations, in different soil types. The tractors were instrumented and the measured parameters were as follows: forward speed, slip, engine speed, draught and fuel consumption per hour. The tractors were also submitted to dynamometer tests measuring PTO power and speed, and engine fuel consumption. The results led to the development of a quadratic equation, that corresponds to an adaptation of the linear model of ASABE, to estimate the draught of the disc harrows in undisturbed loamy soils, not only as a function of implement mass and soil type, but also as a function of speed, working depth and soil conditions. Under these conditions, a ratio of tractor power to implement width of 25–33 kW m1 is suggested. The data also show the existence of a linear relationship between the fuel consumption per hectare and the specific draught, for the range of 4–9 kN m1, valid for dry, undisturbed loamy soils. & 2007 IAgrE. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Historically, the efficient use of energy in agriculture did not have a high priority. However, taking into consideration the diminishing supply of fossil fuels, efficiency was taken more seriously. Fuel is the source of energy for the tractor providing
for the performance of work and propelling the tractor to overcome implement draught (Smith, 1993). Soil-working operations in conventional farming systems involving the use of the tractor are some of the operations that incur the highest levels of energy cost. The sustainability of such systems requires a strictly controlled management of
Corresponding author.
E-mail address:
[email protected] (J.M. Serrano). 1537-5110/$ - see front matter & 2007 IAgrE. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.biosystemseng.2007.08.002
ARTICLE IN PRESS BIOSYSTEMS ENGINEERING
Nomenclature
Ch Cha Cs d m n Ne Nerec P r s
fuel consumption per hour, l h1 fuel consumption per hectare, l ha1 1
specific fuel consumption, g kW
1
h
working depth, cm implement mass, kg 1
engine speed under load, min engine power, kW
recommended engine power, kW drawbar power, kW rolling radius, m
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T Test Tmeas va vt w wr y I Zm Zt Zf
287
draught, kN draught estimated, kN draught measured, kN actual forward speed, km h1 theoretical forward speed, km h1 working width, m work rate, ha h1 power take-off (PTO) utilisation, % specific draught, kN m1 transmission efficiency tractive efficiency field efficiency
slip, %
resources leading to a significant reduction of crop-production costs derived from savings in fuel consumption. Many studies have been conducted to measure draught, power requirements and fuel consumption of tillage implements under various soil conditions (ASAE, 1995; Harrigan & Rotz, 1994; Al-Suhaibani & Al-Janobi, 1997). The draught (draft or drawbar pull) is the force required to propel an implement in the direction of travel (ASABE, 2006a). The availability of data relating to draught requirements is an important factor in selecting tillage implements for a particular situation. Farm managers and consultants use draught and power requirements of tillage implements in specific soil types to evaluate implement performance, energy requirements, and to determine the size of tractor required (Al-Suhaibani & Al-Janobi, 1997). Mathematical models have been developed to predict draught of some tillage tools, but the heterogeneity of the soil, coupled with the complex manner in which soil fails, make the understanding of the complex interactions between a specific tillage tool and the soil medium difficult (Grisso et al., 1994). Draught is primarily a function of the width of the implement and the speed at which it is pulled (Harrigan & Rotz, 1994). However, draught also depends upon operating depth and geometry of tillage tool (Upadhyaya et al., 1984). Tillage draught is further influenced by site-specific conditions including soil type, moisture, density and residue cover (Harrigan & Rotz, 1994). Disc harrows are among the most commonly used tillage implements (Harrigan & Rotz, 1994). Many factors affect disc harrow draught including gang angle, mass per blade, blade type and spacing, operating depth and forward speed (Sommer et al., 1983). Draught data for disc harrows were reported by ASAE (1995) with draught expressed as a function of implement mass and type of soil. More recent studies by Harrigan and Rotz (1994) updated the ASAE data report by including disc harrow draught as a function of speed, working width, working depth and soil conditions (ASABE, 2006b). The ratio between draught per unit of implement width, or specific draught, I, to tractor fuel consumption per hectare, Cha, is a parameter describing tillage energy spent per unit fuel. This parameter is not solely influenced by implementspecific draught, but also by tractor losses. Tractor losses (motion resistance and tractive efficiencies) alter under different loading conditions but in a non-mobilised soil the
