ProMecha Motorcycle Suspension - Crankcase Breather Theory

Motorcycle Chassis and Suspension Specialists

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Graham Byrnes PhD

motorcycle chassis and suspension specialists

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Crankcase Breather Theory

I've been thinking about crankcase breathers for a bit. After reading around the internet, my conclusion (opinionated and arrogant as it may be) is that most of the self proclaimed experts have NFI

So here is some breather theory, in the context of an SV.

Issues

First, a breather has two issues to deal with:

1. Blow by. If this is not vented away, the mean crankcase pressure will increase until oil starts squeezing past seals, through gaskets and generally everywhere.

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2. Variations in crankcase volume. Does not apply to inline fours, or 180° twins. Or inline sixes, for that matter. On a 90° V-twin, the crankcase volume is minimal when the crankpin is pointing halfway between the two cylinders, and maximal opposite that. The difference in volume is the engine swept volume/sqrt(2), or about 450cc for an SV.

First issue is sort of easy. You let the stuff out via a sufficiently large hole to avoid excess pressure.

Second is trickier, especially in conjunction with the first.
If there was no blow-by, you could just seal up the crankcase. The trapped air would act like a spring: some work would be done compressing it, but most would be given back straight away as the air helped push the pistons back up. A little is lost because when you compress air it gets hotter, and some of that heat will be lost through the crankcases and cooling system. Assuming the crankcase volume is about 4.5L, you do about 45J of work per rev to compress the air in the crank. At 11000rpm you are doing it 180 times per second, so if all the heat was lost you'd lose 8kW or about 10hp. In fact, since there is little time for the heated air to lose temperature before expanding again a few milliseconds later, only a very small fraction of this is really lost. I'd guess around 5%, so maybe 0.5hp.

The Formula

The formula is approximately (change in vol in litres) x (mean pressure in bars) x rpm x 1.67. The approximation is valid provided the change in volume is small relative to the total volume. Note that it does not depend on total crank volume. So, despite the confident opinion of various Ducati web experts, increasing crank volume via a huge under-seat breather tank is only a second order effect, and will certainly not give "3 to 5hp at high rpm".

Effects

The first order effect is pressure. If you can halve the mean air-pressure in your crankcase, you will more than halve the power losses.

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There are a couple of ways of doing that. One is to couple the crank breather to a vacuum pump. You could use an electric one, but an exhaust venturi will work very nicely. Even better, most manufacturers thoughtfully provide one on each cylinder as a pollution reduction device

The other way is to simply attach a one-way valve to the crank breather. Then, as the crank volume decreases and the internal pressure rises above atmospheric, some air will be squeezed out. The best you can do is for the crankcase pressure to be 1 atm at minimum volume... i.e. you reduce the pressure by about 10%.

However there is the question of where the one-way valve should live. Standard Ducati practice is to put it at the exit of the crankcase. Ducati race practice is to put it between the airbox and the breather tank, so the breather tank is part of the crank volume.

The bad aspect of this is that it involves pushing air up and down the breather hose each rev. Pushing 450cc of air through a 20mm id hose in 2.7 msec requires that it move at 50m/s, or about 180km/h. That requires a bit of energy which, unlike the stored energy in compressing trapped air, will be almost entirely lost. However it's not much: about 140W (about 0.4hp). About the same as the losses due to heat loss from compressed air in sealed crankcases would be.

If instead the valve is at the crankcase exit, there is no up and down flow and so no power loss.

The other issue is oil entrainment. The crankcase is full of oil mist, so if you start sucking air out, it will take oil with it. There is a fear that connecting the crankcase to a vacuum source will increase the rate of oil loss.

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In fact it should reduce it. Applying a vacuum does not result in an increased flow rate: there is a initial outrush as the c-case is de-pressurized, but once that is done there is only the blow-by flow, plus the same volume of tidal (in and out) flow caused by the volume change. And in fact if the pressure is half, so is the density, so the tidal flow measured in mass is half what it would be without the vacuum. The blow-by flow is mostly unaffected: it's driven by the pressure difference above and below the piston rings. Given 10-20 bar above, changing from 1 to 0.5 below makes little difference.

So, conclusions:

If you want to reduce crankcase pumping losses, better to apply a vacuum to the cases than to vent them to a big catch tank;
Applying a vacuum will reduce oil loss (given the same separator tank system);
The power gain will be 2/3 of bugger all anyway