1. Solar refrigeration has applications in both developing and developed countries for vaccine storage, food preservation, and air conditioning. Previous research on photovoltaic and solar thermal refrigeration systems is reviewed. 2. Research is underway at Warwick University on carbon-ammonia refrigerators driven by steam heat from a thermosyphon heat pipe sourced by solar energy or biomass. A new area of interest is using desiccant wheel technology for solar powered air conditioning. 3. The basic principles of solar absorption refrigeration are described and past experience is assessed. Solar absorption refrigeration shows potential but current systems are still costly.

1. 1
ABSTRACT
Solar refrigeration may have applications in both developed and developing
countries. Applications in developing countries such as vaccine storage or large
scale food preservation have been the subject of much research. In developed
countries the main area of interest is air conditioning. Previous work on photo-
voltaic and solar thermal systems is reviewed. Research at Warwick is
underway on carbon - ammonia refrigerators driven by the heat of steam
condensing in a thermosyphon heat pipe. The heat source can be solar energy,
biomass, or some combination of the two. A new area of interest is the use of
desiccant wheel technology for solar powered air conditioning. The basic
principles are described and past experience assessed.

2. 2
Introduction
The prices of energy have been increasing exponentially worldwide. Industrial
Refrigeration is one of the most energy consuming sector. What if a
refrigeration system is designed which uses no energy or minimal amount of
energy? The solution lies in absorption refrigeration system. By producing an
adsorption refrigeration system we are not only cutting down the energy costs
but also preserving our environment. This refrigeration system doesn’t use any
of the CFCs so our ozone layer is safe. Greenhouse gases and their damaging
effects on the atmosphere have received increased attention following the
release of scientific data by United Nations Environment Programme and World
Meteorological Organization that show carbon dioxide to be the main
contributor to increased global warming(UNEP, 1991). The domestic
refrigerator-freezers operating on alternative refrigerants such as HFC-134a,
contribute indirectly to global warming by the amount of carbon dioxide
produced by the power plant in generating electricity to operate over a unit over
its lifetime. This contribution is nearly 100 times greater than the direct
contribution of the refrigerant alone. Moreover, approximately 62 million mew
units are being manufactured worldwide every year, and hundreds of millions
are currently in. use.(UNEP,1995) it is anticipated that the production of
refrigerator-freezers will substantially increase in the near future as a result of

3. 3
the increased demand, especially in the developing countries. Therefore, in
response to global concerns over greenhouse resorts are being made to produce
refrigerator-freezers with low energy consumption. In most of the developing
third world, adequate supplies of drinking water and water for irrigation are a
scarce commodity. In many places in Africa, India and Central and South
America, adequate supplies of water are found only at considerable depth below
the surface. These locations generally do not have the infrastructure to provide
an electrical grid to pump the water with electricity nor do they have the
infrastructure to provide roads to bring in electrical generators or even the fuel
for those generators.
Description
1.Field The present invention relates to refrigeration system, and more
particularly, relates to a refrigerating system directly or indirectly utilizing solar
energy to achieve an efficient performance.
2. Energy is an unavoidable topic in the new century. It is witnessed that
sciences and technologies related to the energy industry had been significantly
concerned all over the world. Especially, the solar energy, widely considered as
an unexhausted energy source, had been employed in a variety of applications,
such as refrigeration industry. Nowadays, the solar energy refrigeration is
exclusively focused on the air-conditioning refrigeration, in which a plurality of
refrigerating modes had been unveiled, such as, solar absorption refrigeration,
solar adsorption refrigeration, solar mechanic compression refrigeration, solar
dehumidification refrigeration, solar injection refrigeration, and so on.
However, the above mentioned solar air-conditioning devices are so
complicated and costly, which in turn, restricting its prevailed applications in
practices. As a result, it is rarely seen such solar refrigerating means had been
used for household purposes. What is more, such solar refrigerating means are
supposedly operated under sealed or closed chambers, the ventilation is out of
reach thus resulting the air quality really unserviceable. To a worse extent, CFC

