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Automotive Engineering

 Automotive Engineering

Automotive Engineering


To improve scavenging efficiency, a loop-scavenging system which became known as the reverse-flow or (after its inventor, Dr E. Schnuerle) as the Schnuerle scavenging system was developed (Fig. 1.1-6). This layout has a transfer port on each side of the exhaust port, and these direct the scavenging charge  ixture in a practically tangential direction towards the opposite cylinder wall. 

The two separate columns of the scavenging mixture meet and merge together at this wall to form one inward rising flow which turns under the cylinder head and then flows down on the entry side, thus forming a complete loop. With this form of porting, turbulence and intermixing of fresh fuel mixture with residual burnt gases will be minimal over a wide range of piston speeds.

An alternative to the piston-operated crankcase inlet port is to use a disc-valve attached to and driven by the crankshaft (Fig. 1.1-7(a)). This disc-valve is timed to open and close so that the fresh charge is induced to enter the crankcase as early as possible, and only at the point when the charge is about to be transferred into the cylinder is it closed. This method of controlling crankcase induction does not depend upon the piston displacement to uncover the port – it can therefore be so phased as to extend the filling period (Fig. 1.1-7).

A further method of improving crankcase filling is the use of reed-valves (Fig. 1.1-7(b)). These valves are not timed to open and close, but operate automatically when the pressure difference between the crankcase and the air intake is sufficient to deflect the reed-spring.


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