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Number crunching

It’s cheaper to stick with one while three offer more torque. We’re talking of cylinders on your bike, of course

india Updated: Aug 27, 2009 16:03 IST
Grease Monkey

We’ve now learned quite a lot about engines — that they provide motive power through combustion, that the up-down motion of the pistons is converted to rotational motion, and that the engine can have two or four strokes.

This week, we’ll discuss the number of cylinders, their layout and the positioning of the engine.

The benefits of being single
We’ll start off with the least possible number — a single cylinder. Most of our bikes use a single cylinder (which means a single piston or a single combustion chamber.) This ensures low production costs, making the bike cheap to sell as well. Fewer moving parts mean that there’s less to go wrong with the mechanicals as well.

Displacements of single-cylinder engines can vary from 50 cc — remember the Bajaj Sunny? — and usually go up to 650 cc for motorcycles. An example of a big-capacity single that may be brought to the Indian market in the near future by Bajaj is the KTM 690 Duke. Enthusiasts will also recall the 650 cc BMW F650 ‘Funduro’, which was sold for a short while with the help of Hero Honda in our country.

Dirt bikes, supermotards and tourers prefer a single cylinder to keep weight and complexity down, but single-cylinder motors usually have a lot of vibrations and need to be cranked more to start compared to a twin.

A variety of twins
A whole range of bikes use two cylinders, iconic brands like Harley-Davidson, Moto Guzzi, Aprilia, Ducati, KTM and BMW included. The Harleys use a V-twin engine, as do Aprilias and Moto Guzzi. The first two use a longitudinal layout, meaning that the cylinders are arranged along the length of the bike, and the two cylinders make an acute angle, hence the ‘V’ with the ‘twin’. Ducati, most notably, sticks to 90 degrees between the cylinders, which is why it is also called an ‘L-twin’ engine. BMW — think Ewan McGregor in Long Way Around — uses 180 degrees between the cylinders in the R 1200GS. The length required won’t allow the cylinders to be placed front-to-back, so the cylinders are placed ‘transversely’, which means ‘across’ or ‘left to right’. Moto Guzzi also follows the transverse layout, even though their engines are V-twins. The Nano is going to have twin-cylinder engines, as their Ace commercial vehicle does.
Another layout that automotive manufacturers use is the ‘parallel twin,’ which is two cylinders arranged at zero degrees between the two. The Yamaha RD350 had a parallel-twin motor, and the Kawasaki Ninja 250R does as well. A parallel-twin motor is usually placed transversely, and offers simplicity of design over a V-twin, because it needs only a single cylinder head and the fiddly little moving parts that go along with it.

Is three too many?
Triumph Motorcycles are one of the few motorcycle manufacturers who stick to the three-cylinder format. The Rocket 3, a cruiser that can give a Hayabusa a run for its money up to 100kph, uses it in a longitudinal format, but their sport and street bikes like the Daytona 675 and Speed Triple use it in a transverse format. Closer home, the Accent CRDi and Fabia TDI both use inline three-cylinder diesel motors. This layout offers a lot of torque, as anyone who has floored the accelerator in a diesel Accent will testify. However, vibrations rear their head when the motors are revved hard, because the design is ‘inherently unstable’ — jargon that we’ll get our heads around next week.

We’ll also visit the world’s most used engine layout and configuration, visit the big daddies of the automotive engine world, and look at an internal combustion engine that has no pistons!