The biggest difference between a two stroke and four stroke engine has to do with firing timing, which can often be noticed in terms of sound: the two engine often has a high-pitched, very loud rumble, whereas the four engine tends to have a quieter purr. In most cases this is a facet of basic operation and efficiency. Two stroke engines fire once per revolution, which gives them twice the power of a four stroke that generally fires only every other revolution. Four strokes are more efficient, but they’re heavier and more expensive, too. They’re more commonly found in cars and industrial machinery, whereas things like lawnmowers, jet skis, and most lightweight motorboats rely on the smaller two stroke model. The basic fuel burning and combustion properties of these engines are generally the same, but the differences come with respect to how they convert energy and how efficiently that conversion happens.
Understanding the Strokes
Both types of engines burn fuel through four distinct processes, known as “strokes.” How quickly they complete these strokes is one of the biggest differences, but they both will perform all four at some point.
Intake is the first stroke. This is when the piston travels down the cylinder while the intake valve is opened to allow a mixture of fuel and air to enter the combustion chamber. Next comes compression. Here the intake valve is closed and the piston travels back up the cylinder, thereby compressing the gasses. Combustion happens when the spark plug ignites the compressed gas, causing it to explode; this forces the piston down. Finally is exhaust, the final step that happens when the piston rises up the cylinder as the exhaust valve is opened, allowing the piston to clear the chamber to start the process over. Each time the piston rises and falls it turns the crankshaft that is responsible for turning the wheels or propelling a non-wheeled vehicle. This is how fuel is converted into forward motion.
When the Engine Fires
In a standard four-stroke engine the spark plug only fires once every other revolution in synch with a sophisticated set of mechanisms working to create the four strokes independently. A camshaft must alternately tip a rocker arm attached either to the intake or exhaust valve, for instance, and the rocker arm returns to its closed position via a spring. The valves must be seated properly in the cylinder head to avoid compression leaks, and all of this has to happen simultaneously.
Where the two engine is concerned, by contrast, all four events are integrated into one downward stroke followed by one upward stroke; this is where the name “two stroke” comes from. Intake and exhaust are both integrated into the compression and combustion movement of the piston, eliminating the need for valves.
This is accomplished by an inlet and exhaust port in the wall of the combustion chamber itself. As the piston travels downward from combustion, the exhaust port is exposed, allowing the spent gasses to rush out of the chamber. The downward stroke also creates suction that draws in new air and fuel through an inlet located lower in the chamber. As the piston rises again, it blocks off the inlet and port, compressing the gasses at the top of the chamber. The spark plug fires and the process begins again. Significantly, the engine fires on every revolution in these sorts of engines, which gives them a power advantage at least in the short term.
Weight-to-Power and Orientation Differences
Two engines are usually better suited for settings where quick, sudden bursts of energy are important but aren’t expected to be sustained over a long period. A jet ski, for instance, can often accelerate more rapidly with its two stroke engine than a truck could with a four stroke model, but it also is usually only made to ride for a short amount of time, whereas a truck may go for hundreds of miles or kilometers before resting. Some of this short-term efficiency owes to the two stroke engine’s lower weight-to-power ratio; these engines weigh a lot less on average, and as such can get up and going more quickly. It also takes less energy to propel them forward.
In most cases four engines can only be operated in one direction, too, while there’s more flexibility in engines with just two strokes. A lot of this has to do with the complexity of all the moving pieces, as well as the specifics of the oil sump. The oil sump, which provides lubrication to the engine, is typically only present in four stroke models, and it’s important for helping all the processes happen in differentiated ways. Two stroke engines don’t usually have this, though, which means that they’re able to be operated in almost any orientation with no risk of the oil sloshing out or getting displaced. For things like chainsaws, edgers, and other moving tools, this flexibility is often really important.
Efficiency and Pollution Issues
It's also often true that the smaller, faster engines are more prone to pollution and inefficiency. At the lowest point of travel of the piston when the chamber is filling with fuel and air, the exhaust port often allows some fuel to escape the chamber. This is easily seen with an outboard motorboat, for example; people who look carefully are often able to see a multicolored oil slick surrounding the engine. As a result, these sorts of engines are usually considered more inefficient, and are sometimes also cited for environmental pollution. Although four stroke models are heavier and slower, they usually do a better job of making full use of fuel.
Pricing and Expense
The smaller engines are often less expensive, both in terms of initial purchase price and routine maintenance. They aren’t usually designed to last as long, though. Though there are some exceptions, most can't efficiently run for more than a few hours at a time, and most aren’t designed to have very long life spans, either. Their lack of a dedicated lubricating system also means that they may wear out and break down faster as the moving parts disintegrate.
In part because there is no lubricating system, many two stroke engines also require a special sort of oil, called “premix,” with every tank of gasoline. This adds expense and hassle, and can lead to damage over time if it’s forgotten. In most cases four stroke engines require very little maintenance and upkeep.