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A Two-Stroke Intake Primer: Pistons & Reed Valves & Discs, Oh My!

Tanshanomi July 26, 2011 Two-Wheel Tuesday 45 Comments

A traditional 2-stroke with piston-port induction has some geometric limitations that rotary valves overcome.

Many people characterize two-stroke engines as not having valves, but that’s not true. Every two-stroke engine actually employs three different valve functions, but the valve mechanisms are easy to overlook because they do double duty, such as also being part of the piston. And the various methods of configuring these valves is a fascinating lesson in engineering problem-solving.

The 2-Stroke Combustion Chamber

In a traditional two-stroke engine, gasses enter and exit the combustion chamber through ports cut into the chamber wall. As the piston goes up and down, it covers and uncovers these cut-outs, opening and closing the ports. So the crown of the piston is, itself, the intake and exhaust valve. This means that the engine can only perform the sucking part of the suck-squeeze-bang-blow process during the lower portion of the piston’s downward travel (unlike a four-stroke, which draws a deep breath throughout the downward stroke of the pistion). A two-stroke needs to fill the cylinder with unburned oxygen and fuel as rapidly as possible once the piston descends far enough to uncover the transfer ports (the combustion chamber’s intake ports are actually called “transfer ports,” for reasons that will be clear to you a little later). Combustion is still occurring in the cylinder at this point, so the gasses in there are still expanding and creating positive pressure within the chamber — right where the fresh charge needs to go — exactly the opposite of the intake suction we need to do the taking-in. That means we need some method of pressurizing the incoming air-fuel mixture. That pressure needs to be pretty significant, because The incoming gasses must have enough pressure to not only overcome the positive pressure in the combustion chamber, but actually help push those still-expanding hot gases out the exhaust port in a timely manner.

The 2-Stroke’s Secret “Supercharger”

What pressurizes the incoming air-fuel mixture? Two strokes not only cleverly use the sides of the piston as valves, they use the descending piston as a supercharger, pressurizing the crankcase below it as it descends. First, during the upstroke of the piston, vacuum created by the piston draws air through the carburetor, into the crankcase. Then, when the piston starts descending, bingo, positive pressure right when you need it. The trapped gasses are compressed until the piston drops below the tops of the transfer ports, at which time they rush upstairs to be compressed again and ignited. (Using the crankcase as a part of the intake tract is one reason why traditional 2-strokes are an emissions nightmare; the crank bearings need lubricating oil, which ends up mixed with the air and fuel as they all head up into the combustion chamber to be burned.)

“But wait!” I hear you say, “You said the intake mixture gets trapped — how? If the crankcase is open to the carb throat, what stops the gasses from just getting pushed back out into the atmosphere as the piston drops?”

Ah, that is the third valve function in a two-stroke engine, and the specific topic of this post.

This Bultaco roadracer was converted from piston-port to rotary-valve induction to increase power.

Piston Port Induction

If you look at the animation above, you can see the earliest and simplest solution: the bottom of the piston skirt covers and uncovers yet another cut-out in the cylinder wall. This one is low enough that the top of the piston is still above it at the bottom of its stroke. But, as it rises, the bottom edge of the piston skirt slides away from this port, opening the crankcase to the atmosphere just as the pressure in our crankcase-cum-supercharger goes negative. Sucking commences. When the piston changes direction and heads back down, the piston skirt slides back over the port, trapping the the air/fuel mass in there to be pressurized. Voilà! We now have one chunk of cylindrical metal doing five different jobs simultaneously as it moves up and down: exhaust valve, cylinder intake valve, crankcase intake valve, supercharger compressor and, of course, transferring combustion force to the crankshaft through the connecting rod. Brilliant!

But there are two big limitations with this piston-ported crankcase: the first is that the piston must rise partially before it can draw in the fresh charge, and it must drop past the port again before generating any meaningful crankcase pressure. Secondly, intake timing is inherently symmetrical. That is, the piston must close the intake port the same number of degrees after top-dead-center as it open it before TDC.