4. 4
had been sometimes employed in some occasions. The damage to the
environment of the CFC was terrible. The solar cold water air-conditioner had
been introduced in 1980s, wherein the solar energy is converted into heat for
producing cold water. A very common method for producing cold water is the
heat absorption process, wherein the water is utilized as refrigerating medium,
and the lithium bromide is employed as absorbing agent. Unfortunately, the
dehumidification process would be a bottleneck and low temperature chilled
water ranging from 7 9.degree. C. had to be prepared for ensuring such
dehumidification. It is noted that whenever a single centigrade dropped of such
chilled water temperature, the refrigerating efficiency of such refrigerating
process would be decreased by 3%. This is undesirable for most users. In short,
such solar absorption refrigeration is still costly.
Components
1. Solar Panel
2. Battery 7Ah (12v)
3. Inverter 100 VA
4. Absorber
5. Condenser
6. Evaporator
7. Generator

5. 5
ABSORPTION REFRIGERATION SYSTEM
The absorption cycle is similar in certain respects to the vapor compression
cycle. In this system, refrigerant is vaporized by absorbing latent heat of
vaporization from the materials which have to be cooled at low pressure region;
alternately the refrigerant condenses at the condenser by rejecting latent heat of
condensation to the adjacent medium at high pressure region. There are three
types of absorption systems- intermittent absorption system, continuously
absorption system and double intermittent absorption systems. The difference is
in the operating period of cycle. In the continuously operating cycle, heat is
added in the generator for 24 hours by an external heat source and evaporator
maintains it temperature for 24 hours whereas in intermittent system heat is
added for a certain period. Intermittent absorption systems are able to use waste
heat and solar energy. The intermittent process works with ideal refrigerant and
an absorbent. High pressure or heat separates the two elements during the
generating phase and cooling/refrigeration.
takes place through the absorption/adsorption of the pair. Ambient cooling is an
intermediate phase which takes place to reduce high pressure gas/vapor into a
refrigerant working liquid. Double intermittent absorption systems are the
refinement of the intermittent systems that work either in cascade, at a higher

6. 6
pressure and producing refrigeration more than single intermittent absorption
process. The coefficient of performance of the absorption cycle (COP abs) is
calculates as equation.
DESIGN AND EXPERIMENTALSETUP
The solar intensity, temperature, generation period was assumed by considering
the surrounding environment of experiment (Khulna, Bangladesh). The
temperature at day fluctuated between 250C to 350C where solar intensity was
around 700-760 (W/m2) and average generation period was 6-7 hrs. There are
various types of refrigerant and absorber pair are being used at present time.
Among them NH3/H2O is widely used where low temperature is required and
NH3/LiBr systems are used where moderate temperature are required like air
conditioning. Several refrigerant and absorber pairs were studied and it showed
the superiority of solid absorbents over liquid absorbents and calcium chloride
was found better, less expensive and available compared to other absorbents.
Again the boiling temperature between ammonia and calcium chloride is so
distinct that there is no possibility of boiling calcium chloride with the boiling
temperature of ammonia. So the design parameter were assumed as follows

7. 7
System Description

8. 8
Proposed Design of Intermittent vapour absorption system
This system actually was not a true refrigeration system. It was an intermittent refrigeration
system because here the mass flow rate was not constant throughout the system. The mass
flow rate of the ammonia vapor in system increased when the temperature of the generator
increased. The proposed system used-a generator for heating the salt-ammonia mixture, a
condenser coil for condensing the vapor ammonia into liquid ammonia, a storage tank for
storing the liquid ammonia coming from condenser at day cycle and an evaporator where the
materials for cooling were kept. The absorption system operated in a day/night cycle,
generating distilled ammonia during the daytime and reabsorbed it at night. During the day
cycle the valve on the storage tank was opened, the valves at the bottom of the storage tank
and outlet of the evaporator remained closed. Ammonia vapor condensed in the condenser
coil and dripped down into the storage tank .About 100 degrees centigrade, six of the eight
ammonia molecules bound to each salt molecule were available [9]. At night cycle the valve
on the storage tank was kept closed and the valves at the bottom of the storage tank and outlet
of the evaporator were kept opened. The liquid ammonia from the storage tank passed
through the evaporator coils and got vaporized after taking heat from the evaporator box. As
the pressure at the generator remained low at night cycle the ammonia vapor went toward the
generator and reabsorbed in the calcium chloride. The total working cycle of the system is
shown by the following diagram-

