AR

Thermodynamics I

Fall 2004Course # 59:009

Chapter 9, Section 2

Professor Ratner

AR

Outline

1. Air Standard Internal Combustion Cyclesa. Otto Cycle

b. Diesel Cycles

2. Analysis of Both Cycles

AR9.2 IC Engine Cycles

Generalized IC engine power diagram.

4 parts to the cycle:

1. Intake

2. Compression

3. Power

4. Exhaust

Stable (continuous power) operation requires at least 4 cylinders.

AR9.2 Otto Cycle (Gasoline Engines)

Otto Cycle, spark plug ignition:

Qin

Qout

AR9.2 Diesel Cycle

Diesel cycle, ignition by compression:Qin

Qout

AR9.2 Analysis of IC Engine Cycles

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.

AR9.2 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.

AR9.2 Analysis of IC Engine Cycles

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:

Qin

Qout

AR9.2 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.