tractive efficiencies, Zt, presents a rather small variation (ASABE, 2006b), although the transmission efficiency, Zm, is considered to be almost constant for recently manufactured tractors. PTO tests show that the specific fuel consumption, Cs, inverse function of the motor efficiency, is predictable in certain work conditions. Considering these conditions we can accept, with a certain degree of trust, that the ratio Cha =I is predictable. According to Smith (1993), a single line, representing the relationship between the draught and the net fuel consumption per hectare (difference in fuel consumed with and without implement) for various implements at various combinations of gear and speed, suggests that a measurement of draught could be used to estimate fuel consumption. This recorded an exponential increase of the net fuel consumption per hectare with the implement draught. Bowers (1985) and Riethmuller (1989) also carried out tillage field tests in several conditions (implements and soils), and suggested linear relationships between the draught per unit of implement width and fuel consumption per hectare, Eqs. (1) and (2), respectively. These equations are used to validate the results obtained in this paper Cha ¼ 1:2774I,
(1)
Cha ¼ 1:1306I,
(2)
where Cha is the fuel consumption per hectare in l ha1; and I is the specific draught in kN m1 These equations, valid for different implements and soil conditions, consider several theoretical approaches influenced by information given in ASABE standards. These standards have evolved since 1989, namely on total tractive efficiency when we consider Zt with a break of about 10% (Zt ¼ 0.76). The absence of data means that decisions concerning the choice of implement and tractor–implement working set-up are taken on the basis of empirical knowledge or on limited scientific information. Tractors are normally selected according to the power needs of the implement used for heavy tillage operations, usually the mouldboard plough, leaving the tractor over-sized for superficial tillage implements (disc harrows and cultivators). Furthermore, with a general trend towards higher horsepower tractors, implements sized for
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smaller tractors are now being used with larger, more powerful tractors. The questions of correct matching are real, but difficult to answer concerning the variability of soil conditions. In dry farming systems of Southern Portugal, offset disc harrows are very popular among farmers. Within the usual 3-year crop rotation of winter wheat/winter wheat/ sunflower, disc harrows are used as primary and secondary cultivation tools (Serrano et al., 2003). Worldwide research on engines, tyres and fully instrumented tractors has build up a package of valuable information. However, the practical impact of some of these results is difficult to assess due to the particular conditions of the experiment. Tractor Performance Monitors (TPMs) are increasingly being supplied as standard tractor electronic equipment, or factory-fitted option. They provide information to assist tractor drivers and farm managers, and an excellent base to perform experiments in real working conditions. TPMs can be used to gather data and validate the importance of the different variables influencing the dynamics of tractor–soil–agricultural implement. TPMs have the fundamental advantage of being operated by the end user of the research results, making possible, inclusively, to perform demonstration experiments at the farmer’s own premises. The specific objectives of the study of the tractor–implement dynamics in tillage operations, based on the TPMs, were:
to validate the ASABE model estimates of the draught (draft or drawbar pull) of disc harrows;
to evaluate the overall efficiency of different combinations of tractor weight/implement width, building up a matched set and use of gathered data to validate the relation between the fuel consumption per hectare and the specific draught.
2.
Material and methods
2.1.
Tractors and implements
In field trials, various models of trailed offset disc harrows ranging from 20 to 36 discs, 1300 to 3500 kg in mass, were pulled behind four-wheel-drive tractors, ranging from 59 to 134 kW engine power. These tractors were factory equipped with TPMs (‘Datatronic’) providing relevant information such as engine speed, forward speed, slip, and fuel consumption per hour, described by Serrano et al. (2003). The disc harrows had a hydraulic actuator to regulate the angle between disc gangs. Table 1 presents the individual set angle of each gang. The discs of the front gang were notched and the discs of the rear gang were plain. Details of the different tractor–implement combinations are presented in Table 1. These implements were chosen according to the following criteria:
very popular among farmers as primary and secondary
cultivation tool; important implement within the strategy for reduced cultivations;
98 (2007) 286 – 296
well represented within the local farm machinery industry (Serrano, 2002).
2.2.
