• Welcome to The Truck Stop! We see you haven't REGISTERED yet.

    Your truck knowledge is missing!
    • Registration is FREE , all we need is your birthday and email. (We don't share ANY data with ANYONE)
    • We have tons of knowledge here for your diesel truck!
    • Post your own topics and reply to existing threads to help others out!
    • NO ADS! The site is fully functional and ad free!
    CLICK HERE TO REGISTER!

    Problems registering? Click here to contact us!

    Already registered, but need a PASSWORD RESET? CLICK HERE TO RESET YOUR PASSWORD!

scavenging term clarifcation for me and FYI

schiker

Well-Known Member
Messages
3,075
Reaction score
1,961
Location
Pendleton, SC
FYI

So I have been reading several threads on cams, compression ratio, and turbos etc and wanted to post something that may help some people.

I think I have been misinterpretting the term scavanging as it relates to air flow through the cylinder. And it may have different context to some.

I was thinking scavanging was more a flushing or blow through of the cylinder where intake and exhaust opening overlaps. BUT it doesn't necessairly have to blow through or flush through to be considered scavanging.

It can just be the continued movement of air from momentum of air mass independant of piston stroke like this definition taken from wikipedia. This was written from a gasoline context but it clarified "a" definition of scavanging for me. Yes on a naturally aspirated engine this is critical but SOME of the theory applies to turbo diesel as well.

Wikipedia....
" A running engine does not stop taking air & fuel into the cylinder when the piston reaches BDC; The mixture that is rushing into the cylinder during the downstroke develops momentum and continues briefly after the vacuum ceases (in the same respect that rapidly opening a door will create a draft that continues after movement of the door ceases). This is called scavenging. Intake tuning, cylinder head design, valve timing and exhaust tuning determine how effectively an engine scavenges. "

From that standpoint the ATT could very well improve "scavenging" as lower backpressure of the exhaust allows more gasses to exit thus it improves-increases the velocity of the intake air charge by lowering left over gasses in the cylinder on intake stroke. Increased velocity increases momentum of intake air mass thus more momentum more "scavanging".

Hopefully this helps more understanding of the ramifications of more scavanging/momentum and thus swirl/turbulence and how it can take on different contexts and directions of conservations for good cylinder air mix (left over exhaust and intake air) and fuel mix/burn.
 
You bring up a good point , schiker. I had forgotten that the primary purpose of the blowers on old Detroit's was for scavenging and not boost. Later on, GM figured out you could alter the cam timing to produce boost with just a blower. In essence, all we did was advance the cams to close the exhaust valves earlier. We did not have intake valves to worry about as these were 2 stroke engines.
 
I have also read definitions that scavenging is simply air entering and exiting the cylinder, but this is a better definition, using the momentum and context of the vacuum, BDC, etc...

The way I look at our engine is that with the negative overlap the exhaust closes prior to TDC, so there is no momentum of exhaust gas out of the cylinder that wasnt due to the active pushing of the piston. During idle or no boost then the momentum of the intake air does help around BDC, otherwise the positve boost pushing there doesnt need to be any momentum.
 
I think velocity/momentum are key for scavenging/turbulence/mixing and compaction. The way to increase velocity is to change pressure differentials accordingly and reduced friction / tuning etc.

Thinking out loud.....

Maybe the volume change rate at a 1-2-3 5-10 ??? degrees +/- over BDC or TDC is so much smaller volume rate change than at mid stroke is the factor. I don't think the piston would push out or pull in as near as much air over these few degrees even if you looked at one port only and direction of air from a pumping standpoint but from momentum yes.
 
Back
Top