Modern Engineering Thermodynamics

13.18 Otto Cycle 503
T
Q H
1
1
1
v = c
s = c
4s
W out
p 2s p
4
v = c 2s 4s s = c
2
3′ 3
3
Q
3′
3
L
s
V
V
(a) T−s diagram
(isentropic cycle)
(b) p−V diagram
(isentropic cycle)
(c) p−V indicator diagram
(actual cycle)
Combustion (4s to 1)
and power (1 to 2s)
Exhaust
(2s to 3 to 3′)
Intake
(3′ to 3)
(d) The operation of a four-stroke Otto cycle engine
Compression
(3 to 4s)
FIGURE 13.48
The Otto air standard cycle.
the dawn of the automobile era. The German engineer Karl Friedrich Benz (1844–1929) is generally credited
with building the first practical automobile, using a low-speed Otto cycle engine running on liquid hydrocarbon
fuel, in 1885. He used engine exhaust heat to vaporize the fuel before it was fed into the engine.
WHO INVENTED THE “OTTO” CYCLE?
Unknown to Nikolaus Otto, the four-stroke cycle IC engine had already been patented in the 1860s by the French engineer
Alphonse Eugene Beau de Rochas (1815–1893). However, Rochas did not actually build and test the engine he patented.
Since Otto was the first to actually construct and operate the engine, the cycle is named after him rather than Rochas.
In 1878, the Scottish engineer, Dugald Clerk (1854–1932) developed a two-stroke version of the Otto cycle,
producing one crankshaft revolution per thermodynamic cycle (it was like the Lenoir engine but with preignition
compression). In 1891, Clerk went on to develop the concept of IC engine supercharging. This increased
the thermal efficiency of the engine by further compressing the induction charge before ignition.
Although Clerk’s two-stroke engine was inherently less fuel efficient than Otto’s four-stroke cycle engine, it gave
a more uniform power output (which is important only for single- or dual-cylinder engines) and had almost
double the power to weight ratio of the Otto engine. The two-stroke Otto cycle (it never became known as the
Clerk cycle) engine became successful as a small, lightweight engine for boats, lawn mowers, saws, and so forth.
The thermal efficiency of the Otto cycle is given by
ðη T Þ Otto
= ð _W out Þ net Q
= _ H − j _Q L j
= 1 − j _Q L j
_Q H
_Q H
_Q H
where, from Figure 13.48, j _Q L j = _m ðu 2s − u 3 Þ and _Q H = _m ðu 1 − u 4s Þ.
Then, the thermal efficiency of the Otto hot ASC is
ðη T Þ Otto
= 1 − u 2s − u 3
u 1 − u 4s
hot ASC
For the Otto hot ASC, Table C.16a or C.16b in Thermodynamic Tables to accompany Modern Engineering Thermodynamics
are used to find values for the specific internal energies. Since the processes from 1 to 2s and from 3 to
4s are isentropic, we use the v r columns in these tables to find
v 3
v 4s
= v r3
v r4
= v 2s
v 1
= v r2
v r1
= CR