Is GM Bringing Back the Two Stroke?

[Elderberry]

Eric Peters Autos 12/13/2025

There are rumors afoot that GM is developing a two stroke engine, which is a type of engine not seen in cars (outside of the old Soviet Union) since before JFK was elected president.

Two strokes have many virtues, including simplicity – because they have no valvetrain – and low cost (because they have fewer moving parts) and high output for their displacement, relative to a four stroke engine of the same displacement, because two stroke engines make power every time a piston ascends to top dead center within the cylinder. A four stroke engine has four strokes – intake, compression, combustion, exhaust – but only one of those strokes (combustion) results in power.

Two strokes also have some strikes against them. The main one being they are very difficult to make compliant – with emissions standards – because for one thing they burn oil on purpose (it is mixed with the gas, to provide engine lubrication) and for another because the nature of the design allows for contamination of the intake charge with exhaust gasses. which is a function of using ports that are covered and uncovered by the pistons as they go up and down in the cylinders. Four strokes also have ports but they are opened and closed by intake and exhaust valves that seal when closed.

There is also the related problem of piston ring wear caused by the piston going up and down in cylinders that have holes (those ports) in their sides. In a four stroke engine, the piston is surrounded completely by the cylinder wall, which compresses the piston and oil control rings evenly. In a two stroke engine, the open port on the side of the cylinder creates a spot where the rings are not compressed evenly, leading to faster wear and more blow-by (higher emissions). And that is why the only vehicles with two stroke engines that can still be legally sold are for use off-road only.

These being off-road dirt bikes.

But a two stroke may be coming back on-road. The may be part is suggested by a new GM patent for a new-design two stroke engine that appears to have a sleeve-valve or linear system that creates a hole-less cylinder wall when the piston passes by the port, thereby improving sealing and reducing premature piston ring wear – which will (or ought to) help reduce emissions and make the engine compliant.

More: https://www.ericpetersautos.com/2025/12/13/is-gm-bringing-back-the-two-stroke/

[Atruepatriot]

Good Morning... And this is a cool find. I would like to jump into this because it has been my lifelong profession. First I would like to share that they are only describing one type of two stroke design when they mention "no valve train". When in reality there is another well known two stroke design that is a semi hybrid. And that would be the original two stroke diesels designed by GM the Detroit (Jimmy).

They have cylinder ported intake, but valves in the head for the exhaust. This eliminates the "more emissions" issue because they do not put out any more than a four stroke diesel because oil does not need to be introduced into the intake, they have a sealed and closed crankcase. But the problem they did have was fuel efficiency. They were only capable of about 70% of the fuel mileage of a four stroke of the same displacement.

So this fuel efficiency issue is still going to be a problem, sleeve valve or not. But a Gasoline engine could be designed the same as the old Jimmy now that direct fuel injection is a standard.

[Mod2]

Moving this thread to our shiny new Automotive category (it still has that new car smell, it's so shiny new).
And welcome to TBR, @Atruepatriot

[Atruepatriot]

Moving this thread to our shiny new Automotive category (it still has that new car smell, it's so shiny new).
And welcome to TBR, @Atruepatriot

Cool, thank you!

[Bigun]

Good Morning... And this is a cool find. I would like to jump into this because it has been my lifelong profession. First I would like to share that they are only describing one type of two stroke design when they mention "no valve train". When in reality there is another well known two stroke design that is a semi hybrid. And that would be the original two stroke diesels designed by GM the Detroit (Jimmy).

They have cylinder ported intake, but valves in the head for the exhaust. This eliminates the "more emissions" issue because they do not put out any more than a four stroke diesel because oil does not need to be introduced into the intake, they have a sealed and closed crankcase. But the problem they did have was fuel efficiency. They were only capable of about 70% of the fuel mileage of a four stroke of the same displacement.

So this fuel efficiency issue is still going to be a problem, sleeve valve or not. But a Gasoline engine could be designed the same as the old Jimmy now that direct fuel injection is a standard.

I can tell you that there are a large number of Detroit diesels on the bottom of the world's oceans. They get installed on new rigs because they are cheap but soon get replaced with Cats or Cummins.

[Atruepatriot]

I can tell you that there are a large number of Detroit diesels on the bottom of the world's oceans. They get installed on new rigs because they are cheap but soon get replaced with Cats or Cummins.

Yep that is a fact. Need a LOT of oil in the Bilge?

[Free Vulcan]

Otto-Alpha is doing this with a rotary valve design, unfortunately no investors.

[Weird Tolkienish Figure]

Basically weedwacker engines.

[Free Vulcan]

Basically weedwacker engines.

Uh no. If done right it has all the benefits of both two and four stroke engines, and may at some point at least power smaller cars.