An Ode To The Two Stroke

By Eric Peters, Automotive Columnist

I’m in the process of restoring an old Kawasaki motorcycle – a 1975 S1C Triple. I love the bike’s lines and looks – but I also admire its engineering. Which is startlingly simple.

Because it’s a two-stroke.

There is much less to the engine – parts and physical size-wise. The entire valvetrain is eliminated. No camshaft(s), no valves, no pushrods or chains; no keepers or shims or buckets to bother with. The “top end” of the S1’s engine consists of three one-piece cylinder head castings – each little more than a small block of aluminum with a cast-in combustion chamber on the underside and a threaded hole in the center for the spark plug. There is nothing to rebuild. Ever. Provided the block of aluminum is not physically damaged (e.g. warped, or a cooling fin broken off or the spark plug hole’s threads ruined) it is a “forever part” that you can use and re-use almost indefinitely. You can clean it occasionally, if you’re concerned about looks. But function-wise, it never wears out.

The rest of the S1’s “top end” consists of the cylinder barrels, into which are machined the ports that replace the valves you’d find in a four-stroke engine – or rather, on top of a four-stroke engine (usually). These ports are just fixed holes that are covered and uncovered by the piston as it travels up and down and perform the same function as opening and closing valves – only with no moving parts (other than the pistons themselves, of course.)

Other than de-carboning them during the rebuild, they, too, are essentially maintenance free. The upper bore may occasionally need to be machined oversize (and oversize pistons fitted) but otherwise, that’s it. The whole thing, top end-wise.

There’s no oil pan – and no oil pump in a sump. A two-stroke gets its lubricating oil mixed in with the gas it burns. Scratch the need for at least one more part (the oil pump ) and one more routine maintenance job to perform (oil changes).

The bottom end of the engine is just a pair of aluminum cases that hold the crankshaft and transmission and onto which the cylinder barrels are bolted.

With a four-stroke engine, there’s a lot more work, weight, size – and money – involved. The cylinder head(s) is an assembly, not just a single part. There are valves and springs and rocker arms and shims. Usually, machine work is required before worn valvetrain components can be replaced. The resultant package (the engine) is also physically larger because there’s more stuff inside. The S1’s engine is tiny compared to the four-strokes in my other bikes. The complete assembly – the entire engine – weighs less than 100 pounds.

It also packs more punch – on a per cc basis, at least.

That’s because a two-stroke engine produces power twice as often – once for every full rotation of the crank. The piston in a two-stroke is always working, never merely going up and down. As it’s compressing the air/fuel charge on the upstroke, it is also simultaneously drawing in (by negative pressure) the air-fuel charge for the next combustion cycle. Then, as the pistons travels downward, the exhaust port is uncovered, allowing the spent exhaust gases to escape. The next air-fuel charge also enters the combustion chamber at this time, and the cycle is ready to repeat.

Result? A lot of power from not much engine.

My S1’s engine is only a 250 but it produces almost 30 hp. For some perspective, the same-size but four-stroke 250 in one of my other bikes only produces about half as much horsepower. I also have a much larger (nearly 700 cc) four-stroke in another bike that produces about 60 hp, only twice the S1’s output from nearly three times the total displacement.

So, if two-strokes are so marvelous – what happened?

The Smog Police happened.

Because they burn oil along with gas, the resultant exhaust is more politically incorrect than Lawn Darts – and just as illegal. On paved roads, anyhow. You can still buy off-road motorcycles with two-stroke engines and chainsaws and other outdoor power equipment still have them, too.

But they’ve been off the table for on-road use since the ’80s.

That, however, may change. Engineers are working on the exhaust issue and if that can be dealt with, two-stroke engines could make a comeback.

As a fan of the elegantly simple, I hope they succeed.


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