Estimating two-stroke Horsepower

[jaker555]

I am looking for any equations or "rules of thumb" for calculating theoretical 2-stroke engine horsepower. I don't have access to a 2-stroke engine simulation program. I want to know the max power I can squeeze out of this engine, given the engine's bore & stroke, a tuned pipe, and playing with the port sizes and locations.

Thanks!

jake

 

[icrman]

how about .2 per cc

 

[jaker555]

I'm looking at around 2 hp/cc.

But I'm trying to figure out if the port area I have is enough to provide the flow required to produce this amount of power.

In contrast, if the port area is too large, the port velocity will be too low to fill the cylinders in the amount of time they are open. Any advice?

Thanks,
jake

 

[richdubbya]

I dont know what type of engine you are working with, but if it's typical reed valve crankcase induction the actual intake ports in the cylinder wall can not be too big (velocity wise) the only restriction is the strength of the piston and cylinder and the length of the piston. The transfer ports can be too large but most castings dont have the material to make that common. Transfer port direction and being sure velocity isnt too high in the transfers are much more important. Correctly matching the crankcase compression, transfer flow and exhaust timing and flow are where the power is.

200 bhp per liter with larger cylinders, much more with small cylinders.

If you are working with a single cylinder a look and measurement of a good 80cc motorcross cylinder is a start.

There are better designs that were used for gp racing but it is still hard to get info or a look at the insides.

 

[sacem1]

2 HP/cc is an almost imposible output to get. That would mean 2,000 HP per liter not attainable even by blown nitro burning engines.

Cheers

SACEM1

 

[jaker555]

correction: Yes, it was meant to read 0.2 hp/cc. I'm working on an 800cc inline twin two-stroke, and looking at ~170 hp.

What do you guys know about squish band angles? The squish angle of the head should be roughly 1-2 degrees more than the dome angle of the piston, right? How critical is this with respect to detonation?

Thanks,

jake

 

[ctmtwilliams]

Max horsepower with a two stroke will depend on how usable you want your power band to be. Small bore single cylinder can have fairly high hp/cc ratios, but the power band can get so narrow they become hard to use. What are you using the engine for??

If you look at the motorcycle industry, companies like FMF and Pro Circuit make their money off of not necessarily gaining more horsepower from the modifications and pipes they sell, but giving the customer more useable horsepower. The karting guys can get away with narrower power bands and compensate with clutches.

BTW, if you exhaust ports get too big you’ll start catching rings in them.

 

[jaker555]

The engine is for a snowmobile, which has a CVT - primary & secondary clutches, so it will run at a relatively constant RPM under power (around 800cc). So it doesn't need to have a really wide power band at all.

So is port sizing and timing normally done experimentally and from experience, or is there published data to get baseline numbers? What do these engine simulation programs derive their calculations from?

 

[ctmtwilliams]

I doubt very seriously that you will find anything published or from software that can get you above and beyond factory specs. Most aftermarket research is trial and error combined with experience on a dyno. All of the data is closely held. After all thats how they make money, I know, I ground two stroke cylinders for about 5 years.

Port size, as I previously mentioned, is limited to what keeps the top ring from diving into the port. Port shape and flow direction plays a large roll, but with each set of cylinders you are limited with what what you have to work with from the factory. Basic port timing is fairly standard stuff with some room to play around with. You can get fancey with transfer port timing by allowing some to open sooner than others yeilding different flow patterns across the plug.

 

[kenre]

Hello,
only new here so let me know if im out of line.
There are a few software programs available for 2 stroke simulations. The one i have been using is call MOTA, from Ian Williams Tuning. Specificaly written for go-karts engines, but will do any 2 stroke. It is also very good for expansion chamber designs.

Ken