Views on Flat-top VS Higher Crowned Pistons
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.
-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
-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.
-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.