RE: 4 stroke debate

RE: 4 stroke debate
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RE: 4 stroke debate - 6/14/2007 8:42:56 PM

Posts: 1172
Joined: 11/19/2003
From: Auckland, NEW ZEALAND
Status: offline

For anyone here who thinks four-strokes are more "torquey" than two-strokes, try this experiment:

1) Take two engines from your collection of the same capacity - one a two-stroke and the other a four-stroke.
2) Get a range of props - any size within reason that will load the engines from their recommended static rpm down to about 2/3rds of their recommended rpm range. Try props you'd normally run on your four-stroke.
3) Get taching - put each prop on each engine and take a tach reading.

Here is what you'll see: The two-stroke will out-pull the four-stroke on the same prop, every time, no matter how ridiculously large the prop is.

Think about it ...

Will a Saito .72 pull a 13x6 faster than a Tower .75? Not a chance...

Will an OS .52 Surpass pull a 12x6 faster than a Magnum .52 two-stroke? No way...

Will a Saito .91 spin a 14x8 prop faster than an ST G90? Nope, sorry...

Caveats:
1) Only applies to naturally aspirated engines: A YS four-stroke is supercharged and will approach or may even exceed a similar sized two-stroke in torque due to this scheme.
2) May not apply where the two-stroke is a high-timed engine with a narrow band tuned pipe, tuned for much higher rpm that the test band.

There are lots of things four-strokes do well, but, unless it's supercharged, putting out more absolute torque than a similar capacity two-stroke is not one of them.

(in reply to Jezmo)

RE: 4 stroke debate - 6/14/2007 8:54:39 PM

Posts: 1172
Joined: 11/19/2003
From: Auckland, NEW ZEALAND
Status: offline

quote:

ORIGINAL: jeffie8696

Thats OK,my basic question was answered . There is no place to find dyno data for glow engines. Even though they all state their engines give X amount of HP. I searched for the quantifiable and only found the subjective. Too bad really. I think we would all benefit from good dyno data. Thanks to all involved. I didnt mean to stir up a hornets nest.

Do your own "dyno" runs. It's easy and you can share your results with your RCU buddies afterwards.

This is what you do...

1)Select the engine you want to "dyno".

2)Get a range of props of the same brand and style. Don't mix brands/styles. Aim to get a series of props that range from being significantly underpropped to overpropped for that engine.

3) Tach each prop and note the rpm.

4) Use Pe Reivers' "prop-power.xls" spreadsheet to calculate the hp for each rpm peak you recorded for each prop.

5) Plot these data points on a graph of hp versus rpm and voila! You have a "dyno curve" for your engine. Calculate torque at each data point and plot this too if you need to see a torque curve.

(in reply to jeffie8696)

RE: 4 stroke debate - 6/15/2007 2:44:26 AM

Posts: 99
Joined: 6/2/2007
From: Sao Paulo , BRAZIL
Status: offline

I think the two stroke is better to Auto RC than airplanes R/C.. because a prop need torque to have performance. In a car, you can have the same performance with two engines with the same power but diferent torques.. But, in airplane not.. because to you use a high pass prop you need have torque to rev it up, and not only power.

Then, if you have a 2 stroke, with fantastic power at 17.000rpm, but low torque value, and only appears at very high rpm, you will be limited to a low pass prop, and will can´t use this power to push.. will be only "market power".. not "usable" power...

And it´s happens a lot.. You see OS special series engines, that have very high power but at 17.000.. 18.000rpm.. With the prop that they should, the engine don´t rev more than 13.000rpm.. Then, the fantastic power is only market to sell, because you never will use these 17.000.. 18.000rpm.

In the other way, you have a four stroke, that have a low max power, but at 10.000rpm, you will use a prop that you can work at this power with good push.. Then, is a real power result, and not market power.

Cheers,

Matheus S. Almeida

(in reply to Harry Lagman)

RE: 4 stroke debate - 6/15/2007 4:17:22 AM

Posts: 1272
Joined: 5/10/2007
From: coralville, IA, USA
Status: offline

Matheus I think you nailed it.
Horsepower sells engines, torque moves them.
So who can run the numbers for us and get some real world data? I myself do not have the means. I may be able to supply a prop or two but thats about it. To be fair I would recommend that all the engines use the same fuel unless there is some mechanical reeason for not doing so. Perhaps something in the overlap zone of the recomended fuels.
I think we are really getting somewhere.

