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Analysis of Internal Combustion Engine Cycles: Otto vs. Diesel Efficiency

This course outline covers the air standard internal combustion cycles including Otto and Diesel cycles. It delves into the generalized IC engine power diagram, the four parts of the cycle (intake, compression, power, exhaust), stable operation requirements, and energy analysis of both cycles. The ideal representations and assumptions for diesel engines, energy input and output calculations, and the importance of compression ratio and mean effective pressure in engine performance are explored. Efficiency comparisons between Otto and Diesel cycles are highlighted.

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Analysis of Internal Combustion Engine Cycles: Otto vs. Diesel Efficiency

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  1. Thermodynamics I Fall 2004 Course # 59:009 Chapter 9, Section 2 Professor Ratner

  2. Outline • Air Standard Internal Combustion Cycles • Otto Cycle • Diesel Cycles • Analysis of Both Cycles

  3. 9.2 IC Engine Cycles Generalized IC engine power diagram. 4 parts to the cycle: • Intake • Compression • Power • Exhaust Stable (continuous power) operation requires at least 4 cylinders.

  4. Otto Cycle, spark plug ignition: 9.2 Otto Cycle (Gasoline Engines) Qin Qout

  5. 9.2 Diesel Cycle Diesel cycle, ignition by compression: Qin Qout

  6. Otto (gasoline) engines are spark ignited at the smallest internal volume (TDC). Diesel engines auto-ignite due to compression, ideally also at TDC. Otto cycle fuel injection takes several possible forms. 9.2 Analysis of IC Engine Cycles

  7. 9.2 Analysis of IC Engine Cycles An Ideal representation of diesel engine (based on compression ignition) has the following assumptions: 1. heat addition occurs at constant pressure 2. closed system consisting of four reversible processes: (1) isentropic compression, (2) isobaric heat addition, (3) isentropic expansion, (4) constant volume heat rejection. Since only process 2-3 is different from the Otto cycle, let us consider the energy analysis for this process.

  8. 9.2 Analysis of IC Engine Cycles Qin Energy input comes in the form of heat addition, process 2-3. While work produced in process 3-4 as the cylinder volume expands. Hence: Qout

  9. 9.2 Analysis of IC Engine Cycles m∆u = Q - W W12/m = u2-u1 >0 W34/m = u3-u4 >0 Q41/m = u4-u1 >0 η = 1 - Qout/Qin = 1 - (u4 - u1)/(h3 - h2) rc = V3 / V2, is called the cutoff ratio Note that the compression ratio = r = V2 / V1 The mean effective pressure (mep) is defined as an average pressure that Wcycle= mep×displacement volume. For reciprocating engines of comparable size, a larger mep is an indication of better performance in terms of power produced at the same rated speed. With the same compression ratio, Otto cycle is more efficient than Diesel cycle.

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