Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Crankcase Scavenging

  1. Oct 9, 2011 #1
    What does crankcase scvenging mean in terms of an automobile ?? .I searched all around the net but didn't get single satisfactory result.
  2. jcsd
  3. Oct 9, 2011 #2


    User Avatar
    Science Advisor
    Gold Member

    Getting combustion products and oil vapour out of the crankcase, usually (these days) using a breather system which filters these gases and returns them to the inlet tract. A google search gave me dozens of good references, what kind of 'satisfactory' result do you need?
  4. Oct 10, 2011 #3

    Ranger Mike

    User Avatar
    Science Advisor
    Gold Member

    I think your are refering to crankcase vacuum in a race engine.

    Specifically, if you run a dry sump system with a three stage pump (one pressure stage, two scavenge stages), in most cases you cannot achieve a sufficient level (8 "HG) of crankcase vacuum to achieve that power gain. If you go to a four stage pump you can gain 2 HP per cylinder. The reduced pressure ("vacuum") in the crankcase is generated by having a substantial excess of scavenging capacity with respect to the engine's oil flow rate. The "vacuum" increases the pressure differential across the ring package, producing an improved ring seal. The improved ring seal allows the use of a low-tension (reduced friction) ring package, yielding a power increase as well. Further, the reduced crankcase pressure dramatically reduces windage losses at high RPM. In most engines, the expected power gains will occur with 8 to 10 inches HG crankcase vacuum. Beyond that point, more vacuum does not generally produce any measurable power gain until (a) you get more than 20 inches HG of vacuum AND (b) you are operating in excess of approximately 8300 RPM. Generally you size the system to produce around 14 "HG when the engine is fresh. That provides sufficient capacity so that as the engine wears and blowby increases, there will still be sufficient scavenging capacity to achieve the 8"HG minimum, and power does not drop off noticeably. If you want to run a high level of crankcase vacuum (18 inches HG or more), there must be provisions in the engine to supplement the lubrication that is lost due to windage. There will likely be problems with at least wrist pin and cam follower lubrication. The best solution will be the addition of piston oilers and, if your engine has a flat tappet cam, provisions for extra lubrication of the cam lobe-to-lifter interface will certainly be required. If you are trying to achieve over 18 "HG, you will need to install special crankshaft seals (front and rear) which have the sealing lips reversed to hold that higher level.
  5. Oct 10, 2011 #4
    Thankyou both for your replies, especially Ranger Mike. But, i could make the definition for me to understand very well, by seperately reading meaning of "Crankcase" in depth and then "Scavenging". Adding bothe the definions you'll know about Crankcase Scavenging. ^_^
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook