Just wondering about this this morning. The force on the piston
(pressure x area) goes up with the square of the diameter. Also
bigger bore allows for bigger ports, so all makes sense. Only thing
that doesn't quite fit, is that you would think.. a smaller cylinder
bore would transmit pressure change faster into the case.
Did you find the article?
.
Yes I did;
Let me translate;
"In my previous column I promised to tell how I got the idea of a cylinder with FOS-flushing (I don't know where FOS stands for), well here goes;
30 years ago I was working on a 125cc Rotax-engine. You know, the "ancestor" of the Apla-racers where I write about now and then.
That engine delivered 35 HP at 12.500 RPM. I could raise the RPM with bigger ports and shorter exhausts, but that was never good for maximum power.
The Apia RSA125 engine which Faubel, Gadea and Pasini drive with nowadays, deliver allmost 20 hp more at 13000 RPM. The power increased over 50% where as the RPM level where maximum power is deliverd only go up by 4%.
The RPM level allmost didn't change in 30 years!
BIG BANGS
For high maximum power you need 2 things; big bangs in your cylinder and as much as possible in one minute. Why didn't the RPM's go up then in 30 years? Well it issn't like we didn't try or anything. There were built 125cc engines that did 20.000 RPM, but those engines hardly did the 35 hp that I did 30 years ago at 12.500 RPM. For every stroke a 125cc engine makes, it must be filled with 125cc (and preferably more) gasmixture thru the transfers.
If you have a 2-stroke do to much RPM it gets "asthma". The transfers close before the cylinder is properly filled with gasmixture. A short breathing engine produces a lot of bangs per minute, but those bangs aren't that big.
NOT TOO HIGH
If you make the transfers higher, you get bigger holes and they will stay open longer with each stroke. The problem is that the exhaustports needs to be raised alot, because first; old combustiongas needs to flow away thru the upper part of the exhaustport, the part of the exhaustport that opens before the rest of the exhaustport and transfers open and second; because that old gas also needs to be gone in a shorter amount of time if you want to do more RPM.
There is only one thing; the exhaustport cannot be TOO high. I cannot explain this in this column because I don't have enough space for it.
Allright, that's one side of the story; when a 2stroke makes to much RPM, the power gets worse.
Now the other side of this story; 30 years ago there were F1 engines with 3 liter V8 engines, so that makes 375cc per cylinder. Those engines also did 12500 RPM. That made me think; "why can a 375cc 4 stroke cylinder make just as much RPM as my 3 times smaller 2stroke engine?"
BORE
F1 engines have a really big bore and that's for a good reason. If you can make the bore of a 4stroke engine twice the size, you can fit twice the size valves in it, which will give twice the inletsurface, which results in 4 times faster cylinder filling, so 4 times the RPM.
With that you get 4 times more power (well, more or less, not completly true, the combustionchamber gets an ineffective shape).
Now if you want to keep the same CC's with a bore that's twice as big, you need to make the stroke 4 times shorter. That's an advantage, since it will hold up better at high RPM.
KREIDLER
A modern F1 engine has a bore of 98mm (bigger is not allowed by FIA) and a stroke of 39,7mm. That's the same stroke as a 50cc kreidler engine. But you will never get a kreidler engine to make 19000 F1 RPM.
What if we made the bore of the kreidler engine 2x as big and the stroke 4x as small? Then all the ports can become 2x as wide. That's nice. But, the porthightes are a fixed percentage of the stroke and they will be 4x lower as normal. The total portsurface (width x height) will be 2 times smaller as before and the RPM level where the cylinder reaches optimal cylinderfilling will be halved. Same goes for maximum power, so a shortstroke 2stroke engine is not a good idea...
With a 4stroke engine it works the other way around. There, maximumpower, only depends on the valve diameter and so directly from the bore and not the cylindervolume! If you leave the bore unchanged and you make the stroke 2 times as large, the cylindervolume will be twice as large, but that also means it takes 2 times more time to fill the cylinder. The bangs the engine gives will be double as hard, but at a halved RPM level. So the poweroutput stays the same as before, inspite of the bigger cylindervolume.
FOUR VALVES
F1 engines don't have transferports, but they have valvesv (the FIA forbids 2stroke engines, properly because they are affraid it will be to fast).
For those valves you can calculate how big they are and how long they should open in proportion to ports of a 2stroke engine? In other words; what do I have to do to my ports on my 2stroke engine to get just as much RPM as a 4stroke engine without getting the engine asthma?
I did the math (and it was shit) and the outcome was that we need to increase the transferport surface and more important; we need alot more exhaustport surface and to make it more difficult, they cannot be made any higher then they are now...
It's possible, but how would a cylinder like that look like? Nice puzzle he?
I will give you one tip; the difference between a "normal" 2stroke cylinder and mine FOS-flush system is something like 4stroke 2 valve engine and a 4stroke 4valve engine.
Can you picture it?"
I translated by hand, so it might contain some faults in spelling etc. But it's sure is intresting! Also I found a Excel program which let's you calc. time area etc. But all is in Dutch, I have to translate. Will do that later, right now I don't have time.