HEAT ENGINES
A heat engine is a device that continuously converts heat to work (power).
SmartBoard Notes: Heat Engines.pdf Heat Engines.oneHeat 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 CycleA to B: Isothermal expansion heated by source at Th and decreasing pressureB 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 CycleA to B: Isothermal expansion heated by source at Th and decreasing pressureB 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 CycleA to B: Isothermal expansion heated by source at Th and decreasing pressureB 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. |
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 CycleA to B: Isothermal expansion heated by source at Th and decreasing pressureB 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.- Internal_combustion.html
- heat_engines_files/ocean_thermal_energy.html
- heat_engines_files/Solar_Stirling.html
- heat_engines_files/steam_engine.html
- 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)