All engines suffer from a degree of ‘blow-by’, where combustion gas escapes past the piston rings into the crank case. While the movement of pistons and other reciprocating components contributes to crank case pressure, the biggest single culprit is blow-by, and the resulting pressurisation needs to be vented from the engine. If this pressure isn’t relived, it can cause oil to be pushed past the various crank case seals, notably the main crank seal; it will also impact the efficiency of the engine. Engine breather systems are a subject in their own right, so in this article we will look at just one area – oil-air separators and catch tanks.
In the early days of racing it was perfectly acceptable to simply vent crankcase pressure directly to the atmosphere. However, given that the gas being vented contains a mixture of unburnt fuel, combustion debris and oil mist from the crankcase, this approach became unacceptable in the post-war era as environmental concerns started to grow.
The simplest solution to containing these by-products is a closed circuit breather system, where the case pressure is vented directly into the engine inlet. Over time, these have evolved into quite complex systems to manage case venting at varying engine loads and speeds. For example, the systems used on modern roadcars will use valves that prevent oil mist being sucked directly from the engine at low speeds when case pressure is low and inlet vacuum high.
While some race engines feature such systems their complexity is not appealing, and neither is the ingestion of oil mist into the inlet charge. Thus, in racing applications, by far the most popular method of controlling oil expelled through any vents is to use a catch tank with either a separate or integral oil separator.
The most basic type of oil separator consists of a simple volume through which the blow-by gases flow. As the gas enters the volume, its velocity slows, allowing oil to drop out of suspension and pool in the bottom of the volume. The oil can then be either fed back into the engine or into a catch tank. Taking the void approach one step further is the use of what is known as a ‘labyrinth’ system of baffles. These force the blow-by gas to slow down by directing it around tight corners, and again the oil drops out of suspension.
More complex are centrifugal-type separators. These cause the gas to spin through a chamber, with the oil droplets separating out and running down the chamber walls. ‘Driven’ centrifuges, where the cylinder is actively rotated, are one option but they are rarely used because of their complexity. Far more common are non-driven centrifuges, where the gas enters a circular chamber at a tangent and is encouraged to flow in a circular motion, creating enough centrifugal force to throw the suspended oil against the chamber walls.
Most catch tanks with integral separators will use either this type of design or a labyrinth-type baffle. Note though that most dry-sump oil tanks also rely on the centrifugal approach to help de-aerate scavenged engine oil.
The most important design consideration for an oil separator or oil catch is its cross-sectional area. This needs to be large enough to allow for the speed of the blow-by gases entering the device to drop below 1 m/s as they transit the separator/tank to the vent. It is also worth having an inlet angle that is tangential to the wall of the tank, in order to promote a circular motion regardless of whether centrifugal force is destined to be the sole method of oil separation.
Written by Lawrence Butcher