Download Assignment - Gas Turbines and Engine Cycles

Assignment – Gas Turbines and Engine Cycles
MECE E4211 – Energy Sources and Conversion
Read carefully pages 1-30 of “Engines: An introduction”. This material is invaluable material for understanding engines.
Some of the questions here and in quizzes will be from this reading.
Problem 1. Write the expression for the Otto cycle efficiency (ideal) in terms of the compression ratio. Also write the
expression for an ideal Diesel cycle using a compression ratio and a cut-off ratio. What allows the Diesel engine to achieve
to higher efficiency, using the two expressions as an aid?
Problem 2. The assigned text provides a rough value of 0.69 for o /ac at =1.
a.) Provide a simple qualitative explanation for this rather large degradation in efficiency when properties of real
gases are considered as opposed to treating the working fluid as air at standard temperature and pressure (STP)
conditions. Consider your qualitative knowledge of properties of various gases.
b.) Hence for a compression ratio of 8, we are already down to an efficiency of 39% for the Otto cycle. For this same
efficiency of 39%, what is the temperature TH at which one would have to supply heat if one were operating on a
Carnot cycle. Assume heat rejection at 27°C for the Carnot cycle. Compare this temperature TH with the theoretical
peak temperatures in the Otto cycle for a compression ratio of 8 and heat addition of 2000 kJ/kg.
c.) Can you explain why a lean mixture as opposed to a rich mixture for Otto cycle engine could improve efficiency
even though would reduce the peak temperatures? Note that this contradicts our general notion for gas power
cycles that higher the temperature at which heat is supplied, higher the thermal efficiency.
Problem 3. At 2000 rpm the engine of your car has an indicated efficiency of i =0.304 (corresponding to a compression
ratio r = 8.5) at an equivalency ratio (ratio of fuel:air ratio to stoichiometric) of =1.2 (this rich mixture gives maximum
power). Note that the stoichiometric air:fuel ratio is 14.9. Your engine has a volumetric efficiency v = 0.83 and a
mechanical efficiency m = 0.85. The heating value of the fuel is Q = 42.7 MJ/kg. The air density is 1.2 kg/m 3. What is the
BMEP? (Hint: Use the two expressions for brake power.)
Problem 4. In the diagrams below mark arrows on both sets of figures.