Question for the engineers

[TwoSeven]

Inertia,takes time to get things moving....and once moving to stop them again.

Interia doesnt sound right. Doing some (really rough measurements) the surface area of my inlet port is 2.54cm2 and my exhaust port area is 1.77cm2 (its acutally slightly bigger, but I dont yet know how to calculate the volume of an oval).

Because the inlet valve is opening on the exhaust stroke, the exhaust gas is under pressure - the inlet valve is now at 1mm lift (15 deg btdc), that means the exaust gas should in theory try to go out the inlet more so than trying to go out the exhaust port (path of less resistence).

Hence I am confuzzled as to why one would open the inlet valve then. Only thing I can think of is that most of the zorst gas is gone (perhaps because of scavanging) and there is hardly any pressure left (or its less than the inlet charge).

[TwoSeven]

The Goldstar cams I used in my B31 had 120 degrees of overlap - 65-85 - 80-55.So the inlet valve was closed with the piston almost halfway up the bore,the exhaust valve opening just past halfway on the power stroke...

I think your talking about dynamic compression here. Something I am currently trying to do the math on. This engine (a cbr600f2) runs the inlet close at 35 abdc when its on-cam, so losing about 20% of the compression stroke as far as I can work it out. I've been thinking prehaps reducing the duration slightly so its down to about 25 deg.. Less gas charge but more compression.

Zorst valve for me opens at 38bbdc on the power stroke.

[Ixion]

It's actually quite complicated, because you can't think of it in static terms. Remember this is all happening about 50 times per second.

f'instance, on the previous stroke, there was this column of air whizzing at high speed down the inlet tract. Then the inlet valve slammed shut in front of it. But the air has inertia too, so it tries to keep moving and banks up behind the closed valve under pressure. One fiftieth of a second later (that's 20 milliseconds!) the valve opens again. Who knows what the inlet pressure at the valve is then?

Same sort of issues in the exhaust port, made even more complicated by resonance (though you can get that in the inlet , too, when the inlet valve shuts it causes a reverse ripple back through the carb - one reason why hairy race engines backfire through the carb. And inlet valves can be big enough that as they close the heads actually serve as paddle pumps, forcing the air mixture backwards. )

I prefer to classify the whole subject under "magic" myself.

[TwoSeven]

As the exhaust gases are evacuated from the combustion chamber, a small but distinct low-pressure area is created on the surface of the piston head. By opening the intake valve earlier that TDC, the charge begins to fill this low-pressure area while exhaust gases exit. The low-pressure area on the head of the piston assists the fresh charge in filling the combustion chamber to its maximum capacity.

So exhaust is scavanging then (means I have to get the primary header length accurate I guess).

[willy_01]

whats stopping manufactures using rotary valves on a 4 stroke? Can they not handle the combustion pressure or is lubrication their biggest issue (hence why they only seem to be doing big things on 2 stroke engines). It just seems better, not having anything upsetting the flow of air into the chamber ie the valve itself.

[idb]

Interia doesnt sound right. Doing some (really rough measurements) the surface area of my inlet port is 2.54cm2 and my exhaust port area is 1.77cm2 (its acutally bigger coz its oval but not by much).

Because the inlet valve is opening on the exhaust stroke, the exhaust gas is under pressure - the inlet valve is now at 1mm lift (15 deg btdc), that means the exaust gas should in theory try to go out the inlet more so than trying to go out the exhaust port (path of less resistence).

Hence I am confuzzled as to why one would open the inlet valve then. Only thing I can think of is that most of the zorst gas is gone (perhaps because of scavanging) and there is hardly any pressure left (or its less than the inlet charge).

Remember that the exhaust valve opens before the inlet.
This means that by the time the inlet valve opens the exhaust gases are already moving out of the exhaust valve.
These gases have already developed a momentum (inertia if you prefer) and don't just suddenly change direction to head out the newly opened valve.

[TwoSeven]

Port angles come largely into this too. The ideal is to have the incoming slug of mixture moving fast in a sort of spiral (if it moves too slow the fuel falls out of suspension), and shoot off down into the cylinder in a corkscrew direction that takes it away from the still open exhaust port.

And , in a perfect world, the exhaust port is being blocked by a pulse of gas pressure send back from the megaphone.

