Hot air → high pressure → mechanical motion
ideal:
Pressure (lbs/in^2) *dV(in^3) = Work (lbs*in)
actual
PdV = frictional losses, entropy, heat loss, + a little bit of work...
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 and open cycles:
Closed cycles -
.
.
.
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 ideal 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, but #'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:
https://www.grc.nasa.gov/www/k-12/airplane/work2.html
Work = area inside the P-v loop
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.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.
Piston Schematics:
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
