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why can't 2 strokes rev?

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I did a Google search but found no technical explanation.  Can someone educate me on why a production two stroke has a lot lower RPM ceiling than comparative 4 strokes? 

 

Is it something to do with induction through the ports that slows things down that much, the longer skirts on he piston, etc.  Can someone explain why the 2 strokes are limited in the RPM levels compared to 4 strokes.I would like to understand the technical reasons why.  Thank you.

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I did a Google search but found no technical explanation.  Can someone educate me on why a production two stroke has a lot lower RPM ceiling than comparative 4 strokes? 

 

Is it something to do with induction through the ports that slows things down that much, the longer skirts on he piston, etc.  Can someone explain why the 2 strokes are limited in the RPM levels compared to 4 strokes.I would like to understand the technical reasons why.  Thank you.

Port design and usable power band. Two strokes have no problem revving inf they're designed to do so, kart and moto gp engines rev closer to 20k RPM. Since current production two strokes use relatively simple designs( No direct injection, adjustable ports, adjustable exhaust or intake) they have a narrow powerband compared to comparable four strokes. At the same time, four strokes need to rev higher to make competitive levels of power. 

 

In the end, they rev as high as they're design goals required. 

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it is a balance act  of limiting factors...the piston speed  going up and down the cylinder is limited or it will basicly try to weld itself to the cylinder wall. A shorter stroke lowers the piston speed  because it doesn't have to travel up and down the cylinder as far for a given rpm.

 

a combustion chamber likes to be cool other wise the the fuel /air mixture may start to burn in the wrong spot  called detonation. So to fight this a smaller combustion chamber is in order.

 

high rpm means high airflow in and out of the engine  this means larger ports in a 2stroke and bigger valves in a 4stroke.

 

this is just a small tidbit of info  on why things are the way they are    a two stroke fires every rpm so the piston  has less time to cool  so it would like a smaller bore  than a four stroke  A smaller bore means a longer stroke  for a given displacement..   since you can have a larger bore  on the four stroke you can have more valve area in the combustion chamber    compared to safely having port area in the side of a cylinder of a two stroke.

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The two things that primarily limit the two-strokes ability to rev are a longer stroke and harmonic tuning.

 

The single biggest reason is the bore/stroke ratio.  The second event in a two-stroke power cycle is the transfer event, during which the downstroke of the piston forces fuel/air from the crankcase up to the top end for the compression stroke.  The strength of this "crankcase compression" is heavily dependent on the length of the downstroke, so in order to get enough of it to run at a practical RPM, the engines are typically much more likely to be "under square" than a modern, competitive 4-stroke.  A YZ250, for example, has a bore and stroke of 66 x 72mm, whereas a YZ250F is built at 77 x 53.6, way over square. 

 

Gasoline, you see, burns at fairly fixed rate in the combustion chamber under pressure, and this in turn limits the productive range of piston speed at which an engine can produce power.  With that, a longer stroke requires higher piston speeds at any given RPM than a shorter one does.  The same piston speed with a shorter stroke will therefore produce a higher crankshaft speed.  Simple geometry.  This gives the 250F a natural ability to spin up revs that even a 4-stroke with a 66 x 77 bore /stroke would never have, just to begin with.  

 

The second reason is that two-strokes, at least normally aspirated ones, are tuned by timing pressure waves in the exhaust system to prevent the engine from blowing fuel and fresh air out the exhaust port before the port closes.  The pressure wave is timed to get back to the port just as it closes, and stuffs the escaping fuel back into the chamber where it belongs.  Very effective, but also restricted to a narrow range of speed, since the pressure wave will always move at a nearly constant speed, and the timing of it is controlled by the length of the pipe, which is almost always a fixed dimension.  

