Modern Engineering Thermodynamics

494 CHAPTER 13: Vapor and Gas Power Cycles
The fuel-air mixture was ignited by an electric spark inside the cylinder. The spark was generated by a battery
and an induction coil, which was basically the same technique used with all spark ignition engines through the
middle of the 20th century.
The Lenoir engine ran smoothly, but because the air-fuel mixture was not compressed before ignition, the
engine had a very low actual thermal efficiency (less than 4%). 9 Consequently, Lenoir engines became popular
only in small sizes (0.5 to 3 hp) because their fuel consumption was very high.
The thermal efficiency of the Lenoir cycle is given by
ðη T Þ Lenoir
= ð _W out Þ net Q
= _ H − j _Q L j
_Q H
_Q H
= 1 − j _Q L j
_Q H
where, from Figure 13.42a, for a cold ASC with an isentropic expansion from 1 to 2s, we have
j _Q L j = j 2s
_Q 3 j = _m ðu 2s − u 3 Þ + _mpðv 2s − v 3 Þ
= _m ðh 2s − h 3 Þ = _mc p ðT 2s − T 3 Þ
and
_Q H = 4
_Q 1 = _mc v ðT 1 − T 4 Þ
Because the intake air comes from an isothermal source (the atmosphere), T 3 = T 4 . However, the exhaust gas is
confined to a fixed mass, so the condition p 2s = p 3 requires that T 2s /T 3 = v 2s /v 3 ,wherev 2s /v 3 is the isentropic
compression ratio, CR. Then, the thermal efficiency becomes
ðη T Þ Lenoir
cold ASC
ð Þ = 1 − kT T 2s /T 3 − 1
3
T 1 − T 4
(13.21)
v 2s /v 3 − 1
= 1 − kT 3 = 1 − kT
CR − 1
3
T 1 − T 4
T 1 − T 4
= 1 − c pðT 2s − T 3 Þ
c v T 1 − T 4
EXAMPLE 13.11
The small model airplane jet engine shown in Figure 13.43 operates on the Lenoir cycle. It has a maximum temperature of
T 1 = 800. R and an intake temperature of T 3 = T 4 = 530. R. The expansion, exhaust, and intake pressures are all p 2s = p 3 =
p 4 = 14.7 psia, and the engine contains 1.00 × 10 –3 lbm of air. For this engine, determine
a. The combustion pressure p 1 .
b. The isentropic compression ratio CR = v 2s /v 3 .
c. The Lenoir cold ASC thermal efficiency.
FIGURE 13.43
Example 13.11.
9 These engines were often called atmospheric gas engines for the same reason.