9. 9
Working cycle of vapour absorption system
The ammonia gas is toxic and highly corrosive to brass so the pressure gauge,
elbow, valve, T-section, union and the tube for piping were made of stainless
steel. Another concerning thing of the project was the sustainability of high
pressure. The system was made in such a way that can sustain about 1.7MPa
pressure because the ammonia expands greatly. The main components of the
system were designed as follows: The system used a parabolic solar collector. A
cylindrical parabolic shape reflector was designed to use as a reflector in this
project. Firstly a parabola was drawn in a paper and then drawn parabola was
used to construct the structure of the reflector. Reflector had two parts: structure
for parabola and reflecting sheet. GI square bar was used for structure of the

10. 10
parabola due to its low weight and high strength to hold the structure. S .S.
magnet sheet was used as reflector. PVC sheet was used for supporting the
reflector. The structure of the reflector was made by welding. The reflector and
PVC sheet were attached with the structure by screws. The reflector size was
2.2 x 1.59 m. The entire reflector was stood by shaft which was nearly passed
through the centre of gravity of the reflector and receiver.
The proposed generator was a tube made of stainless steel which was 2.45m
long, 76.2mm in diameter and 1.5 mm in thickness. The generator in this system
also acts as receiver. It contained the mixture of Calcium Chloride and
ammonia. The outer surface of the generator was black colored with black so
that it can absorb maximum heat reflected from the solar collector. One end of
the generator was closed and other end was open to the condenser through PVC
pipe. The condenser was a stainless steel tube which had 12 turns and the
diameter of the condenser tube was 9.5mm. The condenser coil had a diameter
of 29.2cm and the total length of the tube was 6m. The condenser was fixed into
a cylindrical box containing water at the day cycle. At the night cycle there was
no water into the condenser containing box. As the water got hotter after taking
heat so hot water was brought out and cool water was supplied to the condenser
box. The condenser box was surrounded by cork sheet box to ensure no other
heating source of water except vapour ammonia. The system used a stainless
steel cylinder for the purpose of storage tank which was about 60.9cm long, 2

11. 11
mm in thickness and 10.16cm in diameter. In the day cycle the liquid ammonia
stored in the storage tank. Here the proposed evaporator was a rectangular box
made of galvanized sheet. The evaporator was surrounded by evaporator coil
made of stainless steel pipe .The diameter of the evaporator coil was 9.5mm and
thickness was 1mm. The evaporator was placed into a box made of cork sheet
so that an even temperature can be maintained all over the box and also
ensuring heat entrapment by keeping felt on the gap between the galvanized
sheet box and cork sheet box.

12. 12
Photovoltaic Operated Refrigeration Cycle
Photovoltaic (PV) involve the direct conversion of solar radiation to direct
current (dc) electricity using semiconducting materials. In concept, the
operation of a PV-powered solar refrigeration cycle is simple. Solar
photovoltaic panels produce dc electrical power that can be used to operate a dc
motor, which is coupled to the compressor of a vapor compression refrigeration
system. The major considerations in designing a PV-refrigeration cycle involve
appropriately matching the electrical characteristics of the motor driving the
compressor with the available current and voltage being produced by the PV
array. The rate of electrical power capable of being generated by a PV system is
typically provided by manufacturers of PV modules for standard rating
conditions, i.e., incident solar radiation of 1,000 W/m2 (10 800 W/ft2) and a
module temperature of 25°C (77°F). Unfortunately, PV modules will operate
over a wide range of conditions that are rarely as favorable as the rating
condition. In addition, the power produced by a PV array is as variable as the
solar resource from which it is derived. The performance of a PV module,
expressed in terms of its current voltage and power-voltage characteristics,
principally depends on the solar radiation and module temperature. Shows
current (solid lines) and power (dotted lines) vs. voltage for a 1.32 m2 (14 ft2)
single crystalline PV module at the reference condition and four operating
conditions.