Data acquisition system
The information provided by the TPMs is volatile. To overcome this limitation a portable computer-based record system was developed (Pec- a et al., 1998) which diverted the signals from the tractor TPMs sensors and also the information from a load cell-based pull sensor. The portable computer was equipped with a data acquisition board capable of handling up to eight single-ended channels, 12 bit resolution and 100 000 samples per second and a terminal board providing the appropriate connection and the voltage excitation for the 50 kN capacity load cell. As the inputs to the terminal board had to be less than 1 V, resistor voltage dividers were used where appropriate. An LabVIEW application was developed to control the dataacquisition process. The following as measured in the field tests (Fig. 1): the actual tractor forward speed, va in km h1; the theoretical forward speed, vt in km h1; the engine speed, n in min1; the fuel consumption per hour, Ch in l h1; and the draught, T in kN. An approximate rolling radius was introduced in the dataacquisition system, DAS, in order to compute slip. The value used was the average of the values of the static tyre radius measured with and without vertical load obtained from the tyre manufacture catalogue for the rear tyre. In each soil condition and tractor ballast, the slip measured by DAS was compared with the slip obtained from the comparison of the distances travelled in classic load/no load slip tests. The results were so close that no corrections on the rolling radius were necessary. With the input of the tyre rolling radius, r in m, and the working width of the implement, w in m, the following performance parameters were calculated on the basis of ASABE standards (ASABE, 2006a): average slip, s in %, Eq. (3), drawbar power, P in kW, Eq. (4), work rate, wr in ha h1, Eq. (5) and fuel consumption per hectare, Cha in l ha1, Eq. (6). s¼
ðvt va Þ 100, vt
(3)
P¼
Tva , 3:6
(4)
wr ¼
va wZf
Cha ¼
10
,
Ch , wr
(5) (6)
where s is the slip in %; vt is the theoretical forward speed in km h1; va is the actual forward speed in km h1; P is drawbar power in kW; T is the draught in kN; wr is the work rate in ha h 1 ; w is the working width in m; Zf is the field efficiency; Cha is the fuel consumption per hectare in l ha1 and Ch is the fuel consumption per hour in l h 1
2.3.
Soils
Field tests were conducted on soil textures ranging from sandy loam soils to clay loam soils, with moisture content,
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Table 1 – Tractor/disc harrow combinations used in field trials Site
Tractor model
Maximum engine power, kW
Work conditiona
Disc harrow model
Disc harrow mass, kg
Disc diameter, mm
Number of discs
Set angle, degb
Maximum working width, m
Working depth, m
1
MF3680
134
M2
3500
660.4
36
12.5
3.95
0.085
1
MF3680
134
M2
3500
660.4
36
17.0
3.95
0.105
2
MF3095
81
M1,M2
1870
660.4
24
26.5
2.93
0.180
2
MF3095
81
M1,M2
1870
660.4
24
15.5
2.93
0.180
2
MF3060
59
M1
1300
609.6
20
23.0
2.07
0.180
2
MF3060
59
M1
1300
609.6
20
18.5
2.10
0.180
3
MF3095
81
M1, M2
2700
660.4
26
22.0
3.01
0.180
3
MF3095
81
M1, M2
2700
660.4
26
16.5
3.13
0.150
3
MF3060
59
M1, M2
1300
609.6
20
23.0
2.06
0.180
3
MF3060
59
M1, M2
1300
609.6
20
18.5
2.13
0.166
4
MF3650
110
M1, M2
2000
711.2
24
22.0
2.89
0.180
4
MF3060
59
M1, M2
1300
609.6
20
23.0
2.08
0.190
5
MF8130
114
M1
2700
660.4
26
21.5
3.19
0.180
5
MF8130
114
M1, M2
2700
660.4
26
13.5
3.31
0.180
5
MF3060
59
M2
1300
609.6
20
23.0
2.17
0.165
5
MF3060
59
M2
1300
609.6
20
18.5
2.10
0.165
6
MF3095
81
M1, M2
1460
660.4
24
27.0
2.43
0.145
6 6 6
MF3095 MF3095 MF3060
81 81 59
M1, M2 M1, M2 M1, M2
1650 1650 1300
609.6 609.6 609.6
28 28 20
21.5 18.5 23.0
3.30 3.36 2.07
0.132 0.156 0.182
6
MF3060
59
M1, M2
1300
609.6
20
18.5
2.11
0.158
7
MF3060
59
M1, M2
1460
660.4
24
27.0
2.52
0.170
7
MF3060
59
M1, M2
1460
660.4
24
19.0
2.60
0.130
7
MF3060
59
M1, M2
1180
609.6
22
27.0
2.20
0.140
7
MF3060
59
M1, M2
1300
609.6
20
23.0
2.08
0.190
8
MF3060
59
M2
1300
609.6
20
23.0
2.18
0.170
9
MF3060
59
M1, M2
1300
609.6
20
23.0
2.09
0.220
9
MF3060
59
M1, M2
1300
609.6
20
25.5
2.20
0.220
10
MF3060
59
M1, M2
1300
609.6
20
23.0
2.34
0.160
11
MF3060
59
M1, M2
Galucho GLHR Galucho GLHR Galucho GLHR Galucho GLHR Herculano HPR Herculano HPR Premetal PLHR Premetal PLHR Herculano HPR Herculano HPR Galucho GSM Herculano HPR Premetal PLHR Premetal PLHR Herculano HPR Herculano HPR Galucho A2CP Halcon Halcon Herculano HPR Herculano HPR Galucho A2CP Galucho A2CP Galucho A2CP Herculano HPR Herculano HPR Herculano HPR Fialho RTM Herculano HPR Herculano HPR
1300
609.6
20
23.0
2.14
0.165
a b
M1—engine at the rated speed and selecting the highest gear; M2—engine at 80% of the rated speed and selecting the highest gear. Set angle of each gang.
