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A Basic Guide To
Two Strokes
I wrote this as just a basic guide for anyone new
to two strokes. Hopefully it helps someone.
What is it? How does it work?
A 2 stroke internal combustion engine (also commonly referred to as
2 cycle or 2T, from the German term 2 takt) operates in a different
manner than the more common 4 stroke internal combustion engine. A 2
stroke engine does not use a camshaft, mechanical valves, or many
other components that a 4 stroke requires to deliver the air/fuel
mixture and dispose of exhaust gases. I'll try to expalin the basics
of both to you.
In a 4 stroke gasoline engine, a carburetor or fuel injection system
delivers a fuel/air mixture into an intake manifold and then to the
cylinder head. A camshaft rotates to operate valves inside the
cylinder head. The most common 4 strokes have 2 valves, one for
incoming fuel and air (intake) and another for allowing the
expulsion of gases (exhaust). The intake valve opens to allow an
air/fuel mixture to be drawn into the cylinder while the piston
moves from top dead center (TDC) to bottom dead center (BDC). This
is known as the intake stroke. The intake valve closes and the
piston then travels from BDC to TDC compressing the mixture. This is
known as the compression stroke. The compressed fuel/air mixture is
ignited by a spark plug near TDC in the compression stroke. The
combustion and expansion of the mixture accelerates the piston from
TDC to BDC while both valves remain closed. This is the power
stroke. The exhaust valve now opens allowing the piston moving from
BDC to TDC to push out the burned mixture. This is called the
exhaust stroke. Now the piston is at TDC again, ready to start the
process all over again. Intake, Exhaust, Power, and Compression... 4
strokes. There are variations of 4 stroke engines, including
gasoline, diesel, different cam and valve configurations but I've
explained the basic principles of their operation. There is only
combustion and therefore power created on every 2 complete
revolutions of a piston in a 4 stroke engine.
Here's an animated gif showing the 4 stroke engine at work.
You will have to bear with me on the 2 stroke
explanation, and understand that multiple events occur at the same
time. This is the best way I can explain it with my knowledge. Like
the 4 stroke, the 2 stroke engine uses a carburetor or in rare cases
fuel injection system to mix and transfer fuel and air into an
intake manifold. A lot of similarities end here. There are
variations on induction methods, but for the 2 stroke engines
commonly found in scooters (case-reed induction) reed valves are
used to control the mixture flowing from the intake manifold to the
crankcase (that's right, I said the mixture enters the crankcase and
not the cylinder or combustion chamber). Reed valves have no
mechanical components directly controlling them, they rely on
pressure created by the engine to open and close. This makes them
amazingly simple, but less efficient than the valvetrain in a 4
stroke engine. The piston traveling towards TDC creates negative
pressure (vacuum) inside the crankcase which causes the reed valve
to open and allow the fuel/air mixture to enter the crankcase. As
the piston travels back down towards BDC positive pressure is
created in the crankcase, which closes the reed valve and there is
compression of the gases inside the crankcase. At some point before
BDC the piston will uncover the transfer port, allowing the fuel/air
mixture to travel into the cylinder. The piston will reach BDC and
begin to travel back up, closing the transfer port and stopping the
flow of fuel/air into the cylinder. As the piston gets closer to TDC
it closes the exhaust port and then begins to compress the fuel/air
mixture. Just like in the 4
stroke engine, the compressed mixure will be ignited by a spark plug
slightly before the piston reaches TDC and the expansion of gases
pushed the piston toward BDC. As the piston travels downward it
uncovers the exhaust port and the pressurized gases exit the
cylinder. The piston will then proceed toward BDC opening the
transfer port and allowing fuel in again so the combustion process
can continue. All the components (Intake, Compression, Power,
Exhaust) are there, like the 4 stroke, but some events are
happenening simultaneously. The 2 strokes are called power/exhaust
and intake/compression. As the piston travels down it creates power
as well as forces out the exhaust gases. As the piston travels up it
pulls in the intake charge as well as compresses it. The 2 stroke
engine will ignite the fuel/air mixture and create power on every
single revolution of the piston.
Here is an animated gif that should help you understand the 2 stroke
a little better.
The 2 stroke and 4 stroke engines differ on their methods of
lubrication as well. 4 stroke engines have a dedicated system for
lubricating essential parts with engine oil. Oil is contained in the
crankcase and/or oil pan and an oil pump sends it through passages
to essential parts and oil is splashed/slung by the crankshaft. The
two stroke engine uses engine oil mixed into the fuel (by premixing
or an oil injection system) for lubrication.
