IDEA: CRANKCASE VACUUM

[Ivan]

Presumably one of the forms of power loss in any combustion engine is caused by the pistons pumping air up and down on the crankcase side (hence the presence of breathers etc).

Are modern engines designed to create a partial vacuum on the crankcase to reduce this effect - or could they be?

[knighty]

I think... it would be cancelled out pretty quick due to the cylinder blow-by

pop your oil filler cap off and there's normally a fair bit of air pumping out....

it's fed back into air intake (so any nasties are burnt up inside the engine) but I doubt there's much of a vacuum there

[SteveH]

 My old Ford Sierra has this arrangement...

 There was a small one way valve were the breather exited the block. This was connected to the throttle body (Injection model) & used the depressed pressure in the inlet manifold to keep the crankcase at lower than atmospheric pressure...

[Minnow]

Presumably one of the forms of power loss in any combustion engine is caused by the pistons pumping air up and down on the crankcase side (hence the presence of breathers etc).

Are modern engines designed to create a partial vacuum on the crankcase to reduce this effect - or could they be?

As far as I'm aware most will be conected in some way to the inlet of the engine to create a partial vacum.... also bear in mind, that the stroke of each piston in a multi cyclinder engine are quite likely to be oppossed (or there abouts), thus as one piston is 'pumping air up' another is 'pumping air down'.

[bram]

Have a quick read of this, newer hdi engines have a slightly different system.
http://en.wikipedia.org/wiki/Crankcase_ventilation_system

[biff]

intresting link bram,
                  brings back some memories,years ago my friend who was a great lover of the ford crossflow 1600s, some of which came to him for the last rites,flickering the oil light and coughing,wheezing steamy oil out the top of the rocker cover oil cap.,sometimes a new oil light switch would help,and sometimes his clients just took out the oil light bulb,but usually he just fitted a plastic telescopic pipe from the heating system of another old car.he attached it to the rim of the oil filler inlet on the top of the engine, the idea was that the oil laden steam would make it way up the pipe and deposit the oil on the cold ribs of the pipe which would then run back down into the engine. we hailed it as a master stroke of invention.it gave us a few more months to source another decent crossflow to replace the patient.we soon got pretty well fed up with the overhead cam replacment. a deliberate disaster.
                                                                                                                         biff

[Ivan]

Well the wiki article mentions positive pressure crankcase ventilation and also weak vacuum crankcase ventilation for the purposes of reducing hydrocarbon emission directly from the crankcase. What I'm talking about is putting say 90% of a full vacuum on the crankcase. So that there is almost no air inside the crankcase. This shouldn't require a lot of work to maintain (unless there is a lot of blow-past fumes from the piston rings or water in the oil) and therefore shouldn't sap too much power from the engine, but it WOULD save the energy required to pump air back and fore from the underside of the various pistons. Bearing in mind that piston speed can be as fast as a bullet, there's a significant amount of energy involved eg. bullets slow down with a force of between 50 and 200g depending on speed and aerodynamics. Pistons are far from aerodynamic and much larger than bullets, so I'd assume that the losses would be fairly significant in the overall scheme of things (I'd guess a couple of percent of the fuel energy is wasted in overcoming drag in the crankcase)

[guydewdney]

what do the F1 boys do? if they dont, its not worth bothering.

[bram]

must be thinking down the same lines, just googling f1 crankcase ventilation as you were posting. only thing I could find was that they were building engines with separate chambers (think this means each cyl has its own crankcase compartment) to reduce 'windage'. Simplistically I would have thought pair it with an opposing piston to cancel it out.
But you can guarantee they will have tried it.

Ivan I can see your rational behind it problem is half a bottle of nice red wine down so its getting harder to think.

[Countrypaul]

One problem that might well occur with reducing the crank case pressure from atmosphericsay 15psi to 1 or 2 psi is that the boilin gpoint of the lubricating oil will decrease also. I have no idea how low the bp would drop at that pressure, but it might necessitate the use of heavier higher bp oil which might put more strain on the oil pump, filter etc.

Paul

[smegal]

Surely it is nothing compared to the friction of the crank weights cutting through the oil in the sump?

[guydewdney]

is it pumping losses - rather than expansion / compression losses? I cant remember the physics that well - carnot cycle describes the loss of a non phase change thermodynamic cycle - ie why your bicycle pump gets hot. Is that minimal in comparision to the pumping of the air around the crankcase? hence the 'separate crankcases' mentioned above?

[Countrypaul]

Surely it is nothing compared to the friction of the crank weights cutting through the oil in the sump?

Isn't that one of teh reasons why dry sumps are used on high performance engines?

http://en.wikipedia.org/wiki/Dry_sump

Paul

[bram]

Think you will find not many modern engines have a crank that splashes through the oil, its pretty well all pumped, older engines certainly used it and maybe some newer small engines (Honda type stuff used to have a little tin flicker type thing to chuck the oil around)
Dry sump is primarily for quantity of oil (keeping it from overheating), sitting engine lower, removes problems of oil starvation in heavy braking cornering etc, that sort of stuff.

[biff]

thinking back to when i was a kid,
                        trucks spent their working lives overloaded to the hilt. the local favoutites were little austins ,bedfords, then the x army bullnosed bedfords,,upmarket were leyland(now we are talking) albion,guy,scammell,foden,achecton,,there were no mercs or hinos but there was a commer 2 stroke diesel supercharged which used to make my old man smile every time he overtook one on a long hill.
         this commer diesel 2 stroke was a real mystry, it could be heard coming miles away,it had a massive blower which made noises that excited every kid in the town as it struggled down through the gears to surrmount any little hill it happened to see.if we heard one coming we could walk up the hill beside it while it wailed away flat out. they were obviously being flogged to death,many were heading off to the dublin market with 14 ton of spuds,200 miles away, it must have been a real expedition for the drivers. my old man trucks were seddons,because he had greater access going through narrow gates and also a crawl gear that could get him out of trouble,, top speed,,45 miles per hour,,in the 60s he switched to leyland comets which he said was a bad move, the little tk bedford could eat them alive muckshifting, and also for reliability,
        so what happened folks, where did it all go wrong,?
                                                                    biff