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Consumption per hour Forward speed En gine (Engine speed)
Transmission (Gear)
Tractor
Tyr e (Slip)
Implement (Width; depth)
(Drawbar pull)
Soil (Type, density; moisture) Fig. 1 – Performance parameters measured in field tests.
Table 2 – Soil physical parameters obtained at the test location in the 200 mm top layers Site
1 2 3 4 5 6 7 8 9 10 11 a
Soil condition
Sand, %
Silt, %
Clay, %
Classificationa
Moisture content (d.b.), %
Bulk density, kg m3
Grazed Crop stubble Grazed Crop stubble Crop stubble Grazed Grazed Grazed Grazed Grazed Grazed
48 68 73 49 73 69 65 75 64 61 39
23 13 9 23 10 13 10 9 20 15 24
29 19 18 28 17 18 25 16 16 24 37
Clay loam Loam Sandy loam Clay loam Sandy loam Loam Clay loam Sandy loam Loam Clay loam Clay loam
4.0 11.5 15.0 12.0 19.0 8.0 8.0 14.0 15.0 17.0 17.0
1.648 1.326 1.592 1.394 1.286 1.528 1.560 1.498 1.543 1.492 1.476
International Soil Science Society.
dry basis, of 4–19%. Undisturbed soil, grazed and crop stubble were the soil conditions used in the field tests. Soil details are presented in Table 2. Test sites were chosen according to the utilisation of the disc harrow in primary cultivation systems. The average depth of the mobilised soil layer was obtained from at least eight values, obtained along the run, being each value, in turn, the average result from three measurements taken across the width of each run. Average working width was obtained from at least six direct measurements across each harrowed path.
2.4.
Prior to each test, various settings were tested concerning the angle between disc gangs, which guaranteed the soil resistance alignment with the working direction, avoiding side pull, and the combinations of engine regime/gear selection that would allow the establishment of the following two work conditions:
settings aiming to maximise the work rate (M1 in Table 1):
Test procedure
The experimental layout included different field tests and power take-off dynamometer tests. The field tests were conducted on private farms, in real conditions of work with 80–100 m-long runs with two replications, either using the farmer’s own equipment and operator or using similar equipment supplied by the university. The results obtained in the tests are the average of the two replications, which correspond to 80–100 sensor readings.
engine at the rated speed; and selecting the highest gear in the transmission at which the work could be performed with the required quality (tilth, buried stubble), within accepted comfort and safety for the operator, and without engine overcharge (no decrease in engine speed of more 10% of the rated speed); settings aiming to compromise between fuel consumption and working rate (M2 in Table 1): engine at 80% of the rated speed; and selecting the highest gear in the transmission at which the work could be performed with the required quality (tilth, buried stubble), within accepted comfort and safety for the operator, and without engine overcharge (no decrease in engine speed of more 10% of the rated speed).
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In dynamometer tests, the tractors were tested on a powertake-off (PTO) dynamometer (Tractor Test Centre XT200) equipped with a strain-gauge torque meter and digital readout for measuring PTO power and speed. The tractor engine was loaded by the dynamometer and the engine governor was set to obtain the desired power output and engine speed. When engine temperature stabilised (after 15–20 min of the operation before each test) the test was begun. Engine speed and torque were held constant during each test by adjusting engine governor control lever position and PTO load. Data recorded included PTO power and speed as well as fuel consumption per hour measured from the TPM’s sensor.
