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Oval Pistons ?

F

FurryCURT

Free Member
So i was talking to a watercraft mech the othe day and he is of the opinion that the reason our big bore (800 +) motors dont hold up is due to the length of the cranks. the bigger the bore, the longer the crank has to be. thats why ( says he ) Rotax was limited to 670 cc ( the drive gear in between cyls weakened the crank too much as it grew in width. Counterbalancing shafts didn't completely fix the issue and added unwanted weight ( skidoos 1000cc ).

Sooooo i was thinking that i read once many years ago about Honda developing a motor for their suerbikes that was an OVAL piston. I'm thinking that would allow bigger ccs without having to spread the center to center on the pistons. Maybe someone has tried it, i dunno. I don't understand indepth the details of reed valves, fuelscavenging, etc on 2-strokes so im not sure if an oval would be better or worse.

All you guys that know stuff, please weigh in....you guys that are just full of it and really don't know anything, don't weigh in...
 
8s and bigger...Polaris' 900 was a bust. Skidoos 1000 had a counterbalace shaft. The point being that the longer the crank, the less durable the motor due to the tendency for the crank to want to flex in the center at high RMPs.
 
Its all about the crank design, rod length, piston design and weight. I talked with Brad from power addiction and he just started production on his 1100cc billet twin for the pro rmk. He said that thing is amazing in both hp and vibration. He told me that thing runs smoother than the stock 800, it dont vibrate hardly at all. That leads me to believe its all about stroke length, rod length, and piston design.
 
piston

Seems like a oval piston would be almost impossible to get the rings to seal. The compression pressure disperses the force evanly on the ring in a round cylinder making it seal better. I am not sure you would get that on a oval piston.

I don't think crank flex in the middle is an issue because there are bearings in the middle. The small block Polaris motors have a gear in the center that drives the oil and water pumps. Their issues haven't been so much crank as cylinder.

The fix trys to address the rod length issue by supporting the piston below the wrist pin. I think this may be part of the issue. The 800 big block motor did not have piston problems and the rods were like 2 mm longer than the new generation 800. This leads me to agree with Brad that it's a rod length issue. He has some kits in testing that he has installed a longer rod on and uses a spacer to get the porting right. I am thinking that may be the answer.

The big block issues were rod pin related on the big block and those were addressed on the small block design. Now it's top end.

The side forces on a piston are much more in short stroke rod than a long stroke rod when the crank stroke is the same. Picture a rod the length of you finger and one the length of you arm. You can see how much more the tip of your finger has to pivot compared to your wrist when your kniuckle or your elbow travel in the same size circle. This is an extreme example but it's pretty representative.

The reason extreme rod lengths aren't used is the case volume would be lots bigger and inefficient. The old term a "square" motor refers to one with the same bore and stroke. These were inherantly more efficient. example is the original 600 small block motor.

I am sure there are other forces that come into play as well. When you start slinging big pistons and rods around, it takes a lot of strength to reverse the direction of travel. That's where the counterbalance comes in. I have done several single cylinder big bore ATV motors and accross the manufacter board they all use counter balance. Twin cylinders help counterbalance each other but it may not be enough.

Trouble is that adds weight and more importantly cost to the motor. Economic trade off.

Indy Dan says the cranks are generically produced and their balance does not come close to matching the rods and pistons on the big block Polaris motors. Cat as well but they are more forgiving. That is his reason for big block failures.

Lots of possibilities, just a few of my thoughts..
 
I don't think so !!! WHO could afford pistons and rings !!!

Real pretty though.

Good Luck

800px-Ovalpiston.jpg
 
It ran great but had a huge number of issues, but it was actually due to the problems honeing the oval cylinder as I recall.
 
Aw yes, the NR 750. Every once in a while you see one for sale on Ebay or around 80k.
 
V-8 smoothness and power from a 4 .... I think there is potential here... Imagine a 1200 cc 2 stroke with a crank width the same as a 600cc. Honda oriented the long side of the oval parallel to the crank. I say put them diagonal. I wonder if the rings are really an issue. Bear in mind Honda pioneered the idea 20 yrs ago. Mfg tech has come along way since then.

The other crazy sled motor idea I lay awake at night and dream about is a Propane fuel, Turbo-charged Rotary. Its clean burning, high HP, high reving and nothing is wasted transitioning from up and down piston energy to a crank. Nothing is trying to tear itself apart. All the rotation and power delivery is linear. High RPM fuel issues are solved by the pressurized propane and the turbo. Top off the oil in the turbo and what the rotary uses every ride and your good to go. Emmision issues are minimal. The tank could be two 6-8" dia tanks nested next to each other and they would form the top of the tunnel and seat base. As for spare gas, Im wondering if one of those Coleman fuel cans for lanterns and stoves would get you a few miles. Mount up an adapter and screw a can on if you run it dry.
 
Another interesting new engine design that has 2-stroke potential. Sorry if you guys have already seen this in another post (that's where I heard of it). Think of how narrow a sled could be with this kind of motor, those snowhawkers would love this motor.

www.ecomotors.com
 
A little off topic but i remember reading in one of my text books about a prototype engine back in the 1920s that used a oval cylinder with a single triangle piston the rotated lateral to create combustion. I dont think it went anywhere though because it caused so much vibration.
 
AKLankford said:
A little off topic but i remember reading in one of my text books about a prototype engine back in the 1920s that used a oval cylinder with a single triangle piston the rotated lateral to create combustion. I dont think it went anywhere though because it caused so much vibration.
Click to expand...

Sounds like the rotary motor, they've used them but have problems sealing around the piston from what I've heard. They can turn some serious rpm for power though.
 
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