Why are two-stroke engines nearly unheard of in aviation?

Question

Almost all aircraft piston engines are four-stroke engines, with one power stroke per piston every other crankshaft revolution and a great deal of intricate moving parts which are only too happy to break. Two-stroke engines, in contrast, have a number of features that would seem to make them excellent choices for aviation use:

• They have one power stroke per piston per crankshaft revolution, twice as many as a four-stroke engine, and also have far fewer moving parts, making them smaller and lighter than four-stroke engines of the same piston size and count; these two factors add up to give them a phenomenally high power-to-weight ratio, much better than that of a comparable four-stroke engine.
• The same mechanical simplicity that contributes to their high power-to-weight ratio also makes them cheaper to build and sell, and (all else being equal) more reliable than four-strokes.
• Most two-stroke engines are lubricated by mixing lubricant into the fuel rather than using a traditional oil sump; this allows them to operate happily in any orientation, including upside down.

The sole disadvantage of two-strokes is their greater fuel consumption and emissions, but:

• Good design can reduce this problem to insignificant proportions;
• Higher fuel costs could easily be offset by the reduced maintenance costs of a more reliable engine¹ and the ability (due to the engine's greater power) to use fewer engines per aircraft, and;
• When it comes to piston-engined-aircraft emissions, there are far worse concerns.

Yet, despite these advantages, to the best of my knowledge, only one two-stroke aircraft engine has ever entered mass-production (the highly-unusual Junkers Jumo 204/205/207 series), with a second (the Rolls-Royce Crécy) being cancelled during static-fire testing.

Why are two-strokes essentially nonexistent as aircraft engines?

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¹: Especially given that maintenance costs are often (though not always) a greater expense than fuel costs to begin with (scroll to page 20); the linked document is discussing jetliners, but the preponderance of maintenance costs would be much more pronounced for piston-engined aircraft, as piston engines are considerably more efficient and considerably less reliable than turbine engines (the latter due mainly to their massive mechanical complexity).

Answer 1

Several issues:

As you mentioned, high specific fuel consumption. About to that of turboprops, but without the reliability. If you're going to live with that SFC, you might as well as go with turbines.

Poor reliability. They are very sensitive to tuning and timing. A 4 stroke piston aircraft engine will run with all kinds of things wrong with it. Cam lobes worn down to nearly nubs. Loose valve guides. Horrible compression. They can have compression readings that will cause most mechanics to ground them, but they will still make rated horsepower (Continental did testing to find this). They are very "under-tuned", like a tractor or industrial engine. You want this in aviation. About the only thing that will make a 4 stroke aircraft engine blow up in your face is something like a thrown rod from a spun bearing or a cylinder that cracks open, and that requires bad maintenance or criminal neglect in the vast majority of cases. 2 strokes, in the ultralight world where they are widely used, have a reputation for letting go at any time (the only 2 stroke aircraft engine that even comes close to 4 stroke reliability is the Rotax 582 - but only close, and they're only reliable for a few hundred hours).

2 strokes require pressed together cranks with ball bearings because they have to depend on lubrication from oil mixed in with the fuel charge. So you can't have a forged one-piece crank (radials get away with ball main bearings but there is only one or two crank throws, and they still have dedicated oil lubrication). With the only oil coming from what's diluted in the fuel in a 2 stroke, piston ring lubrication is marginal and ring life is poor.

Which gets us to longevity. Although it has an official TBO of 2000 hours, if you take decent care of it, a Lycoming O-320 will run 3-4000 hours. The plain bearings in a 4 stroke only see wear on startup; there is zero wear once they are pressurized because there is no metal to metal contact, and if you never shut it down and changed the oil on the go, the bearings would theoretically last forever. Ball bearings lubricated with diluted oil have a finite life, and that and the barely lubricated rings limit longevity. You would never dream of a 2 stroke lasting that long in any galaxy in this universe.

2 strokes are popular in the ultralight world almost totally because they are cheap and have a superior power to weight ratio. You almost get a turbine's power to weight, and minimal cost, and that's worth living with the crappy reliability. Ultralights get away with it because they crash going 30 mph not 70.

Problem is, a lot of the issues are baked into the configuration and can't be designed out. Combine with with a business that is conservative by necessity, because your life depends on the conservatism, and it's no surprise.

Answer 2

The main reason there are few 2 stroke aircraft engines is likely the reason there few gasoline 2 stroke engines above 125cc in general: They have poor fuel efficiency and poor emissions. You note that aircraft engines still use leaded fuel and suggest this is a greater concern, but the concerns mentioned above exacerbate lead pollution, due to the greater fuel requirements and emisions of unburned fuel. You say this can be solved with good design, but that comes with cost and complexity. After a certain amount of time trying to design a 2 stroke cylinder that won't bypass fuel/air mixture from the inlet port to the exhaust port, you give up and design a 4 stroke instead. It'll weigh more but the decreased fuel usage offsets the weight penalty for anything that flies greater distance than an ultralight.