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The pipe is the main factor.  All engines have a VE curve that follows an arc.  Without any external influences they will make peak torque at a certain RPM and the torque curve will begin to fall off as revs increase beyond that point.  Two stroke engines without expansion chambers exhibit the same characteristic and they make boring power like a four stroke of similar displacement.  The expansion chamber provides a boost over a certain power range depending on the design of the pipe.  Big bore pipes are designed to provide a wide power band at the expense of peak power.  Small bore motors are designed to hit as hard as they can so everything is designed to work together at the same RPM.  A 125cc two stroke builds power up to a peak around 11,000 RPM and then the power drops off a cliff.  As soon as the engine outruns the pipe the returned pulse from the baffle cone arrives too late to recharge the cylinder and power plummets.  Big bore motors are tuned with a longer pipe with shallower angles in the pipe.  This has a less pronounced effect on power but over a wider range, so the drop off in power is less severe.

 

Power valves help too.  They change the VE curve of the engine and change the way the pipe works depending on the design.  Kawasaki used drum valves to kill the effect of the pipe at low RPM where it does more harm than good.  These same valves rotate at high RPM to open sub ports in the cylinder in conjunction with a valve in the center exhaust port to increase port area.  This helps broaden the powerband but the pipe is still the primary determining factor.

 

Four strokes don't have to rely on external influences to exchange intake and exhaust gases.  They spend an entire stroke on blowing down the cylinder and then another entire stroke on filling the cylinder.  While two strokes do both at once, four strokes only have a very short amount of overlap between the intake and exhaust cycle.  This results in a very broad VE curve that is limited only in an engines ability to exchange gases.  Modern four strokes are hugely oversquare with large bores so they can fit four or more big valves into the head for maximum valve curtain area.  This also allows a very short stroke which can tolerate very high RPM without exceeding the limits of reciprocating loads on the bottom end.  Now that four stroke technology has evolved to the point where the heads flow as well as they do you can get very high power outputs from fairly small displacement engines.  Two strokes still win on an equal displacement basis but its not as disparate as it used to be.  The smaller the bore, the greater the advantage seems to be for the two stroke. 

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Purely a stroke issue. Most 2-strokes are close to square, i.e. The bore and stroke are the same. Stroke length is proportional to the rpm of the motor. It used to be that 4200 ft/min was an acceptable limit for a street motor. So a 125 with its 51mm stroke had a max rpm of ~ 12,500. Now a 450 pushed to the same limit with 61mm stroke is 10,500. A 250 2-st has a stroke of 72mm so it would have a rpm limit of ~ 9000 rpm. While a 500 would be about 7500.

Now they are pushing 5500 fpm on the 4-st because they use light weight Pistons with short skirts. If you could make the 2-st run at the same mean piston speed you would still have an rpm that was lower than the 4-st because the stroke is longer.

The pipe can be tuned to whatever rpm is needed. On a 125 road racer the pipe worked from 11k to 13k. You had to launch at 9500. Not very good for riding in the woods, but it make lots of hp.

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Root cause : power band size and lack of need

Dirt bike needs some sort of power band

Two stroke power band(which is small to begin with) tends to widen with long strokes and widely spaced ports with high velocity

Both of those things are bad for trying to get a engine to rev

As for the lack of need part ... A 2t 125 makes 30 a 250 40-50 and the 500s putting out 50-60

A non pro rider can't effectively use more than 50 so while there is a jump from a 125 to a 250 going from 250 to 500 doesn't really change much other than widen the power band. I ride my kx500 for the most part in 3rd simply because it just never stops having power and putting it into 1st or even 2nd spins the wheel for a second then the bike is out of revs. However I notice than compared to a 250 it's not a real significant amount faster . I mean of course it is but not miles ahead like the difference between a 125 and 250.

So sure they could rev a 250 a couple 1k rpm more but it would sacrifice rideability for Hp the rider can't even use . 250 4ts wished they could make 250 2t hp but they cant mostly as a result of the limits of materials and the simply fact they have to rev double what a two stroke does per ingnition stroke so that's why you see them trying to rev them to the moon.

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Mean, or average, piston speed is fairly meaningless, even comparing one engine to another, because the limitation piston speed represents is one that relates primarily to the rate of combustion and the optimum piston speed at which useable force will be applied to the crank via the piston and rod by the combustion event.  Metallurgical considerations such as rod or crank strength play into it only in terms of reliability, not as operational limitations to the engine's performance.  

