Explanations on 2 Stroke Operation and Fuel Requirements of ANY 2
In a 2 stroke
engine the REQUIRED amount of F/A needed is dependent and based on
MANY different variables.
to extreme details.. I will try and explain a few key points. Keep
in mind, I will be para-phrasing and will not touch on every
variable just a few key ones.....
1) A huge player is the combustion efficiency of
the engine.. This is a factor that relates to what percentage of the
F/A mix is actually converted to work on the crank. You have a
combustion chamber that has "X" amount of F/A mix to convert to
energy. It will only convert a certain percentage ("Y") effectively
and the rest will be wasted via raw fuel out the exhaust or residual
mix that will linger around the chamber and never do any useful
work. It may perform some cooling functions but nothing more. In
fact, it will take up "space" in the chamber and contaminate the
intake charge on the next stroke.
This is always the case with a 2 stroke engine.. the "key" is to
maximize this efficiency via the combustion process. In other
words...increase ("Y"). This is one reason why the RK Tek
827cc kit can make 172HP on
the 827 and run LESS main jet than a 140Hp stock engine. Also why
the head change alone is worth 4-9 main jet sizes less.
Also one reason, why the RK Tek 827 requires MUCH less main jet than
other big bores out there.
So, in short, the combustion efficiency plays a major part in how
much fuel (jet) an engine requires. It is not solely based on vacuum
from the engine.. Which brings me to the next "factor"
2) Pipes and crankcase vacuum: When the piston is
at BDC, there will be a very strong vacuum pulse at the exhaust
port. This pulse is due to the pipe's wave returning from the
divergent cone.. The strength of this wave is variant on many things
like, angle of the cone, location of the cone with respect to rpms
and efficiency (there's that word again) of the exhaust. In any
case, the fact remains that there is a strong suction pulse at the
exhaust port at BDC and this pulse is a major player in how much
pull there exists at the transfer ports and how much, how efficient
the pipe effects are at getting the F/A mix out of the transfer
ports and into the cylinder where it can be combusted.. So, once
again, if the pipe is very effective in its task then there will be
more mix to convert to power than there would be if the pipe as not
as effective.. Hence the name "tuned pipe" . Crankcase pressure also
aid the pipe in this cylinder filling..
3)Crankcase Pressures: As the piston is travelling up the
stroke to TDC, it is leaving behind a negative pressure that places
a signal on the needle jet of the carb. How strong this signal is
will determine how strong the carb signal is and a stronger signal
will pull more F/A mix fromthe carb in a shorter period of time than
a weaker signal. A few things that determine this signal's strength
are bore size and crankcase volume. Larger bores generally have a
stronger vacuum and looser cases lower primary compression and will
weaken the signal. Other factors include the stroke of the engine
and the size of the carb. Shorter stroke will lessen the time the
signal is present at the carb (Time =Distance (stroke) / Rate (rpms)
)) So a shorter stroke will allow for less time to pull fuel from
the carbs through the reeds. Carb size also plays a role.. A larger
carb will have a slower velocity so there will be less time to get
F/A mix through the reeds.
4)Porting arrangement: I'll touch on this briefly.... The
porting arrangement of ANY 2 stroke engine will greatly effect all 3
of the things listed above...Having the mix flow to the proper areas
(and not to other areas) contributes greatly to how much fuel is
required and dictates the internal engine temperatures. This is
another reason why some big bores require less fuel than others.
This idea that the larger and more powerful the engine the more
fuel it needs is not always correct when you consider the other
players at work like efficiency and internal flow characteristics
(pipe effects and porting arrangement). I can tell you that the more
powerful the engine is.. the less restrictive the exhaust outlet
needs to be will always be true...
Ok, enough rambling.... Point being.. Engine displacement and
Engine HP are not the "big cheese" in determining how much jet and
engine will require. There are many players at work in making this
determination... Many more than listed above..