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Difference between two and four strokes

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Craig Spence06/03/2009 05:45:03
1170 forum posts
615 photos
Hi all,
I was wondering if anyone could explain the difference between two and four stroke engines, more so in performance. Ive read recently that two strokes are more powerfull but when looking at prices four strokes cost more?, im looking at buying a new plane, which I love and want to give it everything lol!, so some advice would be greatly appreciated.
Cheers all.
David Ashby - Moderator06/03/2009 07:15:27
11015 forum posts
1722 photos
616 articles
Craig, I've deleted the duplicate thread - you don't want to split the answers you'll receive. It's a good question but I just wondered what model are you thinking of getting?
Quite often the type of model will suggest the engine type you see.
Craig Spence06/03/2009 11:28:09
1170 forum posts
615 photos
Hi David,
I will be getting the CMPro Lancair, it has the option of two or four stroke.
Cheers David.
andy watson06/03/2009 12:08:41
1942 forum posts
20 photos
I will leave the technical difference to a more knowledgeable engineer, but here is my more pragmatic differences- as they relate to which should I use?
You have it right-
2 stroke- cheaper, more powerful, sound like a wasp!
4 stroke- more expensive, less powerful, sound like a cat purring!
So for me, if you are building a scale model the extra effect of the 4 stroke is worth the increased cost (the lower power is often irrelevant because you end up sticking in a bigger engine to get the same end power).  Scale models are about slow low passes and looking gorgeous, and a 2 stroke can let them down in that respect.
If you are wanting to buzz (literally) around the sky as fast/high/spinny as possible, then the 4 stroke becomes pointless, and better to cram as much power as cheaply as possible in the front with a 2 stroke.
That distinction works for me.
Delta Whiskey06/03/2009 12:57:56
1320 forum posts
90 photos
Andy, I think is dead on, not that I would know much, but after using my 2 stroke heli, the 4 stroke just sounds perfect!
Tim Mackey06/03/2009 13:12:21
20920 forum posts
304 photos
15 articles
To add to the thankfully non too technical answers so far... think of the fourstroke as better at swinging larger props slower ( which is much more efficient anyway ) and the 2 stroke as high revving but having less torque or "grunt" at lower RPM.
Four strokes can by their design be slightly higher maintenance type, as there are valves to adjust occasionally ( well tappets to be more accurate ) no biggy, but they do have many more moving parts than the beuatifully simple design of the 2T
flytilbroke06/03/2009 13:44:28
2083 forum posts
5 photos
The previous answers are a good guide wiyhout being techy.
The Two stroke has a power stroke every 2nd stroke hence the name. More Power at a cost of some wasted fuel compered to,,,,,,,,,,,
The Four stroke has a power stroke every 4th stroke, it is more efficient for fuel use, tends to be quieter and sounds better to most folk. It is heavier than the 2 stroke which can be a plus point on a model with engine/s mounted close to the wings or one which would need  a lot of lead up front for balance. A larger size, .70 instead of a .46 or.53 two stroke is required.
thomas oliver 106/03/2009 22:00:04
96 forum posts
23 photos
Craig, To answer the technical part of your question, this is how they work:-
  Four-stroke:  1. INDUCTION STROKE-  Inlet valve beginning to open at top dead centre--Piston moves down cylinder and causes air to be sucked through carburettor, picking up fuel until valve closes just after bottom dead centre.
    2. COMPRESSION STROKE -- Both valves are now closed--piston moves up cylinder compressing the mixture.
    3. POWER STROKE-- Just before top dead centre, a spark occurrs igniting the mixture which burns and expands forcing the piston downwards- both valves still closed
   4 . EXHAUST STOKE -- Exhaust valve now opens and upward moving piston pushes the burnt gases out of the valve.
      Initially the piston moves up and suction in the crankcase fills it with fuel mixture,  ase
     then the piston moves down compressing  the mixture in the crankcase and forcing it up  a transfer port into the cylinder. This might be called the priming phase.
      The two operative strokes now occurr--
      1. COMPRESSION -- the piston moves up compressing the mixture and at the top of the stroke the plug or spark plug ignites the mixture.
      2. POWER- the expanding burning gas forces the piston down.
      The exhaust escapes through the exhaust port towards the bottom of this stroke.
      Note that while 1. is happening, a new charge is being drawn into the crankcase, and while 2. is happening, this charge is being compressed ready for transfer and a repeat of events
     This is a slight simplification of events.   Tomol.
Craig Spence06/03/2009 22:20:43
1170 forum posts
615 photos
Thank's for the info fella's much appreciated, I think ill go with the two stroke. Even though I want the four stroke, it's just too expensive really and I cant justify spending the money just because it sound's better (i know there are more benifit's than just sound, as laid out in this thread).
I might look for a second hand four stroke.
Cheers all.
Eric Bray09/03/2009 00:55:35
6600 forum posts
2 photos
For a quick audio guide to two or four-stroke engines, listen to a 'rev and rip' scooter, or a big bike engine or a car engine. The r and r will be a two stroker, and the big bike will almost certainly be a four-stroker.
Also, a two-stroke engine 'wastes' the lube oil by burning it and blowing it out of the exhaust, because it is mixed in with the fuel,  while a four-stroker has a sump, and recycles the lube oil via a filter. (Model four-stroke engines also waste the lube oil, as it is not practical to have a sump, filter, and pump just for oil!)
Malcolm Keen27/03/2013 17:04:52
17 forum posts

I have two Saito and two OS four strokes and in some situations I have them fully enclosed within a cowl.

