MDME: MANUFACTURING, DESIGN, MECHANICAL ENGINEERING 

HEAT ENGINES


A heat engine is a device that continuously converts heat to work (power).

SmartBoard Notes:  Heat Engines.pdf     Heat Engines.one

Heat Engine Essentials

  • A Heat Source
  • A Working Substance (working fluid)
  • Mechanical work (power) output
  • A Working Cycle
  • A Heat Sink


Work Output = Heat Supplied - Heat Rejected
W = Qs - QR
So efficiency = W / Qs


where

The dot above the symbol means RATE (or per second)


Perfect Gas Simulator.

Carnot Cycle

The Carnot cycle is the most efficient cycle because the input heat is transferred at the maximum temperature, and output heat is transferred at the minimum temperature. The Carnot Cycle doesn't really work in practice because the area in the PV graph is too thin (so once friction is taken into account the net power = 0 (area inside the cycle).

Carnot efficiency: (Ideal efficiency)

where
Th = the hot temperature (Kelvin) that the heat flows FROM
Tc = the cold temperature (Kelvin) that the heat flows TO

Carnot Cycle

A to B: Isothermal expansion heated by source at Th and decreasing pressure
B to C: Adiabatic expansion decrease in both pressure and temperature back to original values - without heat transfer.
C to D: Isothermal compression  Heat removed at constant Tc, pressure increases
D to A: Adiabatic compression Piston pushed down to increase pressure and temperature - without heat transfer to outside.


Stirling Cycle

The Stirling Cycle is an external heat, closed, 2-stroke cycle. It uses gas as a working substance.
http://www.animatedengines.com/vstirling.shtml Stirling Engine Animation
http://www.stirlingengine.co.uk/index.asp?function=WEBPAGE&id=4 Model Stirling Engines with videos
Stirling Efficiency is the same as the Carnot efficiency.

Stirling Cycle

A to B: Isothermal expansion heated by source at Th and decreasing pressure
B to C: Constant Volume cooling decrease in both pressure and temperature as heat lost to outside.
C to D: Isothermal compression  Heat removed at constant Tc, pressure increases
D to A: Constant Volume heating increase in pressue and temperature as heat added from outside.

Stirling efficiency: (Same as Carnot = Ideal efficiency)

where
Th = the hot temperature (Kelvin) that the heat flows FROM
Tc = the cold temperature (Kelvin) that the heat flows TO

Low temperature Stirling Engine

How it works: http://www.animatedengines.com/ltdstirling.html

Wikipedia page: https://en.wikipedia.org/wiki/Stirling_engine


The 2 Stroke Cycle

The Two Stoke Cycle is a positive displacement, 4 stroke, internal combustion, compression ignition (or spark ignition), open cycle.

2 Stroke Cycle

A to B: Isothermal expansion heated by source at Th and decreasing pressure
B to C: Constant Volume cooling decrease in both pressure and temperature as heat lost to outside.
C to D: Isothermal compression  Heat removed at constant Tc, pressure increases
D to A: Constant Volume heating increase in pressue and temperature as heat added from outside.

2 stroke cycle. An idealised 2 stroke cycle split into 4 gas process stages. Compression (Adiabatic compression), Combustion (Isochoric heating), Expansion (Adiabatic expansion), Exhaust/Intake (Isochoric cooling)


 

About the two-stroke cycle.

Advantages: High power-to-weight ratio. No valves and simpler design.

Disadvantages: Dirtier emissions (lubricant mixed with fuel). Tricky to balance the exhaust to prevent air/fuel mixture going straight down the exhaust pipe, or with too much back pressure the opposite problem - not clearing the exhaust from the cylinder.


The shape of the exhaust expansion chamber produces a pulsating back pressure that ensures the new fuel mixture is contained within the cylinder instead of travelling along the exhaust pipe. This requires geometry that is tuned to the RPM of the engine, so 2 stroke engines tend to have a narrow power band (i.e. they produce maximum power only when they are close to the optimal RPM)

Otto (4 stroke) Cycle

The Otto Cycle is a positive displacement, 4 stroke, internal combustion, spark ignition, open cycle.
Better animation at:
http://www.uwsp.edu/physastr/kmenning/flash/AF_2212.swf
Big list: http://www.educypedia.be/education/carjava.htm

Otto Efficiency


Where V1/V2 is the compression ratio
= cp/cv = adiabatic index (typically 1.4 for air)

Otto Cycle

A to B: Isothermal expansion heated by source at Th and decreasing pressure
B to C: Constant Volume cooling decrease in both pressure and temperature as heat lost to outside.
C to D: Isothermal compression  Heat removed at constant Tc, pressure increases
D to A: Constant Volume heating increase in pressue and temperature as heat added from outside.


Here, we are approximating the cycle with adiabatic compression/expansion, and constant volume (isochoric) heating and cooling. In real life, neither is exactly true, especially the heating/cooling part of the graph. This is why the real life pV diagram shows a curved graph instead of the sharp corners of the idealised pV diagram.

Diesel Cycle

The Diesel Cycle is a positive displacement, 4 stroke, internal combustion, compression ignition, open cycle.


Here, we are approximating the cycle with a slightly different gas process between 2 and 3. In this case it is a constant pressure process, not a constant volume process. Of course, in real life, neither is exactly true.

Tester Special Pages

These web pages are presented during TESTER. They relate to specific Heat Engine questions.
  1. Internal_combustion.html
  2. heat_engines_files/ocean_thermal_energy.html
  3. heat_engines_files/Solar_Stirling.html
  4. heat_engines_files/steam_engine.html
  5. heat_engines_files/Stirling_Low_Temp.html

Questions:

Homework Assignment: Kinksy
Do all questions; Chapter 6: Heat Engines
6.1 to 6.22 (page 146-149)
Relevant pages in MDME

Web Links

Smart Board
  Heat Engines.onepkg     Heat Engines.one     Heat Engines.pdf