Flat-Top VS Higher Crowned Pistons
(Note: These views were formulated with input from other people in the 2-stroke field)
The piston is subject to more abuse then any other part of the 2 sroke
engine. It is constantly being pushed on, sucked on, fired on, squeezed
on.... well, you get my point.
- It is the weakest link in the engine.
- Over the last decade or so, advances have been made in piston technology.
It has been found that adding Silicone to the Aluminum will reduce
piston expansion caused by the extreme heat that is present within an
engine. This reduction in thermal expansion reduced piston seizures.
Silcone also adds strength to the aluminum and reduces wear.
- Pistons are often though of as being a perfectly round cylinder.
In actuality, pistons are tapered from the top to the bottom. Why?...
Well, different parts of the piston are subject to different levels of
heat and since heat tends to expand metals, then it stands to reason,
that the areas which are subjected to more heat will expand more then
the areas that are subjected to less heat. The top of the piston is
obviously subjected to the most amount of heat, as well as pressure, and
therefore; will expand more.
- Pistons are not only tapered, but oval ground at the skirt. Once
again, this is to accomodate the different temperatures that are present
on different parts of the piston.
- So, when measuring a piston for wear, one needs to measure on the
skirt faces at the widest point which is always below the wrist pin.
Measuring anywhere, but the widest point will give an inaccurate
measurement.
- Below are some of the advantages and disadvantages of the two.
Flat Top:-Advantages:
-Minimum surface area, hence lightest with shortest heat path to cylinder wall
-Faster heat transfer to cylinder wall.
-Piston crown is in tension under load
-Ports open faster, not masked at partial opening by chamfer.
-Piston shape does not interfere with the entry and exit angles set by the ports
-Combustion chamber can be a true hemisphere
Disadvantages:
-Aluminium is poor under tension when hot
-Greater possibility of ring over-heating due to more rapid heat transfer
-Nothing to prevent gas crossing piston from transfers to exhaust port. (this is not always a disadvantage with a tuned exhaust)
-Achieving an efficient squish band is not easy.
Dome Top Advantages
-Piston crown is in compression under load
-Dome pushes incoming mixture to top of cylinder and reduces "short circuiting"
-Scavenging tends to be better so less four-stroking at partial throttle.
Disadvantages
-Additional weight
-Compression forces tend to spread the top ring land.
-Port timing is slightly fuzzy.
-Increased surface area of piston and head produce more quench effect in squish band.
-Combustion chamber tends to crescent shape, slowing flame spread.
Thanks to Dave Boothroyd and Mike Gifford for their contribution in this
flat-top vs domed article, they pointed out a few advantages and
disadvantages that I over-looked.
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The Function of the Ring |
What is the real function of the ring?
OK, RINGS?? What are the functions of the piston rings? The most common
answer is " To create a seal for compression." OK, the sealing off of
the gases for the compression portion of the stroke is a major function
of the piston ring, BUT this is not all the ring has to accomplish.
Let's look at at what else is going on with the rings.
- We'll start from Bottom Dead Center (BDC). At BDC, the crankcase
is highly pressurized, the tuned pipe is sucking its hardest ,and the
cylinder is filling with the fresh charge via the transfer tunnels. The
ring has, hopefully, collected a bit of oil from the cylinder wall and
is still transfering heat to the cylinder wall. The piston now begins
its race towards Top Dead Center (TDC), the ring's radial tension is
keeping it as tight against the cylinder wall as it can. The ring is
collecting oil from the cylinder wall and getting ready for the
compression portion of the stroke. Once the exhaust port has been
closed, (controlled by the piston crown edge, or in the case of pistons
fitted with a Dykes ring, the ring itself controls port timing), the
ring is retaining oil to help with the compression seal, as well as
lubrication. The gas pressures from the compressing gases keep the ring
tightly sealed against the cylinder wall.
- As the piston approaches TDC, cylinder pressures are building
rapidly, the piston crown is getting very hot, and the ring is doing its
best not to allow any of the pressures to blow past it. Then combustion
occurs (hopefully at about 11-14 degrees ATDC). The ring now takes on
its role as a heat transfer. The crown temps are very high and rising
and if these temps do not get reduced the aluminum piston will surely
melt. The ring will try and absorb as much of this heat as it can and
transfer it to the cylinder walls where it can be dissipated . Ridding
the piston of these high temperatures is a very important function of
the piston ring.
- The style of ring, number of rings, and its location on the piston
is very important in assuring that this heat dissipation succesfully
takes place.
- Now, the piston is back at BDC and it all starts over again.
The piston is subject to more abuse then any
other part of the 2 sroke engine. It is constantly being pushed on,
sucked on, fired on, squeezed on.... well, you get my point.
It is the weakest link in the engine.
Over the last decade or so, advances have been made in piston technology.
It has been found that adding Silicone to the Aluminum will reduce
piston expansion caused by the extreme heat that is present within an
engine. This reduction in thermal expansion reduced piston seizures.
Silcone also adds strength to the aluminum and reduces wear.
Pistons are often though of as
being a perfectly round cylinder. In actuality, pistons are
tapered from the top to the bottom. Why?... Well, different parts of the
piston are subject to different levels of heat and since heat tends to
expand metals, then it stands to reason, that the areas which are
subjected to more heat will expand more then the areas that are
subjected to less heat. The top of the piston is obviously subjected to
the most amount of heat, as well as pressure, and therefore; will expand
more.
Pistons are not only tapered,
but oval ground at the skirt. Once again, this is to accomodate the
different temperatures that are present on different parts of the
piston.
So, when measuring a piston for wear, one needs to measure on the skirt faces at the widest point
which is always below the wrist pin. Measuring anywhere, but the widest point will give an inaccurate measurement.
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