Every new internal combustion car and truck sold in America today is equipped with a four-stroke engine, as are the vast majority of motorcycles. Today, if you hear the loud, ringing song of a two-stroke, it’s probably because your neighbor has fired up a chainsaw or the local teens are playing with their dirt bikes again (vintage dirt bikes, probably). The two-stroke engine is mostly a piece of history today, but there was a time when some of the coolest developments in engine technology were two-cycle engines. Here’s how two-stroke technology rose to power, and why it’s now largely in the dustbin of history.
History is full of different engine types and even differences within those engines, but what sets a four-stroke and a two-stroke apart is much simpler than you’d think. Some people might talk about cylinder wall ports or scavenging processes, but we can discuss two-strokes and four-strokes even more simply.
During a single full cycle, an internal combustion engine must intake the fuel/air mixture, compress it, ignite it, and allow the exhaust gases created by ignition to escape. As their names imply, a four-stroke engine achieves this with four strokes of the piston, and a two-stroke does it all with just two.
In a four-stroke engine, the piston moves down during the intake cycle (that’s stroke one), pulling in a fresh air-fuel mixture. Then, the valves close and the piston moves up (stroke two), compressing the air-fuel mixture. Once the piston reaches the top of the cylinder, a spark plug ignites the mixture, starting the combustion cycle and sending the piston back down (stroke three). Finally, the piston heads up one more time (stroke four), sending exhaust gases out of the exhaust valve. You only get power every other time the piston reaches the top of the cylinder. Four-cycle diesels work in a similar way, but ignite their fuel through high compression rather than a spark plug.
A Hit With Every Two Strokes
Two-stroke engines combine multiple cycles into the same stroke of the piston. A two-stroke engine combines intake (“scavenging”) and compression into one stroke, then ignites the charge and exhausts it on the next stroke. Unlike a four-stroke and its every-other-stroke power delivery, with a two-stroke you get combustion and thus and power every time the piston reaches the top of the cylinder. The graphics above and below are from Yamaha, a brand once famous for its two-stroke motorcycles.
Critical to this process, at least in the most basic crankcase-aspirated two-strokes, is the operation of the ports. The exhaust port is exposed during the power stroke, and the exhaust begins to exit. As the piston continues down, a fresh air-fuel charge enters the combustion chamber from the crankcase through a transfer port. This charge helps push out the remaining exhaust gases while refilling the combustion chamber.
When the piston begins its upward movement again, the transfer port closes, the exhaust port closes, and the intake port opens. The movement of the piston forces the fresh charge in from the carburetor and into the crankcase, where it will wait for the engine to fire, then to scavenge exhaust gases, and start the process over again. Here’s an animation to help visualize this:
These ports, at least in a very basic two-stroke, aren’t actuated by a valvetrain like the valves in your car. Instead, the movement of the piston is what either closes or opens the ports.
The scavenging process is also one of the two-stroke’s biggest problems. Some unburned fuel is lost during the scavenging process and is sent out through the exhaust, which is bad for the environment. As Cycle World notes, a crankcase-scavenged two-stroke like the one I described above may waste 30 percent more fuel compared to an equivalent four-stroke. The publication correctly points out that such waste would be unacceptable today. However, in decades past, it was seen as a worthwhile trade-off as two-strokes often weighed 40 percent less, had less bulk, and made more than enough power for everything from lawncare equipment to boats.
Here’s an animation of a four-stroke for reference:
Engineers have developed varying ways to increase the power and efficiency of two-stroke engines. Thus, two-stroke designs can often vary wildly. But you’ll notice that two seemingly entirely different designs will still fire each time the piston reaches the top of the cylinder.
As Yamaha correctly notes, the two-stroke engine was favored for decades because firing on every rotation meant that a two-stroke made more power than a four-stroke of the same size. At the same time, two-strokes also responded more quickly. I’ve owned a variety of two-stroke motorcycles over the past seven years, and honestly, Yamaha is actually underselling it. Two-strokes often feel so much more “alive” than an equivalent four-stroke in the same application.
So, if two-stroke engines are so great, what happened to them?
