Modern Engineering Thermodynamics

508 CHAPTER 13: Vapor and Gas Power Cycles
WHAT IS THE SMALLEST INTERNAL COMBUSTION ENGINE?Continued
FIGURE 13.50
Cox Tee engine.
p
1
13.19 ATKINSON CYCLE
In Otto cycle engines, the pressure in the cylinder at the end of the expansion (power) stroke is still 3 to 5 atm when
the exhaust valve opens. The British engineer James Atkinson (1846–1914) realized that the efficiency of the Otto cycle
could be improved if the combustion gases could be expanded to near atmospheric pressure before being exhausted
from the engine. In 1882, he invented a piston engine that allowed the intake, compression, power, and exhaust
strokes of the four-stroke cycle to occur in a single crankshaft revolution. The crankshaft was mounted on a separate
axis from the piston rods and was connected by a series of levers that allowed all four strokes of the cycle to occur in a
single crankshaft revolution (Figure 13.51). The complex crankshaft also produced a power stroke that was longer than
the compression stroke, which allowed the engine to achieve a greater efficiency than an equivalent Otto cycle engine.
The Atkinson cycle engines were slightly more efficient than comparable Otto cycle engines of the day, but they were
also larger and more expensive. Consequently, Atkinson’s engine did not achieve market success and soon
disappeared.
The additional work produced by the Atkinson cycle over the equivalent Otto
cycle is the area enclosed by the area 2a → 2s → 3 → 4 → 2a. Thethermal
efficiency of the Atkinson cold ASC is given by
5 s
2 a
2
3′
4
s
3
FIGURE 13.51
The ideal Atkinson cycle consists of the following
operations: 1–2 s , isentropic (reversible and adiabatic)
expansion; 2 s –3: isochoric (constant volume) cooling;
3–4–3′, isobaric (constant pressure) exhaust and
intake; 3–4, isobaric (constant pressure) cooling; 4–5 s ,
isentropic compression; and 5 s –1, isochoric heating
(combustion).
V
ðη T Þ Atkinson
cold ASC
kðER − CRÞ
= 1 −
ER k − CR k
where ER = v 2s /v 1 is the isentropic expansion ratio and CR = v 4 /v 5s is the isentropic
compression ratio. Note that, as the expansion ratio ER approaches the
compression ratio CR, the Atkinson cold ASC thermal efficiency should
approach the Otto cycle cold ASC thermal efficiency. To be effective, the
expansion ratio should be greater than the compression ratio. Typical values
are compression ratio = 8:1 and expansion ratio = 10:1.
Ilmor Engineering, a firm that is co-owned by Roger Penske and that supplies
HondaenginestotheIndyRacingLeague,isdevelopinganewthree-cylinder
engine that simulates the Atkinson cycle. Two cylinders operate on the conventional
four-stroke cycle and empty their exhaust into a third, low-pressure
expansion cylinder, which allows the expansion and compression processes to
operate independently. A prototype engine was first displayed at the 2009
Stuttgart Engine Exposition.