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2 Stroke Exhaust Pollution: How It Happens
What is a 2-Stroke Engine?
A stroke refers to the upward or downward motion of a piston inside an engine cylinder. If the piston is at the bottom of a cylinder and moves all the way to the top, that is one stroke. So two stroke means one full upward motion and one full downward motion of the piston in the cylinder. This is shown in four stages below that continuously repeat.
All standard internal combustion engines have cylinders that move up and down. The definition of a two stroke engine is one that has a spark and ignition of fuel for every two strokes of the piston movement. This will obviously provide far more power than a four stroke engine which is firing only half as often. Burning Fuel In a 2-Stroke Engine To understand the problems associated with 2 stroke exhaust we need to see what is happening as the fuel is burned inside the piston. The following diagram shows the process broken down into four stages.
Stage 1: Fresh fuel is sucked into the engine through the inlet, shown with the blue arrow. The motion of the piston to the fully UP position causes a vacuum in the cylinder that draws the fuel in. Note that the fuel has oil mixed in with it, and at this point the oil acts as a lubricant for the piston thereby removing the need for a sump and oil pump. Stage 2: The compressed fuel at the top of the cylinder is ignited by the spark plug. The explosion pushes the piston downwards, driving the crankshaft. It is this stage that provides all the power that moves the engine and the machine connected to it. As the piston moves downwards the burnt fuel starts to exit the engine through the exhaust outlet. At the same time the fresh fuel is starting to be pushed into the top of the cylinder. Stage 3: The movement of the fresh fuel into the top of the cylinder helps to expel the old 2 stroke exhaust fumes, but it also means that the two gases are mixing. This results in both some unburnt gases being expelled with the exhaust fumes, as well as some burnt fuel remaining mixed with the fresh fuel. Stage 4: As the piston returns to the top of its cycle the replacement of burnt fuel with fresh fuel is completed. The fresh fuel, with some burnt gas still mixed in, is compressed and the engine returns to the first position in the sequence. 2 Stroke Exhaust Gases The mixing of old and new fuel in the piston chamber is fundamental to the operation of the 2 stroke engine and as such there is no simple remedy that will overcome this problem. Four stroke engines overcome this problem by using an extra cycle of the engine and valves at the top of the cylinder body to fully extract exhaust gases. This prevents gas mixing and gives cleaner emissions. Used sparingly when they are needed such as in snow mobiles, the damaging effect of two stroke engines on the environment can be minimized. However, if we are faced with the prospect of purchasing a new motorized object and have the choice of a two or four stroke engine, it is clear that choosing the four stroke engine will cause less harm to the environment. Return from 2 Stroke Exhaust Pollution to Fossil Fuels or return to the Green Planet home page for more Solar Power Facts.
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The parts that make up a two stroke engine are the same as for all other engines with two significant absences. First there is no water cooling system surrounding the cylinder. The cooling fins on the side of the engine block provide extra surface area for the heat to be lost directly to the air. Secondly there is no oil sump. The oil is mixed into the fuel and burnt with it. The diagram to the left shows the major parts of the standard 2-stroke engine.