3.
Results and discussion
3.1.
Field tests
The results obtained are the average of the two replications. They show a satisfactory approximation (R2 ¼ 0.68) to the ASAE simplest model (ASAE, 1995), Eq. (7), for forecasting the draught on the basis of the disc harrow mass, under normal dry-field-crop conditions on soils with an average texture (Fig. 2). In Fig. 2 it should be noted that a quadratic equation, Eq. (8), provided a better draught estimation on the basis of the disc harrow mass (R2 ¼ 0.80). T ¼ 0:0117 m,
T ¼ 0:000002 m2 þ 0:016 m.
(a) The relation between disc harrow mass and the implement width presented in Table 1, for disc harrows with 60–100 kg per disc and with discs with a diameter between 609.6 and 711.2 mm, allows the establishment of the following equation (R2 ¼ 0.80). m ¼ 965:71 þ 1041:9 w,
(b) (c)
Draught, kN
(d)
20 (e)
10
0
1000 2000 Implement mass, kg
3000
Fig. 2 – Comparison of disc harrow draught data relevant to medium-textured soils with the ASAE (1995) predictions: (J) measured data; - - - - Eq. (7); —— Eq. (8).
Eqn (9)
Implement width
2−4 m
Eqn (8)
Implement mass
1000−3500 kg
P , Zm Zt
(10)
where Ne is the engine power in kW, P is the drawbar power in kW, Zm is the transmission efficiency and Zt is the tractive efficiency; (f) the engine power degree of use is 0.80. This guarantees a certain range of power for slope zones or areas with compacted soil. Eq. (11) defines the recommended
Eqn (10)
Eqn (4)
Draught
(9)
where m is the disc harrow mass in kg and w is the implement width in m Eq. (8), draught estimation considering the harrow disc mass; 6–8 km h1 is the typical working speed considering harrows discs in a primary mobilisation (Serrano, 2002); Total traction efficiency (product of the transmission mechanical efficiency by the tractive efficiency) between 0.65 and 0.70 agrees with the standards of ASABE (ASABE, 2006b); the following equation defines engine power as a function of drawbar power; Ne ¼
0
(8)
where T is the draught in kN and m is the disc harrow mass in kg. The construction of a model for the configuration of the tractor-harrow combination, based on the measurement of the draught required under actual operational conditions, provides the manufacturers with a reliable indication of the recommended power required for each model of harrow produced, and providing farmers with the information required for selecting suitable tractor–implement combinations for their farms. The configuration of the tractor-harrow combination has a great impact on productivity and field efficiency. Under these conditions, a ratio of recommended tractor power to implement width of 25–33 kW m1 is suggested, on the basis of the following assumptions (Fig. 3):
(7)
30
291
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Drawbar power
Eqn (11)
Engine power
6−8 km h−1
Fig. 3 – Recommended engine power diagram according to implement width.
Recommended engine power
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3.2.
engine power Ne rec ¼
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Ne , 0:80
(11)
where Nerec is the recommended engine power in kW and Ne is the engine power in kW
Fig. 4 presents the results of the field tests performed in typical dry conditions on medium-textured soils with the model more recently presented by standards of the ASABE (ASABE, 2006b), Eq. (12), that reports disc harrow draught as a function of speed, working width, working depth and soil conditions. The comparison of draught data measured to medium-textured soils with this ASABE predictions also verify a good approach. The best fit (R2 ¼ 0.79) is obtained by a non-linear equation, Eq. (13).
Tractors were also subjected to tests using a dynamometer brake connected to the power source (PTO). The results in terms of engine speed, torque and hourly rate of fuel consumption, were processed by means of triangulation and linear interpolation of data and engine performance curves were developed. It was shown that specific fuel consumption, the conversion efficiency of the fuel chemical energy into mechanical energy as power at the PTO, is a predictable parameter and has a relatively low variability under certain operational conditions. Those conditions are for a single regime of engine operation, between intervals of power use above 60% (Table 3, Fig. 5), as is usual with traction operations. It is therefore possible to forecast the value of this variable for two typical regimes of use associated with soil-working operations:
(12)
T ¼ 0:88ð364 þ 18:8va Þwd,
where T is the draught in N; va is the speed in km h1; w is the working width in m; and d is the working depth in cm. Test ¼ 0:019 T2meas þ 1:3481 Tmeas ,
a regime of around 80% of the nominal regime, used for
(13)
where Test is the draught predicted by Eq. (12) in kN; and Tmeas is the draught measured in kN.
