What is the purpose of the automatic compression release (ACR) in an engine?

The automatic compression release (ACR) plays a crucial role in the operation of small engines, particularly in assisting with easier and safer manual starting. When an engine is off and the starter is engaged, the ACR temporarily decreases the cylinder compression. This reduction in compression makes it significantly easier for the starter motor to crank the engine over, thus reducing the physical effort required and the strain on the starter system, especially in cold starting conditions or when the engine is under load.

By allowing the engine to start with lower compression, the ACR helps prevent potential kickbacks that could occur if the compression was at normal levels when trying to start the engine. This safety feature is especially helpful for users who may not have the same strength or experience to start an engine with a conventional high-compression setup.

Other options, although they may relate to engine functionality, do not accurately describe the function of the ACR. The ACR does not primarily aim to improve fuel efficiency, increase power output, or enhance cooling efficiency; its main focus is facilitating easier and safer engine startups by managing compression levels during the starting sequence.