13. 13
At any level of solar radiation and module temperature, a single
operating voltage will result in maximum electrical power production from
the module. The module represented in shows the voltage that yields
maximum power ranges between 30 and 35 volts for this PV array.

14. 14

15. 15
The efficiency of the solar panels, defined as the ratio of the electrical
power produced to the incident radiation is between 8% to 10% at
maximum power conditions for the PV array represented in Figure 2. If
the PV refrigeration system is to operate at high efficiency, it is essential
that the voltage imposed on the PV array be close to the voltage that
provides maximum power. This requirement can be met in several ways.
First, a maximum power tracker can be used which, in effect,
continuously transforms the voltage required by the load to the
maximum
power voltage. If the system includes a battery, the battery voltage will
control the operating voltage of the PV module. PV panels can then be
chosen so that their maximum power voltage is close to the voltage for
the battery system The battery also provides electrical storage so that he
system can operate at times when solar radiation is unavailable.
However, the addition of a battery increases the weight of the system
and reduces its steady-state effi ciency. Electrical storage may not be
needed in a solar refrigeration system as thermal storage, e.g., ice or
other low temperature phase storage medium, may be more effi cient
and less expensive. A fi nal option for systems that do not use a
maximum power tracker or a battery is to select an electric motor having
current-voltage characteristics closely matched to the maximum power
output of the module. superimposes the current-voltage characteristics

16. 16
of a series dc motor and separately excited motor on the photovoltaic
module. In this case, the separately excited motor would provide more
efficientoperation because it more closely matches the maximum power
curve for the photovoltaic module. However, neither motor type
represented in Figure 3 is well-matched to the characteristics of the PV
module over the entire range of incident solar radiation. Studies of solar-
powered motors have shown that permanent magnet or separately
excited dc motors are always a better choice than series excited dc motors in
direct-coupled systems that are not equipped with a maximum power tracker.
Solar Mechanical Refrigeration
Solar mechanical refrigeration uses a conventional vapour compression system
driven by mechanical power that is produced with a solar-driven heat power
cycle. The heat power cycle usually considered for this application is a Rankin
cycle in which a fluid is vaporized at an elevated pressure by heat exchange
with a fluid heated by solar collectors. A storage tank can be included to rovide
some high temperature thermal storage. The vapour flows through a turbine or
piston expander to produce mechanical power, as shown in. The fluid exiting
the expander is condensed and pumped back to the boiler pressure where it is
again vaporized. The efficiency of the Rankin cycle increases with increasing
temperature of the vaporized fluid entering the expander, as shown in efficiency
in was estimated for a high-temperature organic fluid assuming that saturated
vapour is provided to a 70% efficient expander and condensation occurs at

