lawnduster
Bronze Member
How does the power band work in two stroke engines
Two stroke
- Thread starter lawnduster
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hobbyfarm
Veteran Member
They tend to go up and up steadliy making more power until they fall off "the pipe" and then drop off suddenly.
Sometimes power can ramp up (read - double or triple) very fast over a couple-thousand rpm.
<font color="blue">This guy</font> has some fair graphs on his website.
<font color="brown">This cat</font> has a cool animation of a running two-stroke, but no graphs.
These websites refer to motorcycles or personal watercraft but they all work similarly, especially the part of losing their power radiply after peaking. Torque is also poor on a two-stroke engine.
Sometimes power can ramp up (read - double or triple) very fast over a couple-thousand rpm.
<font color="blue">This guy</font> has some fair graphs on his website.
<font color="brown">This cat</font> has a cool animation of a running two-stroke, but no graphs.
These websites refer to motorcycles or personal watercraft but they all work similarly, especially the part of losing their power radiply after peaking. Torque is also poor on a two-stroke engine.
Indydirtfarmer
Platinum Member
Just so we're clear, the "power band" isn't anything physical. It is the RPM range in which the engine (2-stroke OR 4-stroke) produces it's best power output. On a typical 2-stroke that band or range is usually narrow. Back when I raced dirt bikes, some older 2-strokes had a "light-switch" power band. It was off, then all the way on, then off again. You'de get a blast of power, then nothing...
Modern 2-strokes are have advantages over older models. They use things such as a variable intake port to keep fuel /air velocity at it's optimum throughout the RPM range.
The worst case I ever saw was an old Maico 450 (dirt bike) I rode in the mid-70's. It was dead flat until the RPM's reached a given point. Then you'd swear a frieght train hit you from behind. MEGA power all at once. It produced such violent power, I never found the "upper hook" where it ran out of steam....
Modern 2-strokes are have advantages over older models. They use things such as a variable intake port to keep fuel /air velocity at it's optimum throughout the RPM range.
The worst case I ever saw was an old Maico 450 (dirt bike) I rode in the mid-70's. It was dead flat until the RPM's reached a given point. Then you'd swear a frieght train hit you from behind. MEGA power all at once. It produced such violent power, I never found the "upper hook" where it ran out of steam....
Bird
Rest in Peace
I never raced dirt bikes, but in 1968, I bought a new two-stroke, 305cc Yamaha, put windshield, plastic saddle bags, and crash bars on it from J.C. Whitney and drove it back and forth to work (it cost money to park a car in downtown Dallas, but I could park the bike free). I also played with it off road quite a bit. One of the odd things to me is the difference in sound between two and four stroke engines and the RPM differences. I had a friend who had a four stroke 305 Honda and the Yamaha, with its two-stroke engine sounded as if it was turning a lot higher RPM than the Honda, when in fact the Honda was turning higher RPM at the same speed. We never actually raced the bikes, but just riding together, and riding each other's bikes, the difference in performance didn't seem to be very big.
SkyPup
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It works by the spark plug detonating a fuel charge every single 360 degree rotation of the crank instead of every other.
Jagmandave
Veteran Member
Waaaaaayyyyyyy back in 66, I had a Suzuki X-6 Hustler, a 2 cyl 250cc 2 stroke with I believe the first 6 speed gearbox on a street bike. The first time I rode it, being used to 4 strokes, I thought it was an abosolute dog. After a few days I got brave enough to open it up, it hit 3500 rpm and jerked the handlebars completely out of my hands! It was redlined at 9000, but started running out of torque at about 7500. In fact, it had a higher top speed in 5th gear than 6th, cause it didn't have the torque to pull it in 6th...........it was fun tho! Still, it didn't seem to be much (if any) faster than my buddy's 250cc Benelli single 4 stroke, but it sure came on hard when it hit the powerband!.............
Highbeam
Super Member
Some motorcycles, Kawasaki's KX line for sure, had a power valve that opened up the exhaust port extra big to induce the lightswitch "powerband" feeling.
Why the exhaust header pipes are shaped with that bulge is beyond me. My chainsaw doesn't have that big header pipe but has a noticebabley narrow powerband.
Why the exhaust header pipes are shaped with that bulge is beyond me. My chainsaw doesn't have that big header pipe but has a noticebabley narrow powerband.
Indydirtfarmer
Platinum Member
</font><font color="blue" class="small">( Some motorcycles, Kawasaki's KX line for sure, had a power valve that opened up the exhaust port extra big to induce the lightswitch "powerband" feeling.
