Four-stroke Engines: A Simple Guide To The Basics

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Have you heard the term ‘four-stroke engine' and wondered what on earth it means? Well, wonder no more, because we're on a mission to educate. In this article, we will try and break down the four-stroke combustion cycle for you as simply as we can.

The majority of petrol and diesel engines today are four-stroke engines, that power everything from mopeds to high-powered race boats. In the past, two-stroke engines were popular, but we'll get into why they are practically extinct today a little later.

The story continues on the next slide.

Most vehicles are powered by internal combustion engines. These kinds of engines work through the ignition of fuel inside the cylinders, which in turn creates a huge amount of energy, turning the crankshaft and powering the car into motion.

A stroke is one full movement of the piston from TDC (Top Dead Centre - the position when the piston is nearest to the valves) to BDC (Bottom Dead Centre - the opposite position of the piston farthest from the valves) or vice versa. In the adjoining image, the piston is seen at BDC.

The majority of internal combustion engines use the four-stroke combustion cycle or the Otto cycle, which was invented in 1867 by Nikolaus Otto.

As the name implies, four individual strokes of the piston (a valve-like moving component in the engine cylinder) constitute one cycle-the intake stroke, compression stroke, power stroke and exhaust stroke.

1. Intake stroke

As the crankshaft turns, it pulls the piston down from its TDC position .The intake valve opens and allows the cylinder to take in a mixture of air and petrol.

2. Compression stroke

During this stroke, the piston moves back up to compress the air-fuel mixture in order to create a larger explosion.

3. Power stroke

The spark plug then generates a spark which ignites the compressed air-fuel mixture, and the energy generated moves the piston downwards.

4. Exhaust stroke

Once the piston hits the bottom of the stroke, the exhaust valve opens and the excess gases leave the cylinder through the tailpipe.

Four-stroke engine advantages

  • Fuel economy: Far more fuel efficient than two-strokes because of more complete combustion of fuel.
  • High durability: Lower operating RPM and a dedicated lubrication system mean four-strokes last longer.
  • Higher torque: Less need to ‘wring' the motor, since four-strokes generally produce more torque, and at lower RPM
  • Lower pollution: Four-strokes burn fuel more completely, with drastically less unburned fuel and oil leaving the exhaust as with two-strokes, resulting in cleaner emissions.

How two-strokes differ from four-strokes

Two-stroke engines, in contrast to their four-stroke counterparts, have a power cycle with every crankshaft revolution, instead of two. This is why they often generate more power than four-stroke engines. ‘Two-strokes' also don't have valves, allowing for simpler and lighter construction.

Two-stroke motors are now rarely seen on the roads since they characteristically don't last as long as four-stroke engines, and pollute a great deal. As a result, India's tightening emission norms in the 1990s meant their numbers dwindled drastically. Still, for some, there is no alternative, and you can thankfully still see proud owners purposely dropping a gear to get their two-strokes to sing their tune.

Four-stroke engine examples

Practically everything on the road today, with the exception of electric vehicles, are powered by four-stroke engines. Even hybrid cars still use four-stroke motors, albeit in conjunction with a secondary electric motor to provide additional power and improve fuel efficiency.

Two-stroke engine examples

Yesteryear saw several shining examples of two-stroke motorcycle engine technology, like the iconic reed-valve RD350 and the notorious Yamaha RX100.

Ours are probably one of the last few generations that will actually see four-stroke engines as a part of daily life, because with all that's happening on the alternate fuel front, four-strokes will be forced to do the backstroke, as all that will be left for air-fuel mixtures is thin air.

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Story first published: Friday, January 9, 2015, 15:19 [IST]
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