17. 17
35°C (95°F). The efficiency of a solar collector, however, decreases with
increasing temperature of the delivered energy. High temperatures can be
obtained from concentrating solar collectors that track the sun’s position in one
or two dimensions. Tracking systems add cost, weight and complexity to the
system. If tracking is to be avoided, evacuated tubular, compound parabolic or
advanced multi-cover fl at plate collectors can be used to produce fluid
temperatures.
Absorption Refrigeration
Absorption refrigeration is the least intuitive of the solar refrigeration
alternatives. Unlike the PV and solar mechanical refrigeration options, the
absorption refrigeration system is considered a “heat driven” system that
requires minimal mechanical power for the compression process It replaces the
energy-intensive compression in a vapour compression system with a heat
activated “thermal compression system.” A schematic of a single-stage
absorption system using ammonia as the refrigerant and ammonia-water as the
absorbent is shown in. Absorption cooling systems that use lithium bromide-
water absorption-refrigerant working fluids cannot be used at temperatures
below 0°C (32°F). The condenser, throttle and evaporator operate in the exactly
the same manner as for the vapor compression system. In place of the
compressor, however, the absorption system uses a series of three heat
exchangers (absorber, regenerating intermediate heat exchanger and a
generator) and a small solution pump. Ammonia vapour exiting the evaporator

18. 18
(State 6) is absorbed in a liquid solution of water-ammonia in the absorber. The
absorption of ammonia vapour into the water-ammonia solution is analogous to
a condensation process. The process is exothermic and so cooling water is
required to carry away the heat of absorption. The principle governing this
phase of the operation is that a vapour is more readily absorbed into a liquid
solution as the temperature of the liquid solution is reduced.
A number of barriers have prevented more widespread use of solar refrigeration
systems. First, solar refrigeration systems necessarily are more complicated,
costly, and bulky than conventional vapour compression systems because of the
Necessity to locally generate the power needed to operate the refrigeration
cycle.

19. 19
Experimental Model

20. 20
Possible refrigeration cycles
There are five classes of cycle that can be used for renewable powered
refrigeration systems. (Desiccant wheel technology for air conditioning is dealt
with later)
1. A standard mechanical vapour compression cycle, requiring an electrical
input to a hermetically sealed compressor. The electricity is generated by
photovoltaic panels. This has the advantage of using offthe- shelf
technology, but the disadvantages of high cost and the probable need for
an electricity storage sub-system.
2. Intermittent adsorption cycles
Adsorption refrigeration cycles rely on the adsorption of a refrigerant
gas into an adsorbentat low pressure and subsequent desorption by heating. The
adsorbent acts as a ‘chemical compressor’ driven by heat. In its simplest form
an adsorption refrigerator consists of two linked vessels, one of which contains
adsorbent and both of which contain refrigerant as shown in Fig. 1 below.

21. 21
Initially the whole assembly is at low pressure and temperature, the adsorbent
contains a large concentration of refrigerant within it and the other vessel
contains refrigerant gas (a). The adsorbent vessel (generator) is then heated,
driving out the refrigerant and raising the system pressure.
The desorbed refrigerant condenses as a liquid in the second vessel, rejecting
heat (b). Finally the generator is cooled back to ambient temperature,
reabsorbing the refrigerant and reducing the pressure. Because the liquid in the
second vessel is depressurised and boils, it takes in heat and produces the
required refrigeration effect. The cycle is discontinuous since useful cooling
only occurs for one half of the cycle. Two such systems can be operated out of
phase to provide continuous cooling. Such an arrangement has a comparatively
low Coefficient of Performance (COP = Cooling / Heat Input). Also, the
thermal conductivity of the bed is generally poor so the time taken for a cycle
could be an hour or more and the cooling power per mass of adsorbent could be
as low as 10 W/kg. This is not a problem with solar powered vaccine
refrigerators which produce a few kg of ice each day and operate on a diurnal
cycle (Critoph [3]). However, a refrigerator producing one tonne of ice in a
diurnal cycle would need 5 tonnes of carbon and contain 1.5 tonnes of
ammonia. When contemplating larger icemakers it is obviously necessary to use
a much faster acting cycle in order to reduce the mass of adsorbent and the cost
of the system. Two beds, similar to the one shown above, can be heated and

