THE HEAT ENGINE

A heat engine converts heat or thermal energy into mechanical energy.  It does this by the combustion of a suitable fuel or energy source, such as coal, wood, biomass, gasoline, diesel, nuclear or direct heat energy from the sun. The heat energy generated from the combustion of these fuels is transferred to a working fluid at high temperature to expand the fluid. The expanded working fluid at high pressure is converted to mechanical energy or to do work in a heat engine or machine.

Heat engines are first classified according to where the combustion process takes place. They are called external or internal combustion engine, if the combustion process takes place externally or internally of the heat engine respectively.

There are many types of external heat engine. The main ones are the steam, Stirling and the Ericsson heat engines.

The steam engine uses water as the working fluid. The fuel usually coal or diesel is heated in the combustion chamber to  convert the water into steam and the expanded steam  at a high pressure is  used to drive or run a turbine or piston generating mechanical work.

The stirling heat engine uses gas as the working fluid. The fuel is burned externally in the combustion chamber to heat the working fluid or gas and this expanded gas at a high pressure is used to drive or run a piston or displacer to do mechanical work.

The Ericsson heat engine also uses gas as the working fluid. The fuel is burned in the externally in the combustion chamber to heat the working fluid or gas  and this expanded gas at a high pressure is used to drive or run a turbine or piston to do mechanical work. The main difference between the Stirling and the Ericsson heat engine is that the Stirling engine uses a closed cycle system, which means the working fluid is sealed within the engine. Whereas, the Ericsson engine uses an open cycle system where the working fluid is not sealed within the engine.

The main types of internal combustion engines are the reciprocating, wrankel and the gas turbine. In reciprocating engines fuel is burned internally inside a cylinder to produce power to drive a piston and crankshaft arrangement, thus converting the linear motion of the piston to rotary motion of the crank shaft.

The Wrankel engine fuel is burned inside a combustion chamber to produce power to drive a rotor directly to produce a rotary motion.

While the gas turbine engine, air is compressed and mixed with fuel to obtain a rich fuel mixture before expansion through a turbine generating mechanical energy to do work.

Heat engines have efficiencies ranging from 20% to 40%, depending on the type and design.

The advantages of heat engines are as follows: Heat engines can generate a large amount of energy from high density fuels like gasoline or petrol and diesel. Heat engines can use a wide variety of fuel sources ranging from fossil fuels to nuclear fuels. Generally heat engines have a high power to weight ratio and are capable of operating for long period of time without the need of maintenance at low cost.

However, their main disadvantages are low efficiency, heat loss potential, vibration, negative environmental impact due to noise and exhaust pollution gases. Also the complexity of heat engines requires frequent maintenance resulting in long non-operating down times.

The application of heat engines is as follows:  They are used as external combustion engines in a variety of industries, including; steam turbines in power plants, Stirling engines for space exploration and as combined heat and power plant (CHP). While heat engines find applications as internal combustion engines as follows; as reciprocating engines in cars, buses, trucks, motorcycles, and generators for both domestic and industrial uses.

The future of heat engines will follow the development of thermo photovoltaic (TPV) cells that is expected to revolutionize solar energy generation. Other futuristic heat engines using advanced combustion concepts, new engine architecture, new materials and optimized exhaust gas systems are already in the works. However the key future challenge regarding heat engines is reducing emissions, increasing efficiency and the integration of heat engines with electric motors and batteries to create hybrid systems that will result in an overall optimization of the heat engine system as a whole.

 

SOURCES;

• Heat engines by M.J Moran.
• Thermal power plants by R.K Singh.
• Heat engines by J.R Senft.
• Heat engines and refrigeration by R. Yadav.
• Thermal engineering by R.K Rajput.