Engine Case Pressurization

[20774]

This idea was brought up on another forum, and probably is an issue with just about any engine where pistons are moving in/out or up/down. When the pistons move from top dead center to bottom dead center, they squeeze the air that is in the interior of the engine case. On the Airheads, there's a crankcase ventilation system that comes up for discussion when it makes noise and/or results in oily mist getting out and into the carbs. This is a fairly simple system with either a disk held against the pressure by a spring or the later style reed valve that pushes open when air pressure increases. On the /2 engine, BMW devised a complicated disk system driven by the engine camshaft (I think) to open a hole at the precise time of the pistons moving to BDC. Excess pressure creates all kinds of problems with seals blowing out or at least the whipping of the oil and creating excess oily mist.

But it's clear to see there is no such system to alleviate the opposite problem when the pistons move from BDC to TDC. This movement tends to create a vacuum and in a small way fight the pistons that are trying to rise to the top. As I think about this, the only way for air to fill that vacuum is to come in through the seals, and mostly the rear main seal.

Really?? Is that the best they could do? How much of an issue is this? What about engines in general? Air coming in around the RMS for millions and millions of cycles throughout the life of the seal. Somehow that just seems bizarre if that is the way for air to get back into the case.

Enquiring minds want to know!

 

[FELAW]

This idea was brought up on another forum, and probably is an issue with just about any engine where pistons are moving in/out or up/down. When the pistons move from top dead center to bottom dead center, they squeeze the air that is in the interior of the engine case. On the Airheads, there's a crankcase ventilation system that comes up for discussion when it makes noise and/or results in oily mist getting out and into the carbs. This is a fairly simple system with either a disk held against the pressure by a spring or the later style reed valve that pushes open when air pressure increases. On the /2 engine, BMW devised a complicated disk system driven by the engine camshaft (I think) to open a hole at the precise time of the pistons moving to BDC. Excess pressure creates all kinds of problems with seals blowing out or at least the whipping of the oil and creating excess oily mist.

But it's clear to see there is no such system to alleviate the opposite problem when the pistons move from BDC to TDC. This movement tends to create a vacuum and in a small way fight the pistons that are trying to rise to the top. As I think about this, the only way for air to fill that vacuum is to come in through the seals, and mostly the rear main seal.

Really?? Is that the best they could do? How much of an issue is this? What about engines in general? Air coming in around the RMS for millions and millions of cycles throughout the life of the seal. Somehow that just seems bizarre if that is the way for air to get back into the case.

Enquiring minds want to know!

isn't there a crankcase vent hole on the upper left side of the motor casting?

 

[20774]

isn't there a crankcase vent hole on the upper left side of the motor casting?

Not sure about such a hole, but there is one at the lower right when viewed from the seat. This is the spot for crankcase breathing and the reed valve. I don't know of any holes that would allow air back in when the pistons are on the way up.

You might be thinking of a small hole above the carbs on the left side. This hole is actually a drain for the starter cavity, so it is not into the engine plenum.

 

[nrpetersen]

I think the crankcase is ported to the carb air intake system thru the one-way check valve which vents to the intake whenever the crankcase pressure is higher than atmospheric. Any blow-by the rings, or other leakage gas flow into the crankcase gets pumped to the engine's air intake whenever the pistons are near BDC.

The overall result is that in operation, the crankcase average pressure will be less than atmospheric. This minimizes any external oil leakage. If the oil cap is loose, leakage air will flow into the crankcase and be pumped to the carb air inlets after the air filter.

Frankly I never thought about this before, but in steady state operation, there must be a substantial crankcase vacuum behind the pistons of an opposed 2 cyl engine. Not sure, but maybe this reduces the "pumping loss" typical of 4 cycle engines?

It is also interesting to listen to what happens whenever the oil cap is removed from an idling 4 cyl in-line engine. The second harmonic breathing caused by the crankshaft/conrod L/R ratio "hoots" very prominently.

Fun to speculate!

 

[20774]

nrpetersen -

I guess I didn't follow...how is the air getting back into the engine case when the pistons are rising up?

 

[beemeruss]

It is also interesting to listen to what happens whenever the oil cap is removed from an idling 4 cyl in-line engine. The second harmonic breathing caused by the crankshaft/conrod L/R ratio "hoots" very prominently.

Fun to speculate!

It is my understanding that in a multi-cylinder inline engine when one piston is rising, another will be retreating. This creates equilibrium, so no issue. I have also pondered this opposed-twin problem and do not envy the engineer(s) who had to deal with the huff'n'puff of the mighty R18. I would certainly not leave the oil dipstick loose on one of those!

Russ

 

[gregfeeler]

It is my understanding that in a multi-cylinder inline engine when one piston is rising, another will be retreating. This creates equilibrium, so no issue. I have also pondered this opposed-twin problem and do not envy the engineer(s) who had to deal with the huff'n'puff of the mighty R18. I would certainly not leave the oil dipstick loose on one if those!

Russ

Yes, boxer engines have significant pumping power loses - there's no free lunch. And you are correct about the equalizing offset of even-numbered cylinder engines. I'm with you about not wanting to be an engineer who has to deal with these issues!

 

[beemeruss]

Yes, boxer engines have significant pumping power loses - there's no free lunch. And you are correct about the equalizing offset of even-numbered cylinder engines. I'm with you about not wanting to be an engineer who has to deal with these issues!

