UNIFLOW SCAVENGING

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 Uniflow Scavenging: Opposed piston Engine                              

1. Uniflow Scavenging: Poppet valve Engine

Air and exhaust gases do not change direction in passing through the cylinder. Usually, the air inlet is from the ports situated at lower end and exhaust goes out form top. The possibilities of short-circuiting or excessive mixing are therefore reduced.

Air inlet, controlled by the piston, may be at lower end of cylinder liner and the exhaust outlet in the cylinder cover may be controlled by poppet valves, mechanically or hydraulically operated from a camshaft.

An alternative uniflow system is to control the exhaust, as well as the inlet air, by pistons, as done by opposed piston engines. The scavenge air and exhaust pistons may be driven by separate crankshafts or by one crankshaft, through suitable gear. By suitable design of ports or by providing special air deflector for incoming air can be given swirl, which intensifies evacuation of gases and promotes turbulence in air, which gives better combustion.

Advantage:

1. Uniflow arrangements permit unsymmetrical scavenging and supercharging. Thus, in opposed piston engines, the two sets of pistons may be out of phase i.e. they do not arrive at the dead centre simultaneously. The amount may be say, 12*, but may be as high as 18* to 20*, depending upon engine.
2. For increases in scavenge pressure, the reductions in scavenge efficiency is less for a uniflow scavenge engine than for either of other two systems. Uniflow scavenging is most suitable for supercharged engines.
3. Final charge is richer is oxygen then that of either of the other systems short-circuiting and excess mixing are avoided.
4. As inlet air and exhaust gas are some distance apart it is possible to provide more air for air inlet and exhaust outlet for the same cylinder diameter; because the ports can extend right round the liner circumference. It assists in rapid evacuation of combustion products even at high-speed rotation.
5. Uniflow scavenging allows greater latitude with the stroke/bore ratio. This will have important bearing on the satisfactory functioning of the engine and also trend to have super-long stroke, slow speed engines for developing higher power.

Disadvantage:

1. Complication. Requires additional valve gear to operate exhausting system.
2. Higher cost.

As the disadvantages are very much offset by advantages and higher specific outputs can be obtained with uniflow scavenging, it is being more and more adopted for fuel efficient present day engines.