_____________________________

Castor, its like Vitamin C for glow engines. I am not Dom from Airwolf but I do resemble him.......Unfortunately

(in reply to MatheusC20XE)

RE: 4 stroke debate - 6/15/2007 2:56:49 PM

Posts: 7249
Joined: 4/9/2003
From: Rosh-Ha'Ayin, ISRAEL
Status: online

Jeff,

This is basically nonsense - not 'logic'....

Torque at RPM is horsepower. This is exactly the definition of HP.

A two-stroke engine, with a normal, expansion-chamber muffler, needs to be timed and designed to be capable of producing good power at some out-of-sight RPM, to be able to spin a usable prop at a respectably good RPM.

The torque produced by these engines declines at a relatively shallow rate, which in turn produces a continually rising output curve, which eventually peaks (with no muffler attached) around 15-18,000 for most sport .40-.61 engines.

Install the muffler and the whole top of the power curve is 'chopped off'...

The engine will be capable of reaching higher RPM, but will produce no additional power.
The OS.46AX engine, for example, with the muffler installed, will spin a 10x6 prop near 14K, outputting just over 1 HP, but if unloaded with a 'toothpick', pylon racing prop, it will exceed 16K and will still produce just over 1 HP...

If the engine is redesigned to only reach its maximum power at 14K, it will make less power all over its usable range, so the manufacturers keep the more radical numbers.

With a four-stroke engine, the torque peak is reached around 80% of peak HP RPM; and drops off soon after peak power, making the right prop, one that is spun over peak torque RPM and below peak HP RPM.

The muffler has little effect on the power produced by a four-stroke engine; much less profound than with a two-stroke engine.

< Message edited by DarZeelon -- 6/15/2007 2:57:27 PM >

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MVVS - Jett - Nelson - Bolly - Mejzlik

(in reply to jeffie8696)

RE: 4 stroke debate - 6/15/2007 5:52:07 PM

Posts: 43
Joined: 1/4/2004
From: Saint Louis, MO, USA
Status: offline

O.S. ENGINES 1.40RX(23 CC) 2 stroke Wt 29 oz w/fuel pump but no muffler or pipe

http://www.osengines.com/reviews/osmg0678-man.html

Peak torque: 443 oz.-in. @ 8,677rpm (Bolly pipe @ 585 mm/10% nitro)

oz/in torque/cc = 19.2

Peak BHP 4.03 @ 9,090 PRM

370 oz.-in. @ 6,700rpm (open exhaust/10% nitro)

oz/in torque/cc = 16.01

Peak BHP = 3.75 @ 6,700 RPM

SaitoFA-125a (20.5 CC) Four Stroke Wt. 26 oz with muffler
MAN July 2006 page 85

Maximum torque 279 oz/in @7,300 RPM 15% Nitro

oz/in torque/cc= 13.6

Peak BHP 2.35 @ 9,700 RPM

Note:The OS FS-120-E & FS-120 III are heavier (30.5 & 32.5 oz)
they also put out less HP @ higher RPMs than the Saito FA-125a

Summary: The OS 2 stroke put out 17.7% more torque per CC at a lower
RPM burning 1/3 less nitro in fuel mix.

The same OS with tuned pipe put out 41% more torque per CC at lower RPM
burning 1/3 less nitro in fuel mix.

When the 2 stroke is designed to put out its power in a comparable range to the 4 stroke it has more torque. Most 2 strokes are designed to produce power at a higher rpm in search of power (torque X RPM =HP), therefore they sacrifice low RPM torque in a trade for high RPM power .
This is something hot rodders know, you "Rob Peter to pay Paul".

Some myths are extremely hard to dispel.

If a guy could design a muffler that could give a 2 stroke the 4 stroke sound he would become rich & famous in short order.

Personally I like the 2 stroke sound, & always have.

Each type of engine has it's plus & minus attributes.

_____________________________

But just remember: If voting could really change things, it would be illegal.
-----Alfred E. Neuman

(in reply to DarZeelon)

RE: 4 stroke debate - 6/15/2007 6:07:24 PM

Posts: 99
Joined: 6/2/2007
From: Sao Paulo , BRAZIL
Status: offline

Not really.. the two strokes get lower efficience when you try make power/torque at lower revs.

