The 2-Stroke Cycle

By Staff
1 / 15
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.
2 / 15
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.
3 / 15
A period photograph of Nikolaus August Otto.
4 / 15
Otto’s first commercial 4-stroke engine was made in 1876.
5 / 15
A period photograph of Sir Dugald Clerk.
6 / 15
The 1880 patent for Clerk’s early 2-stroke engine.
7 / 15
A sectional view of Clerk’s later engine, which used a pump cylinder.
8 / 15
The 1895 patent for Joseph Day’s two-port, 2-stroke engine.
9 / 15
Drawing of a Palmer Bros. 2-stroke engine.
10 / 15
Frederick Cock’s 1895 patent for a 2-stroke engine was assigned to Joseph Day, his employer.
11 / 15
An 1892 Day-Cock engine displayed at the Deutsches Museum in Munich, Germany.
12 / 15
The Coolspring Museum’s two-port with valve 2-stroke Bessemer engine.
13 / 15
A vertical 2-stroke with valve National Transit at the Coolspring Power Museum.
14 / 15
A Mietz & Weiss three-port, 2-stroke oil engine at the Coolspring Power Museum.
15 / 15
A Fairbanks-Morse three-port, 2-stroke twin designed for pumping duty.

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.


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.

Clerk was born in Glasgow, Scotland, on March 31, 1854, the son of Donald Clerk, a machinist. He studied engineering at Anderson College in Glasgow and Yorkshire College of Science in Leeds. A brilliant engineer, he also understood thermodynamics and could calculate engine pressures, temperatures and power. During World War I he was director of engineering research for the admiralty. For this, he was knighted as Sir Dugald. He built gas engines, wrote technical books, and contributed much to the development of the 2-stroke cycle. Many regard him as the father of that engine. He passed away on Nov. 12, 1932, in Ewhurst, Surrey, England.

Clerk’s aim was to build an engine that produced power on every down stroke of the piston, and used compression to increase efficiency. He was successful, but his design required using a separate piston for charging the power cylinder, and he used valves. In 1880, four years after Otto’s breakthrough, Clerk secured a U.S. patent for his machine (Photo 6). It was cumbersome, using two slide valves and a poppet valve, as well as a second cylinder, but Sir Dugald proved his point, with an engine that produced power on every revolution of the crankshaft.

Clerk was an untiring engineer and inventor, and continued improving his engine. Note the sectional view of a later engine in Photo 7. This is a much more sophisticated machine, with the power cycler operating the crankshaft and the pump cylinder attached to a pin in the flywheel. It still used a slide valve for ignition, but now had an automatic poppet valve between the cylinders. Sound familiar, engine lovers? Yep, it is very similar to the Reid engine produced in Oil City, Pennsylvania.

Leaving Sir Dugald working on his engine in Scotland, we cross Hadrian’s Wall into England, stopping in the beautiful town of Bath where we meet a very unique individual, Joseph Henry Day. Born in London in 1855, Day became a well-educated engineer attending the prestigious School of Engineering at the Crystal Palace. He moved to the town of Bath and established the Victoria Ironworks, which became famous for the cranes it produced. He became interested in gas engines, and decided he could make a better one. I will defer from discussing his engine development for a bit, to continue with his somewhat eccentric life and demise.

Day remained a bachelor and lived in his father’s house most of his life. He was brilliant in a way, but tended to get involved in too many projects. Having amassed a fortune with his Victoria Iron Works, he entered the mechanized bread-making business. Due to huge variations in the market, he lost it all. Cast into depression over this tragedy, he slowly realized that he was making another fortune from the license proceeds of his engines, especially those licensed in America, which made dandy boat engines. Once again on the top of the world, he entered into oil speculation in England. This was a folly, and despite his vigorous efforts he again became penniless. He sank into oblivion in 1925, and it is thought that he died in 1946. No one is sure where or when. What a horrible fate for the gent that designed the engine that almost everyone uses today.

So, let’s return to Bath and visit with Day as he toils in his Victoria Iron Works. “It seemed to me that all gas engines as then made were unnecessarily complicated, and therefore expensive to produce, and that the only chance of cutting into the engine market was to devise something very much simpler,” Day wrote.

So Day designed a two-port, 2-stroke cycle engine using just one automatic valve. The engine required an enclosed crankcase, with a poppet valve on the side that was drawn open when the piston went up. As the piston went down, it uncovered openings or “ports” in the cylinder wall that allowed the charge to be transferred from the crankcase to the power end of the cylinder. Wonderfully simple. He obtained a British patent for his design on April 14, 1891, and an American patent on Aug. 6, 1895 (Photo 8).

This design was light and versatile; and it produced power on every revolution of the crankshaft. One of the first American firms to license the engine was the Palmer Brothers Engine Co. of Cos Cob, Connecticut (Photo 9), who quickly realized that with its excellent power-to-weight ratio it would make a perfect engine for small boats. Indeed it did, and still does today, as many “2-strokers” adhere to this identical design. If only Day knew.

The Day two-port engine certainly revolutionized gas engine practice; but there was more to come. Imagine a complete and functional engine with just three moving parts. Impossible? Less than two years after the Day two-port patent, one of his employees, Frederic William Caswell Cock (1863-1944) designed such an engine. Cock was granted an English patent for his design on Oct. 15, 1892, and immediately assigned it to Day. The American patent was obtained in 1895 (Photo 10). Simplicity personified.

By studying the patent drawing a bit, one can see an engine with an enclosed crankcase, and all the intake and exhaust functions controlled by the piston passing the three ports. It makes one wonder why it was not invented earlier. Too simple? Photo 11 shows a Day-Cock engine of 1892 displayed in the Deutsches Museum in Munich, Germany. The descendants of these ingenious machines are still made today in all sizes and descriptions. And, of course, they have only three moving parts: the piston, the connecting rod and the crankshaft.

We’ll end our journey at Coolspring Power Museum to see what might be lurking there. First, we look at that little Bessemer in the Founder’s Building (Photo 12). It’s a two port with a valve. Notice the exhaust comes out through the bottom of the cylinder by means of a port, and the valve is just to its right. It was very successful, and Bessemer’s successors are still building the 2-stroke cycle in some very large machines. Next, we find the neat little National Transit vertical shown in Photo 13. The valve is the device on the left side of the engine. There are many more at the museum, but these two serve as fine examples.

In the Founder’s Building near the Bessemer is the Mietz & Weiss 2-cycle oil engine, a three port without valves (Photo 14). Another fine example is a neat little Fairbanks-Morse fire pump that uses a twin cylinder Waterman three-port engine. It was designed to be as lightweight as possible so a couple of forest rangers could carry it to a fire, start it, and pump water. See Photo 15.

This concludes my brief history of the 2-stroke cycle engine. To keep this short, I have intentionally omitted details, but the reader who delves deeper will be richly rewarded. And the next time you browse through a Lowe’s or Home Depot, look at all those 2-stroke cycle engines on the various yard and garden tools, and try to decide if they are two or three port. They recall Joseph Day. I wonder if he ever guessed how many there would be and how long they would be used?

Paul Harvey is the founder of the Coolspring Power Museum. Contact the museum at P.O. Box 19, Coolspring, PA 15730 • (814) 849-6883

Gas Engine Magazine
Gas Engine Magazine
Preserving the History of Internal Combustion Engines