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2 Stroke vs 4 Stroke Torque / Power Curves

flyingscott said:
It's funny it was a simple question that was answered. Then everybody had to confuse it. A 2 strk will always have more hp and torque given similar cid. Simply because it fires twice as often. You have double the power strokes.
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A two stroke does fire twice as often. But what about the fact that a two stroke has less compression and thus produces less force on the piston per stroke?

Arguably half as much power per stroke.
 
boscoe said:
A two stroke does fire twice as often. But what about the fact that a two stroke has less compression and thus produces less force on the piston per stroke?

Arguably half as much power per stroke.
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It’s not quite that simple, a motor that has twice the compression does not mean twice the power. You are forgetting that an ICE motor is actually a heat engine. High school physics, (Charles Law), says that in a contained vessel, (cylinder in this case), if you double the temperature, you double the pressure.
It is the burning of fuel, resulting in an exponential increase in temperature that results in the pressure which creates the driving force. Increased initial compression means there is more oxygen available, hence more fuel can be burned, (same for turbo or supercharging), & yes increased compression does result in a higher initial pre combustion temperature, (there are limits or you end up dieseling).
Beginning with increased compression, in this theoretical case, double, does not result in double the temperature…if it did it would exceed the limits of common metallurgy, & engine destruction.
Combustion engineering becomes critical, the flame front must be progressive, & of course the fuel used must exhibit a closely controlled range of Reid vapour pressure. It’s a common misconception that high octane fuel, or jet fuel is “much more powerful, nearly explosive”. In fact jet fuel is glorified kerosene.
 
boscoe said:
A two stroke does fire twice as often. But what about the fact that a two stroke has less compression and thus produces less force on the piston per stroke?

Arguably half as much power per stroke.
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Your argument is faulty. Read about 2 strks vs 4 strk
 
All this could be easily resolved if we had curves.

It is interesting that you never see a torque / power curve for a 2 / 4 stroke outboard. I have more chance of finding am amp output curve of the alternator at given RPM. if they did that it would be a lot easier to workout what prop would suit the given boat design and intended speed.
 
Yamaha shows a two stroke VZ250 and a four stroke F250 producing 250 HP at 5500 RPM. Similar bore and stroke numbers. Similar displacement also.

Based on the same HP at the same RPM, the torque has to be the same. It is simple mathematics.

If one model is firing twice as often at the other model at 5500 RPM, then it seems reasonable that the model having half as many power strokes must be producing more power per stroke. Arguably twice the power per stroke. If not, what else could it be?

For the two stroke model Yamaha recommended a vented propeller. To help the two stroke come up to RPM. Same as Mercury Marine did/does. For the four stroke model the vented propeller was not recommended by Yamaha. Obviously not needed. Inference being that the two stroke does not produce the torque at low RPM's that the four stroke produces.

 
The 4 stroke belongs on the family pontoon barge and the 2 stroke goes on the sport or bass boat.

Horses for courses...Way too many variables in the question to have a black and white answer..

If one needs the lightest hp...no govm regs..go 2 stroke. If weight no concern then choose with your other desires in mind..
 
boscoe said:
Yamaha shows a two stroke VZ250 and a four stroke F250 producing 250 HP at 5500 RPM. Similar bore and stroke numbers. Similar displacement also.

Based on the same HP at the same RPM, the torque has to be the same. It is simple mathematics.

If one model is firing twice as often at the other model at 5500 RPM, then it seems reasonable that the model having half as many power strokes must be producing more power per stroke. Arguably twice the power per stroke. If not, what else could it be?

For the two stroke model Yamaha recommended a vented propeller. To help the two stroke come up to RPM. Same as Mercury Marine did/does. For the four stroke model the vented propeller was not recommended by Yamaha. Obviously not needed. Inference being that the two stroke does not produce the torque at low RPM's that the four stroke produces.

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I'll bite...whats the torque at 3000rpm? I see the 4 has cam phasers...curious..

Since ya have enough information here why not plot the curves so we can see them? That would answer alot..

would be better if it was independent info though.
 
Sunkinmess said:
I'll bite...whats the torque at 3000rpm? I see the 4 has cam phasers...curious..

Since ya have enough information here why not plot the curves so we can see them? That would answer alot..

would be better if it was independent info though.
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I only post what I have. If a power curve has been published by Yamaha I have never seen it.

Certainly variable cam shaft timing helps a four stroke.

When I win the Powerball I will pay a reputable company to produce engine test data. We can then disagree as to what it means.
 
leachy said:
All this could be easily resolved if we had curves.

