Posted by Gomoto Media

There are two common types of petrol-powered internal combustion engines: Four-strokes and two-strokes. There are also the rotary engine and Atkinson cycle engine, but these are uncommon as emissions regulations have forced them into extinction.

However, the most widespread type of engine these days is the four-stroke. Two-strokes have pretty much banned except for competition use in closed circuits, such as enduros and motocross. As such, let us start by looking at how a four-stroke engine works, before we move into other subjects such as lubrication.

How does an engine produce power?

An internal combustion engine needs to combust fuel and air to produce power, by converting energies, first in the form chemical energy (burning of the fuel), which then transforms to thermal energy (heat) and lastly, kinetic energy (movement). Hence, the engine must perform four functions – intake, compression, combustion, and exhaust. Those four functions require four distinct strokes (up and down movements of the piston), hence the name “four-stroke” engine.

Before we move on further, a four-stroke engine is just that, using four strokes to create power. It is NOT a four-cylinder engine. There can be four-cylinder four-stroke engines, as well as two-cylinder, three-cylinder, four-cylinder four-stroke engines. There were even four-cylinder two-stroke engines (500cc bikes) in the past.

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

What are the strokes?

Well, no, there is no connection to golf here, nor one’s heart condition. Instead, each upward movement or downward movement of the piston is a stroke.

1. Intake

Also called the suction or inlet stroke, it begins with the piston at top dead centre (TDC). The rotating crankshaft pulls the connecting rod (conrod) downwards. The piston, which is connected to the top of the conrod is also pulled downwards towards bottom dead centre (BDC). The intake valve opens and the piston creates a partial vacuum in the cylinder, pulling in the fuel/air mixture and filling the cylinder.

2. Compression

The piston travels back up from BDC to TDC, pushed by the conrod. The intake valve closes and traps the fuel/air mixture inside the cylinder. The piston compresses the mixture in the combustion chamber. The crankshaft has travelled one full, 360-degree revolution (1 RPM) when the piston reaches TDC.

3. Combustion

Also known as ignition or power stroke. An electrical charge is sent to the sparkplug (or sparkplugs) which creates an electrical arc (not flame) between the electrodes. The “spark” ignites the compressed fuel/air mixture. Pressure in the combustion chamber rises to 3200 to 5000 kPa (32 to 50 bars), and the temperature to 600 degrees Celsius. The pressure exerts a force to the top of piston, pushing it down, which is transferred to the conrod, hence the crankshaft. This is what makes your motorcycle move forward. This rotational force (torque) is transferred to gearbox and clutch, which then makes its way to the bike’s final drive (chain, or belt, or shaft).

4. Exhaust

Also known as outlet stroke. The piston moves upwards and the exhaust valve opens, while the inlet valves remain closed. Burned gasses are pushed past the exhaust valve, and into the exhaust port and exhaust pipe. The crankshaft completes two full revolutions (720o) at the end of this stroke. In other words, a four-stroke engine takes two RPM to produce mechanical power. Thus if you hold the throttle at 8,000 RPM for one minute, the crankshaft had spun 8,000 times, the piston gone up and down by 16,000 times, and there were actually 4000 power strokes.

Valve overlap

Of course, the intake valves open slightly earlier before the piston reaches TDC on the exhaust stroke, because there would be not enough time to fill the combustion chamber fully, and also for the fuel/air mixture to burn completely by the time the compression stroke.

Similarly, the exhaust valves open slightly earlier before the piston nears TDC on the exhaust stroke to evacuate the burned gasses. At the same time, the evacuating gasses help to pull in fresh air/fuel mixture through the inlet valve.

This timing is called “valve overlap” and is one of the ways of creating more engine power, especially at higher RPMs.

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

NEXT: We will touch on how lubricating oil is circulated around a four-stroke engine.