Crankcase pumping losses

[Blackal]

I don't see how a consistant negative pressure can be achieved in a crankcase without some sort of scavenge pump.

theres a set of pistons going up and down pulling a constant vacuum through the air box - its basically an air pump scavenging out of the air box.

the crankcase is one way vented to there - so all positive pressure is dissipated into the engine.

thinking about it - its probably what causes low speed surging.. ?

I still don't see that a regular vacuum is achievable in the crankcase.

The airbox shouldn't be at any appreciable vacuum - it is downstream of the throttle bodies that the negative pressure exists (unless your inlet filter is blocked?). Remember that the crankcase isn't the only source of air to the induction of the engine - the inlet duct also feeds it and is designed to be non-restrictive.

So... any excessive crankcase pressure will be vented to the airbox (which is a requirement for emission controls) so that the vapour is consumed by the engine. On car engines the crankcase breather feeds in upstream of the inlet filter, so I suspect that bike engines are similar? Also, breather systems are designed to deal with oil mist emissions (at positive pressure) and not to maintain the crankcase at a negative pressure.


I don't think we are in total disagreement..... more like degrees of agreement

Al

 

[alimey4u2]

My other bike ( multi cylinder) has perfect primary compression & is vented only to remove vapour & volatiles....
Yup, it's compression as I run ram tubes......

 

[Blackal]

My other bike ( multi cylinder) has perfect primary compression & is vented only to remove vapour & volatiles....
Yup, it's compression as I run ram tubes and ride at full speed into a headwind all the time, regardless of speed cameras ......

Fixed it for ya


Al

 

[rokraider]

I know it is not the same, but to illustrate a point. The old Range Rover EFi V8 engine used to have a filtered,fire trapped breather pipe (approx 12mm bore) than ran from the front of the right hand rocker cover into the engine side of the throttle butterfly on the plenum chamber. The inlet would be sucking hard most of the time. To prevent a total vacuum occurring, there was a 1.5mm approx pinhole with a filter on the back of the left hand rocker cover.You couldn't run a total vacuum as the seals and tray covering the valley would flex with each pistons movement and ultimately would cause the seal to fail or the tray to split (which regularly occurred when the pin hole got blocked with gunge). My understanding is that this achieved 2 goals, 1) To consume the oil vapour and any other noxious emmissions from the crankcase which would be suced into the inlet and burnt. 2) create a negative pressure in the crankcase that was an improvement on the pistons having to displace the air with each stroke ie less air needs less effort by the piston to displace it.
Thats my 2p's worth.

 

[Mouse]

I've stripped a 1200GS engine and I don't remember any valves on the crackcase breather. There is some sort of reed valve between the crankcase and the sump.

Any low pressure in the crankcase could only be caused (I think) by the air being drawn into the airbox up the breather. The breather on the 1200 engine is a bit fat pipe though, much larger diameter than the older engines (again AFAIK).

 

[alimey4u2]

Fixed it for ya


Al

Thanks.........

 

[Dale]

Does the boxer engine suffer from a relatively high level of crankcase pumping losses (due to both pistons moving up and down together) compared to a 4-cylinder engine or a twin with the pistons moving out of phase?

How much is the torque reduced by these losses?

Is anything done to reduce or control these losses?

Does my 1200GS have crankcase breather and filter, or is the crankcase sealed?

I'm just curious about how these things work.

Engineer,

Had a chat to our local workshop manager and a mate who was a design engineer many years ago (mainly atuo gearboxes, but did potter around with motors for a while).

Both pistons are at TDC at the same time. They are in primary balance but not secondary balance - hence BMW's use (and much advertising) of the counter rotating balance shafts in the 1200 motor. Apparently only straight 6 and flat 4 motors have perfect natural primary and secondary balance.

Air is vented from crankcase to top of the airbox through a (roughly) 20mm tube. I forgoit to ask if there is a filter in the breather but the airbox I looked in was not oily in any way so I guess there must be.

Pumpung losses will be higher than for a 4 or 6 cyl boxer motor. It would be very difficult to calc the exact power / torque lost through these losses without suitabe CAD / design expertise. Mate suggested that BMW would surely have done these calcs and perhaps ask them directly.

My personal opinion is that Torque (measured at low revs) would be less affected by losses than power (measured at high revs). At high revs the breather must transport more air per minute and is therefor more of a restriction. Also I guess that if BMW could get more power from the motor by increasing the tube size or restricting these losses in some way then they would surely have done it already.

Regards,
Dale

 

[Engineer]

Engineer,

Had a chat to our local workshop manager and a mate who was a design engineer many years ago (mainly atuo gearboxes, but did potter around with motors for a while).

Both pistons are at TDC at the same time. They are in primary balance but not secondary balance - hence BMW's use (and much advertising) of the counter rotating balance shafts in the 1200 motor. Apparently only straight 6 and flat 4 motors have perfect natural primary and secondary balance.

Air is vented from crankcase to top of the airbox through a (roughly) 20mm tube. I forgoit to ask if there is a filter in the breather but the airbox I looked in was not oily in any way so I guess there must be.

Pumpung losses will be higher than for a 4 or 6 cyl boxer motor. It would be very difficult to calc the exact power / torque lost through these losses without suitabe CAD / design expertise. Mate suggested that BMW would surely have done these calcs and perhaps ask them directly.

My personal opinion is that Torque (measured at low revs) would be less affected by losses than power (measured at high revs). At high revs the breather must transport more air per minute and is therefor more of a restriction. Also I guess that if BMW could get more power from the motor by increasing the tube size or restricting these losses in some way then they would surely have done it already.

Regards,
Dale

Interesting stuff - it is one of the compromises that the a boxer twin has to make I guess.
The BMW press relase mentioned something about a valve with various apertures to control the crankcase pressure. I don't think that it is simply an open pipe all the way from the crankcase to the air-box; but rather a cleverly tuned system to optimise the performance and at the same time meet the emmisssions requirements. I suspect that the ideal would be to 'tune' the crankcase breather as the engine speed changes, but not really practical.

I just wondered how much off an effect these losses have, is it 1%, 5%, 10%? But as you say it probably is not easy to work out. I'll have to get my Physics books out!