Engines are classified in many ways. You are already familiar with some classifications, such as those based on

1. the fuels used (diesel fuel and gasoline),

2. the ignition methods (spark and compres-sion),

3. the combustion cycles (Otto and diesel), and 4. the mechanical cycles (2-stroke and 4-stroke).

Additional information will be given in subse-quent chapters of this manual on some of the fac-tors related to the above classifications as well as to other types of classifications, such as those based on

5. the cylinder arrangements (V, in-line, opposed),

6. the cooling media (liquid and air), and

7. the way air enters the cylinder and the exhaust leaves the cylinder (port-scavenging and valve scavenging).

Classification of internal combustion engines according to combustion-gas action is based on whether the pressure created by the combustion gases acts upon one surface of a single piston or against single surfaces of two separate and op-posed pistons. The two types of engine under this classification are commonly referred to as SINGLE-ACTING and OPPOSED-PISTON engines.

You should understand that the opposed-pis-ton engine is actually a form of a single-acting en-gine since pressure is applied to only one surface

Figure 2-8.Cross section of an Alto 251C 16-cylinder, 4-stroke cycle, single-acting engine.

of the pistons. For the purpose of this rate training manual, we will provide separate discus-sions on the single-acting (one piston per cylinder) and opposed-piston (two pistons per cylinder) engines.