A Simpler, More Efficient Engine
Despite being a much simpler device, the two-stroke engine was actually invented after the introduction of early four-stroke engines. In 1824, French physicist Nicolas Léonard Sadi Carnot proposed the Carnot cycle, a process that education company EBSCO Information Services describes as:
The Carnot cycle is a theoretical model in thermodynamics representing an idealized four-step process that a working substance, such as gas in a piston engine, undergoes to convert heat into work. This cycle consists of two isothermal (constant temperature) and two adiabatic (no heat exchange) stages, culminating in the system returning to its original state. The Carnot cycle serves as a benchmark for the maximum efficiency of real-world heat engines, revealing that no engine operating between two thermal reservoirs can be more efficient than a Carnot engine operating between the same temperatures.
The efficiency of a Carnot engine is calculated by the temperature difference between the hot and cold reservoirs divided by the high temperature, illustrating fundamental principles of thermodynamics, including the second law, which states that entropy in a closed system tends to increase. Although the Carnot cycle is not entirely realistic when applied to real engines, as real-world engines may not achieve isothermal stages, it remains a critical concept for understanding thermodynamic efficiency and has influenced the development of various engine models, including the Rankine and Otto cycles.
Historically, the ideas behind the Carnot cycle were proposed by Nicolas Léonard Sadi Carnot in the 19th century, laying the groundwork for modern thermodynamics and the understanding of energy transfer in various systems. This concept is valuable not only in mechanical applications but also in biological systems, where energy conversion processes can be analyzed similarly.
As noted above, the Carnot cycle laid the groundwork for pioneering work on engines. Many inventors tried and failed to make practical engines out of the Carnot cycle, and the few who succeeded in making a functional engine found them to be noisy, unreliable, inefficient, and sometimes dangerous.
It would take the likes of Nikolaus August Otto in Germany to make the first practical four-stroke engine, in 1876. His name is immortalized in the Otto cycle, which is used today in engines. In 1881, only five years after Otto got his four-stroke working, Scottish inventor Sir Dugald Clerk came into the picture. From Gas Engine Magazine:
Clerk was born in Glasgow, Scotland, on March 31, 1854, the son of Donald Clerk, a machinist. He studied engineering at Anderson College in Glasgow and Yorkshire College of Science in Leeds. A brilliant engineer, he also understood thermodynamics and could calculate engine pressures, temperatures and power. During World War I he was director of engineering research for the admiralty. For this, he was knighted as Sir Dugald. He built gas engines, wrote technical books, and contributed much to the development of the 2-stroke cycle. Many regard him as the father of that engine. He passed away on Nov. 12, 1932, in Ewhurst, Surrey, England.
Clerk, who had already experimented with early engines in the years prior, came up with an idea for a more efficient engine. What if an engine fired on every revolution of the crankshaft? Add in some higher compression figures, and you could achieve much greater efficiency than the other early engines of the late 1800s.
Reportedly, Clerk was a fan of American inventor George Brayton’s “Ready Motor” (above), which was pitched as being better than a steam engine because it could be started immediately without a boiler. The Brayton engine had two cylinders: one was an air pump, and the other was for combustion. Instead of a spark or some other form of intermittent ignition, a pilot flame burned constantly to ignite the fuel/air mixture.
Clerk copied this basic design, but added compression and swapped the pilot light for a spark. Then, he took the idea further. Clerk’s two-stroke engine, as we know it today, utilized two separate cylinders: One piston drew in an air-fuel mixture through a valve, and then sent the mixture out to the second cylinder. When the second piston hit top dead center, the fuel was compressed, and a spark ignited it. Then, this piston pushed down, evacuated the exhaust, and started the process all over again. It was clunky compared to modern engines, but the patented design earned Clerk the title of the father of the two-stroke engine.
The Genesis Of The Modern Two-Stroke
The invention of the more modern two-stroke engine is often credited to British engineer and inventor Joseph Henry Day. Day slapped his name on so many projects, from the cranes built by his Victoria Iron Works to his bread-making machines and even oil speculation. He’s known for inventing impressive devices, only to end up broke after getting chewed up by the market. Yet, his most influential invention was an engine, from Gas Engine Magazine:
“It seemed to me that all gas engines as then made were unnecessarily complicated, and therefore expensive to produce, and that the only chance of cutting into the engine market was to devise something very much simpler,” Day wrote.
So Day designed a two-port, 2-stroke cycle engine using just one automatic valve. The engine required an enclosed crankcase, with a poppet valve on the side that was drawn open when the piston went up. As the piston went down, it uncovered openings or “ports” in the cylinder wall that allowed the charge to be transferred from the crankcase to the power end of the cylinder. Wonderfully simple. He obtained a British patent for his design on April 14, 1891, and an American patent on Aug. 6, 1895.
This design was light and versatile; and it produced power on every revolution of the crankshaft. One of the first American firms to license the engine was the Palmer Brothers Engine Co. of Cos Cob, Connecticut (Photo 9), who quickly realized that with its excellent power-to-weight ratio it would make a perfect engine for small boats. Indeed it did, and still does today, as many “2-strokers” adhere to this identical design. If only Day knew.
What was fascinating about Day’s two-stroke engine was just how simple it was. Day’s engine achieved the same basic goal as Clerk’s, but with far less complexity. This engine would effectively serve as the distant ancestor of two-stroke engines for more than a century.
One of the most famous early two-strokes was the DKW RT-125 motorcycle. After World War II, the tooling and plans for the RT-125 went all over the world as war reparations. Suddenly, everyone from the Soviet Union to Harley-Davidson had its hands on plans for a simple German two-stroke motorcycle engine. Here in America, we got to experience the DKW RT-125 through the Harley-Davidson Hummer.
As Cycle World notes, it wasn’t long before engineers all over the world had discovered a funny limitation in the basic two-stroke motorcycle engines of the 1940s. Cylinder pressure had to be just low enough so that the cylinder would fill itself with a fresh charge when the transfer port opened. Since an early two-stroke’s ports were opened by the movement of the piston, this was achieved by having the exhaust port open sooner. Boom, cylinder pressure drops enough to begin scavenging.
However, the twist is that due to not having any real control over the port, making it open sooner also meant that it would close later than desired. So, the fresh charge would have more time to scavenge, and thus, more fresh fuel would be lost. These limitations meant that the low-displacement motorcycle two-strokes of the 1940s often made single-digit horsepower. If only there were a way to control when to open and close the ports without depending on the position of the piston.
Two-Strokes Get Complex
Engineers have come up with all kinds of wild solutions. Daniel Zimmermann, an engineer in East Germany at the time, incorporated an idea that was used in the 1920s by controlling the opening and closing of the two-stroke’s ports using a rotating disc on the crankshaft. With the rotary disc intake valve, ports can be opened and closed whenever you want them to be.
Austrian engine manufacturer Rotax had famously equipped its motorcycle engines with rotary valves, or flat discs that prevented fresh charges from going back into the intake during the compression stroke. Canada’s Can-Am’s motocross bikes of the 1970s scored countless wins thanks to the mountain of power provided by those Rotax two-strokes. It would take years for the competition to catch up.
Two-stroke power in motorcycles only became even more insane, leading to 500cc class motorcycles with torque curves that ensured power hit like a hammer, sometimes so much power that the bike could break traction at its rear tire, throwing its rider off in violent fashion.
Motorcycles didn’t get to have all of the fun. Here’s the explanation I gave for the legendary Commer TS3 two-stroke diesel engine:
In operation, a supercharger forces 6 PSI of compressed air into the intake manifold. During the compression stroke air enters one end of the cylinder with one piston while the other piston, which is slightly ahead of the other piston, closes the exhaust ports on its way in. A swirl of air is created in the cylinder as the compression stroke continues. As both pistons close on each other and the engine reaches inner dead center (this engine’s equivalent of top dead center), fuel is injected from the middle of the cylinder. The heat and pressure generated during the compression stroke ignite the fuel at about where the pistons meet, starting the power stroke.
As the pistons pull away from each other, the exhaust ports open, blowing exhaust gases out. Once the pistons reach their furthest distance apart, the intake ports open, pushing in compressed fresh air and scavenging the rest of the exhaust gases. Then, the cylinders start coming back at each other, closing the exhaust ports and starting the process all over again.
Rootes TS3 Engine Services notes that opposed-piston engines have several advantages over the typical diesel engine of the era. The Commer TS3 didn’t have cylinder heads, head gaskets, rocker cover gaskets, camshafts, valves, or pushrods to fail. Further, the engine didn’t have the ancillaries necessary to run valves, which means even fewer points of failure. It’s also noted that the TS3 was even further optimized by the fact that sure, it had six pistons, but only three cylinders. That meant fewer injectors. Finally, due to the opposed-piston design, the engine naturally provided its own equal and opposing forces, negating the need for counterbalances in the crank.