And the collector as well (zorst attentuation is next on my list of things to look at)

[Ixion]

whats stopping manufactures using rotary valves on a 4 stroke? Can they not handle the combustion pressure or is lubrication their biggest issue (hence why they only seem to be doing big things on 2 stroke engines). It just seems better, not having anything upsetting the flow of air into the chamber ie the valve itself.

They did. Rotary vales of various sorts (most notably the Knight sleeve valve) were hot stuff in the 30's.

As you indicate, lubrication and gas sealing were their bugbears.

Engineers have always regarded the poppet valve as the most horrible of horrible things. Problem is, no-one's found a better way yet. Except two strokes of course.

[Paul in NZ]

So exhaust is scavanging then (means I have to get the primary header length accurate I guess).

Yes, but remember that a tuned header length is generally only effective at a certain engine speed. The trick is deciding where you want your power to be and making sure inlet length, valve timing and exhaust design all conspire to make it so. Classic example is yamahas EXUP valve that effectively changes the tuned length of the exhaust thus giving a broader spread of power.

In a 4 pot engine, not only is the length of the header important but you need to make sure the collector and muffler are correctly dimensioned.

http://www.carcraft.com/techarticles/0304_merg/

Oh heck the 'net is full of it...

Once you start tuning an engine nearly everything becomes a trade off. I'll sacrifice a little power and torque here to gain a whole lot there... Street bikes usually require a broaded spread of power hence all the trickery...

You need to have a plan and stick with it. Just bolting a bunch of shite on is likely to make the bike run slower....

[Motu]

Right - the exhaust gas has already gone,it has inertia and as it goes out it can help pull in the intake charge.The whole resonance tuning thing works on that,plus a negative pressure wave coming back up the pipe and into the cyl.It's a cycle of events,each dependent upon the other - The exhaust gases go out first under pressure,then the pressure drops,but they are still going out.The inlet side works the same way - by the time you have got the stuff pouring into the cyl it will keep doing it until you tell it to stop.Even with the piston coming up the inertia will keep the charge going in until the forces of the piston overcome it...then you can close the valve.It would be easy if they were sheep and you had a good dog to control them....

[Paul in NZ]

They did. Rotary vales of various sorts (most notably the Knight sleeve valve) were hot stuff in the 30's.

As you indicate, lubrication and gas sealing were their bugbears.

Engineers have always regarded the poppet valve as the most horrible of horrible things. Problem is, no-one's found a better way yet. Except two strokes of course.

That and rotary valves (and rotaries) often end up producing very large combustion chambers that are horribly inefficient on fuel. Not a good thing in this day and age..

[TwoSeven]

Remember that the exhaust valve opens before the inlet.
This means that by the time the inlet valve opens the exhaust gases are already moving out of the exhaust valve.
These gases have already developed a momentum (inertia if you prefer) and don't just suddenly change direction to head out the newly opened valve.

So the inertia of the gas is creating a low pressure area behind it which helps to build the inertia of the inlet charge [conductence ?].

Question is, the zorst gas is under pressure (because its being pushed out, rather than being sucked out as is occuring to the inlet charge which is not under pressure). So I thought that would have an impact as well. Perhaps the piston is now slow enough so that the dwell at tdc is starting to have an effect as well.

I wonder if its to perform attenuation of the inlet port in the same way as the megaphone is used to attenuate the zorst port ?

[Ixion]

Yith. And long and weirdly contorted ports. Which didn't matter so much in the 30's when 6:1 was high compression.

[Ixion]

,,

I wonder if its to perform attenuation of the inlet port in the same way as the megaphone is used to attenuate the zorst port ?

If you look at some of the pictures of the single cylinder racers of the 60's you may see some with honking long tubes on the carb intakes. A foot long in extreme cases. Theory was these acted to attenuate the inlet flow, as you say. Dunno if it ever actually worked, mind.

[idb]

And the collector as well (zorst attentuation is next on my list of things to look at)

It works too.
I made myself some headers for the 750/4 and tuned the lengths from a formula I got from a book.
The power came on exactly where I calculated it should.

There'll be other factors come into play as well (as discussed above) but I don't care-it proves how clever I am at the end and that's all that really matters.