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Two-Stroke Engine – How it Works

Two-Stroke Engine – How it Works

  • A two-stroke engine has very simple workings.

  • It produces one power stroke for every revolution of the crankshaft.

  • Its simplicity and high specific power output makes it a favourite.

Sadly, the two-stroke engine is no more for road-going motorcycles. However, they are still produced for competition use, especially in the off-road arena.

Overview

A two-stroke performs the four functions of intake, compression, power and exhaust in one up-stroke and one down-stroke of the piston, within one complete 360revolution of the crankshaft (1 RPM). Conversely, a four-stroke engine completes all four functions on each separate up and down piston strokes in 720o revolutions of the crankshaft (2 RPM). Consequently, the two-stroke produces (theoretically) twice the power stroke of a four-stroke in every 2 RPM.

Another main difference is the absence of poppet valves unlike in a four-stroke engine. Therefore, there is no cam chain or belt, camshafts, buckets, shims, springs, etc. to activate the poppet valves. That equals simplicity and weight savings.

Here is the basic “crankcase-scavenged two-stroke” to ease our understanding of how it works.


Compression/Intake stroke

The piston rises to compress the charge (combustion mixture of fuel, air, and oil). At the same time, the bottom of the piston creates a partial vacuum in the crankcase . The piston uncovers the intake port on the cylinder wall and the vacuum pulls in the charge 

Combustion/Exhaust

As the piston nears top dead centre (TDC), the spark plug combusts the mixture. The pressure of the combustion drives the piston downwards, transferring the energy to the crankshaft via the connecting rod.

The piston now uncovers the exhaust port on the cylinder wall, allowing the exhaust gasses to exit the combustion chamber. The piston also squishes the mixture in the crankcase simultaneously.

As the piston travels further downwards it uncovers the transfer port. The pressure underneath the the piston pushes the charge into the combustion chamber via the transfer port.

The crankshaft completes one full revolution (RPM).

Lubrication

Because the charge is pumped through the crankcase, this makes it impractical to lubricate the crank and piston with thick oil being pumped around the engine like a in a four-stroke. As such, the lubrication oil (2T) must be premixed with the fuel, or injected into the crank bearings with a metering pump such as Yamaha’s Autolube system.

Why two-stroke motorcycles are no longer produced?

Although mixed with the fuel, much of the 2T oil remains unburned. The unburned oil is ejected into the atmosphere, hence the polluting the environment. This is the bluish-white smoke we are familiar with.

Furthermore, some of the fresh charge gets mixed with the exhaust gases and seeps through the exhaust. Again, another source of pollution.

It is also due to these factors that a two-stroke engine does not produce exactly twice the power of a four-stroke engine of the same capacity.

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