The 2-Stroke Cycle

A look at the development of the 2-stroke engine, a much more efficient invention than the 4-stroke engine before it.

| December/January 2019

  • A basic overview of the 2-stroke cycle: Fuel/air is drawn into the crankcase as the piston rises on the compression/ignition stroke. The next fuel/air charge is compressed in the downstroke and ported to the combustion chamber as the exhaust from the previous combustion cycle is ported out of the cylinder.
    Photo courtesy of Paul Harvey
  • A 2-stroke engine makes power on every revolution, but a 4-stroke takes two full revolutions of the crankshaft to achieve one power cycle, with an intake stroke, a compression stroke, a power stroke and an exhaust stroke.
    Photo courtesy of Paul Harvey
  • A period photograph of Nikolaus August Otto.
    Photo courtesy of Paul Harvey
  • Otto’s first commercial 4-stroke engine was made in 1876.
    Photo courtesy of Paul Harvey
  • A period photograph of Sir Dugald Clerk.
    Photo courtesy of Paul Harvey
  • The 1880 patent for Clerk’s early 2-stroke engine.
    Photo courtesy of Paul Harvey
  • A sectional view of Clerk’s later engine, which used a pump cylinder.
    Photo courtesy of Paul Harvey
  • The 1895 patent for Joseph Day’s two-port, 2-stroke engine.
    Photo courtesy of Paul Harvey
  • Drawing of a Palmer Bros. 2-stroke engine.
    Photo courtesy of Paul Harvey
  • Frederick Cock’s 1895 patent for a 2-stroke engine was assigned to Joseph Day, his employer.
    Photo courtesy of Paul Harvey
  • An 1892 Day-Cock engine displayed at the Deutsches Museum in Munich, Germany.
    Photo courtesy of Paul Harvey
  • The Coolspring Museum’s two-port with valve 2-stroke Bessemer engine.
    Photo courtesy of Paul Harvey
  • A vertical 2-stroke with valve National Transit at the Coolspring Power Museum.
    Photo courtesy of Paul Harvey
  • A Mietz & Weiss three-port, 2-stroke oil engine at the Coolspring Power Museum.
    Photo courtesy of Paul Harvey
  • A Fairbanks-Morse three-port, 2-stroke twin designed for pumping duty.
    Photo courtesy of Paul Harvey

We all know what a 2-cycle engine is, right? It’s that little buzzy-sounding engine powering your weed eater or chain saw! They have been around as long as we can remember, and we don’t give them much thought. Just mix some oil into the gas and they do the job. We take for granted that they are called 2-cycle, because actually, the term should be “2-stroke cycle,” meaning there are two strokes of the piston, one up and one down, to complete a power cycle, with power created on every revolution of the crankshaft. The diagram in Photo 1 shows this quite well.

One might ask then, what is the 4-cycle engine? Again, the correct term should be “4-stroke cycle,” as there are four strokes of the piston to complete one power cycle. First, the piston goes down on the intake stroke, then it pushes back up on the compression stroke, then it’s pushed down again on the power stroke, and finally it goes back up again on the exhaust stroke. The diagram in Photo 2 shows this nicely. Note that the 4-cycle engine has only one power stroke for every two revolutions of the crankshaft versus the 2-cycle engine’s power stroke for every revolution of the crankshaft.

Beginnings

I want to explore some of the varied history of the 2-stroke cycle, including who invented it, and how it came about, but first, let’s take a look at the 4-stroke cycle, as it was invented before the 2-stroke cycle engine.

Perhaps the first thought of building an engine using explosive power (internal combustion) came from someone watching a cannon fire. Picture the barrel as the cylinder and the cannon ball as the piston. Neat concept, and many tried, but few succeeded. Atmospheric engines were inefficient, noisy and dangerous, and the power-to-weight ratio was horrible. So what to do?



In Germany, Nikolaus August Otto (Photo 3) gave it some deep thought. He understood thermodynamics and was aware of the Carnot cycle, a theory of a perfect cycle of efficiency, that can never be achieved. He had patience, and reasoned that if he compressed the charge, he would get more power. In 1876, he was ready to market the world’s first 4-stroke cycle engine (Photo 4). It was a success, and soon hundreds were being built in Germany, the United Kingdom and the United States.

But, as the saying goes, someone will always try to build a better mousetrap, and hence our tale of the 2-stroke cycle commences. Crossing the English Channel to Scotland we meet Dugald Clerk (Photo 5). Just two years after Otto’s invention, Clerk modified a Brayton engine to produce power on every revolution of the crankshaft. Hello, 2-cycle! It was a new idea, one that none had succeeded with before.



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