Chapter 7 - Heat Science for X

Agenda Heat Engines External combustion engine Internal combustion engine Petrol engine Diesel engine Efficiency of heat engine Advantages of Internal combustion engine

Application of Heat Engines It is customary to cool liquids such as fruit juice by adding a piece of ice. The required heat for melting of ice is taken from the juice The high latent heat of steam is useful in working of a steam engine Have you observed that water in an earthen pot gets cooled after sometime? So how does it happen? – The pot has number of poles in its wall. Water oozes out through these pores and evaporates by taking heat from water. Thus the water is cooled

Heat Engines Thomas Savery in 1698 patented an early steam engine Use: His steam engine was extensively used for pumping water out of mines and also to lit water from wells

Experiment So How is heat is converted to mechanical energy? Small Experiment – Take some water in test tube and close it with cork – Heat the test tube and when the water boils the cork flies off The steam produced inside the test tube after some time exerts pressure on the cork and the cork flies off

Heat Engine Heat engine is a system that works on the principle of converting the thermal energy to mechanical energy Types of Heat Engine External Combustion Engine Internal combustion Engine Heat Engine

External Combustion Engine Crankshaft Cylinder Boiler Condenser Outlet Valve Inlet Valve Pump Piston Connecting Rod Steam Engine Parts

External Combustion Engine Operating Principle – The steam engine basically consists of two strokes of operation that is required to convert the heat energy to mechanical energy namely – Expansion stroke Exhaust stroke

External Combustion Engine In this cycle, the water in the boiler is heated The generated steam gets to the cylinder using the inlet valve The expanding steam forces the piston to move outwards as shown During this stroke, the outlet valve remains closed Expansion Stroke Crankshaft Cylinder Outlet Valve Piston Connecting Rod Boiler Condenser Inlet Valve Pump Heat Water Steam

External Combustion Engine As the piston moved inwards, the outlet valve opens The used steam is pushed into the condenser As such this cycle repeats The inlet valve will be closed Exhaust stroke Crankshaft Cylinder Outlet Valve opens Inlet Valve closes Piston Connecting Rod Boiler Condenser Pump Heat Water Steam

External Combustion Engine Steam engines are bulkier Major portion of heat energy wasted Engines cannot be started instantaneously as getting steam at high temperature is time consuming Risk of accidents due to bursting of boiler very high De-merits

Internal Combustion Engine The limitations of the external combustion engine led to the development of internal combustion engine As name suggests, the combustion of the fuel takes place inside the cylinder itself, hence named internal combustion engine Internal Combustion engines – types: Petrol EngineDiesel Engine Internal combustion engine

The Piston moves away from the cylinder head This results in low pressure inside the cylinder and the inlet valve opens The fuel mixture from the carburetor enters the cylinder The outlet valve remains closed during this stroke Intake Stroke Piston Inlet Valve Opens Fuel Mixture enters Outlet Valve will be closed Internal Combustion Engine – Petrol Engine

The fuel mixture is compressed by the piston moving towards the head Both the inlet and outlet valve remains closed Compression Stroke Piston Inlet Valve Closed Outlet Valve remains closed Fuel mixture compressed Internal Combustion Engine – Petrol Engine

The compressed mixture is ignited by the spark generated by the spark plug The resulting product of combustion consisting of carbon dioxide (CO 2 ) and water vapor expands suddenly This causes the piston to be pushed outwards with greater forces Even in this stroke both the valves remains closed Power Stroke Piston pushed outwards Inlet Valve Closed Outlet Valve remains closed Fuel mixture Ignited by spark plug Internal Combustion Engine – Petrol Engine

The exhaust valve opens The piston moves upwards The waste (burnt) gases are through the exhaust (outlet) valve The inlet valve remains closed Exhaust Stroke Piston moves upwards Inlet Valve remains Closed Outlet Valve opens Exhaust gases Internal Combustion Engine – Petrol Engine

Internal Combustion engine - Diesel Diesel engine is similar to petrol engine except that the fuel injunction is fitted in place of spark plug A carburetor is not required in a diesel engine Working cycle of diesel engine also consists of four strokes – Intake stroke – Compression stroke – Power stroke – Exhaust stroke

Internal Combustion Engine Petrol Vs Diesel Fuel mixture enters the valve Fuel mixture is compressed Spark plug ignites the fuel mixture Burnt fuel exits through the exhaust valve Air enters through the inlet valve Air is compressed Fuel is injected to the cylinder and hot air burns the fuel Burnt fuel exits through the exhaust valve

Efficiency of Heat Engine Efficiency of an engine is defined as the fraction of heat that is converted to useful mechanical work It is usually represented as percentage Efficiency of petrol engine is about 40% Efficiency of steam is about 15 % Heat Supplied Work Done Efficiency =X100

Advantages of Internal Combustion Engine over External Combustion Engine Efficiency of internal combustion engine is greater than external combustion engine Internal combustion engine can be started instantaneously Internal combustion engine can be manufactured in different sizes and can be fitted to small machines There is no risk of accidents due to storage of material in high pressure Used in

Back Up slides

External Combustion Engine Steam Engine –Expansion stroke Crankshaft Cylinder Boiler Condenser Outlet Valve Inlet Valve Pump Piston Connecting Rod Heat Water Steam

External Combustion Engine Steam Engine Exhaust stroke Crankshaft Cylinder Boiler Condenser Outlet Valve Inlet Valve Pump Piston Connecting Rod Heat Water Steam

Internal Combustion Engine

Fuel mixture enters the valve Fuel mixture is compressed Spark plug ignites the fuel mixture Burnt fuel exits through the exhaust valve Air enters through the inlet valve Air is compressed Fuel is injected to the cylinder and hot air burns the fuel Burnt fuel exits through the exhaust valve