Suleman Ayub

What are air engines and water engines, and how do they work? 1 Contents Air Engines .................................................................................................................................................. 3 Working Principle of air Engines.................................................................................................................. 3 Design of a Single Cylinder Air Engine ....................................................................................................... 3 Water Engines ............................................................................................................................................. 4 How Airplane Fly? ...................................................................................................................................... 6 How does a Ship Float? .............................................................................................................................. 7 References .................................................................................................................................................... 9 2 Air Engines An air engine is a heat engine that operates on cyclic expansion and contraction of air to convert the thermal energy of air into mechanical work. In other words, air engines are those engines in which air is repeatedly heated and cooled in a cylinder. As a result of heating and cooling, expansion and contraction of air occurs that move the piston to produce useful mechanical work. Air engines are of various configurations, but the most frequently used designs are: I) Single cylinder design II) Two cylinder design In single cylinder design, the working piston and the displacer piston have a common cylinder. Whereas there are two separate, interconnected cylinders for working piston and displacer piston in case of two cylinder design. Working Principle of air Engines The working principle of air engine is based on the fact that air expands when heated and contracts when cooled. In order to avail benefit from this property of air, a fixed quantity of air is enclosed in a cylinder. One end of this cylinder is kept near the heat source while the other end is kept near the cold source or sink. The displacer piston is used to move the air between hot end and cold end in order to make it expands and contracts. Working piston is moved back and forth, in a reciprocating action, due to expansion and contraction of the air. This reciprocating action is transformed into rotational motion by means of the flywheel connected through a crank shaft. Design of a Single Cylinder Air Engine Single cylinder air engine consists of following: i) Single horizontal cylinder ii) A loose-fitting displacer piston and a working piston iii) One end of the cylinder is built into a furnace while the opposite end is cooled by a water jacket 3 The figure below shows the working of these parts when the air engine is in operation. Fig. I shows when air expands and it exerts force on the working displacer while Fig. II shows the condition of gas contraction. Fig. I Fig. II (W. H. Bailey & Co, 1969) Water Engines This is a positive displacement engine, comprising of pistons and valves driven by water pressure. Construction of this engine resembles with that of a steam engines. However as water is an incompressible fluid, the valve gear of water engines are more complicated (than valves of steam engines) and sometimes even small engine is provided to operate these valves. These valves are designed to close slowly as quick shut down of the valve can build excessively high pressure which can explode the pipework. 4 Working of Principle of Water Engines The working principle of water engine is based on the principle that useful work is done when high pressure water enters the cylinder and is then expelled out at low pressure. The amount of this work done is equal to the difference in pressure of water entering and leaving the cylinder. Components of a Water Engine Like steam engine, major components of a water engine consist of a cylinder, a piston and inlet, outlet valves. (Prof William Rankine, 1888) 5 How Airplane Fly? There are four forces acting on the airplane during its flight. These forces are as follows: 1) Lift 2) Weight of airplane 3) Thrust 4) Drag Two of these forces (lift and drag) are produced as a result of relative movement of the air as compared to the plane and are known as aerodynamic forces. The lift is directed upwards and it acts perpendicular to the displacement. It is due to lift that the plane is maintained in the air. Generation of lift can be explained by Newton’s 3rd law of motion. In order to generate lift, the wing must divert a lot of air down. This is called action of the wing on the air. This action is equal to the rate of change in momentum of the air diverting downward. The lift is generated as a reaction of this downward action on the air. The magnitude of this lift is equal to” the product of air mass, diverted down, per unit time and the downward velocity of the air”. It is therefore clear that in order to produce more lift, the wing either to have divert more air per unit time or increase downward velocity of air. This downward velocity of air behind the wing is known as “downwash”. The greater is the angle of attack, the greater