Advantages/Disadvatages
Now that you have a better understanding of what 2 stroke and a 4
stroke engines are, you can better understand the advantages and
disadvantages of a 2 stroke compared to a 4 stroke.
Advantages
Power - A 2 stroke engine will often produce more power than
a 4 stroke engine of the same size (displacement). Since the 2
stroke will produce power on every full rotation of the piston, as
opposed to every other rotation like a 4 stroke, it can produce more
power. The 2 stroke engine also lacks the need to exert any effort
turning an oil pump, camshaft, rocker arms, etc...
Weight - The lack of components in a 2 stroke engine also
translates to an engine that weighs less than it's four stroke
counterpart. The lower weight of the 2 stroke combined with the
often higher output creates a more favorable power to weight ratio.
Simplicity - 2 strokes are usually easier to maintain,
rebuild, and modify. 4 strokes require periodic valve adjustments
and oil changes that 2 stroke engines don't. If you plan to rebuild
or modify a 2 stroke, you'll have less parts to deal with.
Repair / Modification Cost - Less parts to repair and replace
can often lower the costs associated with modifiying and rebuilding
a 2 stroke engine.
Disadvantages
Longevity - The piston, piston rings, and cylinder walls of 2
stroke engines tend to wear faster than those of 4 stroke engines.
This is because 2 strokes don't have a dedicated oiling system like
a 4 stroke.
Fuel Efficiency - The timing of valve events in a 4 stroke is
much more efficient in a 4 stroke. A 2 stroke can actually expell
some unburned fuel for a brief moment while the exhaust port is
uncovered at the same time as the transfer port.
Emissions - 2 stroke engine produce more emissions due to the
burning of lubricating oil and the afore mentioned lack of
efficiency.
Power Band - 2 strokes often have a more narrow power band
than a 4 stroke, meaning they make their peak power output in a
smaller range of engine RPM. Luckily, proper tuning of a CVT
transmission can do wonders for using a smaller power band.
There are other pros and cons, some debatable, but I feel the ones I
have provided are the most relavent to the majority of scooter
riders.
Performance Modifications / Tuning
Before taking a look at any performance modifications, I would like
to make a couple of important points.
Modifying a vehicle often means compromise, though not in all cases.
Don't plan on making big power with the typical 49cc 2 stroke
without sacrificing something along the way. Very often, reliability
and longevity are sacrificed to varying degrees. Not every
modification will cause any major issue, but some will. Sometimes
the sacrifice is easier to deal with, like building an engine that
will scream and pull hard in higher RPM operation often means the
engine will lose some of it's low end power and vice-versa. If you
have a specific goal in mind, it is very important to choose your
modifications wisely. You don't wnat to put together an engine with
compression so high that it requires some form of racing fuel if you
are trying to build a scooter that is well suited for long trips and
trouble free operation. That's kind of an extreme, but you get the
picture.
You should also keep in mind that some components compliment each
other and some contradict each other. For example, you wouldn't want
to add parts to your transmission to keep your engine operating at a
lower RPM range if your engine is designed to achieve maximum power
in the upper RPM range. Just another reason that all modifications
and parts are not suited to all needs, making researching before
modding or buying even more essential.
Jetting & Tuning A Carburetor
I am starting out this section with jetting and tuning a carburetor
because it will be mentioned in other sections and is essential to
properly modifying and maintaing your scooter. I was going to write
this section, but this article explains it better than I can and
with lots of helpful illustrations.
http://hondanighthawks.net/carb14.htm
I should also note that jetting recomendations are very difficult to
make due to variations in altitude, which carburetor and jets are
currently in use, and even riding style. For this reason, you will
find any reference to jetting for other modifications mentioned here
to be vague.
Switching to a larger carburetor is also popular. Using a larger
carburetor on a stock engine will often show little to no power
gain. In some cases the stock carbretors are pretty tiny, so you may
see more of a gain. Don't just run out and buy a new carburetor
because you got a mild big bore kit or something like that either,
as it is not always needed. If your scooter came with a fairly large
carburetor, for instance a 17.5 or 19mm on a 49cc, then you may not
need to swap carbs but merely jets. If you do end up using a setup
that the stock carburetor isn't ideal for, remember that bigger
isn't always better. You will want to find out what size range or
specific size carb is recommended for the mods you are planning.