Then there’s the even crazier Napier Deltic (video above), another two-stroke diesel, which was more or less three complex two-stroke diesels combined into a single, ridiculously complicated diesel. I wrote a whole story on that one, which you can read by clicking here.
But as you can see, as two-strokes got more powerful and more intricate, so did their solutions for the problems that had been known about two-strokes for decades. The Detroit Diesel Series 71 engine family did not use the crankcase for aspiration like the early two-strokes I wrote about above. Thus, these Detroits couldn’t naturally pull in a fresh charge of air and fuel. Instead, a Roots-type blower assisted in sending in charges and scavenging.
But forget about the 8 HP motorcycle two-stroke units of the past. Can-Am’s Rotax engines made over 60 HP, and the Detroits directly above? Series 71 engines got so hot that there was the 16V-71, an 800 HP and 2,150 lb-ft of torque V16 two-stroke diesel. Napier Deltic D18-11B 18-cylinder, 32-piston two-strokes were pumping out a continuous 1,875 HP.
Some of the coolest-sounding buses and trucks on this planet are strapped with two-strokes. Listen to this:
Two-Strokes Are Fun And Weird
My 2005 Genuine Stella, licensed copy of a Vespa PX built in India, makes only 8 HP with its 150cc two-stroke. That’s less power than you’d get out of a Honda Grom with a 125cc four-stroke engine.
Yet, because two-strokes have a different torque curve than a four-stroke, my Stella is somewhat of a wild ride. Two-strokes are generally known for narrow and peaky powerbands that aren’t as linear or wide as you get in a four-stroke. To you, the driver or rider, it’s fun because a two-stroke gives you a whole helping of power right when you hit the sweet spot.
But two-strokes also have some quirks. The simplest crankcase-aspirated two-strokes, like the one found in my Vespa above and some cars, use a total-loss lubrication system where you have to mix oil into the fuel to lubricate the engine’s components. The oil is then burned off as you ride or drive. Sure, there’s no oil to “change” like in a typical car of today, but now you have to worry about getting the mix just right or whether your autolube system, if equipped, is still working.
Of course, there’s also the emissions problem of burning oil by design. Many vintage two-strokes put out visible smoke, something that looks out of place in the modern day. Two-stroke engines in cars also had a funny problem.
When you drive downhill, you may let off the throttle, coast, and let gravity do the heavy lifting. Under normal circumstances, with a manual transmission and a four-stroke engine, the engine speed may increase, but that’s not a big deal. In a two-stroke, this is bad because the engine depends on the fuel and oil flow for lubrication, so now the engine speed is increasing, but the engine is being starved of lubrication. If the downhill stretch is long enough, that could lead to engine damage or even a seizure.
Saab famously had a fix for this for its two-strokes with a freewheel. This was a giant roller clutch, and when you let off the throttle, the rollers free up, allowing the engine to safely idle while you coasted down a hill. The freewheel also allowed Saab owners to shift gears without hitting the clutch pedal. You could also lock the freewheel, keeping the engine engaged, so that you could have engine braking. This is also how common bicycles work. Check it out:
Saab didn’t invent this feature. Cord, Rover, Packard, and even Land Rover used variations of the freewheel in their designs, too.
Once Great, Now Fading Away
Sadly, I’m sure you can see why the two-stroke failed. Over a century ago, the two-stroke engine represented a way to get good power out of a very simple and lightweight design. Then, the two-stroke evolved into something complex, yet still dominating in its fields.
Unfortunately, nothing lasts forever. Four-stroke technology eventually caught up, and did so with lower emissions, better fuel economy, and arguably easier maintenance. Sure, two-strokes have also gotten advanced, today’s manufacturers look to four-strokes to meet demanding emissions and fuel economy regulations. Buyer preferences also changed. The rider of a four-stroke motorcycle could come home not smelling like exhaust smoke and four-strokes eliminate the worry about premix.
Of course, we’re also at a turning point in alternative fuels, where EVs have become mainstream. This only further drives a stake into the two-stroke. That said, two-stroke engines aren’t entirely dead. You can still buy two-stroke chainsaws and other equipment. Two-stroke dirt bikes are still a thing, too. The last time I went swimming I also heard the familiar sound of a two-stroke outboard.