Reed Valve Induction

So engineers decided that adding some other discrete crankcase valve was needed. One option was passive — the reed valve. Like any other one-way check valve, the reed valve is easy to understand: when there’s negative pressure on the proper side, it opens. Positive pressure from that direction forces it shut. It’s simple and has the flexibility of being locatable anywhere on the crankcase. It also allows our crankcase supercharger to “breath in” at least a little bit throughout the entire upward stroke, and at least begin to help pressurize the charge right from the moment the piston starts descending. However, since it is passive, it still is pretty tied to piston travel, and thus symmetrical.

Rotary Valve Induction

That brings us to the third type of intake valve. When some Hoon says “rotary,” you probably think of the Wankel and perhaps Mazda’s “Ours goes Hmmmmmm…” ads. But that’s something totally unrelated. If you have an interest in things two-stroke, rotary means rotary intake valves, aka disc valves, a very effective way to get more power out of the two stroke cycle. A rotary valve is a sort of rotating shutter that is positioned against the crankcase intake port. On motorcycles, this is typically a thin metal or composite disc, mounted to one side of the crankshaft. On smaller engines, a gear driven cylindrical mechanism or even one of the crankshaft flywheels is used as the valve shutter. The valve opens and shuts as a window in this shutter passes in front of the port.

Obviously, you can configure this cut-out in such a way as to open and shut it at any point in the crankshaft’s rotation, allowing even asymmetrical timing for the most accurate fine tuning of the intake. Compared to reed valves and even more-so compared to piston-ports, rotary-valve engines can produce more total power. More importantly, rotary 2-strokes can be tuned to generate that power over a wider rev range, or (in the case of racing motorcycles and constant-speed stationary engines), positively stratospheric maximum output in exchange for a peaky, knife-edged powerband.

For motorcycle use, not everybody thought the added combustion efficiency of a separate crankcase valving mechanism was worth compromising the elegantly simple manufacturing and maintenance efficiency of the piston-port design, and some piston-port engines are still made today. But reed valves and rotary valves have made it clear that you can ask a piston to handle too many functions.

[Image sources: hf1technical.net, ebay.com, wheelsofitaly.com]

  • http://www.muthalovin.com muthalovin


    I did my first wheelie on a 2-stroke. My dad had a NSR250 (that I managed to lowside in California, and we repainted it in Repsol livery). It was like a slingshot, and I will never forget the 2-stoke power delivery. I loved that bike.

    <img src="http://p1.bikepics.com/pics/2004%5C08%5C23%5Cbikepics-200388-320.jpg"&gt;

    It was like ^^^ only less Rossi replica.

    • http://hooniverse.com/ Tanshanomi

      For the record, it had reed valves.

  • sportwagon

    Ten or so years ago, I was lucky enough to see a multi-cylinder two-stroke Japanese bike rip-snorting its way around the outer Richmond District in San Francisco. I remember it had four exhaust pipes (that belched blue smoke) and '80-style sporty plastic. It made an incredible earth-shredding sound. I can't remember who made it or what it was, but I know it was special. Any suggestions on what it could have been?

  • http://www.Hooniverse.com Jeff_Glucker

    Is that lead image .gif the equivalent of car porn DP?

  • Joe Dunlap

    Or, you can do it the way GM did it with their 2 stroke diesel locomotives. Just bolt on a great whacking Rootes supercharger that blows every air molecule with in a 500 ft. radius through a given cylinder on every downstroke. There, I fixed it for ya. :-)

    • sidecar57

      I think that the two stroke will be reinvented for motorcycles one day using this method.No more lubricating oil being burnt has got to help it's cause I'd imagine.

      • joe_bloe

        This was the plan Orbital Engine was working on in the '80's – externally scavenged 2-strokes. It was going to revolutionize small engines. Never came to fruition.

