It has been determined by experiment that the most efficient form of exhaust nozzle is that which keeps the jet in the densest and most compact form.
Tests indicate that the nozzle giving the jet the least spread is the most efficient.
Nozzle a ends in a plain cylindrical portion 2 inches in length. Nozzle c is contracted in the form of a plain cylinder ending in an abrupt cylindrical contraction. It has been common practice, in cases where engines refuse to steam properly, to put across the exhaust nozzles round or knife-shaped bridges as indicated in Fig. The use of bridges accomplishes the desired result but experiments have shown that this method materially affects the efficiency of the engine because of the increase of back pressure in the cylinders. It is, therefore, best not to split up the jet by using a bridge in cases where the draft is unsatisfactory, as the desired results may be obtained by reducing the diameter of the exhaust nozzle.