My latest projects:
Refining the formulas for determining ports top rpm
I ported one of my 55cc cylinders to rev much higher,
using my revised
formulas for determining peak rpm for each port. I ported it and tried
it out and got just a little higher rpm. I was shooting for 9500rpm but
only got around 7500. Looking at the porting the most obvious aspect of
it that was contrary to high peak rpm was the lack of blowdown degrees
(exhaust opening to transfer opening). A normal low for this type
engine is 30 degrees but mine was 23 degrees. Thinking about it on a
long bus ride I figured that any formula could only get away with using
degrees instead of the actual port open time if all the ports to be
calculated were to have nearly the same peak rpm. But that if you want
the formula to work on any peak rpm range that it has to use time and
not degrees. The original formula was formulated just in that way, to
only be applied to road racing engines. But port open time, with the
same amount of degrees, at 7500 rpm is 33% more than at 10,000 rpm. And
I also figured that the amount of pressure (from combustion or
crankcase pressure) should be part of the formula because higher
pressures cause higher gas exit speeds given the same size port. So the
new formula for exhaust ports takes into account the port height so
that an opening at 80*ATDC assumes a blowdown speed 30% faster than
that of a port opening at 100*ATDC. That is because peak pressure
occurs slightly after TDC and lessens as the piston lowers (and expands
the available space for the hot gases). For the transfer ports I figure
the crankcase peak pressure into the formula so that twice the pressure
gives 50% more intake entrance speed. These are both only educated
guesses that no one except the best engineers at the best motorcycle
factories know precisely, but this attempt is better than not figuring
them in at all. I am now in the process of evaluating 4 different
cylinders running at different peak rpm and trying to tweak the
formulas to match them all, to be universal. After all is said and done I truly believe that this is way too big and time consuming a project for me. There are just too many variables involved.
Refining my ideas concerning expansion chamber design
As I continue to build and try out different pipe designs I accumulate
more and more real world knowledge by which I can make more refined
suggestions for different types of powerbands. Pipes are probably 80%
of the main determining factor for what kind of powerband an engine has
and so knowing exactly what to do, especially with the baffle cones,
will enable us all with the needed knowledge to build pipes that
exactly match engines for the desired powerband we want. I now haveExcel files to assist in pipe design that incorporate all I know so
far. I just made a pipe for mine with those files and test results are
friend of mine in Russia shared with me a new design of silencer. I
wanted to share the design with everybody but the guy wants to patent it and sell
the design so I'm not at liberty to share it. So I came up with my own design utilizing his concept. I tried it on my expansion chamber and it was good but not as quiet as I would like. Then I moved the stinger on the pipe to originate from the belly so it would be quieter and then connected the silencer to it. Now it is so quiet that the noise from the cylinder is worse than the exhaust sound. It's so nice to know
that a 2 stroke can be quiet without having to restrict the outflow
(and thus the power) like a street silencer does. I will be making free construction plans available soon.
SOLVING ONE OF TWO STROKES MOST BASIC PROBLEMS:
Two strokes pollution is of burnt oil and of unburnt fuel exiting the exhaust pipe (which also reduces power). The problem of the unburnt fuel happens because at low rpm the fuel mixture has time to enter the cylinder, travel up to the cylinder head, and then loop down and partially exit out the exhaust port. This causes pollution and loss of power. The speed of the mixture flow is basically set by the pressure created by the crankcase compression ratio (CCR). A higher ratio causes more pressure.
SOLUTION SOUGHT: The solution will cause the CCR to be lower at low rpm. A low CCR results in less crankcase pressure at the time of transfer port opening. Less pressure means a slower entrance of mixture into the cylinder which is desirable at low rpm.
SOLUTION FOUND: Apart from the complex solution of automatically manipulating the crankcase volume according to rpm, we can utilize a cheap and simple fix by adding a couple of narrow transfer channels that open 15 degrees before the main area of the transfers open so as to allow the crank pressure to bleed off at low rpm and still have an insignificant effect at high rpm. If the transfers have an open duration of 120 then at 8000 rpm they are open for 1.25 milliseconds before the piston reaches BDC. Bleeding off a sizable amount of the initial crank pressure could happen because at 2000 rpm the time of those "bleed ports" opening is 1.25 ms. Of course, in order to not affect the transfer timing much, the open area of them has to be kept minimal. I would guess that their width can't be more than 10% of the normal transfer(s) width.
CHANNEL DESIGN: Making ascending channels from the transfers or the boost port would cause a waste of that initial fuel mixture because a narrow column of vertical flow would just mix with the exhaust gas and be lost out the exhaust port. Only making the bleed port above the transfers with as flat a roof angle as possible should work (see drawing) because the flow would be nearly horizontal so that once the main transfer flow enters the cylinder they would mix together and rise together.
PROOF OF EFFICACY: If this design works as planned then there will be less fuel mixture lost at low rpm and therefore more power to be felt by the rider. Also there should be little or no change of power at top rpm. If top speed decreases more than 3% then the bleed channels are too big or open too soon.
IMPLEMENTATION OF DESIGN: My 55cc engine has one transfer port on each side of the cylinder and one shallow boost port (which I won't even consider because of its diminished area). Each transfer port is 21mm wide. The narrowest dremel bit is 3/32" wide which is 2.8mm wide. 2.8/21=.13 which means it is 13% of the transfer port width. That is a bit wider than what I wanted but it will have to do.
DRAWBACK OF DESIGN: At top rpm some exhaust gas will push its way down thru the bleed ports causing some dilution of fuel mixture. That's why I believe we have to be willing to lose a bit of top rpm in order to gain a sizable amount of low rpm power. The only way to avoid this drawback is to let the pressure bleed back to the intake side of the reed valve but that would diminish the amount of fuel mixture available for combustion at low rpm. It would solve the pollution problem though.
I will test and report on this new design. stay tuned!