Page 479 - Air Pollution Control Engineering

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Emerging Pollution Control Technologies 451

At the higher compression ratios used in diesel engines, it can be seen from the fol-
lowing formula how greater brake horsepower (bhp) is developed:
bhp ∝ (mep)LAN (1)

where mep is the mean effective pressure, L is the piston movement length, A is the
cylinder cross-section area (displacement), and N is the number of power strokes per
minute in all cylinders.
3.4.3. Steam Engines
The steam or Rankine external combustion engine (ECE) has a higher potential max-
imum efficiency than the ICE but has been plagued by problems with fluid selection and
pumping. The liquid in a Rankine cycle is reversibly pumped into a boiler, where it is
vaporized and then reversibly expanded to produce work. Rankine cycle efficiency η
R
is the net reversible cycle work divided by boiler heat input:

η = W /Q (2a)
R Net Boiler
In a rotating engine, work is being done on or by the power fluid every time it travels
radially, as long as the fluid is constrained to rotate with the engine. Work done by the
pump and added to the fluid per unit weight of fluid is

2
2
W =∆H = (ω /2g R 2 − R ) (3)
p c C B
2
where ω is the angular velocity, g is the dimensional constant (32.174 ft-lb /lb s , or
c m f
2
1 k-m/N-s ), and R and R are the radial distances to liquid levels of the condenser and
C B
boiler, respectively.
Referring to Fig. 2, the liquid pump work ∆H is H − H and vapor pump work ∆H is
7 6
H − H . The net reversible engine work is
4 3
W = H − H (4)
Net 2 4
and the Rankine cycle efficiency can then be expressed as
η = (H − H )/(H − H ) (2b)
R 2 4 1 7
In this cycle using typical organic fluids as shown in Fig. 2, expansion after the boiler
is into the superheated vapor region and the work is less than Carnot cycle work by the
shaded areas. In Fig. 2, T is the fluid critical temperature. Regenerators can be used to
c
improve the Rankine cycle efficiency, as shown in Fig. 3.
The steam produced in the engine is expanded in the turbine to drive the vehicle.
Turbine stage efficiency η is the shaft power divided by energy available to the expander
t
between the nozzle inlets H and the diffuser inlet H , as expressed by
2 3
η = Gross shaft power / [(H − H ) (Mass flow rate)] (5)
t 2 3
The mass flow rate can be calculated using the critical fluid rate in a supersonic nozzle
multiplied by the throat area of all nozzles (assume a discharge coefficient of 0.98).
Currently, steam engines can operate at 25–30 miles/gal on kerosene (depending on
vehicle size and type), start up and run in less than 30 s, operate when the temperature is
−20ºC, and require relatively small boilers. Areciprocating steam engine rated at 100-horse-
power (hp) output would represent a practically sized power plant for automobile utility.