However, two-strokes aren’t where they used to be. You won’t ride a new bus with a two-stroke diesel and you aren’t going to buy a new two-stroke car, either. You also won’t be buying a two-stroke road motorcycle from a mainstream brand in America, either. A lot of folks will see that as a good thing. I’m sure lots of you are fed up with hearing two-stroke lawncare equipment! But it just goes to show how two-strokes were once titans, and now they’re old news.
Top graphic images: Saab; A. Schierwagen via Wikimedia Commons/Attribution-Share Alike 3.0 Unported licence
At least one company is making a rotary valve 2-stroke to try to keep them around for longer.
Germans call the petrol-fuelled engines, Ottomotor, and engines running on diesel, Ölmotor.
My friend had one of those 2-stroke Yamahas. One morning he went out to warm it up and as soon as he turned the key it fired up without kicking it. He just shrugged and went back in side. After he finished his coffee he came out, popped it into 1st, let out the clutch and took off…in reverse.
I’ve got a show room full of brand new two strokes! They’re not dead yet…. Genuine Buddy 50 and Roughhouse are still smoking along. Much better power than a 50cc 4 stroke, they just sound better. In this application at least….
Fun read! I’ve had a few two strokes, Vespas, Suzuki, Honda, and a Penton. It’s sick, but I miss the sound and smell of a 2 stroke, and the power curve. If the deal was right it would not surprise me if one ends up in the garage again someday. I also had a Saab 96, but the v 4, a great car!
Anyone interested in deep diving two-strokes should pick up “Two Stroke Tuners Handbook” by Gordon Jennings. Written in the early 70s. I’m sure Kevin Cameron used it as reference.
What really made the difference in two-stroke performance was learning how to design exhaust systems to create pressure waves that hit the exhaust ports at the correct time to improve scavenging of the exhaust and limit the amount of fresh charge that escaped.
I have a nicely tuned Aprilia RS250. 67hp at the wheel out of 250cc!
I’m not a diesel guy, but I’ve always thought the 2 stroke Detroits were cool. When I was in autoshop, we had a 6-71 V6 on a stand that we’d run, I loved the startup noise and clatter. That thing stunk up the whole shop, even though there was an exhaust hose on it.
My neighbor has a vintage Vespa and I have a ’24 Honda ADV 160. He gets a lot more attention riding around than I do. But the Honda is far more efficient, less polluting and relatively maintenance free.
Don’t forget about the weird exhaust systems that 2-stroke engines require for peak performance. There’s a rabbit hole for you to dive down Mercedes
In a Detroit Diesel, the supercharger is ONLY for scavenging. They use direct fuel injection via cam-operated injectors. And they have exhaust valves. The 71-series was actually made in from 1 to 24 cylinders. There were also Series 53 and Series 92 versions, with the series number being the displacement in cubic inches per cylinder.
Saab’s first solution for the coasting oiling problem was the freewheel – but the later and high performance engines got oil-injection, which was a much better fix.
What a great explanation. I’ve known the basics forever but it was fun to learn all the details, and I’ll definitely be looking up that Can-Am engine, which sounds super interesting.
I’m not an “electric car reactionary”, but I do love the beauty of the mechanical complexity of internal combustion engines.
You omitted to mention the cultural impact of small two stroke motorcycles, without them we would never had had this;
https://www.youtube.com/watch?v=k85mRPqvMbE&t=3s
Oh my, that’s an assault on the senses. I prefer Mr Toad
https://www.youtube.com/watch?v=3Gl0f4wOvQU
2 strokes really hit their peak with Bombardier’s E-TEC engines that first came out as outboard boat engines in the mid-2000s. I worked at a marina at the time, and they absolutely put the industry on it’s ear. Old carbureted 2 strokes that were smoky, smelly, and often finicky to start were giving way to fuel injected 4 strokes. Finally “turn key” starting just like your car, no smoke, no smell, no buying or mixing 2 stroke oil. The problem was the 4 strokes tended to be significantly heavier for the same HP class, and boats are designed with the engine weight very much in mind, so an extra 100 or 150lbs on the back would make the boat porpoise or struggle to plane off or other weirdness. So to match the weight, you’d have to go down on HP with a 4 stroke, which people didn’t want either. So 4 strokes were “a thing”, but definitely not a home run and people were reluctant to “upgrade”.
