API ratings do not apply to which type of engine?

API ratings, established by the American Petroleum Institute, are primarily used to classify and rate motor oils for various types of internal combustion engines. These ratings are designed to provide guidance on the performance and quality of motor oils as it relates to lubrication and engine durability.

In the context of the options given, two-stroke cycle engines are generally not rated with API classifications. This is because two-stroke engines typically do not operate solely on conventional motor oils but use a mixture of gasoline and oil for lubrication. The oil is either mixed directly into the fuel or delivered through a separate lubrication system, and the requirements for lubricants in these engines are different from those in four-stroke or diesel engines that rely on dedicated motor oils meeting API standards.

The other types of engines mentioned, such as four-stroke cycle engines and diesel engines, are specifically designed to use motor oils that must meet API specifications, making them subject to those ratings. Therefore, recognizing the lubrication requirements specific to two-stroke engines clarifies why API ratings do not apply to them.