The following points highlight the five major types of heat engines. The types are: 1. Steam Engine 2. Reciprocating Steam Engine 3. Internal Combustion Engine 4. Petrol Engine 5. Diesel Engine.
Type # 1. Steam Engine:
Steam engine converts heat energy into mechanical energy. The heat energy available due to combustion of coal is used to transform water into superheated steam. The engine utilises it to produce mechanical motion. In order to understand the sequence of operation we consider Fig. 26.1. Water from the condenser is heated to force into the boiler, where it is converted into superheated steam.
This steam is admitted into the steam chest C1 through the opening S which then enters into the cylinder through the opening G1 or G2, where it expands against the piston. At the first part of working stroke, it is connected to the boiler and the pressure remains constant. Then the inlet valve V is closed and the steam expands adiabatically for the rest of the working stroke causing a drop in the pressure and temperature of the steam, and some amount of steam condenses.
On the return stroke the mixture of water droplets and steam are forced out of the cylinder and enter into the condenser where it is fully condensed into water. An amount of heat Q is given out in the process. The water is then forced into the boiler by the feed-water pump, and the cycle is repeated when the openings G1 and G2 are closed periodically by the valve V.
The cycle of operation in a steam engine is known as the Rankine cycle. It is considered as a practical steam engine cycle and is a very close approximation to the ideal thermodynamic cycle. It has got six distinct steps. The indicator diagram in Rankine cycle is presented in Fig. 26.2. The initial state of the working system is indicated by the point d in the figure.
Step 1:
ADVERTISEMENTS:
We consider a unit mass of water represented by the point d. It is compressed reversibly and adiabatically to reach at a along the curve d → a to attain the pressure P2 of the boiler. The point a is on the same isothermal at the temperature T2 of the boiler.
Step 2:
It consists of reversible isobaric heating of water from a to A to bring the water to the boiling point T1.
Step 3:
ADVERTISEMENTS:
This step is an isobaric and isothermal vaporisation when the working substance is shifted from A to B at the same temperature T1 and pressure P2 to saturated steam.
Step 4:
On the indicator diagram this step is represented by B → b when the steam is superheated isobarically to attain the highest temperature T.
Step 5:
ADVERTISEMENTS:
The fifth step consists of an adiabatic expansion from b to c along bc when the steam is converted into wet steam.
Step 6:
The final step of the cycle is a reversible isobaric isothermal condensation. It is represented by b → c in the indicator diagram when the steam is converted into water at the initial state d and the cycle is completed.
The working substance then becomes ready to start the next cycle of operation. It is seen that on the indicator diagram, three isothermals are involved at temperatures T2, T1 and T, where T2 is the temperature of the boiler, T1 is the temperature of the condenser and the value of T is greater than that of T1.
ADVERTISEMENTS:
The work done by the cycle is presented by the area abed. For many reasons, a steam engine cannot follow an ideal Rankine cycle. Consequently, efficiency of an actual steam engine is about 70% of an ideal Rankine cycle.
The efficiency of an engine in Rankine cycle is less than that of ideal Carnot’s engine operating between the same two temperatures. The difference between the two is that the reversible adiabatic compression in Carnot’s cycle is replaced by an irreversible process in which the feed water is heated from T2 to T1.
Thus a part of the heat absorbed in the Rankine cycle is absorbed not at constant temperature but over a range of temperatures from T2 and T1, while all heat absorbed in the Carnot’s cycle is absorbed at constant temperature T1.
Type # 2. Reciprocating Steam Engine:
The mechanical parts of a reciprocating steam engine are shown in Fig. 26.3. Superheated steam enters into the slide valve steam chest (SC) through the steam pipe P. Its further progress is regulated by side valve, which moves backward and forward over three openings known as ports. The outer port leads the cylinder but the central one and the exhaust port (E), goes round the back of the cylinder into the exhaust pipe (EP).
When steam enters into the cylinder through N, its pressure forces the piston P to the left. Then the exhaust steam leaves through the exhaust pipe as the piston moves forward.
The piston controls the position of the slide valve. When the piston moves about one-third, the valve slides and closes the inlet part. The steam in the cylinder then expands and does work in pushing the piston in front of it. It also cools, and heat is converted into work. When the piston moves at the extreme left, the valve slides and opens the left part.
Then the right part is connected to the exhaust. Steam enters to the left and drives the piston in the opposite direction. The cycle is repeated again and again and the piston executes a ‘reciprocating’ motion. This reciprocating motion is converted into rotary motion of the driving shaft and flywheel by a special mechanical arrangement.
