AN INTRODUCTION TO THE HEAT ENGINE

e [Music] energy can be called a capacity to do work there are different kinds of energy for instance energy of movement electrical energy chemical energy in a match head for instance and heat which is also a form of energy we keep up our strength with food which gives our bodies heat energy energy can usually be changed from one form into another and energy of movement can be changed into electrical energy and electrical energy back to energy of movement electrical energy into heat an energy of movement into heat but although energy can change in kind it can never change in quantity the amount of electrical energy being used up here is equal to the amount of movement energy produced or to the amount of heat energy produced this motor changes electrical energy into movement energy it drives a lathe and the energy does useful work cutting the metal what while the work is being done the energy doesn't disappear it changes into heat heat engines are machines for changing heat energy into energy of movement their success depends on how well they can do it the cold burning locomotive is a classic example of the heat engine the modern diesel locomotive is also a heat engine using oil for fuel the diesel engine changes heat energy into energy of movement but does it much more economically and efficiently than the coal burning locomotive men have always struggled to improve their living conditions this is meant dragging heavy loads and raising heavy weights to their own strength then gradually learned to add other kinds of power in their time these were sufficient but as populations grew Society became more complicated trade and commerce widened men were compelled to seek a new kind of power 2,000 years ago a Greek named hero devised a machine which looked rather like this his machine showed for the first time that heat could produce continuous movement but for practical purposes it was useless 1600 years later men had to turn again to heat as a source of Power by the 17th century they were experimenting with machines heat engines to obtain from heat a continuous supply of work they did this by means of steam these early machines were crude and simple but they weren't and by the beginning of the 18th century Nan's engines were giving men greater and more reliable power than they had ever known before by the end of the 18th century steam engines had been much improved during the 19th century they were being used for all kinds of [Applause] purposes they became more complicated as improvements were made [Music] but the steam engine was large and often clumsy so another kind of heat engine was investigated the internal combustion engine Auto's engine of 1864 operated on coal gas this engine together with the introduction of petroleum as a source of powerful and easy to handle fuels led to the development of the gasoline engine and to the diesel engine all these machines old and new have a common purpose to turn heat into work when something is heated it exerts a push because it tries to expand steam pushes up the lid of a kettle it is in this kind of way that all heat engines change heat into work they depend on fundamental laws concerned with heat and temperature temperature measures whether something is hotter or colder than something else we judge it roughly to be a sense of touch and measure it accurately with a thermometer temperature is not the same thing as heat these two burners both give out Heat at the same rate put a very little milk into one pan and a much greater amount in the other after a few moments the milk in the left hand pan has come to the boil the other milk is still quite cool both pans have received an equal amount of heat energy but this heat squeezed into a little milk produces a much higher temperature and the same amount of heat spread out through much more milk temperature then measures the concentration of heat energy how do heat and temperature concern heat engines in a locomotive the steam gets its heat from the furnace at a high temperature the steam changes a part of this heat into work and it then gives up the remaining heat now at a much lower temperature to the surrounding atmosphere as energy can only change in kind not in quantity the heat energy supplied by the furnace is equal to the work done plus the heat given up to the atmosphere the same things happen in Steam turbines such as in power stations Heat at a high temperature from the furnace enters the steam which then goes into the turban inside the turban are rotating rings of blades the steam expands through nozzles in a continuous stream and rushes onto the blades pushing them round to provide energy of movement the steam then gives up its remaining Heat at a much lower temperature to cooling water and eventually to the atmosphere outside and the same in a car engine the source of heat is the burning of fuel in the cylinder here it is in slow motion this Heat at a high temperature enters the gases in the cylinder the gases Turn part of it into work they then give up the remaining Heat at a much lower temperature by way of the exhaust to the atmosphere so all heat engines need the same three things a source of heat at a high temperature an agent such as Steam which can take in this heat and change a part of it into movement or work or it may be other gases as in the car engine finally something cooler than the source of heat to which the agency must give up the remaining Heat at a lower temperature this is in fact the engine surroundings all heat engines have the same purpose to obtain useful work from heat the heat comes from burning fuel but fuel costs money so it is important for any heat engine to get as much work as possible from the fuel that is from the heat the percentage of useful work obtained from a quantity of heat is called thermal efficiency Hero's device of 2,000 years ago had a thermal efficiency of practically nothing it could just about turn itself round and no more even the first practical steam engines had low thermal efficiencies the Nan engines of the 18th century were much less than 1% efficient James Watts engines raised the figure to about 1% while the later Cornish pumping engines may have reached 2% modern engines on the whole do a bit better an Express steam locomotive can reach about 8% larger