Long stroke vs short stroke

[jelys]

In a mono I guess you need more torque than actual top end power compared to a hydro, caused by greater weight and wetted area causing more friction?

With 4-cycle engines, longer stroke will give you more torque, but it is not that simple with 2-cycle from what I understand?

So any suggestions for a type of .21 engine or brand for a relatively light weight mono hull (Kyosho FMR 21V SII). I am interested in a reliable, easy handling but not necessarily the most powerful engine.

Good availability of spares is important.

So the contenders from my point of view are:

Novarossi

CMB

OPS

Picco or Rossi (may be?)

Regarding Novarossi, in Europe they have no less than 16 different .21 models, so it is a bit confusing for a newbe.

I have been thinking of a "long stroke" Bonito or should I rather settle for a 3, 5 or 7 port marine sport version?

Any answer and suggestions are appreciated.

Best regards

Jan Erik

 

[lohring]

The port area on a four stroke, overhead valve engine is proportional to the cylinder head area. The port area on our style two stroke engines is proportional to the cylinder wall area. For the same cylinder volume (stroke times bore squared) you get more cylinder wall area the longer the stroke is relative to the bore.

If breathing was the only issue two strokes would all be long stroke engines. Since other things like inertial load on the bearings are important, most small, high rpm two strokes end up with a bore from equal the stroke to 1.2 times the stroke. Torque is related to average cylinder pressure at a given rpm. A lot of things other than the bore to stroke ratio determine that. Pipe design and port timing are the most important. Scavenging efficiency is also critical and is determined by port design. Piped two strokes usually have a narrow rpm band of peak torque.

The only way to find out what engine is best is comparison testing under controlled conditions. Dynamometers have been the standard way of doing this since the steam engine era. Rpm measurement with a standard series of propellers has been used extensively by model engine builders. The boating version would be running different engines in the same boat with the same propeller and setup. The on board peak rpm would be a good measure if you could keep conditions equal. I've seen a test stand where the engine runs a propeller in a closed tank. That would be an easier to control test setup. See



Lohring Miller

 

[jelys]

Interesting subject Lohring

To me it would look like scavenging efficiency would be higher in a long stroke engine, then.

Othewise I have always looked at the torque as being proportional with the stroke length, as half the stroke acts as the momentum arm for rotating the crankshaft, right?

However by increasing the stroke you have to reduce the bore in order to keep within the engine capacity limit. Assuming we have the same cylinder pressure p, the piston area A will be reduces as will the power P = p x A, forcing the piston downwards, when we increase the stroke s.

The NR .21 short stroke engines have a bore x stroke: ø16.26 x 16.80 mm.

The NR .21 long stroke engines have a bore x stroke: ø15.88 x 17.60 mm.

Performing the momentum calculation for these two engines: M = p x A x .5s we end up with the same values.

So this theory, just increasing the stroke, does not explain any torque increase for a given engine capacity, as I see it.

Jan Erik

 

[lohring]

Actually, scavenging is by far the most important consideration in the narrow range of bore to stroke ratios we use. There has been a lot of research on porting and pipe design for better scavenging. Model engines use relatively poor transfer designs. A long time ago Gordon Blair did a large series of transfer shape research. It turns out that a large radius on the inner transfer wall is needed to accurately aim flow out of the port. Triple exhaust ports or even 360 degree exhaust ports help breathing. Proper pipe design will actually supercharge the engine over part of the power band.

Below are a couple of pictures to illustrate. The YAM 12 is a poor passage shape but represents most model engines. The last show some triple exhaust designs. A more complete discussion is in my articles in the NAMBA Propwash on High Power Two Stroke Design.

http://www.namba.com/content/library/propwash/2012/November/

http://www.namba.com/content/library/propwash/2013/March/

http://www.namba.com/content/library/propwash/2013/October/

Download the .pdf for easier reading.