Oilers/Plant Tenders (HHC) Civil Service Practice Exam 2026 - Free Oilers & Plant Tenders Practice Questions and Study Guide

In a 4-stroke cycle diesel engine, fuel is ignited through a process known as compression ignition. This occurs when air is compressed within the cylinder to a very high pressure, causing its temperature to rise significantly. Diesel fuel is then injected into this highly compressed and hot air. The heat generated from the compression is sufficient to ignite the fuel spontaneously, which is a characteristic feature of diesel engines.

The reliance on high compression in diesel engines differentiates them from gasoline engines, which utilize spark plugs for ignition. In the context of the options provided, the ignition by highly compressed air in the cylinder accurately describes the mechanism by which fuel is ignited in a diesel engine. This process ensures that the fuel ignites efficiently without the need for an external ignition source, making it vital for the operation of a diesel engine.