Then E-TEC came out. The main trick was they were direct injected. Port-injection 2 strokes existed prior, but on 2 strokes that really doesn’t work any better than carbs. They had the lightness/power density of a 2 stroke, but the instant turn-key starting, clean operation, and fuel economy of a 4 stroke. They still did use 2 stroke oil, but so incredibly little of it we had customers literally bring their new boat back thinking something was wrong since they used it all day and the oil tank level didn’t visibly drop (where old carby 2 strokes that would need to be refilled every 2nd/3rd tank of fuel). If there was any visible smoke or smell, something was wrong. And they proved to be hugely reliable too. They ate spark plugs….you really needed to stick to that 100 hour replacement interval since the high pressure direct injection literally fires the fuel straight through the plug gap (you also had to clock the plugs a certain way or they noticeably ran worse). They even had neat tricks like onboard diagnostics(no special cable/computer required, average Joe could pull codes out of the dash like an old OBD1 car) and self-winterizing. They ran shockingly smooth and quiet, “it feels electric” was a common sentiment when trying one for the first time.
They were overnight our #1 seller, whether it was on booze cruiser pontoons or high performance bass boats. Mercury quickly rushed their “me too” OptiMax DI engines to market, although they didn’t quite have the secret sauce. They just weren’t nearly as refined, but were a little cheaper.
E-TEC made it’s way into ski-doo snowmobiles a couple years later, which is the only place you can still get one, and having ridden them, they’re also excellent. The outboards got the ax during covid. 2 strokes still have a niche in snowmobiles, where weight/power density is at a premium and nobody really cares about fuel economy.
My immediate thought was DI would eliminante most 2 stroke pitfalls.
That is cool info. In 1997, Bimota released a motorcycle they called the “V-Due”, which had a 500cc v-twin two-stroke with direct injection of their own design. Unfortunately, it was a massive failure that eventually resulted in the bankruptcy of Bimota. Apparently, the prototypes were brilliant but the production motorcycles were a fiasco and they bought most of them back. An engineer bought all the V-Due stuff out of bankruptcy and gradually sold them off (with a carburetor conversion) for years and years. I’ve been slightly obsessed with them for years but finally decided I really don’t need a science project, at least not one that costs me $35k to start.
2 stroke Force motors on the back of a small boat are the sound of Summer here in SE Wisconsin. I can’t tell you how many friends bought their first boat with a 2 stroke engine, smoking along thanks to not understanding how much oil should go in the boat tank besides “That looks good”.
When I was a teenager, I worked at a summer camp in WI as the boat guy. One of the first things I was taught was how to do the mix, with a strong admonition of “don’t eyeball it, measure it!” The second thing was fiberglass work, to patch those old leaky bastards.
We put a friend’s 96 on the dyno recently. You can’t argue with the noise. We really need you to come out to the next Carlisle Import Nationals to see some cool Saabs, Mercedes.
Where as now it’s about making the power stroke longer than the intake stroke by variable valve timing. So 2 strokes have a 2nd handicap they weren’t even around for.
I owned seven different two-stroke motorcycles over the years. Mostly Yamahas but also a couple of Suzukis. They were wonderful platforms for learning to wrench because there were fewer places for things to go wrong, and at the same time, you needed to be very sensitive to how they were running to know how to adjust them.
Two vital items, two-stroke engines, weren’t mentioned: expansion chambers and reed valves. The reed valve was a dirt-simple one-way valve that helped keep the air-fuel mixture going in the correct direction. Expansion chambers are the ultimate example of science, so complex it ends up being art.
Physically, expansion chambers are nothing more than a shaped exhaust pipe. But the fluid dynamics are insanely complicated. Because they work by reflecting the shock wave of the engine firing to draw out the air/fuel mixture and then stuff it back into the cylinder, they help eliminate unburnt fuel and, in doing so, increase power. However, because this effect is fixed based on the shape of the expansion chamber (the shock wave reflection time isn’t variable), they have a set RPM at which they work well.
Most expansion chambers were built using nothing but feel and experience.
A few years ago, I went down the rabbit hole of designing my own resonator for a car that had a horrible drone at a certain RPM. The “easy” fix is to create a resonator that cancels out the drone at that RPM. To do so, you create a closed pipe branching off the main exhaust of the right length, which bounces a sound wave back to cancel the annoying resonant frequency. The principle is the same as an expansion chamber.
