(140) Auxiliary Exhaust Ports.
To decrease the amount of hot gas and flame passing over the exhaust valve some makers provide their engines with auxiliary exhaust ports, which are similar to the exhaust ports used on two stroke cycle engines.
The auxiliary exhaust consists of a series of holes drilled or cored through a rib on the cylinder wall, the holes being so situated that they are covered by the piston until it is at the extreme end of its outward stroke. The holes are not uncovered until the burning charge has been expanded and cooled to the greatest extent possible in the cylinder. As soon as the piston uncovers the ports the greater portion of the dead gas escapes instantly to the atmosphere, carrying with them the greater percentage of the heat and flame. The small amount of residual gas that remains is forced out through the exhaust valve in the usual manner, thus no flame ever reaches the exhaust valve.
The use of auxiliary exhaust ports produces a cooler cylinder as the gas passes over the cylinder wall only once, and consequently is in contact with the walls only one-half of the time usual with the ordinary system. The cool cylinder lessens the liability of PREIGNITION and decreases the consumption of cooling water and lubricating oil. Auxiliary exhaust ports are particularly desirable on air cooled engines.