Diesel 2 stroke engines generally have too poor a power/weight ratio for aviation, but very sucessful in other fields. The fuel is injected only when the piston is in a raised position, so the engine ingests only fresh air in its intake manifold, and if some bypasses to the exhaust it doesn't waste any fuel.

https://en.wikipedia.org/wiki/Category:Aircraft_piston_engines lists 79 two stroke engines, more than half the number of radial engines at 146, so obviously significant, and for some reason the Junkers engines you mention are listed as diesel but not 2 stroke.

Answer 3

The issues mentioned by the OP and other responders are peculiar to a few specific engines that are not comparable to each other and mostly don't pertain to prop drive applications. Nobody would design a 2 stroke replacement of a Lycoming as described. It would look very similar and be configured very similar to the 4 stroke. The issue is specific power and specific fuel consumption of a prop drive at a given capital expense and operating expense. Power density is not much of a driver, and can be much lower than that desired in the automotive sector, for instance.

A 2-stroke would be a bit more complicated, in that it would need a blower to manage air charge. It would have essentially the same oil system and fuel system as the 4 stroke. It might use poppet valves in the head for intake and exhaust, or it might use ports. There are at least 10 basic variations on how to get air in, light it off, and get the exhaust out of a 2-stroke. But only three or four are worth serious consideration here. And there probably isn't enough difference among those to worry about as far as this question is concerned, so lets just assume a roots blower feeding charge air to a poppet valve headed engine. Maybe it's direct injection or maybe indirect injection.

Specific fuel consumption favors the 2 stroke over a large range of sizes and speeds, but probably not at this small a scale. It's probably a wash at the size of a Lycoming. The savings in internal friction exceeds the blower losses for larger cylinder sizes. But for a light aircraft, the blower wouldn't be the all that great. Gensets operating at comparable rpms still favor 4-strokes.

One of the biggest factors effecting weight is compression ratio. Since we can trade power density for specific power, we go big and low CR and thin walled. This gets the weight down, especially when you can't run high rpms to begin with. This may be where the 2 stroke loses it's advantage. It's good when you need powerful, light and small, but if you don't need the small part, There isn't much of an advantage. It will be a bit smaller, but not necessarily any lighter when built to drive the same prop.

The sort of cylinder design and engine architecture that you want is almost entirely determined by the application, not by whether you are running a 2-stroke or 4-stroke. For the most part, you can take any 4-stroke and convert it to a 2-stroke without all that much trouble. A buddy of mine started converting junked Cadillac 500 cid motors to 2 strokes when he was 14 years old. He bought a house and built a machine shop when he was 16.

So I would say that there isn't any real reason to not develop a 2-stroke for small aircraft, but that none of your preconceived notions about the differences between the two types pertain to this application. I think the dynamics of the intake air might be harder to manage (read more expensive and maintenance intensive) in the 2-stroke compared with the 4-stroke. The requirement for reasonably fixed power at reasonably steady rpm given widely varying charge conditions is a very different problem than that facing the automotive sector, which has steady charge conditions and widely varying power and rpm requirements.

Answer 4

There is nothing inherently inferior in two-stroke engines that would prohibit them from being used in cars or aircraft.

As stated by OP, 2-strokes have many properties that should be of value in aircraft, particularly the higher power-to-weight ratio of generating a power stroke at every revolution. They can also be designed for simplicity and low moving parts count, like a lawn mower: no valves but ports, pre-mix fuel and oil without a separate oil pump, and resulting in an engine with poor specific fuel consumption and high pollution count. But of course for modern day cars and aircraft, that design won't be used. For instance from this article (2nd answer):

In a two stroke the transfer ports and exhaust ports are open at the same time. At some engine speeds, portions of the fuel mix go straight from the transfer ports out the exhaust without being burned. This leads to high fuel consumption and the ejection of unburned hydrocarbons. A solution is to use direct injection in which fuel is injected into the cylinder only after the ports close. This drastically reduces both problems.

The other source of pollution comes from the total loss lubrication in which unburned two-stroke oil passes directly out the exhaust ports. A solution is to replace the total oil loss system with full crankcase lubrication as found in four-stroke engines. To do this, the crankcase can no longer be used for scavenging, therefore a supercharger needs to be used for supplying the scavenging air.

