I think we all know the expression, “It’s run out of breath”, when an engine is asked to do more than its performance will allow. An internal combustion piston engine does actually breathe. The action of the piston going up and down in the cylinder is like our chests expanding and contracting to draw air in and exhale carbon dioxide. It is on the firing stroke the engine does all it’s work to drive us forward on our bikes. It does so because the burning of the fuel and air in combustion chamber creates very high pressures to push the piston down and turn the crankshaft around. A little of the hot gas above the piston leaks past the piston and piston rings into the crankcase because of the high pressure. With the engine running at 7000 RPM you can imagine that it would not take long for a very high pressure to build up in the crankcase. Unless it is given a route out of the crankcase it stays swirling about in the cases. It can be argued that this potentially high pressure may not detract from the engine performance because the work done against the crankcase gas to compress it is returned when the gas expands again as the piston goes on its way back up its stroke. But there are several reasons for having as little gas as possible in the crankcase.
1/. If the pressure gets very high gaskets and seals will rupture. Even with internal pressure only a little about the outside ambiant pressure oil leaks will be encouraged through the tiniest casting porosity.
2/. The oil scavenge pump will prefer to pump gas to the detriment of oil and the crankcase will start to fill with oil as well as with gas.
3/. We also want as little oil as possible swelling around with the gas because it causes drag on the rotating parts. For example, the speed of the periphery of the crankshaft and the big-end of a 500cc Manx Norton rotating at 9000 RPM is around 80 mph and the drag can be considerable so both volumes above and below the piston have to be allowed to breathe as effectively as possible. I mention the Manx Norton because in my experience it was the worst engine for breathing arrangements. In my day the Manx had a crankcase breather pipe at the front, at the drive side and from the timing chest. These various pipes provided nothing more than ventilation with no mechanism to actually expel gas. I blocked all the pipes off and drilled the drive side main shaft and fitted the G50 flapper valve. This is almost the ideal place for the crankcase breather to be because it is at the centre of the whirl-wind in the crank case so almost all the oil is centrifuged away from the breather. While I still had the old exposed hairpin valve springs I needed hardly any oil absorbent felt draped around the engine as many the Manx Nortons of my contemporaries had. Nor did my rear tyre glisten with oil as theirs’ did!
As I said above, a high pressure in the crankcase will damaged gaskets and oil seals so, conversely a depression should help and indeed should actually discourage oil leaks.
The limitation of the G50 flat flapper valve is that it is no more than a non-return valve and has very small hole for the gas to go through. Of course, it is not the only mechanism that can get rid of blow-by gas and creative depression in the crankcase. The normal oil scavenge pump has a capacity of somewhere between one and a half and two times that of the feed pump. That is why you see the oil return to your tank in globs and why the oil tank itself has to breath. The Cosworth V8 DFV racing car engines used the roots type scavenge pumps that were more gas pumps than they were oil pumps and had such pumping capacity that they created a huge gale through the engine which entrained the oil and took it to the oil tank but had to have a swirl pot to separate the oil from the air in the tank.
There is actually a fourth reason for letting the crankcase breath well. The leaked blow–by gas is nasty noxious corrosive stuff and it is best to have as little as possible in the crank case just as it’s bad for us to have smoke in our lungs.
Nowadays it is unacceptable to breathe the crankcase and ventilate your oil tank to the atmosphere. Car and bike breather systems now redirect crankcase gases into the inlet tract to ensure that there are fewer and fewer unburned hydrocarbon emissions as possible.
Peter Williams Engineering 