Twingle engine

The Twingle engine is a small-capacity two-stroke petrol engine. It uses two pistons, one of which controls the inlet ports, the other the exhaust ports. These run in two parallel cylinder bores but share a single combustion chamber, spark plug and cylinder head.

The first Twingle engine was designed by Alberto Garelli, who patented the design in 1912. His design had a forked connecting rod with two small-ends and one big-end, and had a capacity of 346 cc. Garelli produced some motorcycles with this engine, but was more successful with more conventional designs.

Two versions of the Twingle engine were produced by Austrian moped manufacturer Puch. The earlier, based on the Garelli design, was produced from 1923. From 1949 this was replaced by a design by Giovanni Marcellino, with different sized pistons and a more elaborate connecting-rod setup. The Marcellino engine continued in production until 1970. It was complex and expensive to produce compared to a conventional single cylinder engine, and heavier for the same power output. Its only advantage was claimed to be fuel efficiency.

Both the Garelli and Marcellino engines are sometimes described as two-cylinder and sometimes as one-cylinder. Possibly as a result, the Twingle is sometimes confused with the opposed piston two-stroke diesel engine design, which has two pistons per cylinder at opposite ends of the cylinder, and no cylinder head at all. Like the Twingle, the opposed piston design uses one piston to control the inlet ports and another the exhaust, but there the similarity ends.

It is easy to see how a Twingle engine could be mistaken for a single cylinder engine. It looks, sounds and in most ways performs like one, and has only one spark plug, but in fact the Twingle has two pistons each in its own separate cylinder bore.

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, the Clark cycle, is common in truck, railroad locomotive 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. Crossley two-stroke diesels were equipped with "exhaust-pulse pressure-charging" whereby surplus air in the exhaust manifold was forced back into the cylinder by the exhaust-pulse from a neighbouring cylinder.

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.

Two-stroke Engine 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 a transfer 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 transfer port through which an air-fuel mixture (the fuel normally has some oil mixed in) is let into the cylinder. The piston then moves upwards, compressing the mixture which is ignited by a spark plug, driving the piston back down.


In more detail:

Intake and compression

The rising piston creates a partial vacuum in the sealed crankcase. A connection (inlet port) between the crankcase and the carburetor 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 as the piston gradually moves up.


Steps of two-stroke cycle:

Expansion stroke:

The piston is at Top Dead Center (TDC)
Crank is at 0 or 360°.

In real engines the process is completed from 0 to 150° but in this model it is completed at 120°.


Intake/Compression stroke:

The piston moves from Bottom Dead Center (BDC) to TDC.

The intake port is opened and working substance flows in.

Intake gases move inside due to partial vacuum; also, blowers are used to push intake gases in.

The vacuum opens the reed valve (thin flexible sheets made of steel, glass fiber or even carbon fiber) allowing the mixture to enter the crankcase.

The air-fuel mixture already in the cylinder is compressed.
As the piston nears the top of the stroke, the ignition system ignites the charge in the combustion chamber.

In diesel engines, at 11-13° before TDC fuel is injected. Before that point, only air is compressed. Fuel is injected only in the last stage of compression.



Exhaust and scavenging process:

The piston moves from TDC to BDC.

At 120°, the exhaust port is opened and exhaust gases move out of the cylinder due to gas pressure.

After 10-40°, fresh scavenging gases are then let into the cylinder through the transfer port(s).

The air/fuel/oil mixture that was let into the cylinder pushes the exhaust out the exhaust port.

The piston, then, compresses the air/fuel/oil mixture and lets left over exhaust out.



Power and exhaust

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

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

Furthermore, the descending piston closes the inlet port and pressurizes the crankcase. This also pushes 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.



Design issues

A major problem with the two-stroke engine has been 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 minimize 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 typically mix lubricants, two-stroke oil, with their fuel (either manually at refueling or by injecting oil into the fuel stream); this mixture lubricates the cylinder, crankshaft and connecting rod bearings. 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 engine design, since the crankcase is being used to hold the air-fuel mixture.

This problem has been addressed in newer engines which employ gasoline direct injection, similar to diesel two-strokes.