will be the vertical velocity. Similarly keeping the angle of attack constant, the greater the speed of the wing the greater will be the vertical velocity. It is therefore concluded that vertical velocity can be increased by increasing both the speed of the wing and angle of attack. It is this vertical velocity that generates the wing lift. The second important force acting on the airplane is called the drag. This force is exerted in the direction opposite to the displacement of the plane. Drag is generated due to the breaking action of the air on the plane and it retards advancement of the plane. The faster the plane flies, the greater will be this force of friction. Some force of friction is also exerted by the ends of the wings. This force of friction is called induced drag. The upper part of the wing is in depression while the lower part of the wing is in overpressure. The air tries to bypass the wing in order to move from higher pressure to the lower pressure. This results in formation of vortices at the tip of the wings to produce induced drag. A lot of energy is consumed in order to overcome these vortices. 6 It is apparent that there must be a force to compensate the drag acting on the plane. The force which nullifies the drag is called the thrust. The thrust is provided by the system of propulsion of the plane, that is, the engine of the plane. During flight at cruising speed, the role of the engine is therefore to neutralize the force of drag. On the other hand, power of engine is used to bring the plane to the altitude of flight at the time of takeoff. The fourth force acting on the plane during its flight is called force of gravity. The force of gravity, which is the weight of plane, acts in opposite direction to that of the lift. In order to keep the plane at constant altitude, during the flight, the weight of the plane must be balanced by the upward lift. It is the duty of a pilot to regulate the speed of the plane such that the lift force balances the weight of the plane. How does a Ship Float? In order to find that why does a ship float, we have to understand the following terms: 1) Buoyancy Force 2) The force of Gravity, the weight 3) The density When a ship is launched into ocean, it displaces a lot of water. The displaced water tries to return to its original position where the ship is now kept. As a result, the ship is pushed upwards. This force which pushes the ship in upward direction is called buoyancy force. It is according to the Archimedes’ principle which states that “any object, wholly or partially immersed in a fluid, is buoyed up by a force equal to the weight of the fluid displaced by that object”( Archimedes' principle). It means that when an object displaces ten liters of water, it will be subjected to upwards buoyancy force equal to the weight of ten liters of water. The more is the water displaced by an object, the stronger is the buoyancy force acting on the object. When an object is dipped in water, two forces are acted on that object. One of these forces is called the gravity force (acting downwards) while the second force is called the buoyancy force (acting vertically upwards). If force of gravity is equal to the buoyancy force, the object will float. 7 If a stone is thrown in water, it also experiences two forces (i.e. weight of the stone acting vertically downwards & the buoyancy acting vertically upwards). The buoyancy force in this case is much smaller than the weight of the stone. This is the reason that stone sinks when thrown in the ocean. The ship in ocean also experiences these two forces. But as the buoyancy force is large enough (due to design of the ship) to balance the weight of the ship, the ship floats on the surface of water. A ship dips deep enough to displace an amount of water which produces buoyancy force as strong as the weight of the ship. If a ship carries cargo, it will be further dipped into the water in order to generate stronger buoyancy in order to balance its weight. There is another explanation about why the ship floats on the water. According to this explanation, An object will float on the surface of the water if its average density is lower than that of water. As average density of ship is smaller than that of water (due to design of the ship), it floats on the surface of water. 8 References Collier, J. L. (2006). Steam engines. New York, Marshall Cavendish Benchmark. Organ, A. J. (2007). The air engine: stirling cycle power for a sustainable future. Cambridge, England, Woodhead. Organ, A. J. (1993). Stirling engine thermodynamic design: without the computer. Cambridge, Regenerative Thermal Machines. Rankine, W. J. M., & Millar, W. J. (1908). A manual of the steam engine and other prime movers. London, C. Griffin and Co. Rizzo, J. G. (1999). The Stirling engine manual. Volume 2. Rode(Grande-Bretagne), Camden Miniature steam services. Senft, J. R. (1993). Ringbom stirling engines. New York, Oxford University Press. The Museum of Science and Industry in Manchester, (2005) Bailey Hot-Air Engine. [online] Available at: http://www.mosi.org.uk/media/-/baileyhotairengine.pdf2NpZW5jZSBhbmQgSW5kdXN0cnkgfCBNT1NJ [Accessed: 21 Apr 2013]. Urieli, I., & Berchowitz, D. M. (1984). Stirling cycle engine analysis. Bristol, A. Hilger. Wurm, J. (1991). Stirling and Vuilleumeir heat pumps: design and applications. New York, McGraw-Hill. 9