the top of the compression stroke. The closed throttle provides engine braking by forcing the engine to generate a vacuum between the throttle and the cylinders. Typically, the controls for a compression brake consist of an on/off switch and often a selector that controls the number of cylinders on which the brake is active. Throttle and clutch switches are integral with the system. Activation occurs when both the clutch and the throttle are released with the transmission in gear. It is the driver's job to ascertain the correct transmission gear to use, depending on factors such as the steepness of the grade and the vehicle's load. Limitations The use of engine compression brakes may cause a vehicle to make a loud "growling", "machine gun", or "jackhammer" like exhaust noise, especially vehicles having no mufflers, which has led many communities in the United States, Canada, and Australia to prohibit compression braking within municipal limits. Drivers are notified by roadside signs with legends such as "Brake Retarders Prohibited," "No Engine Brake," "No Jake Brakes," "Compression Braking Prohibited," "Please No Engine Brake," "Avoid Using Engine Brakes," or "Unmuffled Engine Braking Prohibited," and enforcement is typically through traffic fines. Such prohibitions have led to the development of new types of mufflers and turbochargers to better silence braking noise. Jacobs claims that the use of the term "Jake Brake" on signs prohibiting engine retarding brakes violates their trademark and discriminates against Jacobs brand products. In the U.S. state of Ohio, state law allows a board of county commissioners or township trustees to prohibit the use of compression brakes within unincorporated areas. These local regulations apply to all state- and locally maintained roads except Interstate highways. The state's standard "No Engine Brake" sign is designed to avoid discriminating against the Jacobs brand name. See also Engine braking Compression release Exhaust brake Retarder (mechanical engineering) Jacobs Vehicle Systems References Vehicle braking technologies de:Motorbremse#Dekompressionsbremse