Many lament the departure of two-stroke road motorcycles. Their lightness hence nimbleness, and screaming top-end rush was an experience one could not forget after riding one. It took many years before four-stroke motorcycles could match that kind of madness.

As we wrote before about the four-stroke engine, any internal combustion engine requires the burning of a fuel-air mixture (charge) to produce power. In order to achieve this, it needs to pull the charge, compress it, burn it, and finally push out the burnt gasses.

READ HERE: How does a four-stroke engine work? 

How does a two-stroke engine produce power?

As the name suggests, a two-stroke engine requires only two, by combining two functions per a single stroke.

Compression/Intake

When the piston rises on compression, the air/fuel mixture at the top of piston is squeezed further and further into a smaller and smaller space, increasing its temperature in the process.

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 combustion mixture rushes in to fill the crankcase.

The crankshaft has rotated 180 degrees at this point.

Combustion/Exhaust

As the piston nears top dead centre (TDC), the an electric arc skips across the sparkplug's terminal and ignites the air/fuel mixture. The pressure of the combustion drives the piston back down, transferring the energy to the crankshaft via the connecting rod.

On its way back down, the piston now uncovers the exhaust port, allowing the burned gasses to exit the combustion chamber. As the piston travels further downwards, it uncovers the transfer port. The piston pushes the charge (combustion mixture) into the combustion chamber via the transfer port. The process then repeats, with the piston first closing the transfer port.

The crankshaft rotates a full rotation of 360 degrees. Hence, a two-stroke engine produces a power pulse in every revolution of the crankshaft. In layman term, if you hold the throttle at 10,000 RPM (revolutions per minute) for one exact minute, the crankshaft had spun 10,000 times, the piston moved up and down 20,000 times and there were 10,000 power pulses. This is why two-strokes make more power than four-strokes given the same capacity.

Comparing a two-stroke to a four-stroke engine

A four-stroke engine requires four distinctive strokes (up and down movements of the piston) to complete the four tasks, requiring 720 degrees of crankshaft rotation.

A four-stroke engine also consists of camshafts (hence a system to drive those shafts, such as chain, belt or gears) to push the (poppet) valves open via buckets, shims or followers. The valves themselves also need springs to close them. Thus, a four-stroke engine has more parasitic losses compared to a two-stroke. A two-stroke engine is also lighter as it is devoid of these parts.

Theoretically, a two-stroke engine produces twice the power stroke of a four-stroke, hence it produces twice the power of a four-stroke. However, some of the fresh air/fuel mixture is mixed with the exhaust gases thus lowering its potential power output. This is also the reason why two-stroke engines cannot idle smoothly.

READ HERE: How does a four-stroke engine work? 

So why are two-strokes banned?

Some of the fresh charge escapes through the exhaust port. Additionally, lubricating oil (2T) needs to be mixed with gasoline and burned but it is not completely burned and again, it is pushed into the environment. These are the factors why two-strokes are banned.

NEXT: We learn how lubricating oil is circulated around a two-stroke engine.