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Monday, July 28, 2008

Thermodynamics and Engines

(1) This picture depicts the so called "Zeroth Law" having to do with statistical mechanics of systems in thermal contact, and the First and Second Laws of Thermodynamics.

(2) This shows some of the equations having to do with heat transfer into and out of the system (Q=TdS) and work being done on or by the system (W=PdV). The graphs show what the various polytropic processes tend to look like when dual plotted on a work (Pressure vs. Specific Volume) and a heat transfer (Temperature vs. Entropy) graphs. The equation for the compression factor "Z" is given. The change in the internal energy "dU" of the system is assuming the system as a whole doesn't gain or lose any potential or kinetic energy. The value "k" is defined as a constant that is proportional to the constant pressure specific heat to the constant volume specific heat, or the ratio of the specific enthalpy temperature gradient "dh/dT" to the specific heat temperature gradient "du/dT" of the system it is experimentally being heated and compressed, respectively.

(3) This shows the definitions for all of the processes whereby one of the parameters are held constant while the other complimentary parameter is being varied.

(4) Here shows a work (Pressure vs. Specific Volume) and heat transfer (Temperature vs. Specific Entropy) graph corresponding to a Carnot Engine cycle that is undergoing adiabatic (meaning "no-energy-go-through) compression and power strokes, and isothermal (meaning "no-temperature-change") combustion and exhaust strokes. The area in the rhombus on the P-V diagram is equivalent to the work being done per engine cycle and the area of the rectangle in the T-S diagram is the amount of heat that is being consumed by the engine on each cycle. The ratio of the work done by the engine per cycle to the heat being consumed per cycle gives the actual engine efficiency value. The maximum theoretical engine efficiency is given by the formula Eff(%)=(T2-T1)/T2. (5) Here is an animation for a 2-stroke piston engine where the intake and compression occur on the up-stroke, and the ignition (power) and the exhaust occur on the down stroke.
(6) This shows a four stroke piston engine where "1" the intake stroke is down, "2" the compression stroke is up, "3" the ignition or power stroke is down, and "4" the exhaust stroke is up. The valves are operated by cams on the camshaft whereby the intake valve cam lags the exhaust valve cam by an angle of 90 degrees.


(7) This shows a V-8 engine operating with all of the pistons firing in a synchronized pattern, and thereby turning the crankshaft which runs the radiator fan and the the power pulley in the front. The power pulley is belted to a few other pulleys for running the oil and fuel pumps, the radiator pump, and the Alternator. The gear in the back that is powered by the crankshaft runs the camshafts and the transmission connects to that from below for running the drive shaft with different gear ratios (all not shown though).


(8) The Wankel Engine is a kind of rotary engine that runs very smoothly because it doesn't require pistons and it is relatively continuous. Note how the inner gear's diameter is 1/3 that of the outer gear's diameter, meaning that the inner gear has 3 revolutions per every 1 revolution of the outer gear.


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Images courtessy of Wikipedia.



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