8/16/2019 Bonus Prob

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(Bonus Problem: 20 pts) In a four-stroke direct-injection stratified-charge (DISC) engine, fuel is

injected into the engine cylinder at the top dead center like a diesel engine; a spark discharge is

then used to initiate the constant-pressure combustion process. The exhaust from the DISC engine

has the following composition (in mole fraction):

CO2 = 0.12; H2O = 0.14; CO = 0.01; H2 = 0.005; N2 = 0.7247; C8H18 = 0.0003Let’s assume that the fuel is C8H18 gas; as shown above, a small fraction of the fuel escapes from

the cylinder unburned. The engine has the compression ratio of 12 (i.e.,1 2

/ 12v v   = ), and the

volume after combustion becomes double the volume before combustion (i.e.,3 2

/ 2V V    = ).

Assume that the injected fuel temperature is the same as that of the compressed air. The inlet

condition of the air to the engine is 25°C and 1 atm. When the engine operates at 3000 RPM with

the air mass flow rate of 3 kg/s, calculate (1) the net engine power in [kW] and (2) the thermal

efficiency of the engine using EES.

Hints:

1. You may want to study the EES code for Homework 10 (Prob. 3) carefully.

2. Although the fuel is gasoline (octane), the cycle itself is identical to the diesel cycle.

3. At the end of expansion, the volume (4

V   ), not specific volume, should be the same as the volume

at the beginning of compression ( 1V  ).

4. Do not freak out when you have a warning message saying “C8H18 property correlations may

not be accurate for temperatures outside of the range 200