01-24-2010, 01:34 AM
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Join Date: Jun 2008 Location: Glendora California
Posts: 4,076
| Re: PWC 2 stroke engine technology
Michele,
Sorry to make you wait so long for my reply. I got sidetracked and was hoping to get back to you sooner.
Two stroke port timings are specific to the type of application that the engines are designed to be used for. The quick answer to your puzzel, is that a motorcycle engine needs it's torque over a much wider band because the engine's task to turn a very large diameter tire that is fairly well connected to the surfaces that they are used on, to do so, and they need some type of transmission device to carry them up to speed, and a torque band that is wider in order to do so.
When engineers decided to apply the two stroke engines to drive waterpumps, they realized that a completely new set of engine design rules we needed. They basically had to rewrite the book, so to speak, in order to port the engines for a completely different enviroment of turning a small diameter direct drive impellor running in a fluid enviroment made up of liquid water.
If the pwc engines were to leave from a standstill and accellerate in a linear fashion using a direct drive system spinning a relatively small impellor housed in a waterpump, if they used the rules needed to turn a sometimes very large diameter motorcycle tire and rim on a hard surface, the direct drive small impellor would just sit stationary and burn water cavatating at redline engine rpm instantly, and the hull of the pwc would not move forward. Because of the slippage of the small pwc impellor turning in liquid, if they applied too much torque upon take off at the initial hit of the throttle, it required a completely different mapping of the torque curve of the pwc engines.
Kawasaki engineers used the original pwc engines to power water pumps on the huge ships they built, so although those first engines were relatively small displacement producing a much lower torque and horsepower engine that ran constantly at pretty much one rpm, they ported those motors, changing the torque curve to turn a direct drive waterpump, not too much different than what is used in a pwc as what we know as a jetpump. As the pwc engines evoloved in the generations to follow, they made the engines larger and producing much more torque and power, and built them to withstand prolonged high rpm as the impellor driving the watercraft forward progressively connected with the liquid passing through them as the speed of the hull increased across the surface of the water and the parasitic drag of of the hull decreased more and more as the hull rose up and out of the water.
With a transmission, you can sustain similiar or higher rpm with a torque curve that requires lower port timings to produce torque at a much lower rpm, but still run at very high rpm. A small diamenter jetpump's max efficency falls off drastically at much over 6000 rpm. We do use some slightly larger jetpumps in the larger displacement pwc stock and racing boats to allow us to raise that threshold of efficency, but only enough to turn the engines somewhere right below or right over 8000 rpm without completly losing efficency of the impellors to process water through them.
A set of the newer snowmobile 800 cc Rotax two stroke power valve clyinders, right off the shelf as new, measure around 208 degrees ex. timing with the power valves retracted, very similiar to some of the highly modified large displacement pwc power valve racing engines. But power valve engines have the alility of lowering their ex. port timing at the lower rpm to supply enough torque down low, but not too much for running an impellor in a jetpump or a clutched belt on a sled.
Different applications require different port timing parameters. The intake tracts also require a different design on pwc engines as the power increases and larger venturi carbureators are installed to feed more air and fuel into the motors.
Thank the more daring thinkers for allowing us to have two strokes in our pwc's today that still do not produce 28-48 hp. We now have sled and pwc motors well under 1200 cc's that are capable of producing in excess of 200 hp. and are much less mechanically complex than their larger displacement 4 stroke non aspirated cousins that produce similar or less power at a much heavier weight and cost to build and maintain penality.
Last edited by Mr. Bill; 01-24-2010 at 01:51 AM.
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