30 Draught estimated Test, kN
Power take-off dynamometer tests
operations that does not require full tractor power and where the aim is to optimise fuel consumption without adversely affecting work rate, where a minimum and relatively stable specific fuel consumption is obtained (which, for the range of tests carried out, was 270710 g kW1 h1); a nominal regime, used in operations that are demanding in terms of power or where the aim is to optimise work rate, with a lower degree of efficiency in terms of specific fuel consumption (which, for the range of tests carried out, was about 300715 g kW1 h1).
25 20
Two regression equations between the specific fuel consumption and the PTO power for six MasseyFerguson tractors, 3060, 3065, 3085 and 3095 models were obtained: Eq. (14) to 80% of the nominal engine speed and Eq. (15) to nominal engine speed. These equations presented, respectively, a determination coefficient of 0.97 and 0.98.
15 10 5 0 0
5
10 15 20 Draught measured Tmeas, kN
25
30
Fig. 4 – Comparison of disc harrow draught data measured to medium-textured soils, Tmeas with the ASABE (2006b) predictions, Test : (J) measured data; - - - - Eq. (12); —— Eq. (13).
Cs ¼ 266:4 þ 884:5 eðy=12:4Þ ,
(14)
Cs ¼ 289:8 þ 1166:3 eðy=15:97Þ ,
(15)
where: Ch is the fuel consumption per hour in l h1 and y is the PTO power utilisation, % of maximum power.
Table 3 – Specific fuel consumption of Massey–Ferguson tractors (3060, 3065, 3085 and 3095 models) (Serrano, 2002) PTO power relative to nominal PTO power, %
Average specific fuel consumption, g kW1 h1
Standard deviation of specific fuel consumption, g kW1 h1
80
60 90
270 266
10 9
100
60 90
316 294
21 11
Engine speed relative to rated engine speed, %
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Specific fuel consumption, g kW−1 h−1
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600 575 550 525 500 475 450 425 400 375 350 325 300 275 250 225 200 0
10
20
30 40 50 60 70 PTO power, % maximum power
80
90
100
Fig. 5 – Specific fuel consumption of Massey–Ferguson tractors (3060, 3065, 3085 and 3095 models): (J) 80% of the nominal engine speed, 1750–1800 min1; (W) nominal engine speed, 2200–2250 min1.
3.3. Fuel consumption per hectare: indicator of the overall energy efficiency The overall energy efficiency is the ratio of the specific energy transferred from the tractor for operating the implement to the energy equivalent of the fuel consumption required to perform the operation (Smith, 1993). The efficiency of the energy transformation of the fuel supplied to the tractor engine under operational conditions involving the use of the harrow can be related to the pertinent variables, namely tractor engine, power transmission, interaction of tyres with soil and interaction of the moving parts of the harrow with the soil, by Eq. (16). Each of these performance factors is dependent on the efficiency of the operator. The interaction of tyres with the soil implies the definitive influence of the soil as a major factor on the overall energy efficiency. This is the reason as to why different authors (Bowers, 1985; Riethmuller, 1989; Smith, 1993) were cautious concerning the domain of application of their results Cha
Cs I ¼ , 360Zm Zt
(16)
where Cha is the fuel consumption per hectare in kg ha1; Cs is the specific fuel consumption in g kW1 h1; Zm is the efficiency of tractor transmission; Zt is the tractive efficiency and I is the specific draught in kN m1. In Eq. (16), the ratio (Zm Zt/Cs) represents the overall efficiency of the conversion of fuel into useful work (Serrano et al., 2003). The measurement of the following parameters: draught, tractor forward speed and hourly rate of fuel consumption, in field tests involving the use of tractors and disk harrows under different operational conditions combined with the measurement of specific fuel consumption in tests carried
out at the power source for a range of operational conditions enabled the calculation of overall energy efficiency. The mean value of 0.7070.05 for the total tractive efficiency (ZmZt) was obtained considering 100 observations in undisturbed soils (Serrano, 2002). These values confirmed the ASABE standards (ASABE, 2006b) of 0.76 for the tractive efficiency (Zt) and assume a range of 0.85–0.90 for the transmission efficiency (Zm). Assuming that the ratio (ZmZt /Cs) is maintained somewhat constant and is predictable, the tractor fuel consumption per hectare can be determined by its disc harrow draught requirement. Two regression equations between these variables were obtained: Eq. (17) to 80% of the nominal engine speed and Eq. (18) to nominal engine speed. These equations presented coefficients of determination of 0.87 and 0.90 respectively. Cha ¼ 1:2097I 0:2474,
