Equipment Notebook: Jan. 1999, Two-Stroke vs. Four Stroke Engines

As the pressure from regulatory groups continues to mount against hand-held power equipment – which is commonly powered by two-stroke engines – some manufacturers have begun to explore four-stroke engine technology as a solution.

Other manufacturers, however, see faults with four-cycles that they believe will keep it from being as effective as two-stroke engines have become.

Lawn & Landscape invited two engine manufacturers to share their thoughts about why the technology they have chosen to focus on is better:

TWO-STROKE ENGINES
Nick Jiannas
product manager, power tools
Stihl Inc., Virginia Beach, Va.

The two-stroke engine is a typical internal combustion engine in the sense that energy is released when the air/fuel mixture in the cylinder burns rapidly. This energy is converted into motion via the piston and crankshaft. The stroke of the piston is its movement in the cylinder from one end of its travel to another. As the name states, two-stroke engines require two strokes of the piston to complete an operating cycle. By comparison, four-stroke engines require four strokes of the piston to complete an operating cycle.

An operating cycle consists of the following four functions: intake, compression, power and exhaust. This operating cycle is the same for four-stroke engines, but the method in which it is completed in both engines is quite different. Simplistically speaking, in two-stroke engines, the gas/oil mix is dispersed into the air stream via the carburetor. This air/fuel mixture then travels from the carburetor to the crankcase of the engine and from there, up to the top of the cylinder. The spark plug ignites the mixture and the resulting explosion forces the piston down the cylinder, giving the engine its power.

Two-stroke engines complete all four functions of this cycle in two strokes, meaning that two functions are completed with every stroke of the piston.

The first stroke of the piston is called the induction and compression stroke. As the piston moves up the cylinder, it compresses the air/fuel mixture in the firing chamber (compression). It also closes the exhaust port, minimizing any scavenging loss of fresh unburned mix, and closes the transfer ports as well, essentially sealing the firing chamber. At the same time, this upward motion of the piston opens the inlet port, allowing new fresh mix to enter the crankcase below (induction). Once the spark plug ignites the compressed mixture, expansion of the combustion gases forces the piston down. Thus begins the second stroke, or power and exhaust stroke.

This downward motion of the piston imparts power to the crankshaft (power), and, at the same time, compresses the fresh air/fuel mixture in the crankcase. As the piston moves down the cylinder, it opens the exhaust port and transfer ports. The fresh mixture in the crankcase, which is being compressed, travels up to the top of the cylinder via the transfer ports. As it enters the firing chamber, it helps to expel the remaining gases from the previous stroke (exhaust). While the exhaust and transfer ports are open, the inlet port is closed. Once the fresh mix reaches the top of the cylinder and the piston begins to move upward, the inlet port is once again opened. This creates a vacuum in the crankcase that brings in a fresh air/fuel mixture. And so begins the next upward stroke.

Following are some characteristics of two-stroke engines:

• Two strokes per operating cycle.
• One power stroke on every revolution of the crankshaft.
• No valves necessary.
• No camshaft necessary. Ports and the position of the piston control flow of gases.
• Few components (only three major moving parts) resulting in minimal maintenance and a compact engine of simple construction.
• Engine lubrication is achieved through a mixture of gasoline and specially blended two-stroke engine oil. This gas/oil mixture is passed through the engine and constantly surrounds the moving parts in the cylinder (piston and crankshaft). The oil is burned with the gasoline in the fire chamber.
• Two-stroke engines do experience some scavenging losses – approximately 20 percent. This means that about 20 percent of the fresh air/fuel mixture coming from the crankcase is lost through the exhaust port before the piston moves up far enough to close this port. This reduces the efficiency of the engine. However, this effect is offset to an extent by the fewer energy-consuming strokes compared to a four-stroke engine.
• Precisely tuned exhaust system prevents excessive scavenging losses that could be caused by exhaust backpressure in the firing chamber. However, if a two-stroke engine has a carbonized muffler, enough backpressure could be created to prevent burned gases from being completely expelled, thus reducing engine power and necessitating occasional cleaning of the muffler.
• Two-stroke engines are typically lighter than four-stroke engines due to the overall smaller engine size. However, because of their design, two-stroke engines require a smaller cylinder bore size to achieve the same horsepower.
• Two-stroke engines burn more gallons per hour than four-stroke engines and are also loud-er than four-stroke engines. However, two-stroke engines exhibit faster acceleration, require less general maintenance, and have less initial cost.
• Since the oil for engine lubrication is mixed with the gasoline, two-stroke engines can operate at any angle. However, two-stroke engines do run hotter than four-stroke engines. Many people do not realize that fuel is the most important coolant in two-stroke engines. Therefore, the fuel supply from the carburetor must be precisely calibrated to ensure that there is enough fuel to cool the engine, but not too much to reduce performance of the engine and/or increase emissions.