Otto cycle

The Otto cycle ( constant volume process ) is the comparison process for the combustion engine, named after the German inventor Nicolaus Otto. He is a thermodynamically quite running cycle, which converts heat into work ( heat engine ).

The term constant volume based on the assumption that the supply of heat at a constant volume ( isochoric ) takes place. This is in contrast to the direct printing process (including diesel cycle ), in which the supply of heat at a constant pressure ( isobaric ) is carried out.

It was the beginning of the 20th century the constant-volume gas turbines, who defended the constant-volume process with cyclic combustion of the gas mixture. These named after its designer Hans Holzwarth turbine did not need a compressor. They were displaced by the continuous constant-pressure gas turbines.

The comparison process

Consists of four state changes of an ideal gas within a closed system. It therefore does not include chemical reaction and therefore no gas exchange.

The area enclosed by the polyline 1-2-3-4 face in the graphs corresponds to the specific process work w.

Efficiency

To illustrate and easy calculation of state variables of an ideal gas is assumed to be independent of temperature, specific heat capacity as a working medium. The thermal efficiency of the ideal Otto process then does not depend on the amount of heat supplied and can be determined as follows:

The higher the compression ratio V1/V2 and the higher the isentropic exponent, the higher the efficiency.

The thermal efficiency of the process is the same space with the same compression ratio higher than that of the direct printing process.

The equations for the changes of state

The specific heat or heat energy qzu determines the pressure and temperature increase. For efficiency it does not matter.

The ideal Otto engine

The ideal engine has no dissipative losses, mechanical friction, auxiliary equipment, cylinder cooling and leakage losses. The working gas has over the entire cycle the same features and no flow losses. There is no mixing of the charge mixture with exhaust gas.

There are two-and four - stroke engines. A clock consists of a piston or one half revolution of the crankshaft, respectively. When 4- stroke gasoline engine, the state changes can be as follows assign the work cycles:

The real spark-ignition engine

When real gasoline engine knock resistance of the gas mixture limits the compression pressure. The state changes of the constant-volume process correspond only very inaccurately the real engine, as for the combustion time is required ( see below). With an adapted Seiliger cycle process to obtain a better approximation. The air-gas mixture is not an ideal gas, the material properties are strongly dependent on temperature ( isentropic exponent smaller and larger heat capacity at high temperatures), so that the equations given for accurate calculations can not be used. Compounds according to the oxygen of the combustion (primarily water vapor and carbon dioxide ) have different thermodynamic characteristics than the oxygen content in the fresh air.

Compared to the process of the real process in the engine is also a smaller work from, because:

• Sucking in and out is connected to friction losses ( left-handed loop between 0 and 1 in the pV diagram, charge exchange work )
• The incineration of non- isochoric done, but requires time in which continues to rotate the crankshaft. Therefore, the ignition takes place before the top dead center and the combustion is after TDC completed. The peak on the graph at 3 is thus rounded down and to the right.
• A portion of the supplied by the chemical reaction energy is lost (in addition to incomplete combustion and endothermic formation of nitric oxide ) without doing work by heat transfer to the cylinder walls. Therefore, the expansion curve is below the ideal curve.
• The exhaust valve is opened before the bottom dead center. The process area is rounded at point 4 down.

The ratio of released in the engine to theoretical work of the process is called a grade. Real motors have an additional mechanical power loss from friction and the power required for ancillary and auxiliary drives (valves, pumps for oil and cooling water, fan ), which can be about 10 % of the rated power and reduce the efficiency.