The cycle experienced in the cylinder of an internal combustion engine is very complex.First, air (CI engine) or air mixed with fuel (SI engine) is ingested and mixed with the slight amount of exhaust residual remaining from the previous cycle.
This mixture is then compressed and combusted, changing the composition to exhaust products consisting largely of COz, Hz 0, and Nz with many other lesser components. Then, after an expansion process, the exhaust valve is opened and this gas mixture is expelled to the surroundings. Thus, it is an open cycle with changing composition, a difficult system to analyze. To make the analysis of the engine cycle much more manageable, the real cycle is approximated with an ideal air-standard cycle which differs from the actual by the following:
1. The gas mixture in the cylinder is treated as air for the entire cycle, and property values of air are used in the analysis. This is a good approximation during the first half of the cycle, when most of the gas in the cylinder is air with only up to about 7% fuel vapor. Even in the second half of the cycle, when the gas composition is mostly CO2, H20, and N2, using air properties does not create large errors in the analysis. Air will be treated as an ideal gas with constant specific heats.
2. The real open cycle is changed into a closed cycle by assuming that the gases being exhausted are fed back into the intake system. This works with ideal airstandard cycles, as both intake gases and exhaust gases are air. Closing the cycle simplifies the analysis.
3. The combustion process is replaced with a heat addition term Qin of equal
energy value. Air alone cannot combust.
4. The open exhaust process, which carries a large amount of enthalpy, out of the system, is replaced with a closed system heat rejection process Qout of equal energy value.
5. Actual engine processes are approximated with ideal processes.
(a) The almost-constant-pressure intake and exhaust strokes are assumed to be constant pressure. At WOT, the intake stroke is assumed to be at a pressure Po of one atmosphere. At partially closed throttle or when supercharged, inlet pressure will be some constant value other than one atmosphere. The exhaust stroke pressure is assumed constant at one atmosphere.
(b) Compression strokes and expansion strokes are approximated by isentropic processes. To be truly isentropic would require these strokes to be reversible and adiabatic. There is some friction between the piston and cylinder walls but, because the surfaces are highly polished and lubricated, this friction is kept to a minimum and the processes are close to frictionless and reversible. If this were not true, automobile engines would wear out long before the 150-200 thousand miles which they now last if properly maintained. There is also fluid friction because of the gas motion within the cylinders during these strokes. This too is minimal. Heat transfer for anyone stroke will be negligibly small due to the very short time involved for that single process. Thus, an almost reversible and almost adiabatic process can quite accurately be approximated with an isentropic process.
(c) The combustion process is idealized by a constant-volume process (SI cycle), a constant-pressure process (CI cycle), or a combination of both (CI Dual cycle).
(d) Exhaust blowdown is approximated by a constant-volume process.
(e) All processes are considered reversible
Here are the some general formulas that will be used in this cycle as well as in my next post for analysis of different cycles.
Pv=RT
PV=mRT
P=pRT
dh= CpdT
du=CvdT
Pvk=constant (isentropic process)
Tvk-1=constant (isentropic process)
TP(1-k)/K=constant (isentropic process)
w1-2 =(P2v2-P1v1) /(k-1)
where
P=pressure
v=specific volume of gas
V=volume of cylinder
R=gas constant of air (0.287 kJ/kg-K)
T=temperature
m=mass of air
p=density
h=specific enthalpy
u=specific internal energy
Cp, Cv = specific heats ( for air Cp=1.108 kJ/ kg-K and Cv=0.821 kJ/kg-K
k=Cp / Cv ( 1.35)
w=specific work