Manufacturers may offer advice as well as searching for information
on forums and websites from others with your basic setup. If you
find that perhaps 19-21mm carbs are optimal for your setup and both
should work well, here is something to think about. The larger
carburetor would be capable of supplying more fuel/air to support
better power in the upper RPM range. The smaller carburetor will
offer better low end power and throttle response. Intake manifolds
should be matched to carb size.
Air Filter
Installing an aftermarket air filter or modifying the existing air
box can give you a small gain in power, but don't expect a big power
boost unless your current setup is extremely restrictive. Any major
changes are likely to require adjusting and re-jetting the
carburetor. Some people like to drill large holes or a series of
holes in their stock airbox to allow increased airflow across the
filter. Be careful doing this if you do not wish to install larger
jets. You can probably get away with adding some small holes or
slightly enlarging the current inlet, but major changes are likely
to leave your engine running lean. If your scooter was in tune
before the filter or mod, you should only need a slightly larger
jet.
Exhaust Pipe
Putting an aftermarket pipe on a 2 stroke is different than choosing
an exhaust for a 4 stroke. When dealing with 4 strokes, especially
of a very small displacement, aftermarket exhaust systems don't
always offer a good gain and differences in pipe layouts and
backpressure changes (within reason) don't generally have dramatic
effects. If your stock exhaust is very restrictive, you may see more
of a gain than a scooter with a less restrictive exhaust system.
That's the (very simplified) gist of it for a small 4 stroke.
Simply changing to a larger exhaust pipe and a less restrictive
muffler will probably offer marginal benefit similar to the effect
it would have on a four stroke. However, even small displacement 2
stroke engines can show excellent gains from properly designed
exhaust systems featuring expansion chambers. Expansion chamber
exhausts (sometimes referred to as tuned pipes) can do suprising
things. They are designed to provide the largest power increases
within a certain RPM range. Changing parameters such as the length,
diameter, or angle of sections of the expansion chamber pipe can
change it's operation drastically.
When the engine reaches the RPM range that the expansion chamber
pipe has been designed to improve, there is often a very sudden and
apparant increase in power. This sudden boost is often called
"hitting the pipe." Some designs make the power come in more
gradually and other designs can make the pipe hit so hard that it's
more of a jolt. On some heavily modified 2 strokes, the rapid
onslaught of additional power can be enough to pull the front of the
bike off of the ground (don't expect your stock 49cc scooter to do
power wheelies with only the addition of a pipe). It's a very
impressive thing, especially the first time you witness it.
These are all reasons to choose a pipe that suits the demands of
your engine and transmission setup, or modify the engine and
transmission to make the best use of the pipes effective range. You
may need to change jetting in your carburetor for a pipe, but not
necessarily.
Spark Plugs
Some scooterists look for power by switching to spark plugs with
different heat ranges, iridium plugs, and spark plug indexing. In
most cases, you won't find much power in these areas. Swapping heat
ranges is really only necessary if the current plug is showing signs
of being cold like excessive fouling or deposits or showing signs of
being too hot such as damaged electrodes. The main goal in selecting
a spark plug heat range is to have the spark plug operating hot
enough to burn off deposits, but cool enough to avoid pre-ignition
(when something inside the cylinder is so hot that it ignites the
fuel mixture before the spark) and/or other damage. The stock heat
range usually works fine, but it never hurts to do an occasional
plug read.
Iridium plugs can be a great upgrade for your scooter, but
significant power gains are doubtful. Iridium spark plugs do offer
benefits over standard plugs however. The properties of iridium
allow manufacturer's to use smaller electrodes to better concentrate
spark energy and create a stronger spark. This is said to aid in
more complete combustion, leading to improved efficiency and power.
This may be true, but when you are dealing with very small
displacement engines you often don't see much or any noticeable
improvement in power output or fuel efficiency. One advantage that
you are more likely to notice is that iridium plugs are less prone
to fouling, which is an excellent feature when oil is present during
combustion as it is in 2 stroke engines. The other great aspect of
iridium plugs is that they may actually allow easier starting. Some
2 stroke owners, myself included, have reported much easier cold
starting after the addition of an iridium plug.
