Thermo Test 2
What four processes make up the ideal otto cycle?
(1) isentropic compression, (2) v= constant heat addition, (3) isentropic expansion, and (4) v=constant heat rejection
what are the air standard assumptions?
(1) the working fluid is air which behaves as an ideal gas, (2) the processes are internally reversible, (3) the combustion process is replaces by the heat addition process, and (4) the exhaust process is replaced by the heat rejection process which returns the working fluid to its original state
n(otto)
1-(T1/T2)=1-(1/r^y-1) .....r=V1/V2
real life efficienciencies for otto and diesel
20-30% otto and 30-40% diesel
specific heat constant equal to its value at .....
25 C
How does a diesel engine differ from a gasoline engine?
A diesel engine differs from the gasoline engine in the way combustion is initiated. In diesel engines combustion is initated by compressing the air above the self ignition temperature of the fuel whereas it is initated by a spark plug in a gas engine.
do diesel or gasoline engines operate at higher compression ratios? why?
Diesel engines operate at high compression ratios because the diesel engines do not have the engine knock problem (autoignition)
Enthalpy
H (J)
How does the thermal efficiency of an ideal otto cycle change with the compression ratio of the engine and the specifc heat ratio of the working fluid?
It increases with both of them
Define the compression ration for reciprocating engines
It is the ratio of the maximum to minimum volumes in the cylinder
what does the area enclosed by the cycle represent on a p-v diagram? How about on a T-s diagram?
It represents the net work on both diagrams
Entropy
S (J/K)
How is the mean effective pressure for reciprocating engines defined?
The MEP is the fictitious pressure which, if acted on the piston during the entire power stroke, would produce the same amount of net work as that produced during the actual cycle.
what is the difference between spark-ignition and compression ignition engines
The SI and CI engines differ from each other in the way combustion is initiated; by a spark in the SI engines, and by compressing the air above the self-ignition temperature of the fuel in CI engines
for a specific compression ratio, is a diesel or gas engine more efficient?
The gasoline engine
How do the effeciencies of the ideal otto cycle and the carnot cycle compare for the same temperature limits? explain
The ideal otto cycle involves external irreversibilites and thus it has a lower thermal efficiency
is the fraction of the thermal energy supplied to a heat engine that is converted to work.
Thermal efficiency of a heat engine
Are the processes that make up the otto cycle analyzed as closed-system or steady-flow processes?Why?
They are analyzed as closed system processes because no mass crosses over the system boundaries during any part of the process
Compression Ratio
Volume of combustion chamber at end of compression stroke as compared to volume of cylinder and chamber with piston on bottom center.
In gas power cycles ........ is negative for the fluid and ........ is positive for the fluid
Wnet, Qnet
no heat exchange
adiabatic
As a car gets older, will its compression ratio change? How about the mean effective pressure?
assuming no accumulation of carbon deposits on the piston face, the compression ratio will remain the same (otherwise it will increase). The mean effective pressure, on the other hand, will decrease as a car gets older as a result of wear and tear
diameter of the cylinder
bore
minimum volume (when piston at TDC)
clearance volume
no intake or exhaust
closed system
heat addition from external source
combustion
In a diesel engine, air alone is compressed above the autoignition pressure of the fuel, then the fuel is injected and combustion starts automatically.
compression ignition real cycle
isentropic
constant entropy (s)
difference between max and min volume
displacement volume
Diesel cycle Pv and Ts
draw it
heat rejection that returns the fluid to its initial state
exhaust
In gas power cycles....... throughout the whole cycle (no mixture)
fluid is a gas
when comparing two engines with same v(displacement), higher MEP is equivilent to .........
higher power
adiabatic(no heat exachange) + internally reversible
isentropic
ideal otto cycle TS
isentropic
ideal otto cycle pv
isentropic
constant volume
isochoric
constant temp
isothermal
How does the thermal efficiency of an ideal cycle, in general, compare to that of a Carnot cycle operating between the same temperature limits?
it is less than the thermal efficiency of a carnot cycle
which is more efficient otto or diesel?
otto (in a diesel engined only air is compressed, (without risk of uncontrolled autoignition),hence the compression ratio is higher for diesel engines)
compression ratio
r=vmax/vmin
reciprocating engines can be
spark-ignition type (otto, diesel) or compression ignition type (diesel, gasoil)
largest distance that the cylinder can travel, between top dead center and the bottom dead center
stroke
Define the following terms related to reciprocating engines: stroke, bore, top dead center, and clearence volume
stroke is the distance between TDC ans BDC bore is the diameter of the cylinder , TDC is the position of the piston when it forms the smallest volume in the cylinder, and clearance volume is the minimum volume formed in the cylinder
what is the difference between air standard assumptions and the cold-air standard assumptions?
the cold air assumption involves that air is ALSO and ideal gas (air standard assumption) and it can be treated as an ideal gas with constant specific heats at room temperature
Can the mean effective pressure of an automobile engine in operation be less than the atmospheric pressure?
yes
