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S E C T I O N 2 2 . 5 • Gasoline and Diesel Engines 681 Using these equations and relying on the fact that A ! V D ! V 1 and V B ! V C ! V 2 , we find that Subtracting Equation (2) from Equation (3) and rearrang- Substituting Equation (4) into Equation (1), we obtain for (4)
T D " T A T C " T B ! " V 2 V 1 # *" 1 (3)
T D ! T C
" V 2 V 1 # *" 1 T D V 1
*" 1 ! T C V 2
*" 1 (2)
T A ! T B
" V 2 V 1 # *" 1 T A V 1
*" 1 ! T B V 2
*" 1 which is Equation 22.7. We can also express this efficiency in terms of tempera- tures by noting from Equations (2) and (3) that Therefore, Equation (5) becomes During the Otto cycle, the lowest temperature is T A and the highest temperature is T C . Therefore, the efficiency of a Carnot engine operating between reservoirs at these C ! 1 " (T A /T C ), is greater than the efficiency of the Otto cycle given by Equation (6), as expected. (6)
e ! 1 " T A T B ! 1 " T D T C " V 2 V 1 # *" 1 ! T A T B ! T D T C (5)
e ! 1 " 1 (V 1 /V 2 ) *" 1 Application Models of Gasoline and Diesel Engines We can use the thermodynamic principles discussed in this Two important quantities of either engine are the displacement volume, which is the volume displaced by In a diesel engine, only air (and no fuel) is present in the cylinder at the beginning of the compression. In the C (B : C). At C, the fuel injection is cut off and the power stroke is an D ! V A (C : D). The exhaust valve is opened, and a constant-volume output of energy To simplify our calculations, we assume that the mixture in the cylinder is air modeled as an ideal gas. C and assume constant values for air at 300 K. We express V ! 0.718 kJ/kg ( K, c P ! 1.005 kJ/kg ( K, * ! c P /c V ! 1.40, and R ! c P " c V ! 0.287 kJ/kg ( K ! 0.287 kPa ( m 3 /kg ( K. A 3.00-L Gasoline Engine Let us calculate the power delivered by a six-cylinder gasoline First, let us calculate the work done in an individual cylinder. Using the initial pressure P A ! 100 kPa, and the initial temperature T A ! 300 K, we calculate the initial volume and the mass of the air–fuel mixture. We know that the ratio We also know that the difference in volumes is the V A V B ! r ! 9.50 Adiabatic processes A B C D P V Q h Q c V 2 = V B V C V 1 = V A Figure 22.14 PV diagram for an ideal diesel engine. |