Decades ago I read a treatise that, to sum up, stated that a horizontally opposed 4 cylinder engine has 10% of the vibration of an inline. At least NASA engineers realise that there is no such thing as a free launch....

 

[ragtoplvr]

To minimize crankcase pumping losses many high rpm racing engines use a Vacuum pump to make a negative pressure in the crankcase, the thinking is the less total air, the less pumping around. None of those are boxers, so when some pistons are going down, others are going up, and the vacuum forces balance, except of course for the slight difference from rod ratio.

How a crankcase vacuum would work out on a boxer is unknown to me. I suspect it would still be advantageous.. Of course where you would house the vacuum pump, the vapor separator and plumbing. Some race vehicles get crankcase vacuum from the dry sump scavenge pump, and others harness the exhaust with a check valve, some thing that emissions would NOT like.

I do recall reading of a drag race Harley that did pull a vacuum on the crankcase and saw a increase in horsepower on the Dyno. When they lost a piston the pressure blew apart lots of hoses and stuff. How such a system would work on the street for lots of miles is not known by me. In general, extra parts fail and overall reliability decreases.

Rod

 

[melville]

Yes, the goal is actually to use the breather to get some of the effect of a dry sump system and NOT replace the air that gets pushed through the turkey gobbler or reed valve on the downstroke. Pulling a slight vacuum in the crankcase reduces pumping losses and slightly improves the ring seal.

Some drag racers use vacuum pumps on their wet sump motors for the same effect.

 

[nrpetersen]

I suspect that the vacuum in the crankcase after a few revolutions is quite substantial - like maybe 10 to 15 inches of mercury.

To others - Of course the pistons extend at the same time. It is an alternate firing engine. And each of the pistons spend more of their time at the bottom of the stroke than the top because the connecting rods are comparatively short compared to the crankshaft throw. This is the source of second harmonic (twice engine rpm) huffing and puffing in an inline 4 cyl engines. Our our boxer twins have a massive huff and puff at the crankshaft rotation frequency. When the crankcase is sealed and only venting out through the check valve (I like that turkey gobbler designation!), the crankcase will soon end up running at reduced pressure. How much reduced pressure depends on the piston displacement vs the crankcase total volume.

Engines are a hobby to a lot of us....!

 

[lkchris]

20 - Crankcase vent valve
21 - Stabilizing chamber for crankcase vent
22 - Return bore in stabilizing chamber

 

[20774]

Kent -

I think the system you're referring to was on a limited run of bikes. On them, they had the small hole to let puddled oily mist from the breather to drip back into the engine case. The rest of the mist circulated through the tubing before routing the oily mist air back into the carbs.

That is the only hole that is in the engine case by which air could get back into the case. It is quite small, maybe 3-4mm in diameter. No way that would suffice for all the vacuum being creating by the pistons returning to top dead center.

As I said in my original post, Vech indicates that the air gets back into the engine case via the rear main seal. It would certainly scavenge any oil trying to leak out of the seal and help to keep the seal seated. But just the thought of how often that happens over time when cruising down the road at 4000 RPM is mind boggling!

 

[88bmwjeff]

Kent -
But just the thought of how often that happens over time when cruising down the road at 4000 RPM is mind boggling!

This thread has been interesting to follow. On a side note, what I find really mind boggling is how many times the valves open and close per second at 4,000 RPM. The valves open and close once every other rotation; as such, that's 2,000 times per minute or about 33 times per second. And, it's about 42 times per second at 5,000 RPM and 50 times per second at 6,000 RPM. I find that absolutely mind boggling.

Sorry about the tangent. Now back to the engine case pressurization discussion.

 

[lkchris]

Kent -

I think the system you're referring to was on a limited run of bikes.

Illustration is from official BMW service manual from mid 1980s, blue binder and all.

When I die, my estate will sell it on eBay for at least $1k. (!)

 

[20774]

Illustration is from official BMW service manual from mid 1980s, blue binder and all.

When I die, my estate will sell it on eBay for at least $1k. (!)

Well my 1978 model has that system. I don't believe it was part of the engine casting until the /7 series.

 

[20774]

I'm thinking the majority of the air gets back in by going past the rings. During the power phase, the explosion in the chamber forces the rings against the cylinder walls on the way down. But coming back, possibly the rings relax somewhat and air seeks the vacuum when the pistons are rising. That's much more volume at the rings then can get past the rear main seal.

 

[pglaves]

I'm thinking the majority of the air gets back in by going past the rings. During the power phase, the explosion in the chamber forces the rings against the cylinder walls on the way down. But coming back, possibly the rings relax somewhat and air seeks the vacuum when the pistons are rising. That's much more volume at the rings then can get past the rear main seal.

All at 66 times a second at 4Krpm.

 

[Omega Man]

As far as I know, all crankcases have to be vented in one way or another. In the old(r) days, it was just a tube vented to the ground. As emissions evolved, the tube was vented into the air box.
On my Honda 750, the low tube was “plugged” for emission purposes. This plugging and resulting sealing of the crankcase resulted in the valve cover being blown out (large H shaped gasket on the 4 cylinder engine) and oil on my leg.
It was suggested that if I removed the plug at the end of the tube, the problem would be solved. I did, it was.
OM

 

[20774]

OM -

The BMW Airhead is vented but only "out". There is no purposeful orifice to allow air in...or so it seems. Air has to find its way back in by hook or crook!!