Spark plug indexing is simply aiming the section of the gap not
shielded by the ground electrode where you desire it to be. Where
exactly the open spark should be aimed is often debated on 2
strokes. For that reason, I suggest reading more on the subject and
coming to your own conclusion. Indexing is done by using special
washers called indexing washers. They usually sell in kits including
different thicknesses. You mark the spark plug (somewhere that will
be outside of the cylinder) where the gap is not blocked by the
ground electrode. You then use the appropriate washer to align that
mark with the location of your choice when proper torque is applied.
Indexing will usually produce between 0 and 2 percent gains in power
in 4 stroke multi-cylinder engines. I've never seen numbers for a 2
stroke single cylinder.
Ignition Coil
Installing an aftermarket ignition coil should produce a stronger
spark. You really shouldn't need a stronger spark than what your
stock ignition coil provides unless you are increasing cylinder
pressure or running at very high RPM. It's not a bad upgrade to do,
but don't expect any gains if your current ignition system is
functioning properly. One great advantage of the aftermarket coils
is that they often include a better spark plug boot that is less
likely to vibrate off of the spark plug. Don't buy one just for that
reason though, because you can purchase better boots made by NGK and
install them on most stock plug wires.
CDI
Aftermarket CDIs (Capacitive Discharge Ignition) are frequently
claimed to provide a substantial increase in performance by their
sellers. This can be true in some cases, but not every scooter will
benefit from a "racing" CDI. The two big selling points for an
aftermarket CDI are usually the lack of a rev limiter and altered
timing or "re-mapping". Many scooters don't have a rev limiter, so
the first selling point is null to some riders. If your CDI does
have a built in rev limiter that cannot be disabled, then a CDI with
no limit may benefit you. This depends what the RPM limit is on the
CDI. Some stock CDIs have rev limiters built in that a stock engine
would be very unlikely to reach anyway.
The altered timing of aftermarket CDIs may or may not show an
increase in power. The effect of an ignition advaned/retarded CDI is
also made debatable by variances in different engines and different
styles of timing control. Some CDIs provide a standard or fixed
timing while others use a variable timing curve which allows
differnt timing at different RPM. A lot of tuners find fixed timing
CDIs to be pointless, because the timing can often be adjusted in
ohter ways for the same effect. I've actually tried the same
aftermarket advanced timing CDI on three different bikes belonging
to myself and two friends. Two of the bikes showed no noticeable
gain, while the third benefited from improved throttle response.
Each bike was tried by each owner, and we all agreed. All of the
bikes had the same style of engine and similar modifications.
Results like that are what make opinions on aftermarket CDIs
diverse.
I would advise that you leave the stock CDI in place until one of
two things happen. One, you can verify that you have a rev limiter
in your stock CDI that cannot be disabled and will limit
performance. Two, you have already done a lot of engine work and are
trying to squeeze every last ounce of performance out of your
scooter. The only other thing that makes a CDI swap compelling, is
the thought of increasing performance with such little effort. CDI
installation and removal only involves locating the CDI, unplugging
it, removing any mounting hardware if so equipped, plugging the new
CDI in, and re-attaching mounting hardware.
Cylinder Kits
Cylinder kits can be an excellent choice for anyone trying to make
more than a slight improvement in performance. They can also provide
a great "bang for the buck" gain, especially with some of the
moderately priced kits that are available. Kits may include a
cylinder, piston, piston rings, circlips, cylinder head, gaskets, a
wrist pin, needle bearing or other acessories. Check the details of
specific kits to be sure. You will commonly find two general types
of cylinder kits, big bore and standard bore. Big bore kits are much
more popular because they increase the displacement of the engine by
using a larger bore than stock. Standard bore kits will maintain
your stock displacement, but can still increase performance through
other means. Standard bore kits are a good choice for someone
wishing to compete in a racing class or league that limits
displacement, but you can often find more performance for the same
price by using a big bore cylinder kit. There is also no need to
stick with a standard size bore kit if you think you will remain
100% legal that way, because in most states 49cc scooters and mopeds
are only allowed a certain amount of horsepower. Many stock scoots
are already rated at the limits of the law.
Big bore cylinder kits will come in varying bore sizes to create
different displacements depending what engine you are using. The
most common big bore cylinder kits for most 49cc 2 stroke engines
will end up around 70cc displacement, but larger bore kits are out
there. The added displacement is great, but there are other things
to consider with a big bore cylinder kit. Compression ratio, timing,
and other factors can make one 70cc kit entirely different than
another 70cc kit. One 70cc cylinder kit may be well suited for the
average scoot rider looking for a little extra power without
sacrificing much reliability or longevity of the engine. A kit on
the other end of the spectrum could be intended for racing use only,
used for making the absolute maximum power in an engine that will be
rebuilt very often. Research any cylinder kits available for your
engine carefully, and decide which one will help you fulfill your
goals.
