Environmental Engineering Reference
In-Depth Information
that soon there was no shortage of comprehensive over-
views or detailed case studies (Pimentel 1980; Fluck and
Baird 1980; OECD 1982; Smil, Nachman and Long
1983; Stanhill 1984; Stout 1990). A series of six topics
entitled Energy in World Agriculture, published between
1986 and 1992, covered the entire field exhaustively
(Singh 1986; Helsel 1987; McFate 1989; B. F. Parker
1991; Peart and Brook 1992; Fluck 1992), but subse-
quently there was a major decline in publishing on these
topics (FAO 2000).
Only the internal combustion engine, a powerful
but light prime mover, made it possible to mechanize
field farming. The first U.S. enterprise specializing in
tractor manufacture was set up in 1905. Subsequent fun-
damental innovations included power takeoff in 1919,
mounted-type implements in 1924, power lift in 1930,
low-pressure rubber tires in 1932, and hydraulic lift in
1935. Diesel tractors were introduced in 1931, LPG as
a fuel a decade later (Dieffenbach and Gray 1960). In
the United States the capacity of gasoline tractors sur-
passed that of horses by the late 1920s, and by 1950
there was nearly 1 OM difference in the total capability
of these prime movers. In Europe the rapid mechaniza-
tion of fieldwork began only after WW II. The process is
still underway throughout the poor world, which in
2000 had less than 30% of the world's 27 million trac-
tors, whereas the United States alone had nearly 20% of
the worldwide total (FAO 2006) and even a larger share
of rated tractor power.
Compared to early heavy machines (up to 450-500
g/W), modern tractors are much lighter and larger
(70-80 g/W for sizes over 100 kW). In North America
average maximum belt power rose from 11 kW (15 hp)
in 1920 to 20 kW in 1945, 33 kW in 1970, 50 kW in
1985, and more than 70 kW by the late 1990s. The larg-
est machines working in the huge fields of the U.S. Great
Plains and the Canadian Prairies are now all diesel-
powered and rate about 300 kW, with diesel tractors
between 120 kW and 220 kW being fairly common (fig.
10.8). In Europe, and particularly in Asia, the sizes
remain much smaller. The average annual fuel con-
sumption of diesel-fueled tractors (in L/h) can be easily
approximated from the annual Nebraska Tractor Test
Data by multiplying maximum power (in kW) by 0.3. A
large 175-kW (235-hp) tractor will consume roughly 55
L/h, and its annual consumption (working 500 hours)
adds up to about 900 GJ. Consumption for specific oper-
ations is a function of soil, resistance, and speed.
Functional draft (soil and crop resistance) has been
measured for scores of tasks with different soils. Mold-
board plowing is generally the most demanding task, fol-
lowed by rotary tilling. Specific needs for these tasks may
be several times higher in silty clays than in sandy loams.
Rolling resistance is highest with heavy implements in
soft or loose soils. Typical speeds with tillage, fertiliza-
tion, and seeding implements are 1.5-2.5 m/s, in grain
harvesting 1-1.5 m/s. Field efficiencies are commonly
up to 90% in tilling and cultivating, 65-70% in fertilizing
and grain harvesting. Small plots and heavy yields will
lower these levels and demand much time off for adjust-
ments or repairs. Actual fuel consumption is thus highly
variable. With diesel-fueled machines the typical ranges
(in MJ/ha) are moldboard plowing 600-1200, disking
200-4900, planting 80-160, ammonia application 150-
300, cultivating 100-200, and grain harvesting 250-
500. For gasoline-powered machines the rates are about
one-third higher.
No-till farming (row crops planted into narrow slits in
undisturbed sod) and conservation tillage, which leaves
at least 30% of plant residues on the soil surface and
