2 - Background

2.2 - Specific examples

2.2.1 - Thermodynamics of heat engines

2.2.1.4 - Classification of heat engines

Combustion taking place inside an engine is known as internal combustion. There are two options and in both cases, the circuit is open:

- the work process can be cyclical and after each cycle a fresh charge has to be injected and the burnt gases discharged, as in the case of reciprocating piston engines;

- or the work process can be continuous and the charge is replaced without interruption, as in the case of gas turbines.

Combustion which takes place outside the engine is known as external combustion. In this case, the work process can be continuous and the circuit closed because the motive fluid can remain chemically unchanged and undergo changes of state without having to be replaced.

Preparation of the air-fuel mixture outside the combustion chamber helps to create a more uniform mixture. We call this external or uniform mixing.

Mixing inside the combustion chamber is less satisfactory because the mixture is less uniform. This type of mixing is known as internal or non-uniform mixing.

Combustion ignited by an electric spark is known as spark-ignition,but if combustion is started by the mixture reaching the spontaneous ignition temperature, it is called auto-ignition.

2.2.1.5 - Thermodynamic cycles

Thermal energy can only be converted to mechanical energy if the motive fluid changes state.

These changes in state are generally represented by diagrams which express two state magnitudes:

- temperature increase or decrease;

- pressure increase or decrease;

- volume increase or decrease.

Diagram coordinates generally express these relationships:

- pressure-volume in a p-V diagram;

- temperature-entropy in a T-s diagram;

- enthalpy-entropy in an h-s diagram.

The study of thermodynamics has led several scientists to propose engine cycles. Below is a list of those which are still in use today:

- A. Beau de Rochas (1815 – 1893) invented the 4-stroke compression cycle in 1862. This is the standard cycle for modern-day four-stroke petrol engines.

- N.A. Otto built an engine based on this cycle in 1867, but had to abandon his work after legal action was brought against him for breach of patent.

- In 18... J. Joule . (1818 – 1889) invented the constant pressure cycle. This is the standard cycle for gas turbine engines.

- Sabathe, whose first name is not known, was the first to recommend a mixed cycle which combined the A. Beau de Rochas and R. Diesel cycles. This is the standard cycle for gas-oil engines which many people still wrongly call diesel engines.

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