At what point is cylinder compression dumped into the exhaust when a Jacobs internal engine compression brake is actuated?

The correct point at which cylinder compression is dumped into the exhaust when a Jacobs internal engine compression brake is actuated is around top dead center (TDC) on the compression stroke. This action takes place just before the engine reaches TDC during the compression stroke.

When the compression brake is engaged, it effectively opens an exhaust valve at TDC in order to release the pressure that has built up during the previous compression stroke. This release of pressure reduces the engine's power output, slowing the vehicle down in a controlled manner without the need for conventional brakes.

By discharging the compressed air at such a crucial point—just as the piston is about to hit TDC—this method provides significant braking force, leveraging the engine's own mechanics to enhance deceleration. This technique is particularly useful in heavy-duty vehicles, where traditional braking systems may be less effective due to the vehicle's weight and momentum.