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

Qh – Heat the gas, gas expands, gas does work

Qc – heat output, into environment.

Heat transfer from the hot object (or hot reservoir ) Qh

Into

( cold reservoir,) Qc

Model of an Engine

The temperatures of the hot reservoir(Th) and the temperature cold reservoir (Tc)

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= Qh –Qc

W net work done by system

Since ΔU=0

0 = Q – W

W = Q

But not all Q is converted to Work, thus

W = Qh – Qc (cyclical process)

W = Qh – Qc (cyclical process)

When work is done by a heat engine running between two temperature, only some of the input heat (Qc) can be converted to work (W), and the rest is expelled (Qc)

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 (Qh) from Th,

thus increases - internal energy

2) Convert some of the energy into mechanical work (W)

3) expel the remaining energy Qc as heat to Tc

Gasoline Engine 1) -Burning fuel I combustion

chamber provides high Temperature reservoir (Qh)

2)- Hot gases do mechanical work on the piston (W)

3) – Heat is expelled to the environment via the cooling system and the exhaust (Qc)

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

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.