How does the Jacobs Exhaust Brake work and why do only diesel engines use them?
A diesel engine, unlike a gasoline engine, has no intake throttle plate. When the truck is coasting and there is no fueling in the cylinders, a diesel engine with no exhaust brake produces less retarding (slowing) power than does a gasoline engine. By adding a Jacobs Exhaust Brake, restriction is created in the exhaust system during the coasting condition. This increases pressure in the exhaust manifold, slowing the pistons during the exhaust cycle when the exhaust valves are open. This produces improved retarding power that helps slow the vehicle. The Jacobs Exhaust Brake is actually superior in performance to an intake throttle in a gasoline engine. An intake throttle produces vacuum in the cylinder during the intake stroke, which can reach a maximum level of about 14 psi. The Jacobs Exhaust Brake produces a backpressure somewhere in the neighborhood of 60 psi, resulting in enhanced retarding power.