My question is how can I best protect them from deteriation because I can't get after run oil into them or can I ?

Any suggestions gratefully recd.

Many thanks Malcolm keen.

Steve Hargreaves - Moderator27/03/2013 18:04:44
6763 forum posts
197 photos

Which models are they Malcolm.....? Your best bet is a squirt of after-run oil into the breather (put a bit of tube on so it reaches outside the cowl) if the engines have one. The latest OS motors have internal breathers though so that kind of wrecks that idea.

Another option is to arrange a length of brass tube so it exits just over the carb mouth.....again extend this tube so you can get at it from outside the cowl & then use this to introduce a bit of after run oil into the carb.....this should then find its way to the crankcase. Not as good as squirting it in via the breather but your only option I'm afraid.

Also make sure you use a fuel with synthetic oil & ensure you "run the engine dry" at the end of a flying session....wink 2


ceejay27/03/2013 23:20:30
481 forum posts
410 photos

whilst not wanting to get technical here i have another question on the same subject, i have often wondered why the model 2 stoke is deemed to be less tourquey than a 4 stroke, i dont know how it works at smaller engine sizes but in my past life i road motorcycles and my sport was trials, and if we wanted tourge and lots of it a single cylinder low reving two stroke was the man for the job, and if you needed hp that comes from high revs a four stroke was your engine of choice, as a four stroke will rev far far higher than a two stroke, F1 is the ultimate user of extreme high revving 4 strokes, so now i fly larger models and what do we use 2 strokes, and they can be tuned to turn huge props yes


Steve Hargreaves - Moderator27/03/2013 23:30:38
6763 forum posts
197 photos

Well I think you've put your finger on it comes down to how the engine is "tuned" can have torque-y 2 strokes & screaming high rev 4 strokes...or vice versa....depends on valve/port timing....dimensions (under or oversquare).....

For two similarly tuned engines though a 4 stroke will generally be the more torquey as a) the cylinder filling is more efficient & b) the "power stroke" of a 4 stroke is longer.

Basically the 4 stroke cycle is just more efficient & extracts more energy from the fuel....where the 2 stroke scores of course is in its lighter weight...useful in trials bikes & aero engines....teeth 2

John Olsen 128/03/2013 04:19:40
446 forum posts
23 photos

OK, a curious thing about the weight of two strokes and four strokes...My son and I recently put an OS 56 four stroke into a Hangar 9 Sundowner 35, for which the recommended engine is an Evolution 46 two stroke. As part of the research we checked the weight of the two engines...according to the advertised figures the OS is slightly lighter than the Evolution, desite having more capacity. I suspect that the OS has an ABC type of liner, while the EVO probably has a steel or cast iron liner...but anyway, in that particular case the four stroke, being larger, has about the same power, is no heavier, and will use less fuel. So the all up weight for a given flight duration will be less, especially if longer flights were needed.

Mind you, shoehorning the OS into the cowl was not a trivial exercise. It is taller, so we mounted it sideways with the head and barrel coming out through one of the side cheeks. Works fine but it is not easy to assemble and disassemble.

The resulting model flies really well, the four stroke sound is deceptive since it does not sound all that fast...but it is actually going along pretty quick.


Former Member28/03/2013 10:03:38

[This posting has been removed]

MikeS28/03/2013 16:36:51
818 forum posts
240 photos

Intresting read this thread.

I always understood the four stoke principle and hows things work. However I never understood the two stroke principle until this thread. I can tune a four stoke better as I understand how it all works.

I myself prefer four stokes in my models. I like the sound, look and power. However my P40 has a two stoke in it and when in flight it sounds much better than a four stoke would due to how the P40 flys. Most of the time it goes like a scalded cat.


Edited By MikeS on 28/03/2013 16:39:48

Phil Claridge28/03/2013 17:25:59
1924 forum posts
32 photos

four stroke, suck, squeeze,bang, blow. two stroke, sucksqueeze, bangblow, simpleswink

Myron Beaumont28/03/2013 17:38:05
5797 forum posts
51 photos


Are we talking engines?wink 2

Phil Claridge28/03/2013 18:11:48
1924 forum posts
32 photos

i was, myron. mandy says two strokes are too quickwink

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