  • craigsu

    Hmm, so where does the Saab 2-stroke engine fit into this scheme? A Saab 96 is on my short list of desired vintage cars and I'm trying to decide between a 2-stroke (pre-1968) and a V-4 (1968 on) engine.


      They are all piston-port engines, although a few people have made aftermarket kits for adding reed valves, mostly for racing purposes. I've owned both 'stroker and V4 SAABs and prefer the former, but if you get one you should expect to become familiar with rebuilding the engine as a regular aspect of its maintenance schedule. My two current sedans are 'strokers, one with a premix engine and the other with an oil injection engine, both stock.

      SAAB made a few 'stroker cars during 1968, too, during the transition to the V4. These have smaller "Shrike" engines (820cc instead of 850cc) to take advantage of an exemption from US emissions laws. The company sold the Shrikes with a 100% guarantee on engine parts for the lifetime of the original owner, which was still being honored the last time I heard. Hard to say what'll happen in the near future, but that doesn't really matter for anyone who didn't buy one new in '68 anyway.

      • craigsu

        I've heard that some 96 owners carry an extra engine in the trunk on long road trips just in case. It's so light and small it can be handled by one person.

        Thanks for the info. I didn't realize the transitional 2-strokes for 1968 were a little smaller in displacement. I must admit I've wondered how difficult it would be to perform a Hayabusa transplant on one.

        Just wondering, what is it about the stroker that makes you prefer it over the V4?


          Oh, that's easy. I prefer it because I am completely and utterly insane.

          • http://killerbeeracing.blogspot.com/ ZomBee Racer



              What? That still doesn't settle the question of whether, of the two of us, you're the crazy one.


          To expand upon my previous answer: The Taunus V4 is a perfectly fine engine, albeit with a few quirks such as a balance shaft with problematic bearings and a set of easily worn/destroyed fiber gears for timing, but overall it is quite robust and serviceable and is straightforward to hop-up if one is so inclined. The Mighty Three, on the other hand, is a device for producing incomparably wonderful sounds and smoke, and usually even will do an adequate job of propelling the car, until something inevitably goes wrong. I prefer the overall experience of the latter, but won't begrudge anyone the former. Either way, I've never seen much point in swapping an altogether different engine into a 95/96/97. There are those who have experimented with the Cologne V6, various Mazda rotaries, and even more exotic choices, however, so you should be able to find some advice on this out there somewhere.

          • Alff

            "Taunus V4" as in Ford Taunus V4 from the Cologne plant?


              Yup. That's why the V6 is such a tempting bolt-in swap, despite the fact that it makes the car even more nose-heavy and doesn't do any favors to the already overtaxed 'stroker-era transmission. That's also why V4 owners in the US can still get some parts through Ford industrial channels (sometimes even at the local feed-and-seed) because it was used over here as a stationary engine for a few applications.

          • suju89

            +1… I mean the best of SAABs 3 cylinder two-strokse was pushing 100bhp/per litre specific output.


            That sounds like the very reason that I loved my Suzuki GT550 two stroke triple. That bike sounded like a bunch of angry bees, but ran like hell everywhere I rode it. Of course, I kept the oil reservoir filled, and the oil pump cable that was attached to the throttle was always adjusted to factory settings, and the drive chain was adjusted regularly, and I knew how to synchronize the three Keihin carburetors. That was a nice little bike, and the two stroke triple didn't produce much smoke unless I was riding around town for a week or so, and then entered a freeway on ramp. Once the pipes would heat up, that bike would send up a smoke screen that would kill mosquitos and Escalades for like a half mile behind it. Great stuff.

    • http://hooniverse.com/ Tanshanomi

      Saab 2-strokes were piston-ports.