Type # 3. Internal Combustion Engine:
Combustion of fuel in internal combustion engines, occurs inside the cylinder of the engine as in the boilers of steam engine. The engine occupies less space and is suited for small power purposes. Thermal efficiency and speed of internal combustion engine is higher than that of steam engine.
Principle:
In the engine the fuel is supplied in the form of vapour which is mixed with air. This mixture of fuel with air is burned to produce large explosive force generated by the combustion of the fuel which puts the piston in motion. The fuel used is either a gas, like coal gas or a liquid, like petrol, benzene, alcohol, etc., which are readily vaporised or a heavy oil like diesel oil, etc. When the fuel is vaporised, it forms an explosive mixture with air.
In general, internal combustion engines are four-stroke engines and they require four strokes of the piston to complete a cycle of operations within the cylinder.
Type # 4. Petrol Engine:
This is an internal combustion engine. Basically, there is no difference between a petrol engine and any other gas engine. But a petrol engine is more compact and light. They are generally used in motor cars and aeroplanes. It is a four-stroke engine and completes one cycle following auto cycle.
Description:
In Fig. 26.4 the diagram of a petrol engine is shown. P is a piston made of iron. This can move up and down in an air tight cylinder. Above the cylinder there is a chamber called combustion chamber, where the mixture of air and petrol vapour is ignited by electric sparks from sparking plugs fitted in the chamber. Then entry of the fuel inlet pipe and exit of the burnt gases by exhaust pipe are controlled by two valves, V1 and V2.
The explosive mixture of petrol vapour with air is done in an arrangement called carburetor. The toothed wheel C1 and C2 opens and closes the valves properly. C1 and C2 are connected to rotating shaft which is driven by the engine. Petrol vapour mixed with air from the carburetor enters the engine through the inlet tube I. When it is compressed sufficiently by the piston, it is ignited by spark plug. Both temperature and pressure of air increase to a high value to move the piston.
The action of the petrol engine consists of four strokes over a complete cycle.
(a) First Stoke (Charging Stroke):
The piston moves to draw into the cylinder an explosive mixture of air and gaseous fuel through the inlet valve, V1 [Fig. 26.5(a)] which then opens. The operation is carried out at a pressure slightly higher than the atmospheric pressure.
(b) Second Stroke (Compressive Stroke):
The piston makes its return stroke, moving inwards and compresses the explosive mixture. Both the valves V1 and V2 [Fig. 26.5 (b)] remain closed. The piston compresses the mixture to about one-fifth of its initial value when the temperature becomes 600°C (approx.) and pressure about 5 atmospheres. At the end of the stroke, electric sparks are produced.
(c) Third Stroke (Working Stroke):
Just after the explosion, temperature and pressure increase to a large value. In fact, temperature becomes about 2000°C and pressure about 15 atmospheres [Fig. 26.5(c)]. Then the piston is driven outward violently, subjecting the gaseous mixture to an adiabatic expansion until initial volume is attained.
But that time the pressure and temperature of the mixture fall. This stroke is known as working stroke, as during such stroke heat energy is converted to mechanical energy. At the end of this stroke, the valve V2 opens.
(d) Exhaust Stroke:
The piston further moves inwards and force the spent gas out of the valve V2. During this operation the valve V1 remains closed [Fig. 26.5(d)].
The spent gas escapes out of the cylinder and the outlet valve V2 closes. The piston starts to move outward and the initial condition is restored. The cycle is repeated again the again.
The four strokes of a petrol engine follows a cycle called Otto cycle. Fig. 26.6 shows the indicator diagram of the Otto cycle.
(a) In the figure, AB represents the first stroke or the charging stroke. At this stroke gas mixture (petrol vapour and air) enters the cylinder at atmospheric pressure. At B, temperature of the mixture is T1 which is same as surrounding temperature.
(b) BC in the indicator diagram represents the second stroke or the compression stroke. At C, the temperature of the gas mixture is 600°C and pressure is about 5 atmospheres. At this point the vapour is ignited by spark plug and consequently the pressure and temperature of the mixture increase rapidly but the volume remains the same. The change of state of the gas is drawn by CD. At D, pressure is about 15 atmospheres and temperature 2000°C nearly.
(c) The third stroke or the working stroke is indicated by DE. The gas then expands adiabatically and performs external work. At the end of the stroke, the condition is represented by the point E.
