Fellows, I am a newbie. I know that a 4cycle engine does twice the effort over a 2cycle engine for more:

power?
speed?
torque?

When or why would I want or need a 2cy over a 4cy or a 4 over a 2?


Thanks for your knowledge and imput

Kinahora,

A single cylinder 2cycle engine has a power stroke every revolution. A 4cycle has one every second revolution.

In theory a 2cycle engine would have twice the potential power of a 4cycle. It doesn't.

You got to read a book about it. In 2 paragraphs in this forum, you will get very partial answers. You can get books about engines in most hobby shops and book stores.

Sincerely,

For a newbie
2 cycle produces very reliable power at low cost and easy operation.

4 cycle produces cool sounds at high cost.
IMHO

that aught to get em going

I may not know anything about engines but I know an instigator when I see one!

I don't know of any theory that says a 2 stroke engine has twice the power. The effective displacement of a two stroke is just over 1.5 or so. This is because the ports open well before the piston is at BDC, and this volume is not used except as an induction device. Two stoke engines do indeed have more power at all speeds for the same displacement 4-Stroke. However the larger mufflers make them heavier for the same displacement. This sometimes makes the four stroke a better choice when high speed isn't the primary requirement.

The way I see it, a 4cy is a fully pumped engine, the entire length of the stroke is used to pull fuel into the cylinder, and the entire length participates in the combustion downstroke. They get more power per "pop" useable. Another big plus is that the power range is wider because the RPM range is smaller. Thus the larger, higher pitched props are used. Anytime you can turn a prop at a lower RPM, and still be in the power range of the engine, you have less drag on the individual prop blades for each lb of thrust produced. Even if a prop had zero pitch, there is a drag involed in pulling something with a non zero thickness through the air. This drag increases exponentially with speed through the air. This makes a slower turning prop more efficient.

If you compare a two stroke to a four stroke same displacement; a 91FX to a 91 surpass for instance, the two stroke will turn the same prop faster and turn a bigger prop than the fourstroke. the four stroke sounds cooler.

LORD_RC, you are correct, but my 91FX drinks a LOT more gas turning the larger prop. More than would be accounted for with the power increase. Select an equivalent powered 2 stroke as a four stroke and it will use more glow fuel to produce the same power. This translates into flight time to me.

Not to mention having to carry extra fuel thus more weight! I like my fourstrokes better than my two strokes in almost every situation the exception being if I have a plane Im looking for maximun rpms from the engine. I dont fly too many of them anymore. Two strokes do like to rev higher than fourstrokes thus most people like to use smaller props on them. I think its a misconception that a four stroke can pull a bigger prop than a two stroke displacement being equal. I also like the smaller muffler size of a fourstroke, its easier to fit in most planes.

A two stroke is like a whiney little four banger Honda. You must wind it up high in order to get usable power. On the other hand, a four stroke is like a throaty V-8 or V-12. It has lots of low end torque and sounds good no matter what muffler you put on it. It's hard to make a Honda sound like a Corvette or a Ferrari.

All my engines are four strokes, and I love them all.

only ferrarris are small displacement v12s that rev like crazy! But I get what you mean.

My Saito 100 is the finest engine I have ever heard, and used. The engine generates so much power for the sound that it puts out, if the club lawnmower is going, I get afraid that it has stopped running on me! With a lot of two strokes going at the field, you can close your eyes and swear that a motocross event is taking place.

Sport_pilot

...And a 4 stroke's exhaust valve begins to open 40-70 degrees before BDC and its intake begins to close about the same duration after BDC. And there is an overlap period when both intake and exhaust are open, around TDC...

A sport two stroke has about 140 degrees of exhaust timing and about 120 degrees of intake timing. Sounds similar?

I wrote "theoretical", didn't I?

The most fuel efficient engines are Turbocharged Diesel, Sleeve intake, overhead exhaust valve, two-strokes.
They exist on some Allison/Detroit Diesel busses and military vehicles and on the Orbital. They are twice as powerful and 20% more fuel efficient as convetional turbo-Diesels of the same cubic displacement.
They sound very strange, however.

Since we are talking about model engines, the late YS53 4 stroke used to spin 2stroke 60 vintage 12x6 props (and larger sizes too) about 1000 RPM faster than good two stroke 60s.

I was just telling the guy he should read more about it.

Sincerely,

2 Strokes:

1 , Initially CHEAPer
2 , Usually need HUGE ,HEAVY mufflers
3 , Even with #2 they still aggravate noise problems more than 4 Strokes
4 , Burn a lot more fuel
5 , Need to swing smaller less efficient props to develop full power
6 , Are an ever shrinking percentage of the new engine sales.
7, Do not work well with smoke systems.

4 Strokes :

1 , Initially cost a little more (but return the difference in fuel savings alone)
2, Use small ,light mufflers and often run straight pipes without complaints from neighbors
3 , Less bothersome to most people who happen to live near fying sites.
4 , Burn less fuel for similar power levels.
5 , Swing larger props at more efficient speeds that result in strong realistic flyong.
6 , Are increasing their share of the market.
7, Are putting out power approaching a similar disp. 2 stroke.
8 , Make GREAT smoke with a muffler heat smoke system.