Why the exhaust header pipes are shaped with that bulge is beyond me. My chainsaw doesn't have that big header pipe but has a noticebabley narrow powerband. )</font>
Honda developed the "Honda Power Port" in the mid '80's. It was a couple "slides" on the upper side of the exhaust port. It served a two-fold purpose. By enlarging the port, it gave better breathing ay high RPM's. Closing the exhaust port down gave it better loe end torque. Also, when the slides opened, it effectively "raised" the exhaust port (in relation to the piston. That gives more high RPM horsepower. The 1986 Honda CR 250 had a motor that was "electric smooth" compairedto 250 moto-crossers prior to that. We would add wieghts to the flywheel to tone them down and add MORE low-end grunt.
The reason your chain saw DOESN'T have an expansion chamber and most 2-stroke dirt bikes DO is your chain saw doesn't need such a wide range of power...... It doesn't accelerate. You generally use it at an almost constant speed. The "reversion" (I believe that's what it's called) caused by the expansion chamber generates increased air/exhaust flow FROM the motor as needed at various RPM's
Why the exhaust header pipes are shaped with that bulge is beyond me. My chainsaw doesn't have that big header pipe but has a noticebabley narrow powerband. )</font>
Honda developed the "Honda Power Port" in the mid '80's. It was a couple "slides" on the upper side of the exhaust port. It served a two-fold purpose. By enlarging the port, it gave better breathing ay high RPM's. Closing the exhaust port down gave it better loe end torque. Also, when the slides opened, it effectively "raised" the exhaust port (in relation to the piston. That gives more high RPM horsepower. The 1986 Honda CR 250 had a motor that was "electric smooth" compairedto 250 moto-crossers prior to that. We would add wieghts to the flywheel to tone them down and add MORE low-end grunt.
The reason your chain saw DOESN'T have an expansion chamber and most 2-stroke dirt bikes DO is your chain saw doesn't need such a wide range of power...... It doesn't accelerate. You generally use it at an almost constant speed. The "reversion" (I believe that's what it's called) caused by the expansion chamber generates increased air/exhaust flow FROM the motor as needed at various RPM's
Highbeam
Super Member
Honda will discontinue the CR250 after this model year. Pretty crazy. 4 strokes are where it's at.
Unfortunately they also discontinued my current bike, the infamous XR400.
Unfortunately they also discontinued my current bike, the infamous XR400.
zio
Member
</font><font color="blue" class="small">(
The reason your chain saw DOESN'T have an expansion chamber and most 2-stroke dirt bikes DO is your chain saw doesn't need such a wide range of power...... It doesn't accelerate. You generally use it at an almost constant speed. The "reversion" (I believe that's what it's called) caused by the expansion chamber generates increased air/exhaust flow FROM the motor as needed at various RPM's )</font>
I'm only regurgitating what I've read elsewhere, so I may have a few details mixed up somewhat. Bear with me.
The shape of the pipe also adds backpressure to aid in scavenging. In the two stroke cycle, not all the charge is completely detonated. Since there are no valves, the piston is what closes the exhaust port. As the charge ignites and forces the piston down, some unburned mixture gets expelled out the exhaust port. The expansion champer creates "waves" of backpressure that forces some of this unburned mixture back into the cylinder.
There's no need for these bulbous expansion chambers on a 4 stroke since the overhead valves completely seal off the chamber for the entire duration of that ignition cycle.
My nephew's little Yamaha PW80 (beginners' 80cc two stroke) doesn't really have much of an expansion chamber. But that's because they want to keep the power down on that bike.
I'd guess chainsaws don't have them just because they'd be too cumbersome. The manufacturers have already managed to get respectable power out of the engines as they are.
The reason your chain saw DOESN'T have an expansion chamber and most 2-stroke dirt bikes DO is your chain saw doesn't need such a wide range of power...... It doesn't accelerate. You generally use it at an almost constant speed. The "reversion" (I believe that's what it's called) caused by the expansion chamber generates increased air/exhaust flow FROM the motor as needed at various RPM's )</font>
I'm only regurgitating what I've read elsewhere, so I may have a few details mixed up somewhat. Bear with me.
The shape of the pipe also adds backpressure to aid in scavenging. In the two stroke cycle, not all the charge is completely detonated. Since there are no valves, the piston is what closes the exhaust port. As the charge ignites and forces the piston down, some unburned mixture gets expelled out the exhaust port. The expansion champer creates "waves" of backpressure that forces some of this unburned mixture back into the cylinder.
There's no need for these bulbous expansion chambers on a 4 stroke since the overhead valves completely seal off the chamber for the entire duration of that ignition cycle.
My nephew's little Yamaha PW80 (beginners' 80cc two stroke) doesn't really have much of an expansion chamber. But that's because they want to keep the power down on that bike.
I'd guess chainsaws don't have them just because they'd be too cumbersome. The manufacturers have already managed to get respectable power out of the engines as they are.
