Internal combustion engines can be classified in a number of different ways:
1. Types of Ignition
(a) Spark Ignition (SI). An SI engine starts the combustion process in each
cycle by use of a spark plug. The spark plug gives a high-voltage electrical discharge between two electrodes which ignites the air-fuel mixture in the
combustion chamber surrounding the plug. In early engine development,
before the invention of the electric spark plug, many forms of torch holes
were used to initiate combustion from an external flame.
(b) Compression Ignition (CI). The combustion process in a CI engine starts
when the air-fuel mixture self-ignites due to high temperature in the combustion chamber caused by high compression.