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In order to understand how the Jacobs Engine BrakeTM retarder provides its strong retarding power, let's compare the engine cycles with and without a Jacobs Engine BrakeTM. The illustration pertains to a four-cycle engine. However, the Jake Brake is effective on both two- and four-cycle diesels.
(1) The intake valve opens and air is forced into the cylinder by boost pressure from the turbocharger.
(2) Air is compressed to approximately 500PSI by the engine piston. The energy required to compress this air is produced by the vehicle's driving wheels. Near top dead center, the Jacobs Engine BrakeTM opens the exhaust valves, venting the high pressure air and dissipating the stored energy through the exhaust system.
(3A) On the downward stroke, essentially no energy is returned to the piston (and to the driving wheels). There is a loss of energy. This loss is how the retarding work is done
(4A) Normal exhaust stroke.
Information provided by Jacobs Vehicle systems