IDEA: CRANKCASE VACUUM

[bram]

The Commer two stroke was infamous for decoking itself when working hard uphill, sending large showers of sparks/soot and c**p out of the exhaust  . Not sure what was actually going on within the engine but it certainly happened. Something else that was said could happen on these engines was for them to run backwards if when stopping the engine you changed your mind and let it run back up just on the point of stalling. Never seen this but it was well talked about.

[biff]

seen this with my own two eyes bram,
                                     the engine in question was a p6 perkins in our seddon, we were parked on a steep hill and loading out of a bank of gravel on ballyliffin strand.the load was almost complete,(loaded by hand,,shovel) when i notice the wheels beginning to turn,next thing i saw was the truck heading backward towards the water and the old man running like mad to catch it,which he did but the strangler would not work,so he hauled on the handbrake(4ft long) and both feet on the footbrake,and finally she stopped hung in 2nd gear,he said if he had knocked it out of gear it would have blown up, the truck was by now in about 2 ft of water,a push of the button and she started up.when we got onto hard ground he discoverd that the engine was very low on oil, his face was black and the inside of the cab was full of black fumes. with the oil topped up everything returned to normal, to this day i am not sure wheither it was a combination of the rear exhaust being submerged in the tide and the old man hauling on the brakes but any intake of brine would have scuppered the engine,.sucking through the exhaust and exhausting through the breather is terrifying stuff.
                                                                                                        biff

[bram]

Spent an embarrassing length of time trying to start a dumper with a Lister engine, almost had the thing running, then noticed it was puffing smoke out of the inlet Didn't realise you could get left and right handed cranking handles took handle apart swapped tooth around to face other direction and she started on half a turn  it was a long time ago, honest.

[oliver90owner]

When you think about it, fluids take time to start moving and time to stop.  By the time the opiston has reached TDC the gases behind will, by having inertia (and likely velocity in the opposite direction) only just be starting to move by the time the piston is descending.  It will pull it's own little vacuum behind it and the energy used for that will be recovered on the down stroke.  At slower engine speeds (Lister CS ilk) this will not happen of course, but the engines revving ten times as fast as that it may not be particularly wasteful, energy wise, thinking adiabatic and isothermal expansion as well as actually pumping gases.  I would think the rotating big end assembly will be losing more energy, needing to move the gases aside (or driving them in front of itself).

Finally there is always some oil mist, and as a liquid fluid, will have vastly higher inertia or energy of motion than the gases, per unit volume.

Too complicated for me to even think about it any more than considering the underlying principles.

Regards, RAB

[camillitech]

Surely a vacuum in the crankcase would have a detrimental effect on the piston rising  still it would discourage oil leaks, must devise a system for my Land Rover 

[spaces]

Citroen flat twin engines as in 2CV have crankcases under a partial vacuum - being twin opposed cylinders which come together at the same time like here: http://www.animatedpiston.com/BMW.htm so there is a pressuring of crankcase unless the gases are vented. The oil filler neck has a series of one way valves which create the slight depression - when these are tired then there is a power loss of about 10%.

[knighty]

kind of off topic a bit....

but I always thought it would be a good idea to run a generator inside a vacuum.... so it's nice and quiet

then you'd only have air intake and exhaust noise (which are easy to deal with)


I was wondering how much of a vacuum a supercharger could pull - it should be pretty efficient once it's pull'd the vacuum ?

but I didn't think there would be enough suck without using up lots of engine power

[biff]

bram,
         dkw,auto-union or the modern today audi, the front grill had 3 hoops instead of the present day 4, anyhow,,dkw did this van,it was 3.6 ltr, some models were supercharged  complete animals.the shape was like the present day renault trafic and about the same length as the l.w.b models.
    they were front wheel drive like the saloon,again they were 2 stroke each cylinder had its own coil. my old man used to drive one.he had a healthy respect for them. these were not a van that you would use for work,they were more of a smugglers van or a black market trader,s van,they could outrun anything on the road, the ton was no trouble and roadholding  was incredible. however two-stroke was just too much trouble and the old man got a vw microbus which he used to marvel at.,he could get 17 adults into it and off to football matches.it could go anywhere in snow and was very easy run, not like the later versions.back then the skoda was one of the best built cars on the road,a bit like the volvo 21.the worst car was the NSU prinz, you were guaranteed not to reach your destination or be lucky that it did not burst into flames.there was still plenty of standard ensigns and vanguards on the roads as well as standard tens,austin a30 was the best small car.the small fords were smelly with vacuum operated wipers and only 3 forward gears.
          there were loads of american v8s and v12s.henry ford was still in cork.you could get a lift down to cork and for 600 pounds buy a ford v8 customline(still only 3 forwards)straight out of the factory.
                                                             biff

[CWatters]

Bit late to this thread but...

Crankcase volume is roughly constant isn't it? When one piston is going down another is going up etc. So no air is being compressed.

Sure you have a mass of air moving back and forth but that's nothing compared to the mass of the piston and connecting rods etc.

[marktime]

what he said +1

MT

[SimonHobson]

I recall going to a talk a few years about the engineering behind F1 engines - the numbers were mind boggling. I forget the figures, but at high speed, as the piston approaches TDC and is slowing down there is a tension of several tons in the con-rod. Then the spark fires, and suddenly there is several times that in compressive force in the con-rod. The cylinder heads move relative to the block as under the forces involved, the head bolts act like very stiff springs. And the whole thing (block, heads, etc) has to be modelled as a flexible structure.

Yes, they do in fact run the crankcase under a partial vacuum because it does reduce drag on the whirly dirty bits. However, they don't run a particularly large vacuum as there comes a point where it takes more power to run the pump than you save from the lower pressure. They don't use a separate vacuum pump, just arrange that the dry sump scavenge pumps do the job by sucking more air and oil out than the lubrication pump puts oil in. By using the scavenge pumps, the exhaust for the vacuum system becomes the vent on the oil tank.