(d) At E, the outlet valve opens. Then the temperature of the working substance is T1 and pressure is same as atmospheric pressure. Volume remains unaltered. The change is shown by EB.
(e) In the indicator diagram, BA represents the fourth stroke or exhaust stroke.
In order to calculate the efficiency of the cycle we assume:
i. During charging stroke and during exhaust stroke the pressure in the cylinder is atmospheric.
ii. The compression and expansion curves BC and DE are adiabatic and follow the relation PVγ = constant.
iii. The specific heat of the gas always remains constant.
iv. Heat is received by the gas at constant volume V2 when pressure increases due to explosion. Heat is rejected also at the constant volume V1 at the end of the working stroke.
We assume that at the points B, C, D, E of the indicator diagram, the pressures and volumes are P1, P2, P3, P4 and V1, V2, V3, V4 respectively. The corresponding absolute temperatures are T1, T2, T3 and T4.
Let the mass of the working substance be 1 g. Due to combustion the amount of heat received by the gas between C and D is Q1 = CV (T3 – T2) (CV = specific heat at constant volume).
It rejects an amount of heat Q2 between E and B where Q2 = CV (T4 – T1).
Hence, thermal efficiency,
Type # 5. Diesel Engine:
The oil engine devised by Rudolf Diesel is called diesel engine. It is similar to petrol engine in construction. The engines are widely used in heavy lorry, bus, pumps and factories.
In this engine the piston exerts very high pressure on the enclosed air. The sparking plug here is replaced by extra valve called fuel valve and is used to inject liquid fuel.
In Fig. 26.7 of the diesel engine, C is a cylinder or combustion chamber of the engine which is provided with three openings, all fitted with valves. There are air admission valve V1, fuel valve V2 and exhaust valve V3. Air is passed through the valve V1, oil is injected through the second and the spent gas is allowed to escape through the third.
All the valves are controlled by levers operated by crankshaft. There is a further valve behind V2 (not shown) which allows the fuel to enter into the cylinder. This is known as starting valve. Compressed air kept in a bottle is connected to the air admission tube, while a regulating tap connected to the bottle controls the entry of air into the cylinder.
The fuel valve V2 remains initially closed, while the regulating tap of the store bottle is opened. As a result, compressed air is sucked into the cylinder and the engine performs a few cycles. At normal speed the regulating tap is closed and the fuel valve is opened. The fuel then enters into the cylinder and the engine begins its usual operation.
The working of the diesel engine consists of four stroke.
(a) Suction Stoke:
Both the exhaust and fuel valves are closed but the air admission valve is open and the piston moves outward. At atmospheric pressure air is sucked in.
(b) Compression Stroke:
All the three valves are kept closed, the piston moves inwards and the air is compressed adiabatically to about 1/17th of its initial volume. As a result pressure and temperature increase largely.
(c) Working Stroke:
The air valve and the exhaust valve are closed, while the fuel valve is opened at the beginning of the stroke. Due to compression in the previous stroke the temperature of air ignites the fuel as soon as it is injected into the cylinder. The combustion continues so long fuel valve is kept open and it occurs at constant pressure. By combustion the gaseous mixture expands adiabatically forcing the piston outward. At the end of the stroke, the exhaust valve opens and spent gas goes out of the cylinder.
(d) Exhaust Stroke:
The air valve and fuel valve are closed, while the exhaust valve remains open and the piston moves inwards, it drives away the spent gas through the outlet valve.
At the end of this stroke, the initial condition is restored and the cycle is repeated again and again.
In Fig. 26.9, the indicator diagram of diesel cycle is shown.
(a) In the diagram, AB represents the first (or suction stroke), when air at atmospheric pressure is sucked into the cylinder.
(b) In the second (or compression stroke), the air is compressed adiabatically as represented by BC. At the end of the stroke the pressure is about 35 atmospheres and temperature is nearly 1000°C. At this time the fuel valve opens and fuel enters the cylinder at constant pressure as represented by CD. By combustion of the fuel, the temperature of the mixture at D reaches at about 2000°C.
(c) In the third stroke (or working stroke) the gas expands adiabatically and external work is done by the engine as represented by DE. At the end of the stroke, the outlet valve opens and the pressure drops to B, which is now nearly the atmospheric pressure.
(d) The fourth stroke (or exhaust stroke) is represented by BA when the spent gas mixture goes out.