The perfect example of it are the street motorcycles. Here in Brazil, we have a Yamaha 180cc 2 stroke engine, that makes 16,6hp @ 8.000rpm. And the Honda 4 stroke 190cc makes 19,2hp @ 7.800rpm. Then, in this case, the 4 stroke is much more efficient.

then, when you compare higher rev 2 stroke motorcycle engine, like Cagiva 190cc, it´s makes 27,5hp, but appears only in 9.700rpm, and with peak torque above 8.000rpm.

Only the high rev strokes are efficient.. when you try make a 2 stroke get power at low rev, the result in most cases are very bad.

If it´s not true, why the OS didn´t made it? If much people are going to 4 stroke because it´s have more plane torque curve, then, why the OS continuos making 2 stroke high reving? it´s no sense..

And about the power figures of manufacters, is very dificult to believe.. in my opinion have much strange info on manufacter tables..

I will make some tests with diferents props, nitro and rpm in both engines..2 stroke and 4 stroke.. and i will post here the results of it for the friends see.

But one thing i agree 100%, 2 strokes engine is a fantastic efficient to power.. but only in high rev and with very bad torque curve.. For auto r/c is perfect.

Cheers,

Matheus S. Almeida

(in reply to max-nix)

RE: 4 stroke debate - 6/15/2007 6:18:37 PM

Posts: 7828
Joined: 1/21/2002
From: Acworth, GA, USA
Status: offline

quote:

But one thing i agree 100%, 2 strokes engine is a fantastic efficient to power.. but only in high rev and with very bad torque curve.. For auto r/c is perfect.

Two strokes are actually less efficient than a four stroke. Though, they can be made efficent for a certain power range. They do give more power for the same displacement. That is by having one moderately weaker power strokes to in one revolution to one somewhat more stronger power stroke for every two revolutions. This is more force applied to the crankshaft. If you time the engine for low speeds the two stroke will have more torque than the same size four stroke timed for the same speed, if you time it for higher HP then it will have more HP than the equivalent timed four stroke. The backpressure is more of an issue with the two stroke, but the broader power band has more to do with the lower speed timing than the number of strokes. None of this power differance has anything to do with efficiency, that is another matter about how much fuel is burned for the same power output.

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(in reply to MatheusC20XE)

RE: 4 stroke debate - 6/15/2007 6:47:01 PM

Posts: 99
Joined: 6/2/2007
From: Sao Paulo , BRAZIL
Status: offline

Sorry.. i mean "volumetric efficience"..

2 strokes usually is more volumetric efficient than a 4 stroke.

I was no mean BSFC efficience.. in this case, i agree perfect with you.

About the higher power 2 stroke comparing the same rev of 4 stroke.. not always.. the example of motorcycle engine that i was add, show it.

And other importante thing.. two stroke to make high power, need have a high overlap.. and FANTASTIC fuel consumption.. it´s mean bigger fuel tank capacity, and it´s mean more weight..

The two stroke need have long mufler exhaust type(it´s uses the mufler like inlet pump, then, need the backpressure in exhaust to take more inlet.. inverse of 4 stroke), that is heavier.. then.. is very important to see power to weight ready to fly the same both time..

But i want see more examples of low rev 2 strokes very volumetric efficent and with large torque band.. is really hard to see one..

Cheers,

Matheus S. Almeida

< Message edited by Lightfoot -- 6/18/2007 7:46:22 PM >

(in reply to Sport_Pilot)

RE: 4 stroke debate - 6/15/2007 6:54:33 PM

Posts: 43
Joined: 1/4/2004
From: Saint Louis, MO, USA
Status: offline

MatheusC20XE
Yes really.... OS does make it and you can see for yourself at this web site. Buy one & test it yourself as you suggested.

http://www2.towerhobbies.com/cgi-bin/wti0001p?&I=LXBY54&P=0

you can see Prop-RPM test figures available @ http://www.osengines.com/reviews/osmg0678-man.html

_____________________________

But just remember: If voting could really change things, it would be illegal.
-----Alfred E. Neuman

(in reply to Sport_Pilot)

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