So a two-stroke engine not designed for simplicity and low cost, will have no unburnt fuel and no total loss oil exhaust. It will still have double the power strokes of a comparable four-stroke. And it will still have a smaller parts count than a four-stroke with an oil sump and a supercharger. Some examples:

1. Image above is from an article in the NYTimes, on a modern design of a 2-stroke automobile engine. With valves instead of ports. With a compression ratio that can be varied: lower compression for petrol due to knocking prevention, high compression for diesel for higher Carnot cycle efficiency.
2. The Junkers Jumo 205 mentioned in OP was a 2-stroke diesel with intake and exhaust ports, with a clever arrangement allowing for better timing of the fuel/exhaust cycle. Designed and manufactured in the 1930s(!). Then abandoned, according to this article because of the two crankshafts connected to one propeller creating complexity, plus it had to be installed vertically to accommodate the oil scavenging system.
3. Marine 2-stroke diesels, with injectors, valves, and a twin piston layout, as used and operated for decades.

So why are two-strokes not used for aircraft propulsion anymore?

• The aviation industry is super conservative and new developments take a long time and investment in order to be accepted in the mainstream. As illustrated by the Thielert insolvency - if applying mature diesel engine technology to GA aircraft results in bankruptcy, where would that leave companies developing a new 2-stroke technology?. A newly developed 2-stroke piston engine would be used for smaller aircraft, which simply cannot generate the revenues and numbers for significant Return On Investment. Development of car/GA aircraft size petrol 2-stroke was simply stopped many decades ago, while 4-stroke continued being refined.
• There is an inherent limitation in two-stroke technology: there is much less time to expel the exhaust gas and draw in fuel, because each stroke is a power stroke. This limits either the cylinder volume, or the max. RPM. Not a problem for diesels, ship diesels run at a very leisurely 150 RPM. Four-stroke petrol does not have this limitation. Diesel can run at super high compression ratios generating an enormous amount of torque per stroke, petrol cannot. So petrol 2-strokes run into a power limitation per cylinder. Which can also be solved with new technology - another Thielert saga?

Answer 5

Many UAVs use two stroke engines due to their low weight and high power output. The resulting high RPMs are matched with relatively small diameter propellers which is fine for the power, speed and size constraints involved. Examples include the Boeing / Insitu Scan Eagle and RQ-21A Blackjack / Integrator. Their specific fuel consumption is 33% to 50% worse than a well-tuned automotive four stroke engine but similar to the smaller four stroke engines that would otherwise be used.

Answer 6

Because 1-cylinder engines aren't powerful enough

2-stroke engines have a thing happen with the cylinder at bottom dead center. Exhaust gas is pushed out of the cylinder by intake air, which is actively pumped into the cylinder. This is called scavenging.

2-stroke engines, as you imagine them, use the crankcase for an air pump. The piston going down compresses air in the crankcase, and when the intake port is exposed, scavenging occurs.

Guess how many cylinders you can have in a 2-stroke engine. ONE.

Becuase if you have 2 cylinders, then the air volume of the crankcase does not change, and it's not usable as a pump. (Well if it's exactly two, you can have a 180 degree crank so they both top out at once, but that pretty much defeats all the advantages of two cylinders.)

2+ cylinders require blowing

With 2 or more cylinders, the crankcase is useless as a pump because crankcase volume does not change. (On the upside, you can use a normal oiling system, and you no longer need to mix 2-cycle oil; of course now more complexity has crept in.)

You must use an air pump of some kind to accomplish scavenging. So now you've added a bunch of complexity. And worse, the blower will perform quite poorly at altitude.

"Oh, we know the fix! We'll just use a turbocharger". Great kid, but how are you going to start it? The engine needs to be blown at all speeds, even during cranking - so a turbo is useless at low power settings. You need an overrunning clutch to the engine can mechanically drive the turbocharger at low speeds/start. More complexity.

Further, if the intake and exhaust are both cylinder-side ports, it creates some serious issues with ring wear and carbonization. Further, the funny shape of the piston (to get air to flow the right way) greatly complexifies fuel burn, and limits maximum compression. So real soon, as your 2-cycle engine gets bigger, you need to move intake or exhaust to valves, and because of rings and carbonization, exhaust valves make more sense than intake. So now you're right back where you started with valves in the cylinder head.

However, intake ports have their own problems: the fuel/air mix will come in direct contact with the piston rings, and unevaporated fuel will tend to wash the oil in the rings, increasing ring wear and diluting the oil. This is not a problem for a diesel, since they direct inject into the cylinder.

So congratulations. You have an engine powerful enough to sub for a Continental or Lycoming, and you have

• An oil pump
• Cams and exhaust valves
• A blower
• Direct injection to solve the ring-wash problem

So that's a simplicity fail.