Besides the apparent differences in fuel-injection, ignition, and valve timing, are there any other differences in the design/build of two-stroke and four-stroke engines?

  • 1
    Lubrication is an obvious one. – Brian Drummond May 29 '17 at 13:07


Tiny little model airplane and chainsaw 2-strokes are godawful studies in expediency: how light and simple can we make the engine and have it still run? They use the backside of the piston as a scavenging pump, so the oil in the fuel wreaks havoc on everything from poor crank lubrication to making the exhaust heavy with oily carbonized gunk.

The larger 2-cycle engines are magnificent. They often have 4 valves per cylinder (all exhaust) - intake is via the usual ports uncovered by the piston. Obviously this does nothing to reduce complexity, but what it does do is give it twice as many power strokes for more power in the same engine.

Since a multicylinder** engine cannot use the backside of the piston for scavenging, these engines use a proper and normal oiling system. They also must be supercharged - no option. The very fancy ones use a turbocharger instead, but it is crankshaft-driven with an overrunning clutch (with a whooper of a gear-up) - functioning as a centrifugal blower until the engine develops enough power for the turbine to spin up.

A Fairbanks-Morse and some of the big marine engines are the best of both worlds, because it gets rid of the valvetrain and adds a second crankshaft, being opposed piston (not opposed-cylinder like a boxer motor). The two pistons mash together. Seriously. One piston uncovers intake ports and the other uncovers exhaust ports.

The British Deltic adds a third crankshaft - seriously - and now there are three sets of cylinders in triangle fashion. A pair of opposed pistons in each cylinder, for six per cylinder row. Really. Like the Fairbanks-Morse, the intake and exhaust ports are on different pistons.

The Fairbanks and Deltic have cams, but only to operate the fuel injectors, being diesel.

** well, two is possible with piston scavenging, but they must fire simultaneously, which largely defeats the point of two cylinders.

  • actually multi cylinder ( 2 and 3 ) cylinder 2-strokes are common (or they were a few years ago). The crank case is partitioned and sealed so each cylinder has its own backside scavenging chamber. – agentp May 29 '17 at 21:06
  • FTR, “supercharged / ... use a turbocharger instead” doesn't really make sense as such: a turbocharger is a supercharger. – leftaroundabout May 30 '17 at 7:24
  • split hairs much? turbo = exhaust driven, super = crank driven – agentp May 30 '17 at 11:29
  • @leftaroundabout you are correct. A turbocharger is a type of supercharger that doesn't work on a (large) 2-cycle engine. The reason is the turbine generates no useful power except at high engine power levels. A 2-cycle engine needs scavenging all the time including during cranking, so a turbo on a 2-cycle is a non-starter unless it has a separate, independent scavenging pump such as the back-of-piston or a roots blower. – Harper May 30 '17 at 16:42

For Two Stroke engine:

  1. One working stroke in each cylinder per revolution of crankshaft.
  2. Light in weight.
  3. Moving parts are few.
  4. Mechanical efficiency is more.
  5. Thermal efficiency is less because a part of air fuel mixture gas as waste with the exhaust gas.
  6. Noise is more.
  7. Wear and tear is more due to the smaller size of the same power.
  8. Construction is simple and easy to manufacture.

For Four Stroke engine:

  1. One working stroke in each cylinder per two revolution of crankshaft.
  2. Size and weight of the engine is heavy and larger.
  3. Moving parts are more in number.
  4. Mechanical efficiency is less.
  5. Thermal efficiency is more.
  6. Produces less noise.
  7. Wear and tear is less.
  8. Construction is more complicated due to valve mechanism.
  • Good comparison, but I would be interested to understand what mechanical efficiency is and how it differs from thermal efficiency. – juhist May 28 '17 at 15:00
  • Item 1 should read "of crankshaft" not "in crankshaft". – Solar Mike May 28 '17 at 18:55
  • 1
    The thermal efficiency issue (loss of fuel-air mix to the exhaust) is true for carburetted petrol engines. Fuel injection eliminates this - such as in the large marine 2-stroke diesels. – Brian Drummond May 29 '17 at 13:12

Not sure if you count that under “besides the apparent”... but IMO the single most striking peculiarity of a two-stroke is that it uses both sides of the pistons to work on gas volumes. That's what gives it such high power for a given size: it doesn't “waste” the space that the crankcase necessarily takes up anyway. On the other hand, this means the crankshaft has to operate under less than ideal conditions: instead of being coddled with a good supply of freshly-filtered oil, specially chosen for that purpose, it has to make do with the little bit of oil mixed into the fuel. And that oil just causes trouble for the actual combustion, which is in an Otto engine not hot enough to properly burn the less volatile components in the available time. The remainders of the oil contribute to making the two-stroke so dirty, compared to a four-stroke which nicely separates the lubrication-hungry parts from the gas stream: the crankshaft below the piston and the camshaft out of the engine chamber.

This doesn't hold for large ship Diesel engines, which are also two-stroke but built differently. But I don't suppose those are what you have in mind.

...except when it isn't. The standard wet-sump four-stroke engines have the disadvantage of requiring a constant down-orientation of the gravitational/accelerational pull. This is not given for high-performance racecars (which therefore tend to use complicated dry sump systems), nor for small, portable engines like in RC planes or chainsaws (which could hardly afford much extra complexity). A two-stroke has little heed for orientation since the gas stream readily distributes the oil everywhere it's needed, regardless.

  • Ahh, indeed it seems the two-stroke is like a completely different design! The piston itself as a valve here, necessitating quite a different design altogether... – ManRow May 31 '17 at 8:27

Yes, number of power strokes per revolution, less complicated : fewer moving parts, emissions or pollution levels. Have a look here : http://mechstuff.com/differences-advantages-disadvantages-of-4-stroke-2-stroke-engine/

Also the TS3 engine - two stoke, horizontally opposed pistons used by Rootes / Commer and had a blower.


It is important to understand that a Blower as used on a Detroit Diesel 2 stroke truck or industrial engine is NOT a supercharger. The Blower is used to provide a positive air flow to the crankcase, at relatively low pressure, but more than atmosphere. A Roots blower, when used as a 'Supercharger' as on a dragster is used at an over speed rate to provide a high pressure boost to the fuel intake. On a Detroit Diesel 2-stroke, the air intake comes through the ports in the cylinder boosted by the blower. the fuel comes in via injectors towards the top of the compression stroke, and the valves are used for exhaust only. Four strokes work under the common 1. Suck, 2. Push, 3. Bang, 4. Blow principle, or 1. Intake, 2. Compression, 3. Power, 4. Exhaust..

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