What I found was that since the speed of sound is very temperature dependent and the pipe also expands and contracts with temperature, the formulas I found for the length of the required pipe were very approximate and it took a few iterations to get it right. It was a fun project (looking back, anyway – it was pretty aggravating at the time).
Figuring out how to make an expansion chamber put the power where you want it within the packaging constraints of a motorcycle would be an art form.
Not just their bikes but DKW cars were hugely influential. Sweden having been neutral in WW2 they were neither obliged to respect prewar German IP in 1945-8 nor in line for reparations, so they straight copied the DKW while the Trabant was a development of it built in the original factory in Zwickau. Before the war DKW had also licensed their system – 2-stroke engine set inline driving the front wheels – to Aero in Czech and Amilcar in France among others.
2-stroke cars that didn’t use the DKW Front driveline layout were mainly found in Japan, the rear engine Subaru 360 while Honda covered the bases doing front engine/rear drive for the S360/600/800 and mid-engine under the cargo bed for the T series kei trucks before settling on transverse FWD for the N- and Z-series sedans.
As an aside, Trabis in old footage of the DDR and its’ collapse are more smoky than modern preserved ones but that’s due to a failure of the country’s system, not the cars. The planned economy under Brezhnevian stagnation made engine-rebuild parts and shop time hard to get, to say nothing of new cars, so a cheap insurance policy was to make the mix extra oil-rich well in excess of factory recommendations.
Among my favorite engine sounds are 2-stroke Detroits for the heavy stuff and OMCs for the small stuff.
Love the big Detroits. Was in Sea Scouts as a teenager and the two boats we had while I was involved had 6-71’s and 8V-92s. Cool machines to get an opportunity to get hands on with.
Thanks for a well researched article. So many YouTubers talk about a transfer port 2 stroke gas engine while showing a blower scavenged diesel engine (or vice versa). Makes my OCD engineer brain explode.
I have three of the things and still don’t really get them. I have no idea how a premix two-stroke crank survives without a flow of oil to it. I just presume it’s dark magic.
Mercedes if you ever get a chance, I’d love one of your deep dives on the Bimota V-Due. The audacity of them trying to build a fuel-injected, emissions-legal two-stroke v-twin as their first in-house engine effectively bankrupted them.
Bimota always felt like a company teetering on bankruptcy and blind optimism.
I can only hope that Kawasaki can keep it together that we can still dream of seeing a Tesi out on the road.
Last I read, they are still selling bikes here, but down to only two dealers in the country. Your odds of seeing one are not that great!
The two stroke fuel injected emissions legal dream also bankrupted Evinrude.
The bearings on that crank are ball bearings, so the oily mist of premix passing through is enough to give them lubrication. Ball bearings don’t require much lubrication unlike plain bearings where the oil film carries the load.
That makes a lot of sense now, thank you.
I just mentioned the V-Due in another comment. I’ve been lusting after one for years. I finally saw one in person earlier this year but it wasn’t running. I have a db4, which I absolutely love. It came out just after the V-Due.
Many years ago I found one for sale online that had almost no miles, and still had the factory fuel injection. I wanted it so badly, but even single me at the time didn’t have the money they wanted for it.
Now that I think about it, I don’t believe I’ve ever seen a Bimota of any type in person. I’d like an original Tesi as well, but again that’s a pipe dream.
There was an original, no-km V-Due for sale near me. The guy wanted $40k for it, which isn’t even market for a sorted bike and a truly original V-Due is a decoration. The crankcases were improperly machined, causing air leaks which are a disaster with a two-stroke. A Bimota engineer bought all the V-Due stuff out of bankruptcy and had new crankcases cast, then sold them with carbs for quite a few years.
A friend took a ride on my db4 and almost immediately bought a db2. Later, he got an sb6 so I get to see 3 all the time. 🙂 They are a delight to look at and ride but working on some of them can try your patience. Parts availability is surprisingly good.
Nothing compares to the smell of High Octane 2 stroke exhaust out of an old Cr250.
Ah man, I miss my desert camping days, CR250 by day and Banshee by night – absolutely nothing beats the sweetness of Castor 927 exhaust and hitting the powerband of a two-smoke…