4-Stroke Basics
 

Hello all,

I'm pretty new to understanding internal combustion engines. From my basic understanding of the 4-stroke process, here is what I've written out in terms of what is happening with the cams / valves in crankshaft degrees of rotation:

0 degree = tdc - intake stroke starting
(intake and exhaust valves both open here, intake opening, exhaust closing)

180 degree = bdc - compression stroke starting
(intake valves finished closing)

360 degree = tdc - power stroke starting
(all valves closed, spark fires btdc)

540 degree = bdc - exhaust stroke starting
(exhaust valves opening)

One of the reasons this came into my head was in terms of understanding how you would specifiy where camshafts should be oriented in crankshaft degrees. How exactly is this done? How is this specified usually for the stock cams, and then for aftermarket camshafts as well?

Another thing that I sort of realized in talking this through in my head, is the cams must be geared so that they spin precisely 180 degrees for every 360 degrees of rotation of the crank. Is that right? This allows the camshaft lobes to define unique valve action for 720 degrees of crankshaft motion. Am I understanding this all correctly?

Well, a couple different resources:

If you can find them online, Mike Kojima's series called, "Suck, Squish, Bang, Blow" that was published in Sport Compact Car. Also his series, "Revenge of the Nerds" that was on www.nissanperformancemag.com.

Otherwise, go visit the library, preferablely the engineering library on a college campus, and look up the many books on engines and combustion :)

Thanks for the ideas on resources. I will check them out when I get a bit of time. However I have read three books on engine building and performance tuning, so I believe I have a decent idea on fundamental theories. But what I was curious about was to begin to apply this knowledge specifically to the evo.

My first question is, how specifically, do aftermarket camshaft manufacturers specify their recommendation of where the intake and exhaust cams should be positioned in relation to crankshaft rotation? It seems to me they would need to specifiy a degree number, and also which stroke the degrees refer to?

This is where it gets a little tricky, but in short, cam events are always expressed in crankshaft degrees. Cam events take place in fairly restricted regions of crankshaft rotation, so the stroke need not be specified. For example, the lobe centerline (LC) of the intake cam is expressed in deg ATDC, the intake valve opening (IO) is expressed in deg BTDC, the intake valve closing in deg ABDC, etc. Turning the cams far enough such that these default values don't work pretty much means the engine wouldn't even run.

The only time we speak of 'cam degrees' (because cams turn at 1/2 crank speed) is when we are discussing cam gear settings (which are indexed in cam degrees).

Hi Ted,

Thank you for that. Ok this is new terminology for me, but I would assume "lobe centerline (LC)" is the center of the dwell at peak lift. Your explanation makes sense I think.

It is always the case that you want peak lift to occur for intake cam ATDC on the intake stroke obviously. Therefore if a cam manufacturer says put LC of intake at 100 degrees, then this means 100 degrees after top dead center.

And likewise, can I infer that it seems rational that peak lift of the exhaust cam should always occur during the exhaust stroke, so am I correct in inferring it would be measured in degrees BTDC? So if a cam manufacturer says 100 degrees for LC of the exhaust cam, that they mean BTDC? Or could they mean ABDC?

Correct.

Correct. Example: The LC for all HKS intake cams is 110deg ATDC when the cams are installed straight up.

Correct. Example: The LC for all HKS exhaust cams is 110 deg BTDC when the cams are installed straight up.

If we add both LCs and divide by two, we get the angle between the intake and exhaust cam LCs, which is the Lobe Separation Angle (LSA). For HKS, (110 + 110)/2 = 110 deg LSA.

Where the LCs fall with respect to crank rotation and the degree of LSA has a direct influence on the mechanical efficiency of the engine with respect to rpm.