Otto Cycle

The Internal combustion engine (Otto Cycle)

The Otto cycle is a set of processes used by spark ignition internal combustion engines (2-stroke or 4-stroke cycles). These engines a) ingest a mixture of fuel and air, b) compress it, c) cause it to react, thus effectively adding heat through converting chemical energy into thermal energy, d) expand the combustion products, and then e) eject the combustion products and replace them with a new charge of fuel and air. The different processes are shown in Figure 3.8:

  1. Intake stroke, gasoline vapor and air drawn into engine ( clip_image001).
  2. Compression stroke, clip_image002, clip_image003increase ( clip_image004).
  3. Combustion (spark), short time, essentially constant volume ( clip_image005). Model: heat absorbed from a series of reservoirs at temperatures clip_image006to clip_image007.
  4. Power stroke: expansion ( clip_image008).
  5. Valve exhaust: valve opens, gas escapes.
  6. ( clip_image009) Model: rejection of heat to series of reservoirs at temperatures clip_image010to clip_image011.
  7. Exhaust stroke, piston pushes remaining combustion products out of chamber ( clip_image012).

We model the processes as all acting on a fixed mass of air contained in a piston-cylinder arrangement, as shown in Figure 3.10.

clip_image013

Figure 3.8: The ideal Otto cycle

clip_image014

Figure 3.9: Sketch of an actual Otto cycle

clip_image015

Figure 3.10: Piston and valves in a four-stroke internal combustion engine

The actual cycle does not have the sharp transitions between the different processes that the ideal cycle has, and might be as sketched in Figure 3.9.


Efficiency of an ideal Otto cycle

The starting point is the general expression for the thermal efficiency of a cycle:

clip_image016

The convention, as previously, is that heat exchange is positive if heat is flowing into the system or engine, so clip_image017is negative. The heat absorbed occurs during combustion when the spark occurs, roughly at constant volume. The heat absorbed can be related to the temperature change from state 2 to state 3 as:

clip_image018

clip_image020

 
 

clip_image021

 

The heat rejected is given by (for a perfect gas with constant specific heats)

clip_image023

Substituting the expressions for the heat absorbed and rejected in the expression for thermal efficiency yields

clip_image024

We can simplify the above expression using the fact that the processes from 1 to 2 and from 3 to 4 are isentropic:

clip_image025

clip_image026

clip_image027

The quantity clip_image029is called the compression ratio. In terms of compression ratio, the efficiency of an ideal Otto cycle is:

clip_image031

clip_image032

Figure 3.11: Ideal Otto cycle thermal efficiency

The ideal Otto cycle efficiency is shown as a function of the compression ratio in Figure 3.11. As the compression ratio, clip_image033, increases, clip_image034increases, but so does clip_image006[1]. If clip_image006[2]is too high, the mixture will ignite without a spark (at the wrong location in the cycle).


3.5.2 Engine work, rate of work per unit enthalpy flux

The non-dimensional ratio of work done (the power) to the enthalpy flux through the engine is given by

clip_image035

There is often a desire to increase this quantity, because it means a smaller engine for the same power. The heat input is given by

clip_image037

where

  • clip_image039is the heat of reaction, i.e. the chemical energy liberated per unit mass of fuel,
  • clip_image041is the fuel mass flow rate.

The non-dimensional power is

clip_image042

The quantities in this equation, evaluated at stoichiometric conditions are:

clip_image043

clip_image045

 

clip_image046

clip_image047

 

so

clip_image048

clip_image049

 

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