ENGINES

Types of Engines

Types of Engines

A) Square engine

The square engine is an engine configuration used on some 4-cylinder motorcycles like the Ariel Square Four. It's easiest to think of a square-four engine as a pair of straight-twin (AKA parallel-twin) engines with their crankshafts mated together using gears to give a common output.

This design was revived as a two-stroke version on some racing Suzukis, and their subsequent road-going version the Suzuki RG500. Although some racing success was achieved, the road bikes didn't sell in great numbers, and the design was phased out in favour of in-line, four-stroke designs, as at the time two stroke engines were quickly being superseded by more economical and reliable four strokes.

The engine is only referred to as "square" when four cylinders are utilised, otherwise the engine is a U engine. An engine can also be referred to as "square" when the bore and stroke are exactly equal.

Undersquare

A piston engine is undersquare or longstroke if its cylinders have a smaller bore (width, diameter) than stroke (length of piston travel). This can be a negative trait, since a longer stroke usually means greater friction, a weaker crankshaft, and a smaller bore means smaller valves which restricts gaseous exchange; however, with the aid of modern technology, these are not the large problems that they used to be. An undersquare engine usually has a lower redline than an oversquare one, but it may generate more low-end torque.

In addition, a longstroke or undersquare engine can have a higher compression ratio with the same octane fuel compared to a similar displacement engine with a much shorter stroke ratio. This also equals better fuel economy and somewhat better emissions. An undersquare engine does not overheat as easily as similar oversquare engine. Engines can be modified with a "stroker" crankshaft, which increases an engines stroke from stock, increasing torque.

Undersquare engines are typically shorter in length, but heavier and taller than equivalent oversquare ones, which is one of the reasons why this type of engine is not generally used.

Many British automobile companies used undersquare designs through the 1950s, largely because of a motor tax system that taxed cars by their cylinder bore. Therefore, many of the most famous cars of that era use this design. This includes the Austin A-Series engine and many Nissan derivatives. Willys also used mostly undersquare engines, in fact the L- and F-134 engines, with their fairly small 3.125 inch bore and outrageous 4.375 inch stroke, are probably the most undersquare engines ever built.

Nowadays, undersquare engines tend to be quite rare, but this form of engine is still used in some aplications. For example, a modern 8.4 liter Valmet 645 inline-6 tractor diesel engine is a longstroke/undersquare engine, but has an output of over 300 horsepower (224 kW) with turbocharger and intercooler.

Oversquare

A piston engine is oversquare or shortstroke if its cylinders have a greater bore (width, diameter) than stroke (length of piston travel). This is generally considered to be a positive trait, since a longer stroke means greater friction ...