Many riders, racers, and builders have different reasons as to why single or dual ring 2-stroke piston designs are better. But who to believe? We've put together some information so you can make an educated decision.
Single and dual compression ring two-stroke pistons have been in service for decades, and since their inception, many have wondered if there are advantages to one or the other. If you have been involved with dirt bikes, jet skis, or snowmobiles long enough, you’ve probably noticed different manufacturers have chosen to use one or two compression ring piston designs for their engines. Furthermore, you may have noticed some aftermarket piston companies offer single ring pistons that replace dual ring pistons and vice versa. So, as a consumer, what do these design differences mean, and which one should you choose?
Wiseco has been manufacturing two-stroke pistons since 1941. In fact, the company started with two-stroke racing pistons being built in Clyde Wiseman’s garage. There’s no replacement for experience, so we want to take this opportunity to shed some light on the advantages and disadvantages of single and dual compression ring two-stroke pistons designs.
Compression Ring Function
We’ll start with a quick review of what a compression ring is designed to do. First and foremost, the compression ring provides a seal that allows the piston to compress the air/fuel mixture as the piston travels upward, then during the combustion event, it seals the rapidly expanding hot gases that form. The effectiveness of the compression ring seal has a significant effect on the power and efficiency of the engine. Should the compression ring lose its ability to seal, combustion pressure will sneak past causing a loss of power and efficiency. This is often referred to as blow-by.
The piston rings play a vital role in transferring heat from the combustion process to the engine's liquid or air cooling systems. During combustion, the piston crown absorbs a portion of the extreme temperatures it is exposed to. If left unregulated, the piston would become so hot that it would melt. Thankfully, the piston rings transfer heat from the piston to the cylinder liner. From the liner, the heat finds its way to the water jacket or to the cooling fins (on an air-cooled engine). Engine designers optimize the size, shape, position, and the number of rings to influence how the piston and rings transfer heat.
The piston ring’s conformability refers to how well it adheres to the shape of the cylinder bore. The conformability of the ring will have a direct effect on how well it seals combustion pressure as well as transfers heat to the cylinder liner. Factors that influence a ring’s conformability are shape, material, and thickness. For example, thicker rings will be less conformable than thinner rings because ring thickness has a significant influence on ring stiffness.
Wiseco’s Research & Development Manager, Dave Fussner, comments, “Racing applications tend to favor single rings for a lower friction penalty. Also, thinner single rings have better conformability to the cylinder and are less susceptible to flutter at high RPM. Even when specified with lower tension, thinner rings can still have good unit pressure which promotes sealing without a high friction penalty.”
Single Versus Two Ring Applications
While many have speculated that certain types of two-stroke powered vehicles—whether it be ATVs, dirt bikes, jet skis, or snowmobiles—need one or two ring pistons, it isn’t so much the specific vehicle application that drives the selection, but more the intended use for the vehicle. The big differentiator is whether the vehicle’s intended use is for racing or not. Two-stroke engines designed and developed for racing typically utilize single ring pistons. When designers optimize an engine for racing and select a single compression ring design, several advantages and disadvantages arise when compared to a two-compression ring design. Let’s take a look at the pros and cons of each.
Single Ring Pros:
- Lowest friction design for increased power and throttle response
- Lowest weight
Single Ring Cons:
- Shorter service life due to heat dissipation and ring wear.
Two Ring Pros:
- Improved heat transfer due to the addition of the second ring
- Engine performance durability due to 2nd ring’s ability to seal if first ring’s seal becomes compromised
Two Ring Cons:
- Increased friction and weight
“Since a good portion of the piston heat is transferred from the piston to the ring and then to the cooler cylinder wall, one advantage of a 2-ring system is that the second ring would provide a second heat transfer path,” adds Dave Fussner on the topic of two-ring designs.
The last point worth mentioning is that racing piston ring applications are optimized for excellent ring control at high RPM. A condition called “flutter” occurs when a ring becomes unseated from the piston’s ring groove. Flutter occurs around top dead center as the piston transitions from upward motion to downward motion, in part, because the inertia of the ring, which is a function of the ring’s mass, overcomes the gas pressure, pushing the ring against the bottom of the ring groove. When this happens, the ring’s sealing ability is compromised, and engine performance degrades both in terms of performance and durability. Engine designers combat flutter by optimizing the ring's weight so that the ring’s inertia forces cannot induce flutter within the intended RPM range. This is why in many single ring applications the rings are relatively thin.
These are the factors that drive single-ring and two-ring designs in two-stroke pistons. If you are considering a switch from a single ring to a two ring piston, ultimately, how you intend to use your machine should dictate whether the decision is sensible or not.