(17)
Cha ¼ 1:4350I 0:5939,
(18) 1
where Cha is the fuel consumption per hectare in l ha and I is the specific draught in kN m1. The general relationship between fuel consumption per hectare and the effort required by the harrow depends on the overall energy efficiency of the transformation of the energy supplied to the engine during harrow operations. Maximum overall energy efficiency corresponding to minimum consumption per hectare can be used as a guide in the assessment of the configuration of the tractor-harrow combination. Maximum overall energy efficiency will occur when firstly the minimum specific fuel consumption of the engine, secondly the maximum mechanical output of transmission from the engine to the wheels and thirdly the maximum traction output of the tyres interacting with the soil are all
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Table 4 – Minimum specific fuel consumption of tractors in DLG tests at 60% of the maximum power PTO and 60% of the nominal engine speed (Serrano, 2002) Specific fuel consumption, g kW1 h1
Profi International reference
223 236 225 228 234 236 246 223 235 252 235 242 242 232 246 240
12/98 (No. 12, December 1998, p. 15) 1/00 (No. 1, January 2000, p. 15) 4/00 (No. 4, April 2000, p. 17) 5/00 (No. 5, May 2000, p. 15) 6/00 (No. 6, June 2000, p. 14) 8/00 (No. 8, August 2000, p. 15) 11/00 (No. 11, November 2000, p. 15) 12/00 (No. 12, December 2000, p. 15) 3/01 (No. 3, March 2001, p. 15) 5/02 (No. 5, May 2002, p. 15) 6/02 (No. 6, June 2002, p. 15) 7/02 (No. 7, July 2002, p. 14) 7/02 (No. 7, July 2002, p. 15) 8/02 (No. 8, August 2002, p. 15) 10/02 (No. 10, October 2002, p. 17) 12/02 (No. 12, December 2002, p. 16)
John Deere 6910, 99 kW New Holland TN75S, 53 kW Massey Ferguson 6290, 99 kW Lamborghini Champion 150, 110 kW John Deere 5500, 59 kW Valtra-Valmet 8350 HiTech, 99 kW Deutz-Fahr Agrotron 120 MK3, 88 kW Fendt Favorit 712 Vario, 92 kW New Holland TM150, 104 kW Deutz-Fahr Agroton 110 MkIII, 79.7 kW Landini Mythos DT100, 66.7 kW Case IH CS150, 98.4 kW Case IH CVX150, 106.9 kW Renault Ares 715 RZ, 102.9 kW Fendt Farmer 309 C, 73.1 kW McCormick MTX 125, 82.6 kW
achieved simultaneously. With regard to the first, the minimum specific fuel consumption was 23679 g kW h1 (Table 4). With regard to total traction efficiency for four-wheel drive tractors on undisturbed soil, ASABE (2006b) indicate a maximum value of 0.76 for the tractive efficiency (Zt) and assume a range of 0.85–0.90 for the transmission efficiency (Zm) as mentioned above. The expected maximum overall energy efficiency for soil-working operations under these optimised conditions provides the consumption per hectare reference value (the reference line in Fig. 6). Any operating tractor-harrow combination will produce higher fuel consumptions per hectare the further it is away from the reference value. The less the tractor-harrow combination is suitable, and the more it is out of correct adjustment. Following adjustment of the tractor and the harrow, it was possible to obtain reference equations that relate fuel consumption per hectare to the specific draught. These enabled a comparison of different soil-working systems, with a range of different traction requirements, corresponding to a range of energy costs, translated in terms of fuel consumption per hectare. It was expected that soil-working systems for the sowing of crops based on soil-working operations that require traction force should be associated with high levels of fuel consumption per unit of area worked as compared with superficial soil-working operations or soilworking operations carried out for the purposes of soil conservation. Fig. 6 shows the relationship between fuel consumption per hectare and the specific draught based on the results of field tests carried out, with the reference line corresponding to maximum overall output. It should be remembered that the particular soil conditions from which the relation resulted are dry, undisturbed loamy soils presented in Table 2, as commonly found in primary cultivations with trailed disc harrows in Southern Portugal. Heavier clay soils, particularly in wetter conditions, may not fit into the present results, since the expected higher slip in the interaction of tyres with
