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TM 5-685/NAVFAC MO-912
3-1. Mechanical energy.
(4) In the Otto cycle, the fuel/air mixture is
compressed and ignited by a timed spark. The exact
A prime mover is an engine that converts hydraulic,
ratio of fuel to air is achieved by carburization of a
chemical, or thermal energy to mechanical energy
volatile fuel. Fuel injection is also in use in the Otto
with the output being either straight-line or rotary
cycle to achieve more precise fuel delivery to each
motion. Rotary mechanical energy is used to drive
rotary generators to produce electrical energy. Over
(5) Four-cycle SI gasoline engines are used as
the last 125 years, the internal combustion engine,
prime movers for smaller portable generator drives
steam turbine and gas turbine have displaced the
(see fig 3-l). The advantages are:
steam engine. Auxiliary electrical generators are
(a) Low initial cost.
today usually driven by either reciprocating engine
(b) Light weight for given output.
or gas turbine. These are available in wide ranges of
(c) Simple maintenance.
characteristics and power rating, have relatively
(d) Easy cranking.
high thermal efficiency and can be easily started
(e) Quick starting provided fuel is fresh.
and brought on line. In addition, their speed can be
(f) Low noise level.
closely regulated to maintain alternating current
(6) The dis ad vantages of using four-cycle SI
system frequency.
gasoline engines are:
a. Fuel is burned directly in the internal combus-
(a) Greater attendant safety hazards due to
tion engine. The burning air/fuel mixture liberates
use of a volatile fuel.
energy which raises the temperature of the mixture
(b) Greater specific fuel consumption than
and, in turn, causes a pressure increase. In the
compression ignition (CI) engines.
reciprocating or piston engine this occurs once for
(7) Reciprocating CI engines. These operate on
each power stroke. The pressure accelerates the pis-
the Diesel Cycle principle typical for all CI engines.
ton and produces work by turning the crankshaft
The-events are:
against the connected load.
(a) Intake stroke. Air is drawn into the cylin-
(1) Reciprocating spark ignition (SI) engines.
These engines operate on the Otto Cycle principle
(b) Compression stroke. Air is compressed,
typical for all reciprocating SI engines. The events
raising the pressure but `also raising the tempera-
ture of the air above the ignition temperature of the
(a) Intake stroke. A combustible fuel/air mix-
fuel to be injected.
ture is drawn into the cylinder.
(b) Compression stroke. The temperature
(c) Power stroke. A metered amount of fuel at
and pressure of the mixture are raised.
greater-than-cylinder-pressure is injected into the
cylinder at a controlled rate. The fuel is atomized
(c) Power (expansion) stroke. Ignition of the
and combustion occurs, further increasing pressure,
pressurized gases results in combustion, which
thus driving the piston which turns the crankshaft.
drives the piston toward the bottom of the cylinder.
(d) Exhaust stroke. The burned gases are
(d) Exhaust stroke. The burned gas is forced
forced out of the cylinder.
from the cylinder.
(2) Four strokes of the piston per cycle are re-
(8) As with the SI four-cycle engine, the four
cycles of the CI engine occur during two revolutions
quired (four-stroke cycle or four-cycle). One power
stroke occurs in two revolutions of the crankshaft.
of the crankshaft, and one power stroke occurs in
(3) The output o fan engine can be increased
every two revolutions.
with some loss in efficiency by using a two-stroke
(9) The CI or d'
diesel engine may also use two-
cycle operation with increased output but at lower
(two-cycle) Otto process. During the compression
stroke, the fuel/air mixture is drawn into the cylin-
engine efficiency.
der. During the power stroke, the mixture in the
(10) In the Diesel cycle, only air is compressed
and ignition of the fuel is due to the high tempera-
cylinder is compressed. Near the end of the power
stroke, burned gases are allowed to exhaust, and
ture of the air. The CI engine must be more stoutly
constructed than the SI engine because of the
the pressurized new mixture is forced into the cyl-
higher pressures. The CI engine requires high-
inder prior to the start of the next compression
pressure fuel injection.

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