All,

I'm trying to understand the exhaust principles of 2 and 4 stroke engines. Got a question:

The 2 stroke ignition cycle fires one every revolution. The 4 stroke fires once every other revolution. Using these facts 2 stroke should generate more heat than 4 strokes, but they don't. Why? (This questions relates to a smoke system that I am designing for a 2 stroke engine)

If I have stated anything that is untrue, feel free to correct me. Any further elaboration is greatly appreciated.

Thanks in advance,

Mods

No, youre wrong The two cycle engines produces more heat then the four bangers. If you got a 70size four banger and compare it to the 2cycle engine you will see that the two stroker is much more warm after a run.
The reason why the foru bangers produces better smoke is probably beacause of the lack of heating drain from the muffler.

From my experience, the four stroke exhaust temperature is much hotter than the two stroke because the four stroke is much more efficient in its combustion process. Due to its inefficiency, the two strokes exhaust is cooled by unburned fuel and oil.

Enjoy,

Jim

Thank you again for your opinion

Mods

I think there is a point in the timing of most two cycles where fresh air is mixed with exhaust--just before the exhaust port closes. This would tend to cool off the exhaust some. Also, two cycles have huge mufflers--more volume for the exhaust to spread out in, and therefore cool off before exiting through the end of the pipe.

Don't know if any of that was correct, but I tried.....

WOW sure glad you explained your Mods-R-Me name
With an attatude like yours and since your an engineer I will keep what I know to myself .
I dont want to word it wrong.[] Besides chances are you would just Modifiy the facts

A 2 stroke's porting is a compromise because, of necessity, it has to be symetrical around BDC so there's very little difference between the time the exhaust port opens and the time the transfer ports open, usually only around 10 degrees of rotation. This means there's still quite a bit of hot exhaust gas still in the cylinder when the cool fresh mixture begins to flow so there's a heat transfer between the two.

Contrast this with a 4 stroke which has assymetrical timing where the exhaust stroke is almost complete before the inlet opens.

There are a couple of reasons the exhaust is hotter with our model sized 4strokes.

The 2stroke engines flow a bunch of fuel out the exhaust thanks to how long the intake and exhaust stay open. And some of that amount of unburned fuel comes from when the exhaust is open in the stroke. And since a 2stroke sees twice the number of firings, it's seeing more heat that needs to be exhausted, so the 2stroke designs consider that. So they usually pass through more fuel than less in order to carry off that "extra" heat. And the idea is that if the unburned fuel temp isn't somewhat cooler than hotter, there isn't enough being passed through so the designers tweak the timing to make sure they get that "cooler" exhaust. (A bunch of Control Line speed guys look at EGTemps. It tells them what their timing changes are doing, and when they've reached max or beyond.)

A 2stroke is also passing unburned fuel every time the exhaust port dumps into the muffler. And that unburned fuel is still cooling things as it goes out the muffler. It is actually carrying some raw alcohol that's still cooling things as well. A 4stroke only usually needs to dump exhaust into the muffler. And since the 4stroke has less combustion to cool in the cylinder, the cylinder fins wind up being "good enough" for the job, so it's exhaust doesn't have to carry out unburned fuel, so will actually wind up being hotter for that little detail.

And the timing on a 2stroke isn't controlled by a cam. It's more linear and because of that is less "accurate". When you run a 2stroke at less than it's optimum design rpm you wind up with a lot less efficient combustion. (Control Line stunt flyers discovered long ago that running their engines with a 2cycle/4cycle break would make them run LOTS cooler and last LOTS longer.) The cam on a 4cycle is profiled and can open at one rate and close at another.

The intake and exhaust timing for a 2cycle often has to be longer in order for the timing to work for all the functions the fuel charge does. You often have to have the exhaust open longer to help get the incoming charge into the cylinder. Sometimes the boost intakes, by their location, will spill undiluted fuel out the exhaust. With a 2cycle, you have the exhaust open while the piston is both going down and coming up. With a 4cycle, you open the exhaust only when you want and only for how long you want it and you don't have to have it open during compression or whatever...... So the 4cycle exhaust is really just exhaust.

---and that's the right answer

Allow me to explain that. He corrected me on a few key facts and I am grateful to him for that, but I need another question answered that was out of his knowledge scope. Instead of answering the question, he just replies that I'm wrong (which i am often) but does not tell me the reason why (doesn't know). After 3 or 4 replies like that I kind of became irritated and thus you see the results.
Maybe I should of PM that response. Woops

All,

Feel fee to answer the question anyway possible.

Thanks Ahead of Time,

Mods

Thank you all for increasing my knowledge.

Mods