Fuel consumption per hectare, l ha−1
Tractor model, maximum PTO power
10 9 8 7 6 5 4 4
5
6 7 8 Specific draught, kN m−1
9
10
Fig. 6 – Relationship between fuel consumption per hectare and specific draught in undisturbed medium-textured soils: (J) measured data 80% of the nominal regime; (W) measured data nominal regime; —— reference line;- - Riethmuller (1989). Bowers (1985);
soil will affect negatively and to a greater extent the overall energy efficiency. Results also confirm the models presented by Bowers (1985) and Riethmuller (1989), who reported that fuel consumption per hectare as a linear function of specific draught. Furthermore, the results (Figs. 5 and 6) still confirm the advantage of setting engine speed towards the maximum torque regime, approaching a more favourable range of engine thermal efficiency, and therefore improving the overall fuel efficiency of the tractor. As would be expected, the test situation with an operational engine regime 80% of the nominal regime presents results that are nearer maximum overall output, the reference line, than the nominal regime test situation.
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Disc harrow model
Implement mass (m in kg)
Working width (w in m)
T = −0.000002m2 + 0.016 m (R2 = 0.80)
Specific draught
Draught (T in kN)
( ℑ in kN m−1) 80% of the nominal engine speed
P = Tva / 3.6
Cha = 1.2097 ℑ −0.2474 (R2 = 0.87)
Ne = P/(m t )
Nominal engine speed Nerec = Ne /0.80
Cha = 1.4350 ℑ −0.5939 (R2 = 0.90)
Recommended tractor engine power (Nerec in kW)
Fuel consumption per hectare (Cha in l ha−1)
Fig. 7 – Scheme for calculating the recommended engine power and fuel consumption per hectare in tractor-disc harrows systems, valid for undisturbed loamy soils.
Fig. 7 summarises, step by step, a qualitative outline, including the equations proposed and the expectable accuracy levels, for predicting the recommended engine power and fuel consumption per hectare in tractor—disc-harrows systems, valid to undisturbed loamy soils. More field tests are recommended in several soil conditions to extend the ASABE model of draught prediction to forecast fuel consumption.
4.
Conclusions
If we consider the objectives of this study, validate the mathematical model presented by ASABE to estimate the draught of disc harrows in medium-textured soils and the relation between fuel consumption per hectare and the specific draught, the results allow us to define a quadratic equation that corresponds to an adaptation of the linear model of ASABE to estimate the draught of the disc harrows to undisturbed loamy soils, as a function of implement mass and soil type. Under these conditions, a ratio of tractor power to implement width of 25–33 kW m1 is suggested. This provides the manufacturers with a reliable indication of the recommended power required for each model of harrow produced, thus providing farmers with the information required for taking decisions concerning the selection of a suitable tractor–implement combination for their farms. The data demonstrate that fuel consumption in tillage operations
can be minimised by selecting an engine speed approximately 70–80% of the nominal speed, and using a higher gear. The degree to which this can be done depends upon implement draft requirements and adequate pull from the tractor. The results also show the existence of a linear relationship between the fuel consumption per hectare and the specific draught, for the range of 4–9 kN m1, valid for dry, undisturbed loamy soils. This relation, representing various tractors and disc harrows models, various combinations of gear and engine speed, various tractor ballasts and tyre pressures, suggests that a measurement of draught could be used to estimate fuel consumption, to demonstrate correct tractor–implement set-up and to select the crop production system. Although a step by step calculation scheme for predicting the recommended engine power and fuel consumption per hectare for tractor-disc harrows systems that is valid for undisturbed loamy soils has been developed, more field tests are recommended in several soil conditions to extend the ASABE model of draught prediction to forecast fuel consumption.
Acknowledgements The authors acknowledge the funding provided by the programme supporting the modernising of Portuguese agriculture and forestry-PAMAF-8.140.
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