CROSS SCAVENGING

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There are 3 systems of airflow scavenging in common use for Two Stroke Engines.

1. Cross Scavenging:

Exhaust and inlet ports are arranged around the periphery of the lower end of the liner opposite to each other and in so doing eliminate the necessity for cylinder head valves and their operating gear.  This considerably simplifies engine construction and can lead to reduction in maintenance. As shown in the figure above there is usually a deflector-head type of piston, the object being to avoid short-circuiting by directing the airflow.  This deflector head is troublesome from heat flow considerations; because of the metal thickness required and the difficulty in cooling it. Combustion space is not thermodynamically best for oil engines.  An alternative construction is the angling of the inlet ports towards the top of the cylinder to direct the airflow.  This method is not so effective and it increases cylinder liner costs.

Usually, top edges of exhaust ports are higher than those of scavenge ports. Cross scavenging systems may not be very effective and the b. m. e. p. may not usually exceed 3 to 3.5 bar, but their simplicity and low cost may often be more important than high specific output. This system was popular in early days before turbo-charger introduction, to keep weight of engine balanced on either side, by scavenge and exhaust trunkings. When turbo-chargers were introduced both these trunkings has to be on one side to facilitate connections. Old type Sulzer (RD) Engines used this system.