Ed Cregger
Posts: 8007
Joined: 1/31/2002
From: Ringgold,
GA, USA
Status: offline
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quote:
ORIGINAL: jeffie8696
I did a search and found some very interesting information on the 4 stroke versus 2 stroke debate. Having studied 2 stroke and 4 stroke engines in college I really need to ask how some of the information came to being.The following points are generally considered standards of the industry.1) 4S engines are more efficient due to the intake charge not being diluted as much by exhaust gasses . 2) 2S engines make about 80% more power per cubic inch since it is firing on every down stroke. 3) Since 2S engines have no valves to hit the pistons at high speed they are generally more capable of high RPMs. 4) 4S engines are more complicated therefore have more things to go wrong. 5) 4s engines have(generally) more torque at lower RPMs and 2S engines have a more narrow power band placed at higher RPMs. I heard someone state that 4S engines are lighter for the same displacement and that makes no sense to me since the 2S has less parts therefore less weight. I welcome the ensuing debate.
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Our glow two-strokes, in modeldom, are exceptions to some of the rules that you learned in college.
In general purpose utility engines, four-strokes are nearly always heavier than equivalent two-strokes. However, front rotor, shaft induction two-strokes are an oddity that is specific to model applications. It is easy for a model glow two-stroke, or Diesel, to be considerably heavier than a model four-stroke glow or Diesel.
Why? Two factors come into play (probably more, now that I think of it).
As mentioned previously, shaft induction (front rotor) two-stroke glow engines have much heavier crankshafts than four-stroke glow engines. The added girth is there to accomodate a large intake pathway and to provide extra strength to accomodate the extra power being produced by such an engine. Also, the two-stroke's muffler must be larger in capacity in order to quiet the engine sufficiently. A four-stroke glow engine, while producing a much lower pitched exhaust note, is not perceived as being as loud as a two-stroke glow exhaust note.
There are two-stroke exceptions to the high rpm rule, just as their are exceptions for the four-stroke low rpm rule. Intended usage and design timing and porting have more to do with whether an engine is a screamer or a torquer. The 1972 Yamaha 360 Enduro was a torquer, not a screamer. The more recent four cylinder inline 250cc four-stroke Honda redlines at 17k rpm, not exactly a torquer, by anyone's standards.
Ed Cregger
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