Power Cycles

Part of the Point of 1304?
Spacial visualization!  It's not just learning computer software, it's learning how to visualize and better understand engineering systems.  



Review:
http://www.grc.nasa.gov/WWW/k-12/airplane/engparts.html

Vocab:

Engine - System used to produce a net power output


Mechanical cycle - a sequence of processes that begin and end at the same state.
Gas cycle - working fluid remains a gas through the entire cycle.
Vapor cycle - fluid alternates between liquid and vapor through the cycle

Closed cycle - working fluid is returned to initial state, and recirculated, heat crosses boundaries, moving parts produce work, but fluids do not cross boundary.
.
.
.



Open cycle - fluid comes in, gets used up, then exhausts out, the working fluid does not go through a complete thermodynamic cycle.




External combustion engine: Steam power plant, energy is supplied from an external source, such as a furnace, geothermal well, nuclear reactor, the sun, etc.


Internal combustion engine: Fuel is burned inside of the system.
Analysis of Power Cycles:

Start with an Ideal Cycle
Assume reversible process, ignore friction, assume system is in thermodynamic equilibrium, ignore unwanted heat loss to surroundings.

Reversible process: adiabatic (no heat loss out of system), you can go back and forth between state 1 and state 2 either in forward or in reverse, the path does not matter.





Idealized model - study major parameters without being bogged down with details.
Trends in ideal cycles match trends in actual cycles
#'s in ideal cycles do not match #'s in actual cycles

Ideal assumptions:
No friction between moving parts
Quasi-equilibrium expansion and compression processes
Negligible heat transfer in connecting parts.
4 strokes:

Work done by a Gas:



https://www.grc.nasa.gov/www/k-12/airplane/work2.html


Work = area inside the P-v loop

https://www.grc.nasa.gov/www/k-12/airplane/otto.html

Carnot - most efficient ideal cycle

isothermal = constant T
Adiabatic = no heat transfer/loss, perfectly insulated walls.


https://www.grc.nasa.gov/www/k-12/airplane/carnot.html


http://en.wikipedia.org/wiki/Carnot_heat_engine

The area enclosed by the cycle on a p-V diagram is proportional to the work produced by the cycle.


What do these ideal cycles teach us?
Thermal efficiency (gas mileage) increases with compression ratio.

Compression ratio - CR

, where

= cylinder bore (diameter)

= piston stroke length

= clearance volume - the volume of the combustion chamber (including head gasket). This is the minimum volume of the space at the end of the compression stroke, i.e. when the piston reaches top dead center (TDC). Because of the complex shape of this space, it is usually measured directly rather than calculated.

Diesel engines - better compression ratios, Why?

Consider fire pistons:

Diesel engine - no spark plug, direct fuel injection, fuel ignites through compression.

 Fuel injected after air is compressed so no worries on compression ratios = better fuel economy. (Trick is injecting fuel to mix homogeneously with air)

Gasoline engine - carburetor mixes air and fuel, then fuel is ignited by spark plug.  (If it ignites on it's own, the timing will be off, and you get engine knock, so there's an upper limit to the compression ratio you can use.)

Higher octane fuels - combustion at higher temp - increased compression ratios.

Gasoline - C9H20
Diesel - C14H30 - longer chain, heavier more oily fuel, less refined so cheaper to make, evaporates more slowly than gas, higher energy density, greater MPG,... pollutes more than gas.




http://www.hedelin.se/drawings_engine.html
 
http://www.animatedengines.com/otto.html

Internal Engine Project: 
Start with a piston & Connecting rod

 Read through this:
http://confident-instruments.com/Piston_Study.htm

Piston:
http://en.wikipedia.org/wiki/Piston
Piston Rings:
 http://en.wikipedia.org/wiki/Piston_ring
Wrist Pin:
http://en.wikipedia.org/wiki/Wrist_pin
Connecting Rod:
 http://en.wikipedia.org/wiki/Connecting_rod
Crank Shaft:
 http://en.wikipedia.org/wiki/Crankshaft
Cam Shaft:
 http://en.wikipedia.org/wiki/Camshaft



Choose your materials, and include tolerances in your views.

http://www.mahle-aftermarket.com/MAHLE_Aftermarket_NA/en/Products-&-Services/Engine-components/Light-Vehicle/Pistons

Piston Schematics:

AA..... Distance between bosses
F...... Top land height
GL..... Total length
KH..... Compression height
MO..... Combustion chamber diameter
MT..... Combustion chamber depth
MV..... Combustion chamber offset
UH..... Dome height
VT..... Valve recess depth





Do a Google image search, choose a make/model, dimension everything out!

Read through:
http://courses.washington.edu/engr100/Section_Wei/engine/UofWindsorManual/Piston%20and%20Piston%20Rings.htm


http://courses.washington.edu/engr100/Section_Wei/engine/UofWindsorManual/Piston%20Design.htm