Thread: Thinking two stroke
08-30-2006, 08:17 PM #1
Thinking two stroke
Its interesting. So far one of the biggest challenges in my 30 yr career of racing 4 stroke engines has been to fully understand how the 2 stroke engine actually works, and the many ways it differs from a 4 stroke engine. If you know 4 stroke engine theory, and want to learn 2 stroke engine theory, take literally everything you have learned from the 4 stroke class, throw it out the window and start over.
I found out that a 2 stroke engine is far more sensitive to intake restrictions than a 4 stroke is. On a 4 stroke engine the downward motion of the piston sucks the air into the cylinder and minor changes in the intake system go pretty much un noticed, except at wide open throttle. Not much is going to stop that piston from pulling in the same chunk of air, the mechanical force is overwhelming. Not the same on the sensitive little 2 stroke engine. With the exiting exhaust gases being the driving force in pulling the incoming charge back in the cylinder, these minor changes as little as using an outerwears cover over your flame arrestor can change everything.
The incoming charge and the exiting exhaust gases engage in this delicate little dance with one another that is so easily upset. Slow down the exiting gases, and slow down the incoming charge also, and vise versa. The intake and exhaust variations on 2 strokes seem to react very similar to making valvetrain, camshaft changes on a 4 stroke. It seems they in essence are like fraternal twins, they sure dont look alike, but they do pretty much the same things.
With any engine though, it seems the trick is to be able to visualize airflow in your mind, and understand exactly what is happening inside that engine. That can be tough to do. Only a small percentage of talented mechanics i have known seem to possess this voodoo like ability. The ones who are able to visualize this are the best engine tuners in the land, or, in our case, on the water.
Still learning after 30 yrs and lovin every minute of it...
Thanks for reading,,, Rossnemo
08-30-2006, 08:36 PM #2If you know 4 stroke engine theory, and want to learn 2 stroke engine theory, take literally everything you have learned from the 4 stroke class, throw it out the window and start over.
06-28-2007, 06:56 AM #3
- Join Date
- Jun 2007
I like your post. As a lover of two strokes I have always found their temperamental nature frustrating. You have hit the nail on the head. If they finally get rid of 2-strokes on the water it will be a sad day. Nothing creates the power-to-weight ratio advantage like a 2-stroke. It's awesome to line up a couple of big bore 4-strokes and let em have it with almost half the displacement. Running a 160 horse 2-stroke against that 250 horse Kawi is such a thrill.
06-28-2007, 10:26 AM #4
- Join Date
- Feb 2006
You are correct. Air parcel theory doesn't apply as well to the 2-strokes. There is a gap in the process as the gas and oil go into the crankcase and then get pulled into the chamber by the negative evacuation pressure.
It is a delicate balancing act...
06-28-2007, 10:34 AM #5
Got to love them air pumps!
Spent 9 years learning N/A N2O BB chevys... then another 9 learning 2 strokes in skis. Back to the 4stk world, now in a SC ski, is even another steep learning curve. I'm so confused.
06-28-2007, 10:41 AM #6
06-29-2007, 12:56 AM #7
- Join Date
- Nov 2006
Is this why the size and shape of the expansion chamber of the exhaust so important/specific to the torque/hp characteristics on the engine? It's all about the positive and negative pulses that are created in the chamber; right?
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