Two-stroke Cycle Basic Operation Intake & Compression

Two-stroke engines are used most among the smallest and largest reciprocating powerplants, but less commonly among medium sized ones.

The smallest gasoline engines are usually two-strokes. They are commonly used in outboard motors, high-performance, small-capacity motorcycles, mopeds, scooters, karting, and motorized garden appliances like chainsaws and lawnmowers. In each application, they are popular because of their simple design (and consequent low cost) and very high power-to-weight ratios (because the engine has twice as many combustions per second as a four stroke engine revolving at the same speed). For handheld devices, they also have the advantage of working in any orientation, as there is no oil reservoir.

Two-stroke cycles have also been used in diesel engines, notably opposed piston designs, low speed units such as large marine engines, and V8 engines for trucks and heavy machinery.

1 Basic operation

The two-stroke engine is simple in construction, but complex dynamics are employed in its operation. A typical simple two-stroke contains a piston whose face is shaped, an exhaust port on one side of the cylinder, and an intake port on the other side. The downward movement of the piston first uncovers the exhaust port, allowing most of the exhaust to be expelled, and then uncovers the intake port through which an air-fuel mixture (the fuel normally has some oil mixed in) is let into the cylinder. The exhaust port does not allow any air in by means of a valve. The piston then moves upwards, compressing the mixture which is ignited by a spark plug, driving the piston back down.

1.1 Intake & compression

The rising piston creates a partial vacuum in the sealed crankcase. A connection (inlet port) between the crankcase and the carburettor is uncovered by the piston as it rises, and the air-fuel mixture is sucked into the crankcase. At the same time, the air-fuel mixture already in the cylinder is being compressed.

1.2 Power & exhaust

When the piston reaches the top of its stroke, the mixture is ignited, and the piston is forced down by the rapidly expanding gases of combustion.

As the piston descends, a hole in the side of the cylinder connected to the exhaust pipe (exhaust port) is opened, and the burned gases can escape.

Furthermore, the descending piston closes the inlet port and pressurizes the crankcase. This pushes also some mixture from the crankcase back to the inlet tract, causing the reed valve to close and preventing the mixture from entering the air filter.

The air fuel mixture is forced into passageways that connect the crankcase to the cylinder. Holes connecting these passages to the upper cylinder (transfer ports) are uncovered by the descending piston and air-fuel mixture is forced into the upper cylinder. The transfer ports are just a bit lower than the top of the exhaust port, so there is a period of time when fresh air-fuel mixture is coming in while exhaust is leaving. The incoming fresh charge assists in forcing the exhaust gas out.

As the piston reaches the bottom and then starts to rise again, the transfer ports are closed by the piston and the air/fuel mixture is compressed. The next cycle starts.

1.3 Design issues

A major problem with the two-stroke engine is the short-circuiting of fresh charge from intake to exhaust which increases fuel consumption and emissions of unburned hydrocarbons. The cylinder ports and piston top are shaped to minimise this mixing of the intake and exhaust flows. Furthermore, a tuned pipe with an expansion chamber provides back pressure at just the right time to push fresh air-fuel mixture sneaking out the exhaust back in again.

The major components of two-stroke engines are tuned so that optimum airflow results. Intake and exhaust pipes are tuned so that resonances in airflow give better flow.

Two stroke engines mix lubricant with their fuel (either manually at refueling or by injecting oil into the fuel stream); this mixture lubricates the cylinder. The lubricant is subsequently burned, resulting in undesirable emissions. An independent lubrication system from below, as is used in four-stroke designs, cannot be used in the above-described two stroke engine design, since the crank case is being used to hold the air-fuel mixture.

2 Two-stroke diesel engines

A two-stroke cycle has also been used for some diesel engines. As the fuel is injected directly into the cylinder, the lubrication of the crankshaft must be independent in these engines. There is no mixing of lubricating oil into the fuel.

There are three patterns. Some modern designs differ from the gasoline two-stroke cycle in that they have intake and exhaust valves in the cylinder head, exactly like a four-stroke engine. In these engines, the two-stroke cycle is used to improve power-to-weight ratio and/or reduce the engine speed to increase reliability. This pattern is common in truck, railroad locomotiveA locomotive is a vehicle that provides the motive power for a railway train. Traditionally, the locomotive or locomotives are positioned at the front of a train, pulling passenger carriages and/or freight vehicles. This requires the locomotive to be move and machinery engines.

Other engines have used the same ported arrangement as the gasoline two-stroke, although the charge air is generally delivered under pressure from a blower through ducting rather than through the crankcase. Examples are the Junkers Jumo 205 and Napier Deltic high-speed opposed piston engines.

A third pattern uses the induction method of the gasoline two-stroke, but with an exhaust valve in the cylinder head. Large marine diesels commonly use this arrangement. These engines commonly also use a crosshead bearing, which together with a sliding seal on the piston rod allows the air path to be separated from the crankshaft while still using the piston movement as an air pump.

The simpler stroke in the fully valved diesel two-stroke cycle is the compression stroke; both valves are closed, and the rising piston compresses the air, heating it. At the top of the stroke, diesel fuel is injected into the cylinder, where it ignites and burns. The hot, high pressure gases produced by the combustion push against the piston as it descends in the initial part of the second stroke, delivering power. At this point, both valves are still closed. When the piston nears the bottom of the stroke, the exhaust valve opens, and the exhaust gases, still under pressure, rush out. The intake valve then opens. Air under pressure rushes into the cylinder, blowing out the remainder of the exhaust gases. The exhaust valve closes at that point, and shortly after that, and at about bottom dead center, so does the intake valve.

If the crankcase is not used as an air pump, some other means of forced induction is required, and is often used for efficiency in any case. The intake air must be under pressure, since the engine does not have an induction stroke and cannot suck the air in by itself. A low-pressure supercharger (blower) is needed at minimum, but many are turbocharged.

The diesel two-stroke generally lacks the inefficiency and pollution problems of the gasoline two-stroke, since no unburned fuel, only air, can get blown out of the exhaust valve before it closes. Also, there is no mixing of lubricant with the fuel.

3 Compared with four-stroke engines

Two-stroke engines have several marked disadvantages that have largely precluded their use in automobiles (although there was some use, such as in historic Saabs and DKWs and until recently in Trabants) and are reducing their prevalence in the above applications. Firstly, they require much more fuel than a comparably powerful four-stroke engine due to less efficient combustion. The burning oil, and the less efficient combustion, makes their exhaust far smellier and more damaging than a four-stroke engine, thus struggling to meet current emission control laws. They are noisier, partly due to the more penetrating high-frequency buzzing and partly due to the fact that muffling them reduces engine power far more than on a four-stroke engine (high-performance two-stroke engine exhausts are tuned by computing their resonant frequencies, essentially). Finally, they are considered less reliable and durable than four stroke engines.

There are more elaborate possible two-stroke engine configurations, but these often have enough complications that they do not outperform comparable four-stroke engines. New two-stroke designs rely on electronically-controlled fuel injection, oil injection and other design tweaks to reduce pollution and increase fuel efficiency. However, such systems increase the cost of the engines to the point that for small systems simple four-stroke engines are most cost-effective. Many large manufacturers, including Ford and Honda are still actively researching ways to build practical and clean two strokes for automotive use.

4 External links

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