2 - Background

2.2 - Specific examples

2.2.2 - Mechanical design of heat engines

2.2.2.3 - Gas turbine

The gas turbine uses an non-uniform air-fuel mixture and a continuous flame.

It is a multi-fuel engine which can take liquid, gas or emulsified fuels.

The air-fuel mixture is prepared in a separate combustion chamber where an injector delivers a fine spray of fuel to obtain a high combustion rate with the mixture.

Compression takes place and the work is obtained with the help of blade turbines which turn without rubbing against their respective housing.

This configuration can be used to obtain very high revolution speeds, with gas flows reaching and sometimes exceeding the speed of sound in certain parts of the system, although the calculations are based on a maximum gas speed of around Mach 0.8 to 0.9.

The intake air is first compressed to a value of around 4 to 6 bars, then passes through a heat exchanger where it increases in temperature, reaches the combustion chamber where it combines with the fuel to form a gas which increases in temperature as it mixes with the fuel to form a gas that increases in temperature as it burns, thus increasing in volume. The gases transfer part of their energy to a power turbine, and where needed, another part into the heat exchanger and dissipate the remainder into the atmosphere.

As working pressures are not very high, power can only be generated by the revolutions which obviously depend on the size of the turbine. Thus, a gas turbine for a car would operate at between 8,000 and 70,000 rpm.

Advantages of the gas turbine:

- multi-fuel operation;

- smooth running;

- good p_max/p_me;

- favourable emission values without other equipment.

Disadvantages of the gas turbine:

- Poor performance: -less than 30% of the thermal energy in the fuel is transformed into mechanical energy;

- low working pressures;

- engine speeds often too high;

- high manufacturing cost;

- high fuel consumption;

- unsuitable for low powers;