Many 4 stroke flyers started with 2 strokers . They could easily go back.........if they wanted . The sales trends confirm most don't

Good point. But the exhaust valve timing of even 40 degrees is on the high side. 70 degrees would be a lopey high perf engine. And 150 degrees of exhaust would probably be more common on a two stroke. I don't know about our model engines but I suspect they are around 35 degrees. And the exhaust valve opens very slowly at first, the pressure in the cylinder is probably providing usable power for about 10 to 20 degrees of that, don't know just a guess. The ports of a two stroke open suddenly all pressure is relieved in just a few degrees. Good point however, hadn't thought of that. Of course you are right, the poorer exhaust scavenging of the two stroke is part of the reason it doesn't produce twice the power also.

I agree with you for the most part. but.


1. Most two strokes have a much heavier muffler than necessary, the newer tuned expansion mufflers are much lighter, still large though. Some of the pitts mufflers are lighter as well.

2. The noise problem is workable, but with 98 Db being the usual limit there is not a lot of them below that. Course the hight RPM is partly responsible.

3. They burn more fuel but I disagree that it is a lot, especially with the more powerful four strokes. You would have to fly a lot to pay for the difference, more than I currently fly.

4. With the timing of the ports detuned the two stokes can perform better than the four strokes for the same displacement, supercharged YS engines being the exception. The OS FX 160, large Moki's, and large SuperTigres are an example of this. But still have that large muffler, though a rear exhaust and light weight pipe gets around this somewhat.

5. Maybe in competition and your part of the woods the four strokes are gaining in popularity. It was here too till the economy turned down. Now I see people showing up with two strokes, complaining about the high maintenance of four stokes, maybe but I suspect that they are not making as much and cost is a big factor. I myself make less than I did, and I have to support a wife and three kids.

But other than that it's whatever floats your boat!

To put the difference as simply as possible, a 2 stroke's porting is at best a compromise and is far from being ideal because, of necessity, it has to be symmetrical around BDC. This drastically reduces it's efficiency but it's made up for by the fact it fires every revolution. Because it's not effecient it uses fuel faster. A 4 stroke on the other hand has the opening and closing points for it's porting system tailored much more closely to what's really required. This makes it much more effecient in the way it controls the available pressure inside the cylinder.

However, a 4 stroke has a far lower potential maximum rev limit because of (mainly) valve float and flow limitations through the valves/ports. Below this point though it develops quite a lot of torque (higher average cylinder pressures or BMEP) which is useful for driving big props.

A 2 stroke doesn't have any mechanical limitation on porting so generally they're designed to be able to flow large volumes of mixture at much higher revs. Their compromise porting sacrifices lower rev torque but continues to give reasonable torque at high revs which in turn means high BHP to drive smaller props at high RPM.

So in general from an efficiency point of view, a 4 stroke is a better choice for a model needing a relatively large prop at lower revs (ranging from trainers, sports and scale). The downside is the added complexity, cost and maybe weight.

A 2 stroke is the better choice for a small, clean model that's intended to fly fast (pylon or similar).

In practise though, either type of engine will fly just about any average model equally well.

The big difference is the muffler.

If you bump a 4 stroke muffler on a bad landing, the threaded boss in the cylinder/head breaks out and you just bought a new engine.

Sport_Pilot,

The sleeve valves in a two-stroke, unlike popet valves in a four-stroke, (but like a rotory valve in a four-stroke) open very quickly. The piston is speeding down and unlike a closed popet valve, it doesn't have to accelerate to open (the valve). That would put things closer to your description.

Still, a two-stroke doesn't start the comperssion with a full load of mixture, due to bad scavanging.

Sincerely,

Want to go fast? Get a two-stroke

LOONY BOY

Want to go fast and sound good doing it? Get a YS four stroke.

I have seen many P-51s going fast with YSs

The now defunkt YS .53 FS would outrev all sport two-stroke 60 engines and even some piped ones. All on 60 size 12x6, 11x8/7 and 13x6.
It was replaced by the more expensive YS .63.

Sincerely,

"The now defunkt YS .53 FS would outrev all sport two-stroke 60 engines and even some piped ones. All on 60 size 12x6, 11x8/7 and 13x6. "

I have both YS53s and 63s and they are fabulous, but out rev, out turn, out power my good 60 2 strokes. I doubt it. In a direct comparison my OS 60 FSR on a 13/6apc prop would only do as well as the YS63 if I stuffed the OS60FSR exhaust with rags so it wouldn't shriek at me.

You must have some sick 2 strokes. Just kidding

I agree the The YS 53/63 and Saito 72 are wonderful user friendly engines that certainly are competitive with 60 2 strokes in many if not most applications AND WILL FIT where a 40 2 stroke goes. And they sound so cool.

Geez I guess I did instigate something.

Denis

hey richard L want to go really fast get a YS 2 stroke. I like the sound of 18K.