22. 22
cooled out of phase to provide continuous cooling Good heat transfer is required
to reduce the cycle time to a few minutes and thereby increase the power
density of the adsorbent to the order of 1 kW/kg. We can also achieve a higher
COP by maximising the quantity of heat regenerated. The heat rejected by one
bed when adsorbing can provide a large part of the heat required for desorbing
in other bed. This also requires good heat transfer.
Intermittent absorptioncycles
These are thermodynamically identical to adsorptionsystems but use liquid
absorbents rather than solid adsorbents. Typically the pair used is ammonia-
water, but ammonia-NaSCN, methanol-LiBr and other pairs have been used
experimentally.
A continuous absorption cycle with an electrically driven feed pump eliminates
the problems of bulk, but if electricity is available to drive a solution feed pump
then it could be argued that it would be better to use a conventional vapour
compression cycle. The use of a small amount of photovoltaic electricity to
drive a feed pump might be justified.
The Platen-Munters diffusion absorption cycle is continuous and does not use a
mechanical pump. It is used successfully in small gas or kerosene refrigerators
and freezers but has proved difficult to adapt to larger sizes and to irregular heat
sources such as solar energy.

23. 23
Energy Efficiency
No mention of Coefficient of Performance (COP), the ratio of cooling to energy input has
been made in the above discussion. This is partly because of the difficulty of comparing
systems under different conditions, but also to avoid the inference that COP is the most
important parameter. Commercialisation will depend on the total cost of capital equipment,
maintenance and fuel over the lifetime of the plant. In a solar thermal system where the
collectors probably dominate the capital cost, and the fuel cost is zero, it can be argued that a
high COP reduces collector area and thus the total cost. However, if the consequences of a
high COP are complexity and unreliability then other costs may rise. As a rough guide
one may say that internal COP (cooling / heat to generator) of thermal systems can range
from 0.1 (typical) to 0.5 (best) for a Platen-Munters machine, 0.2 to 0.5 for intermittent
sorption systems and up to 0.7 for intermittent regenerative cycles. Vapour compression
machines with an electricity input would have typical COP’s of 1.0 to 1.5.
The most expensive part of all solar refrigeration systems is the collector array.
If the system is to be economic then ways must be found to minimise the cost.
The total area of collector can be reduced by utilising a back-up energy source
during periods of low insolation. The source can be a renewable one such as the
combustion of agricultural waste i.e. bagasse, rice husks, etc. In some locations
there may be enough biomass to dispense with the solar input completely and
use a biomass source. If collectors are used in what are bound to be large arrays,
there is a case for using an intermediate heating fluid in order to reduce the cost.
Many thermally driven refrigeration cycles use ammonia as the working fluid
and the cost of constructing solar collectors which heat the fluid at pressures of

24. 24
up to 25 bar could be prohibitive. Other types of machine use water or methanol
as refrigerants but in these cases the collectors suffer from the problems of
having to be hermetically sealed against air ingress. Any inward leakage of air
can stop the system working. The solution preferred here is to use collectors
which will boil water in a thermosyphon heat pipe arrangement which does not
require a pump and yet provides excellent heat transfer between the collector
and refrigeration unit. Such a heating arrangement is also ideally suited to the
use of burning biomass as a heat source, whether as a back-up or as sole source.
Our research is concentrating on the use of adsorption cooling systems which
receive heat either from a thermosyphon heat pipe or from a pumped thermal
fluid. The former is probably more suited to remote applications.

25. 25
Vapour Compression Refrigeration System Using Solar
Theoretical cycle
Idealized cycle can be used to describe the operation of system. The
consideration of these cycle gives a better understanding of the system and
provides a basis of evaluating the theoretically expected performance of the
system for comparison with experimentally determined data.
Theoretical cycle of the system
The teoritical cycle for the operation of the solar power is the

26. 26
Superheated Rankine cycle
A temperature- entropy diaagran of the theoretical cycle is shown below:
Beginning at point 1, the worl=king fluid goes into the pump as a saturated
liquid, it is compressed isentropic ally to a sub cooled liqid condition at sate 2
and enters the vapour generator. In the genetor 2, the fulid is heated at constant
pressure to a super heated condition at sate 3. It is then introduced into the
engine, where it expands isentropic ally to sate 4. The vapour is the condensed
to a saturated liquid in condenser, completing the cycle.
For the quantitative evaluation of the theoretical cycle, one must select a
working fluid and establish certain criteria for operation of the cycle. These
criteria must be selected from experimental work and from limitations
established for the components of the system.