      Interestingly, Saab used a scavenging system called "loop charging" that used directional ports to manage the incoming charge, but didn't use an expansion chamber exhaust that really made loop charging effective. An expansion chamber exhaust system makes loop charging more efficient by reflecting the exhaust pressure wave back toward the exhaust port from the other direction, using residual exhaust energy to "slam the door" on the escaping gas prior to the closing of the exhaust port. The upside of not using a tuned exhaust is that you can fab up any sort of random exhaust pipe to a Saab without ruining what power was there.

      Many earlier designs (including the majority of vintage 2-stroke outboards) used cross-charged scavenging. Asymmetrical, dished piston crowns were designed to divert the incoming fresh charge long enough to keep it from heading out the exhaust along with the burned gas from the prior stroke. It was far inferior to a tuned expansion chamber, but it worked better than loop charging without a tuned exhaust, á la Saabs.

      • http://hooniverse.com/ Tanshanomi

        Edited that last response extensively, had my terms backward. It's been a long day; time to go home.


          You're much closer after the editing, but SAAB engines do rely on tuned expansion chambers. They're perhaps not the most efficient design (and there are people who have done wonders in offering improvements), but they do require more than "any sort of random exhaust pipe" to achieve even the factory levels of performance.

          • http://hooniverse.com/ Tanshanomi

            All my knowledge of Saab 2-stokes is randomly passed-on bits of 3rd-hand knowledge, so consider me schooled.

  • brazilreporter

    As important the intake valve may be, don't underestimate the supercharge function of a well dimensioned expantion chamber exhaust. The biggest boosts are done with more or bigger cilinder porting a bigger carb and an exhaust dimensioned for what RPM you want to run. The recepy was good to get my Kreidler doing 12 to 13 hp at 14.000 RPM

  • Joe Dunlap

    If youre gonna do a 2 stroke Saab, be advised that the connecting rods have no caps. they are one piece. Guess how they get them on the crankshaft. Ohhh yeahhhh. :-)

    • BlackIce_GTS

      I'm guessing the process is something like this?
      [youtube wT2abf3tByE http://www.youtube.com/watch?v=wT2abf3tByE youtube]

    • http://hooniverse.com/ Tanshanomi

      I had a blown-up Kawasaki S1 250 Triple a long time ago, and discovered that they no longer sold crank assemblies for it. I was working at a Kaw deaker, so I did some digging through the parts books and learned that I could order suitable crank pins, bearings and rods from another single pot trail bike that used them. I pressed the crank apart and then back together using a 10-ton press…then spent several maddening days trying to get all the pieces aligned. Once I had it as close as I was going to get it, I put the motor back together and sold it cheap before it blew up again (I was honest about the dubious crank rebuild.)

  • fede6882

    Great post! Very enlightening, before this I really didn't knew much about the 2-strokes, other than the piston-port thing and the existence of something called rotary valves. I hope this is a kind of comeback of the don't be afraid to ask questions posts

    • skitter

      Don't ever be afraid to ask questions.
      Some of the answers might be frightening, but that's the nature of the hoon.

  • buzzboy7

    I prefer GM's Detroit Diesels which cheat, using the piston as the intake valve and a HUGE(automotively) honkin' Turbo blowing through that MASSIVE(automotivley) Rootes Group supercharger. They then have a standard exhaust valve in the head to feed said turbo. They sound great and love revving upwards of 3500rpms!

  • McQueen

    Here is a fun fact 2 Stroke motors provide 1.7 x's the horse power of a 4 Stroke . If there was an easy way to recycle the oil used for lubricating the bottom end bearings , instead of burning it , I'm sure we would see alot more 2 Stokes in use . If you want to be amazed take a look a the HP figures on the newer snowmobiles , manufacturers are getting 160+ HP out of 800cc engines . Not all piston port engines had there carbs feeding the crankcase solely , many were ported so that the crown of the piston was exposed and as the expanding exhaust gasses were being pushed out , the intake port drew
    directly from the carburetor .

    • Capt. Klutz

      Where does the 1.7 figure come from? There are so many different two-stroke designs that have such varying outputs for a given displacement, and with different dyno curves, that I think it would be hard to slap any single figure on it. And the same is true for four-strokes.

