The 4-stroke myth
 

Quote from a Mr Dar Zeelon, on that other Forum.

From a book published by RCM, "The r/c engine" page 24, Quote by Clarence Lee after some testing.

Which is it folks?

Ed S

If I can remember my auto mechanics from Waaaay Back, I believe a four stroke fires on every stroke; a stroke being every time the piston reaches top dead centre, or simply on every full revolution of the crankshaft

A four stroke fires every other stroke. The one in between is the exhaust stroke.

Ed S

Ed,
My friend and well know engine writer has also done scientific testing, and he has concluded (which I also believe) that the two cycle glow engines will indeed "misfire" every 2nd or so stroke and "4 stroke".

I forget the exact wording but I have it in a book somewhere...

Like with most things, even the engine experts sometimes can't agree! :)

I have no knowledge on the subject. I found the two articles interesting.

I do wonder though, is it possible that a two stroke engine would fire every alternate stroke reliably, without missing a stroke just because it is rich?.


Ed S

Ed,
On a four stroke engine it will fire every second stoke. Which will give you 1: intake 2:compression(firing) 3: power stroke down 4: exhaust up.
Hope I have the terminology right. It has been awhile since I taught some on the subject.
As for a four stroke on a two cycle I believe since it fires every stroke that the terminology is because it is rich You don't get complete combustion and the sound is uneven. It probably doesn't fire all the time which would account for some of the roughness but the plug would go out if it was regular. That is the main reason for the 4 stroke plug.
Just my thoughts anyways.
John
PS: Ed are you still racing and what engines do you use now?

As far as I know, Dave did some pretty detailed testing and did in fact determine that on every 2nd stroke (average) there was a buildup of both fresh fuel charge, and also exhaust gasses that did not scavenge, and this caused the engine to "miss", ie, not fire, but clean itself out on that particular stroke, and then fire on the next one. I guess it happens fast enough still to keep the plug lit. But as you and I know, if you richen up an engine too much, it will go from a nice 4 stroke to a misfiring situation and a bit more and its too rich to continue running without the glow battery connected.

I find this stuff really interesting, and wish there were more to read on these type of topics.

Yes John I am still racing. Is there anything else??
I use the Nelson engines for both Q40 and Quicky. Is there anything else??



My only reason for posting it was to get other opinions. The major part of this hobby for me is playing with engines/props. My attitude is that I do not race model airplanes. I race engines. The airplane is a necessary encumbrance to carry the engine around the course. I race my engine/prop combinations against the others.

I have said this before. There is nothing to compare with the sound of four .40 size tuned pipe racing motors at 30,000rpm going through the Nos 2 & 3 pylons just 15 feet away.

Ed S

Ed,
I agree with you about the sound of the engines but the big thing I miss and you don't get anywhere else that I know of is the smell of burnt nitro when standing between 2 & 3 on launch. I still play with engines although not the newer ones. I still have a few of the St's and Vr's and I like to still rework them. If they break I am not out because I got lots of enjoyment from them a long time ago.
Keep up the tradition and turn left.
John

C/L stunt grunts often use the 4-stroke term to describe what is called the "stunt run" for many non-piped engines. They use a venturi with a needle valve, not a variable carb. The engine is retimed (if it is a modern RC engine by design) and the needle is set in such a manner as to give a rich setting in level or downward flight and a leaner (clearly a 2-stroke) run when the model is flown upwards or put under conditions demanding more power. This is a result of the timing and tank placement/configuration. They refer to this as a "4-2-4 stunt run." In practice, it does sound like just that, although "misfire-fire-misfire" might be a better description. It works great when set up properly. Dzl