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Air-Standard Otto Cycle. Lecture – 33 11/19/08. Spark Ignition vs Compression Ignition.

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Spark Ignition vs Compression Ignition

Spark-ignition: mixture of fuel and air are ignited by a spark plug. Have advantages for applications requiring power up to about 225 kW (300 hp.). Relatively light and lower in cost, suited well to automobiles.

Compression ignition engines: Air is compressed to high enough pressure and temperature that combustion occurs spontaneously when fuel is injected. Preferred for applications requiring large power and high fuel efficiency (trucks and buses, locomotives and ships). Recently diesels have become popular for automobiles. Require pollution controls for particles and NOX.

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Fig09_01

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The Four Strokes

Intake

Compression

Ignition/Power Stroke

Exhaust

Mean effective pressure = (Net work for one cycle) / (displacement volume)

Air Standard Analysis

A fixed amount of air modeled as an ideal gas is the working fluid.

The combustion process is replaced by a heat transfer from an external source

There are no exhaust and intake processes as in an actual engine. The cycle is completed by a constant volume heat transfer process taking place while the piston is at the bottom dead center position

All processes are internally reversible

In a cold air standard cycle, the specific heats are assumed constant at their ambient temperature values.

Air Standard Otto Cycle

The Otto cycle is shown on p-v and T-s diagrams. It consists of four internally reversible processes in series:

Process 1-2 is an isentropic compression of the air as the piston moves from bottom dead center to top dead center

Process 2-3 is a constant volume heat transfer to the air from an external source while the piston is at top dead center.

Process 3-4 is an isentropic expansion (power stroke)

Process 4-1 is a constant volume heat rejection process while the piston is at the bottom dead center

Cycle Analysis

The air standard Otto cycle consists of two processes in which there is work but no heat transfer and two processes in which there is heat transfer and no work. Processes 1-2 and 3-4 have no heat transfer but only work. Processes 2-3 and 1-4 have only heat transfer and no work.

Assuming no KE and PE effects, we can write the following balances:

Net Work and Efficiency

The net work and efficiency of the cycle can be evaluated by the following relations:

Isentropic Compression and Expansion

For the isentropic expansion and compression, the relations can be written as:

When the Otto cycle is analyzed on a cold air standard basis, the following relations can be used.

Effect of Compression Ratio

The efficiency of the Otto cycle depends on the compression ratio r. This can be seen by the following relations.

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Diesel Cycle

The air standard Diesel cycle is an ideal cycle that assumes heat addition occurs during a constant-pressure process that starts with the piston at top dead center.

The cycle has four internally reversible processes. The four processes are:

1-2: isentropic compression

2-3: constant pressure heat addition and part of the power stroke;

3-4 : isentropic expansion, remainder of the power stroke;

4-1: heat is rejected from air while piston is at the bottom dead center.

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Cycle Analysis

Process 2-3 involves both work and heat addition

Cut-off ratio

The cut-off ratio is defined as: V3/V2

For the constant pressure process 2-3, we can write

Isentropic Compression and Expansion

For the isentropic expansion and compression, the relations can be written as:

When the Diesel cycle is analyzed on a cold air standard basis, the following relations can be used.

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Fig09_06

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