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#4331598 - 02/16/17 11:33 PM The best explanation on how a 2-cycle engine works
Merkava_4 Offline


Registered: 01/30/07
Posts: 15387
Loc: Clovis, CA
THIS is it!

I've always wanted to understand how they work, but until now, never had the right explanation.

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#4331615 - 02/17/17 12:39 AM Re: The best explanation on how a 2-cycle engine works [Re: Merkava_4]
Ducked Offline


Registered: 10/25/12
Posts: 1957
Loc: Taiwan
I think they're like the Bumble Bee is supposed to be. Impossible.


Edited by Ducked (02/17/17 12:39 AM)

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#4331650 - 02/17/17 05:00 AM Re: The best explanation on how a 2-cycle engine works [Re: Merkava_4]
ZeeOSix Offline


Registered: 07/22/10
Posts: 13812
Loc: PNW
This is a pretty good explanation.

https://youtu.be/hV3LImCslpo?t=45s

Next engine to ponder is the Wankel.

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#4331664 - 02/17/17 05:42 AM Re: The best explanation on how a 2-cycle engine works [Re: Merkava_4]
Panzerman Offline


Registered: 12/16/06
Posts: 3392
Loc: Port Orange, Florida
Truly a light power producing motor. Stupid simple. Produces about 1.5 times the power of the same size four stroke and will run at any angle. I love em, unfortunately environmentalists/ EPA doesn't and they really don't like people like me that encase themselves in smoke. I once argued with a shop teacher in high school because he said two stroke longetivety is shorter than a four stroke. I disagree. Run good oil, at a rich ratio and a two stroke will run as long as a four stroke in the same service.
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#4331698 - 02/17/17 07:00 AM Re: The best explanation on how a 2-cycle engine works [Re: Panzerman]
JetStar Offline


Registered: 10/02/08
Posts: 1467
Loc: Flatlands of Indiana
And they really make a lot more power with one of these;
https://en.wikipedia.org/wiki/Expansion_chamber
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#4331731 - 02/17/17 07:48 AM Re: The best explanation on how a 2-cycle engine works [Re: Merkava_4]
RedOakRanch Offline


Registered: 11/10/13
Posts: 613
Loc: Central Coast California
I have always loved the simplicity. I think of them as a pressurized crank area on the down stroke (piston becomes a super charger), then from the bottom of the cylinder the fresh mix pushes out the exhaust. Rinse and repeat.
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#4331792 - 02/17/17 09:11 AM Re: The best explanation on how a 2-cycle engine works [Re: Merkava_4]
CT8 Offline


Registered: 10/09/14
Posts: 8096
Loc: Moving,not there yet.
I miss the tow stroke motorcycles.

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#4331801 - 02/17/17 09:17 AM Re: The best explanation on how a 2-cycle engine works [Re: CT8]
HerrStig Offline


Registered: 08/24/11
Posts: 8795
Loc: Boston, MA
Originally Posted By: CT8
I miss the tow stroke motorcycles.
AKA "crop dusters".

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#4331803 - 02/17/17 09:19 AM Re: The best explanation on how a 2-cycle engine works [Re: Merkava_4]
HerrStig Offline


Registered: 08/24/11
Posts: 8795
Loc: Boston, MA
Even with a fancy exhaust and reed valves instead of ports there isn't a lot of low end torque to be had.

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#4331807 - 02/17/17 09:21 AM Re: The best explanation on how a 2-cycle engine works [Re: Merkava_4]
beanoil Offline


Registered: 10/17/04
Posts: 2415
Loc: Midwest, Illinois
Love the 2 stroke. Light, instant power, very tune-able with exhaust, reed tension, port size and position in the cylinder. And simple. When my boy raced MX, if needed, we could install the spare piston, rings, and cylinder between races. Tuning for short straights and supercross style jumps was done with different pipes and silencer lengths, and race fuels. EPA killed them because of the unburnt hydrocarbon emissions.
Lots more info in Two Stroke Tuning by A. Graham Bell.
https://www.amazon.com/Two-Stroke-Performance-Tuning-Bell/dp/1859606199
One of my best memories is a long ride on a Suzuki GT380, an in line 3 cylinder two stroke.


Edited by beanoil (02/17/17 09:24 AM)
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#4331839 - 02/17/17 09:48 AM Re: The best explanation on how a 2-cycle engine works [Re: HerrStig]
Cujet Offline


Registered: 02/15/03
Posts: 5860
Loc: Jupiter, Florida
Originally Posted By: HerrStig
Even with a fancy exhaust and reed valves instead of ports there isn't a lot of low end torque to be had.


You might be surprised at the dyno curves of big bore 2 stroke dirt bikes. 500cc two strokes make a bunch more low end than a comparable 4 stroke. It's a mistake to think that a well designed 2 stroke engine has less low end torque. What really happens is that 125cc two strokes produce almost as much peak power as 250cc 4 strokes. When comparing the two, the 125 falls very short on low end.

Better to compare by displacement.




Edited by Cujet (02/17/17 09:50 AM)
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#4331862 - 02/17/17 10:12 AM Re: The best explanation on how a 2-cycle engine works [Re: HerrStig]
IndyIan Offline


Registered: 09/23/08
Posts: 7874
Loc: Ontario, Canada
Originally Posted By: HerrStig
Even with a fancy exhaust and reed valves instead of ports there isn't a lot of low end torque to be had.

Normally they aren't tuned for low end torque, but could be. I think also the fact that 2 stokes fire twice as often tends to make people think they are revving higher than they are, compared to 4 stroke.
I used to ride an '84 IT200 (2T) and my buddy had an XR200 (4T) and even off idle my bike was quicker, got a much quicker once on the pipe. The XR would rev out more though, and in the woods the XR had nice and smooth power.
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#4331891 - 02/17/17 10:51 AM Re: The best explanation on how a 2-cycle engine works [Re: Merkava_4]
DoubleWasp Offline


Registered: 05/21/12
Posts: 3974
Loc: Fort Lauderdale, FL
I love 2-stroke outboards. A lot of 4 strokes are just a mess. Lot less to go wrong on a 2.
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#4331912 - 02/17/17 11:19 AM Re: The best explanation on how a 2-cycle engine works [Re: DoubleWasp]
SteveSRT8 Offline


Registered: 10/10/08
Posts: 18105
Loc: Sunny Florida
Originally Posted By: DoubleWasp
I love 2-stroke outboards. A lot of 4 strokes are just a mess. Lot less to go wrong on a 2.


I don't care what the peanut gallery says, my 2 stroke outboards came out of the hole much quicker than comparable 4 strokers do. My old Polaris jetski would absolutely throw you off if you weren't pretty strong and holding on well when we took off.

Love me a couple strokes...
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#4332044 - 02/17/17 01:32 PM Re: The best explanation on how a 2-cycle engine works [Re: Merkava_4]
Kawiguy454 Offline


Registered: 06/01/15
Posts: 503
Loc: Arizona
I had 2 Yamaha RD350's that would scream on the street a Yamaha IT465 enduro that had boatloads of torque. Yes a 2stroke can be made for low end torque as the IT465 was exactly that. All the magic happens in the exhaust as the expansion chamber is tuned to use the moving hot gasses volume to PULL in the fresh fuel charge. The misconception of peaky horsepower IMO came from dirt bikes that were always run on the ragged edge around a track.

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