 

At 9000 RPM, the peak piston speed of an engine with a 72mm stroke (longer by far than the typical 450cc MX single) will reach roughly 6995 FPM, depending to some degree on rod length.  (The calculation was run with an assumed rod of 4.4" (112mm) )  The 250F with the 53.6 mm stroke only reaches 5100 at the same RPM, and doesn't clear 7000 FPM until 12000 RPM.  

 

At present, most normal gasoline performance engines seem to run out of spin at just about the 7000-7200 FPM piston speed mark.  Facing that same "limit", an R6 at 42mm would spin up 16500 before hitting that point. 

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HP is just X amount of torque at high RPM.  A 250 4t will never make the same kind of torque as a 250 2t.  You can make the four stroke rev high enough to make similar horsepower but it'll never make the same torque.

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A classic deflection.  Your statement was that 4T's couldn't make 2T power, you were shown otherwise, and you immediately go to "can't make torque".  Whoopie.  Since we've previously established that 2-strokes can't rev, and both types can produce similar power, it's sort of a wash, isn't it? 

 

The whole concept of radically over square four-strokes, something that bobbed around on paper for years, was finally brought forward by Ducati in their efforts to compete with the European two-strokes on an inch-for-inch basis. Since it's impossible to do in 4 strokes what a two-stroke does in two, they decided to do what a two-stroke can't; work the HP math by running the RPM up as far as possible. It worked. 

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The two things that primarily limit the two-strokes ability to rev are a longer stroke and harmonic tuning.

 

The single biggest reason is the bore/stroke ratio.  The second event in a two-stroke power cycle is the transfer event, during which the downstroke of the piston forces fuel/air from the crankcase up to the top end for the compression stroke.  The strength of this "crankcase compression" is heavily dependent on the length of the downstroke, so in order to get enough of it to run at a practical RPM, the engines are typically much more likely to be "under square" than a modern, competitive 4-stroke.  A YZ250, for example, has a bore and stroke of 66 x 72mm, whereas a YZ250F is built at 77 x 53.6, way over square. 

 

Gasoline, you see, burns at fairly fixed rate in the combustion chamber under pressure, and this in turn limits the productive range of piston speed at which an engine can produce power.  With that, a longer stroke requires higher piston speeds at any given RPM than a shorter one does.  The same piston speed with a shorter stroke will therefore produce a higher crankshaft speed.  Simple geometry.  This gives the 250F a natural ability to spin up revs that even a 4-stroke with a 66 x 77 bore /stroke would never have, just to begin with.  

 

The second reason is that two-strokes, at least normally aspirated ones, are tuned by timing pressure waves in the exhaust system to prevent the engine from blowing fuel and fresh air out the exhaust port before the port closes.  The pressure wave is timed to get back to the port just as it closes, and stuffs the escaping fuel back into the chamber where it belongs.  Very effective, but also restricted to a narrow range of speed, since the pressure wave will always move at a nearly constant speed, and the timing of it is controlled by the length of the pipe, which is almost always a fixed dimension.  

 

Agreed   I will not pretend to understand the math but I beleive I understand the concept.  basicly since a four stroke does its suck squeeze bang blow  once every 2 revolutions compared to a two stroke suck squeeze bang blow every single revolution there is twive as much time for each event to happen.   Of identical bore and stroke  you could probably make a 2stroke  spins just as fast as a four stroke  but with less time for things to happen makes the powerband narrow.. too narrow  for an average transmission and a human too operate.

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Wonderful;!!!  These were the explanations that I was hoping to get, thank you.

 

A quick couple nieve questions:  So is it understood that the two stroke engine design needs that much stroke designed into it in order to properly execute the needed amount of intake and exhaust gas needs?  In other words we could not make a shorter stroke two stroke with a larger bore?

 

I assume the long skirted pistons used in two strokes are due to the port delivery of the gasses through the cylinder and the skirts cannot be done away with since they are needed to cover the ports.  Is this correct?  If so, we will always be limited by piston weight (relative to 4s) on the two stroke due to the needed skirts, correct?