27. 27
The use of water at approximately 72°F for cooling in the condenser makes. the
minimum temperature which can practically be expected in the condenser 80'F.
Also a reasonable maximum vapour temperature that can be attained with solar flat plate
collectors is about 160'F. We will suppose that the vapour temperature is 140'F or 160'F. And
from experimental evaluation of the engine it was discovered that the optimum pressure for
operation of the engine is about 35 psig using compressed air.
Inshort the criteria are:
(1) condenser temperature of 80'F
(2) vapor temperature of 140°F in vapor generator
(3) pressure rise of about 35 psig in the pump.
After basic design criteria is established, it is possible to examine a theoretical
cycle operating on a particular fluid.
Ezample: Theoretical Cycle of R-11
If we consider the desigi criteria and apply to the Refrigerant 11 as working
fluid, the cycle can be represented on a P-H diagram in Figure 3. From
ASHRAE Handbook of Fundamentals 1967, p. 286, we can get the following
results: For R-11

28. 28
The cycle for R-11 is described. The condensing pressure at 8j°F is 16.2 psi and
state C falls on saturated liquid at an enthalpy of 24.52 Btu/lb. The process from
D to A is an isentropic compression of liquid to a compressed liquid state at
45.1 psi and the same values of temperatures and enthalpy.
At Point B the fluid is a superheated vapour at 140°F with and enthalpy of
108.5 Btu/lb. After the isentropic expansion to sate C. He enthalpy os
102Btu/lb.
Thus the Following calculation can be made:

29. 29
If W = work a1loutput = HD -HC 6.5 Btu/lb Heat input = (108.5 -24.5) Btu/lb =
84 Btu/Ib
11thermal = net work output = 6.5 7% heat input 84
The thermal efficiency of this theoretical cycle is7%.
If the temperature in the generator is 160 0F,
Tthermal = 9.6%.

30. 30
Theoretical Analysis of the Engine Cycle
The theoretical cycle of the V-2 vapour cycle engine is represented on a
pressure-volume diagram in figure 4. The solid-in identifies the cycle operating
on air and the broken line describes operation with R-11 as the working fluid.
Following the solid line, the cycle for air begins with the opening of the intake
part at point F. Air at constant pressure flows into the engine driving the power
piston back. The intake valve closes at point B, and A isentropic expansion
occurs at point C', where the exhaust valve opens and the pressure drops to the
exhaust pressure. A constant pressure exhaust stroke from C to D takes place,
and the cycle is completed by an isentropic compression of the vapour
remaining in the cylinder at the end of the exhaust back to the initial pressure
and volume. The cycle is similar for R-11.
Determination of Engine Volumes
A factor which has considerable effect on the results of the theoretical
evaluation of the engine cycle is the determination of the engine volumes. This
is done by relating the diagram of valve opening and closing to the position of
the piston within the power cylinder.

31. 31
The povrer piston leads the valve piston by 450; therefore a 450 rotation of the
valve diagram makes it possible to determine the position of the piston within
the power cylinder at the time of valve opening and closing, and subsequently
calculate the engine volumes. The clearance volume of a cylinder is about 0.298
cubic inches and the valve opens for intake 0.0625 inches from the top of its
stroke, which yields a volu-Me at point A of 0.496 cubic inches. But this
volume is not equal to the volume calculated by an isentropic compression of
air from point D to point A of 5.91 c-1. This difference is due to errors om
measuring the piston, in calculating theoretical performance it will be supposed
that the cylinder volume at point A is ).591 C.i.