It is interesting that you never see a torque / power curve for a 2 / 4 stroke outboard. I have more chance of finding am amp output curve of the alternator at given RPM. if they did that it would be a lot easier to workout what prop would suit the given boat design and intended speed.
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There would be something within those curves to disagree with.
 
While on the subject of two stroke versus four stroke power curves, why is it that drag racing cars are using four strokes and not two strokes?
 
boscoe said:
Yamaha shows a two stroke VZ250 and a four stroke F250 producing 250 HP at 5500 RPM. Similar bore and stroke numbers. Similar displacement also.

Based on the same HP at the same RPM, the torque has to be the same. It is simple mathematics.

If one model is firing twice as often at the other model at 5500 RPM, then it seems reasonable that the model having half as many power strokes must be producing more power per stroke. Arguably twice the power per stroke. If not, what else could it be?

For the two stroke model Yamaha recommended a vented propeller. To help the two stroke come up to RPM. Same as Mercury Marine did/does. For the four stroke model the vented propeller was not recommended by Yamaha. Obviously not needed. Inference being that the two stroke does not produce the torque at low RPM's that the four stroke produces.

Click to expand...

I didn't realize that the 2 stroke engines were so low in compression, I guess that is necessary for the oil lubricating the piston. That would have a large influence on the power of the engine, and the efficiency due to the laws of thermodynamics. My guess is that the 2 stroke motors have a much narrower band for optimal operation, hence 4 strokes being a lot more fuel efficient overall as not many people spend 100% at WOT. I guess the manufactures would have all this information somewhere, and yes if I won Powerball I'd couple up a few motors to dynos for the fun of it.

I still wish the were manufacturing DI 2 strokes as developed that technology further.

Sorry for opening up such a large can of worms
 
Compression ratio has nothing to do with the lubrication of the piston.----I have stated it many times ---" Oil / lubrication on 2 strokes is not well understood "
 
racerone said:
Compression ratio has nothing to do with the lubrication of the piston.----I have stated it many times ---" Oil / lubrication on 2 strokes is not well understood "
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I was thinking that the higher compression means higher cylinder temperatures at combustion. Id like to understand the lubrication part of a 2 stroke a whole lot more. I remember my Yamaha ran at 100 to 1 ratio.
 
Lubricating oil ( conventional 2 stroke ) gets onto motor parts via the gasoline.-----Oil is burned off eventually.-----Remember that on some 2 strokes they run on just gasoline injected into the cylinder.----Oil is not mixed with the gasoline at all.
 
racerone said:
Lubricating oil ( conventional 2 stroke ) gets onto motor parts via the gasoline.-----Oil is burned off eventually.-----Remember that on some 2 strokes they run on just gasoline injected into the cylinder.----Oil is not mixed with the gasoline at all.
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Are they lubricated with an oil bath similar to a 4 stroke. Im assuming you mean something like E-tec .
 
racerone said:
Lubricating oil ( conventional 2 stroke ) gets onto motor parts via the gasoline.-----Oil is burned off eventually.-----Remember that on some 2 strokes they run on just gasoline injected into the cylinder.----Oil is not mixed with the gasoline at all.
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In some two strokes, yes. In some two strokes, no. As you sort of allude to in your latter sentences.
 
The E-tec and others operate like a 4 stroke for part of the revolution of the crankshaft.----In that part , only air is compressed.----Then fuel ( gasoline only ) is added. ------Lubrication is done separately with help from sophisticated electronics and sensors.
 
boscoe said:
How so? Does a two stroke not fire at every stroke? 5500 times at 5500 RPM. Whereas a four stroke fires every other stroke? 2750 times at 5500 RPM. What am I missing?
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Well that is exactly twice as many power strokes? How do you not understand that.
 
leachy said:
I didn't realize that the 2 stroke engines were so low in compression, I guess that is necessary for the oil lubricating the piston. That would have a large influence on the power of the engine, and the efficiency due to the laws of thermodynamics. My guess is that the 2 stroke motors have a much narrower band for optimal operation, hence 4 strokes being a lot more fuel efficient overall as not many people spend 100% at WOT. I guess the manufactures would have all this information somewhere, and yes if I won Powerball I'd couple up a few motors to dynos for the fun of it.

I still wish the were manufacturing DI 2 strokes as developed that technology further.

Sorry for opening up such a large can of worms
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2 strks build compression with Rpm. I don't get how little is known about 2 strks.
 
The guy above that said 2 stroke will ALWAYS have more torque, speed and power is right. Yoy see many high end boats today with 4 or 6 outboards on them, that 2 would do with 2 stroke, not to mention that the Evinrude 8 still holds the worlds speed record!
 
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