more elaborate steam engines reach about 177% a Motorcar engine is higher 22% a modern steam turbine gives 28% a diesel engine gives [Music] 35% patrolling fuels have helped increase the efficiency of today's engines however they can still only turn about onethird the heat they consume into useful work the reason is fundamental it has to do with the behavior of heat itself in doing its work the movement energy provided by the lathe motor becomes heat energy this change movement into heat is much more common than all other kinds of En energy change it happens all the time not only in engineering but in everyday life sometimes the heat produced is obvious sometimes not but whenever anything moves when feet tread on Pavements when snooker balls Collide movement changes inevitably into heat always this way never this if it did heat could come out of the air of its own accord change into movement and drive the cart backwards pulling the horse after it heat engines put the cart before the horse because they try to do this very difficult thing to turn heat energy into energy of movement the opposite direction to the Natural change and the efficiency with which they do it is low they can't even do it at all unless there are two different temperatures a hot and a cold this is because of another property of heat it always flows from hot to cold put something hot in contact with something cold and heat flows naturally out of the hot thing into the cold one in the same way the heat from a furnace if we let it would all pass straight into the cooler room and eventually out to the atmosphere a heat engine Depends for its very working on this flow of heat from hot to cold we make the heat flow through the engine which can intercept a part of the Heat and change it into useful work but the rest of the heat must continue on its downward flow the engine must give it up at a lower temperature to its surroundings the heat given up is always a high percentage of the heat taken in this then is why heat engines have such low thermal efficiencies how can they be increased heat engines depend on the flow of heat from hot to cold so the longer this flow the more chance there is to change heat into work so the bigger the temperature difference in the engine the better for instance here is a compound steam engine the steam comes from the boiler at 160° Centigrade into the cylinder here it expands pushing the Piston to supply useful work and cooling from 160° down to 110° if the engine ended here the steam would now give up its remaining heat to the surrounding air but this is much cooler and this temperature difference can provide some extra work so the steam is actually LED from the cylinder into a second one here it expands further and supplies this extra work and cools further down to 85° finally it is Led away and gives up its remaining heat through cooling water to the surroundings so a bigger temperature drop in the steam has been used and more work is produced from the same original quantity of heat which means higher thermal efficiency and this is a general rule for all heat engines the bigger the temperature drop in the working agency as it goes through the engine the higher the thermal efficiency so thermal efficiency can be increased by taking in the heat at a higher temperature and by giving up the heat at a lower temperature first let's deal with a low temperature end adding on a second cylinder allows the steam to be used down to 85° the thermal efficiency of this particular compound engine is about 10% this engine goes one better it has three cylinders the steam goes through each in turn and the total temperature drop is bigger than in the two-cylinder compound the steam can be used down to 60° and this increases the efficiency to to 15% the steam turbine goes many times better each Ring of Blades is like a separate cylinder the steam expands onto each in turn cooling as it does some turbin have a second complete set of rains and this allows the steam to expand and cool still more work is gotten from the steam right down to 45° efficiency is much higher 28% so thermal efficiency is increased by using steam down to lower and lower temperatures the same applies in other engines for instance diesel engines have an efficiency of about 35% much higher than than gasoline engines this is chiefly due to the Diesel's high compression the gases in the cylinders are compressed relatively more in a diesel than in a gasoline engine so the hot gases expand more in the diesel and do work down to a lower temperature the exhaust gases leave the diesel engine at about 400° centigrade in the gasoline engine they are never less than 750° put a drop of oil onto a diesel exhaust pipe nothing much happens but a gasoline engine's exhaust is much hotter and the same oil [Applause] burns so much for increasing thermal efficiency by pushing the low temperature down but it can also be increased by pushing the high temperature up the steam turbine is much more efficient than the compound steam engine because the steam leaves at a much lower temperature and also enters at a much higher temperature 500° Centigrade has against 160° internal combustion engines are more efficient than steam engines because their upper temperature is far higher than in Steam Engines over 2,000 de Centigrade at such a high temperature the whole engine would melt but luckily it only lasts for a very small fraction of the running time in the newest heat engine the gas turban there is not this intermittent cooling so the development of the gas turbine depended on finding the right Metals Metals which could rotate very fast without Distortion while in continuous contact with gases at a temperature 6 or 700° Centigrade by 1940 new metallic Alloys had been developed which could stand these continuous stresses at high temperatures the development of these Alloys helped to make the first gas turbines possible research will allow still higher gas temperatures in the future this will mean more efficient gas turbines for industry and transport since Men first succeeded in turning heat into useful work Engineers have been aiming at higher thermal efficiencies which means saving in fuel and money today civilization runs on heat engines and they all depend on the simple fact that heat must flow from hot to cold