    • http://hooniverse.com/ Tanshanomi

      When you say "2 Stroke motors provide 1.7 x's the horse power of a 4 Stroke", are you saying on average? And are you comparing them based on displacement, number of cylinders…what? Because as we've just been discussing, there are a lot of variations in power even among individual configurations, whether they're two stokes and four strokes. Yamaha's 2-stroke RD250LC made about the same 40 peak horsepower as Honda's CBR250 four, while a 2-stroke Montesa 250 makes well under 30. It's impossible to make a blanket statement like that.

      • McQueen

        Yes sorry I should have said of equivalent displacement the reason this happens is because on a 2 Stroke there is no " wasted " stroke like a 4 Stroke engine , another reason is the frictional loss due to valves , cams , litters , and timing apparatus for cams robs alot of power . That's why if you remember back when 4 Stroke bikes started to race Moto Cross they were 400cc vs 250 and such .

      • McQueen

        And that could also be the way those examples that you have provided are tuned , I'll have do do some digging but I know it's in one of my school books from years ago . I'm not blowing smoke up your ass , ive been working in the small engine industry for over 15 years and have gone to school for 6 just to achieve the certificates I have hanging on my wall ( Mercury Marine of Canada , Polaris Industries , Honda power equipment of Canada , just to name a couple ) the reason 4 Strokes have a greater appeal is because of their broad " power band " which starts out at idle and works it's way to top RPM 2 Strokes on the other hand can usually only perform from mid rev range to WFO which makes them more difficult to tune .

  • SSurfer321

    Am I the only one that stared, mesmerized at the opening .gif for 10 minutes before following the jump?

  • RKC

    Many of the small early inboard marine engines were piston port 2 strokes. They were were called make-and-break's for their ignitions, where instead of a high voltage coil, points, etc, they used a purely mechanical connection where low voltage electricity ran across points (the "make" part of the name) that at the right moment "broke" open and had a spark to ignite the charge. The advantage in a marine environment is they can wet and still work. With only 3 moving parts, a piston port is extremely simple (think about that, only the piston, rod and crank move) and so robust with early technology, and also had the fun feature that such an engine runs equally well in either direction so most of the early versions had no reverse. The operator would kill the engine and restart in the opposite direction. If you were good with the timing, you could catch it at the right moment and it would fire right over the other way. To manage both intake and exhaust at the same time, the piston typically had a large fin on top between the intake and exhaust ports to make the frest charge travel the farthest and not straight out of the engine unburned.

    • McQueen

      That is how Ski-Doo and Polaris snowmobiles reverse is set up , it's just another trigger coil mounted a few degrees off the main trigger coil that is used when you push the reverse button and the ECU switches over to it ( you also need a double sided Helix in your secondary clutch for this )

  • bill

    I like the idea of these, not quite for two stroke, but still: http://www.coatesengine.com/uploads/7/1/4/1/71411

  • http:///user/view/468038 Learn about any Australian Accent

    Interesting post. Were did you bought everything from.

  • Tom

    Can someone help me get my head around piston ported induction?

    It seems to me the intake port is open while the piston is rising for just as long as it is open when the piston is falling. So why does the mixture stay in the engine??

    • http://hooniverse.com/ Tanshanomi

      Because the transfer ports are blocked on the upstroke, the rising piston creates negative pressure in the crankcase. Air/fuel mixture comes rushing in when the intake port opens and continues to fill the vacuum even as the piston starts descending. Ideally, the pressure in the crankcase equalizes just as the intake port closes, and positive pressure builds as the piston continues to descend. At the bottom of the stroke, the pressure is relieved when the transfer ports open. As the piston rises, the transfer ports shut and the intake suction begins again.

      • Rod

        Ski-doo 500, 583, & 670 rotary valve engines were incredible and got many miles compared to the new reed motors!


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