Crankshafts
Aftermarket crankshafts can be used to create power in different
ways. Better construction with tight tolerances, full circle
designs, increased displacement, reduced weight and more. The first
thing I mentioned, better construction and tight tolerances, is
pretty simple to understand. A better made crankshaft, and bearings
for that matter, will relieve as well as handle stresses better and
help increase longevity, especially in engines reaching very high
RPM. Some cranks are also made lighter to help the engine spin up
faster. Another trick that you won't see unless you're purchaising
race parts it polishing the crank. Polishing the crank is meant to
reduce every bit of surface friction possible, and is more common in
4 strokes to help them cut through oil.
Full circle crankshafts can alter the primary compression by
reducing the volume of the crankcase. Reducing crankcase volume
leads to higher velocity and attempts to push the fuel/air mixture
into the cylinder at an increased rate. This effect can be made
greater by "stuffing" the crankcase using a full circle crank that
also has chunks of nylon or other material attached to it. More
isn't always better. The amount of crankcase volume desired is not
universal to all applications.
Crankshafts with a longer stroke (stroker crankshafts) are used to
add displacement. This means the piston actually travels farther in
the cylinder, enabling it to pump more fuel/air. Your piston can't
just travel farther wihtout something being altered. Some stroker
crankshafts use a shorter connecting rod, allowing the use of stock
components or widely available aftermarket cylinder kits. Other
crankshafts rely on the use of special shorter pistons to make up
the connecting rod length. Crankshafts need to be a little larger to
accomodate a longer stroke, so you will need to know your
limitations when stroking your engine. If the crankshaft is too
large it can hit parts of the case, so clearancing may be necessary
in some applications. It is absolutely essential to know what you
are getting or you can have an engine that won't run or trys to
destroy itself for lack of proper clearances.
Port Work
Port work can be an excellent way to improve the performance of your
2 stroke engine. Port work can also be a great way to hinder the
performance of your 2 stroke. There are a lot of areas that can
benefit from some attention inside a two stroke engine, but it is
not as simple as it is with 4 strokes. Although not ideal, you can
get into a 4 stroke engine and port the intake and cylinder heads
with the basic goal of enlarging and smoothing so air has a better
path. It is actually a bit more complex than that, but that's the
basic idea. If you were to enlarge all the ports on your two stroke
with the sole purpose of making them bigger to flow more, you would
be likely to affect the engine in a negative manner. Since 2 strokes
lack a dedicated valvetrain like a 4 stroke, the piston and ports do
the job of the intake and exhaust valves. Making a port larger will
increase the time duration that the port remains uncovered by the
piston, similar to using a larger camshaft in a 4 stroke engine with
longer duration. You should have a very good understanding of 2
stroke operation before you attempt doing your own port work, for
that reason I suggest you have it done by a professional or start
reading up on more adnvanced theory and operation of 2 strokes. You
may alos be able to find guides online detailing what works well for
your style of engine and/or cylinder.
Reed Valves
Using different materials, larger ports in the reed block, mounting
angles, and the amount of reed petals can change the behavior of
your engine. The size of the ports in a reed valve (also called a
reed cage) can be sized to allow a greater volume or greater
velocity. More volume is beneficial for high RPM power while more
velocity is good for low end power and throttle response. Changing
the angle that reed valves are mounted also affects the power band.
A lesser angle is better for lower RPM and a greater angle is best
for high RPM. Selecting the proper reed material is also important.
Flimsy fiberglass reads can react faster to pressure changes while
stiffer carbon fiber reeds are less likely to flutter in the upper
RPM range. Using less reed petals is better for lower RPM power and
response and more petals help high RPM power. With all of these
variables reed blocks and reed petals can be tailored to suit many
needs.
If you would still like to learn more, there is a wealth of
knowledge out there. Try searching ScootDawg and the web or pick up
a book about two strokes. "The Two Stroke Tuner's Handbook" by
Gordon Jennings and "Two Stroke Performance Tuning" by A. Graham
Bell are excellent choices for much more technical research.
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