Heat Engines

The Heat Engine  A heat engine typically uses energy provided in the form of heat to do work and then exhausts the heat which cannot be used to do work  Example of heat engines: steam engine (for example in trains),Gasoline, diesel engines, jet engines, steam turbines  What do these engines have I common?  They all are powered by the expansion of heated gases.

 THE SECOND LAW OF THERMODYNAMICS (FIRST EXPRESSION)  Heat transfer occurs spontaneously from higher- to lower-temperature bodies but never spontaneously in the reverse direction.  OR  It is impossible for any process to have as its sole result heat transfer from a cooler to a hotter object

 Q h – Heat the gas, gas expands, gas does work  Q c – heat output, into environment.  Heat transfer from the hot object (or hot reservoir ) Q h  Into  ( cold reservoir,) Q c

Model of an Engine  The temperatures of the hot reservoir(T h ) and the temperature cold reservoir (T c )  The heat engines a cyclical process  Cyclical process - brings the system ( the cylinder) back to its original state at end of every cycle.

 In a cyclical process ΔU=0  ΔU= Q – W  Q  net heat transfer during the cycle  Q= Q h –Q c  W  net work done by system  Since ΔU=0  0 = Q – W  W = Q  But not all Q is converted to Work, thus  W = Q h – Q c (cyclical process)

W = Q h – Q c (cyclical process) When work is done by a heat engine running between two temperature, only some of the input heat (Q c ) can be converted to work (W), and the rest is expelled (Q c)

 THE SECOND LAW OF THERMODYNAMICS (SECOND EXPRESSION)  It is impossible in any system for heat transfer from a reservoir to completely convert to work in a cyclical process in which the system returns to its initial state.

Using the model  1)-Gain heat (Q h) from T h, thus increases - internal energy  2) Convert some of the energy into mechanical work (W)  3) expel the remaining energy Q c as heat to T c Gasoline Engine  1) -Burning fuel I combustion chamber provides high Temperature reservoir (Q h)  2)- Hot gases do mechanical work on the piston (W)  3) – Heat is expelled to the environment via the cooling system and the exhaust (Q c)

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Crankshaft

 Paths AB and CD are adiabatic and correspond to the compression and power strokes of an internal combustion engine  Paths BC and DA are isochoric and accomplish similar results to the ignition and exhaust-intake portions  Work is done on the gas along path AB

 Heat Engine Cycle

 If the cycle is operated clockwise, the engine uses heat to do net work. Counterclockwise, it uses work to transport heat and is therefore acting as a refrigerator or a heat pump.

 Simplest Engine Cycle 1

 Simplest Engine Cycle 2

 Simplest Engine Cycle 3

 Simplest Engine Cycle 4

The Diesel Engine  Diesel internal combustion engine differs from the gasoline powered Otto cycle by u ("compression ignition" rather than "spark ignition")

Diesel Engine  -Air is compressed adiabatically,  -With compression ratio (15-20)  -Compression raises the temperature to the ignition temperature fuel mixture.

 (a –b)Adiabatic compression,  (b –c)Constant pressure combustion  (c-d) Adiabatic expansion ( power stoke  (d-c) Isochoric exhaust

 A glow plug is heating device used to aid starting diesel engines.  In cold weather, it can be difficult to start, mass of the cylinder block and cylinder head absorb most of the heat for compression, preventing ignition.