 

 

So as I understand it the expansion chamber is a prime operating area RPM that it is designed for, once outside that RPM area it is inefficient and actually causes the power to plummet.  So without auto changeable/tunable dimensioned exhaust chambers we will be limited by their design.  And therefore limited to an RPM level.  Is this correct?

 

For me the reason I think the 4s work so efficiently compared to the 2s is that they have a much broader RPM level in which to operate and therefore have much more area under the curve on a dyno chart compared to 2s.  Would having a larger bore (bigger than 300cc) that has enough torque out of the powerband, combined with a high RPM pipe not give us a lot more area under the curve to work with than standard configurations we currently have?  Can a two stroke function reliably spinning to say 10,000 RPM, if a pipe was designed to operate there?

 

I love the 2s simplicity, reliability, and easy maintenance as well as top end replacement.  However I am now riding 4s because of their wider operating rpm powerbands.  There are obviously design reasons on why the two stroke cannot keep pace in regards to efficient power delivery and I wanted to understand them.  Thanks to the info discussed in this thread the light is everso slowly turning on for me, thank you.

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 So is it understood that the two stroke engine design needs that much stroke designed into it in order to properly execute the needed amount of intake and exhaust gas needs?  In other words we could not make a shorter stroke two stroke with a larger bore?

 

One could probably build a functional, even relatively high revving 2T with a more square, or even over square, bore/stroke ratio, but the penalty for shortening the stroke would be weak performance at low speeds, difficulty in starting, stuff like that. Probably also an even narrower power curve. 

 

I assume the long skirted pistons used in two strokes are due to the port delivery of the gasses through the cylinder and the skirts cannot be done away with since they are needed to cover the ports.  Is this correct?  If so, we will always be limited by piston weight (relative to 4s) on the two stroke due to the needed skirts, correct?

 

The biggest reasons that the piston has to be that long do relate to the ports, and there are two: First, as you say, the piston must cover the ports that need to be closed the whole time that they need to be closed.  With reeds, the intake isn't an issue any more, but you don't want the exhaust port peeking into the crankcase at TDC, either.  

 

The second reason that 2T pistons need the length is stabilization.  A low-drag "slipper" type such as used in modern racing 4T's would get tripped up crossing the port windows and tilt out of square with the bore, probably with disastrous results.

 

So as I understand it the expansion chamber is a prime operating area RPM that it is designed for, once outside that RPM area it is inefficient and actually causes the power to plummet.  So without auto changeable/tunable dimensioned exhaust chambers we will be limited by their design.  And therefore limited to an RPM level.  Is this correct?

 

Yes.  At speeds lower than the tuned speed range, the reflected pressure wave gets to the port too soon, missing the opportunity to return the fuel/air overshoot back to the combustion chamber.  In fact, it can interfere with clearing the exhaust from the cylinder.  At speeds above the range, it simply gets there too late.  Expansion chambers are de facto superchargers in the sense that they can end up having stuffed more fuel and air into the cylinder than would have flowed there under atmospheric pressure, but their effect is limited to that narrow RPM range 

 

 

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Really?

I don't advise staying with that one. A YZ250F from Harris Performance (much wider than a 2T power zone, also):

Comparing a built 250 4t versus a stock 2t is not a relevant comparison dollar for dollar the 2t wins . Nowadays 4ts have gotten a - 10-15 years worth of r&d 2ts havent and coming out of the factory the 250 2t makes more power still and is cheaper.

2ts achieved a negative emissions image so there was a switch to more "environmentally friendly 4ts" in an evironentally irrelevant segment otherwise there would have been no push for high hp 4t mx bikes other than in the enduro sector possibly

What's done is done and eventually 4ts will exceed 2ts In power but it's not because they're better for the task fundamentally

500cc moto gp bikes were making 370 hp/liter that's tough to do with for a 4t even with forced induction today and this is 20 years ago

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A 250 2t comprised of a 2x125cc platform would punch out 70whp with ease.  Would love to see a thumper pull that off.

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