Stroke ratio

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In a reciprocating piston engine, the stroke ratio, defined by either bore/stroke ratio or stroke/bore ratio - are terms which are used to describe the relationship ratio between the diameter of the cylinder bore, and the length of the piston stroke - within its cylinders. This can be used for either an internal combustion engine, such as a petrol- or diesel engine, where the fuel is burned within the cylinders of the engine - or external combustion engine, such as a steam engine, where the combustion of the fuel takes place outside the working cylinders of the engine.

The stroke ratio gives a general indication of the characteristics of the engine.^{[citation needed]}

Conventions

In a piston engine, there are two different ways of describing the stroke ratio of its cylinders, and these are often mixed together causing confusion. These are: bore/stroke ratio, and stroke/bore ratio.

Bore/stroke ratio

Bore/stroke is the most commonly used term, which is mainly used in the North America, Europe, United Kingdom, Asia, Australia, and some other countries.

The diameter of the cylinder bore is divided by the length of the piston stroke to give the ratio.

Stroke/bore ratio

Stroke/bore ratio is generally more rare than bore/stroke ratio, but is used in some countries, like in Finland for example.

The length of the piston stroke is divided by the diameter of the cylinder bore to give the ratio.

Stroke/bore ratio is similar to the bore/stroke ratio with the following exception:

When stroke/bore value is over 1:1 the engine is long-stroke or undersquare and when the stroke/bore value is under 1:1 the engine is short-stroke or oversquare. However the square engine has a value of 1:1 in both cases.

For example an engine with 110 millimetres (4.33 in) stroke and 80 millimetres (3.15 in) bore, stroke/bore value 1.375 - is an undersquare or long-stroke. An engine that has 70 millimetres (2.76 in) stroke and 100 millimetres (3.94 in) bore, stroke/bore value 0.7, is oversquare or short-stroke.

Square, undersquare and oversquare engines

The following terms describe the naming conventions for the various configurations of the relationship ratio between the diameter of the cylinder bore, and the length of the piston stroke within the cylinders of a piston engine.

Square engine

An engine is described as a square engine when it has an equal bore and stroke dimensions, giving a bore/stroke value of exactly 1:1.

For example an engine which has 95 millimetres (3.74 in) bore, and an identical 95 millimetres (3.74 in) stroke, has a bore/stroke value of:

95 mm / 95 mm = 1.00

Usually engines that have a bore/stroke ratio of 0.95 to 1.04 are referred as square engines.^{[citation needed]}

Square engine examples

This section requires expansion.

The Volkswagen Group W16 engine as used in the Bugatti Veyron is an example of a square engine - with an identical bore and stroke of 86.0 millimetres (3.39 in).

Oversquare, or short-stroke engine

An engine is described as oversquare or short-stroke if its cylinders have a greater bore diameter than its stroke length - giving a ratio value of greater than 1:1.

For example an engine which has 100 millimetres (3.94 in) bore, and 80 millimetres (3.15 in) stroke has a bore/stroke value of:

100 mm / 80 mm = 1.25:1

This is generally considered to be a positive trait, since (all things equal), a shorter stroke results in less friction on the cylinder wall, and less stress on the crankshaft.^{[citation needed]} An oversquare engine is generally more reliable,^{[citation needed]} wears less,^{[citation needed]} and can be run at a higher speed. In oversquare engines, low-speed torque tends to be lower, since torque is relative to crank throw - the distance from the crankshaft main bearing centre to the crankpin (big end) centre, as well as the lowering pressure of the combustion gases as the piston descends.

Engines can be modified by being "de-stroked", shortening the stroke to increase maximum rpms and top-end power, at the expense of low-end torque.

Oversquare engines tend to be lighter^{[citation needed]} and shorter^{[citation needed]} than undersquare engines of similar engine displacement along the direction of piston travel, but they are wider in directions perpendicular to piston travel. As the length is not a large problem, these engine types are highly favored by many manufacturers because of their power and compact size.

Some Formula One engines have a bore to stroke ratio of approximately 2.5:1, and are capable of revving to 19,000 rpm.

Oversquare engine examples

One very popular example of an oversquare engine is the chevy 350 small block, used and still used for many decades.

Undersquare, or long-stroke engine

An engine is described as undersquare or long-stroke if its cylinders have a smaller bore (width, diameter) than its stroke (length of piston travel) - giving a ratio value of less than 1:1.

For example an engine which has 90 millimetres (3.54 in) bore, and 120 millimetres (4.72 in) stroke has a bore/stroke value of:

90 mm / 120 mm = 0.75:1

This can be a negative trait,^{[citation needed]} since a longer stroke usually results in greater friction on the cylinder walls, more stress on the crankshaft,^{[citation needed]} and a smaller bore requires smaller valves which restrict gaseous exchange. An undersquare engine usually has a lower redline than an oversquare one, but it generates more low-end torque.

Engines can be modified with a "stroker" crankshaft, which increases an engines stroke from stock, increasing torque.

There are examples where undersquare engines have been used for good effect for fuel efficiency, since it improves low rpm performance and allows for gearing changes to further reduce rpm at cruising speed, i.e. Matsu Matsuzawa's 470mpg motorcycle [1] that was a 125cc engine stroked to 185cc.

Undersquare engines typically are, proportionally, shorter in length, heavier,^{[citation needed]} and taller than equivalent oversquare ones, which is one of the reasons why this type of engine is not generally used.^{[citation needed]}

Undersquare engine examples

Many British automobile companies used undersquare designs through the 1950s, largely because of a motor tax system that taxed cars by their cylinder bore.^{[citation needed]} Therefore, many of the most famous cars of that era use this design. This includes the Austin A-Series engine, and many Nissan derivatives.

The Chrysler Slant-6 engine, in its most common 225 cubic inch (3.7 litre) version, is a massively undersquare engine, with a 86 millimetres (3.39 in) bore and a 105 millimetres (4.13 in) stroke, producing most of its power right on the peak of its torque curve. The achilles heel of this engine, otherwise known for its exceptional durability, is being over-revved by inexperienced drivers. Red line for a factory engine is under 4,500 revolutions per minute (rpm); red line with aftermarket connecting rods is about 5,500 rpm. On the other hand, a well-maintained Slant-6 can be made to idle as low as 75 rpm (though this is not a recommended speed - neither the alternator nor the oil pump will function adequately).^{[original research?]} In some circles, the Slant-6 is nicknamed "The Stump-Puller" for its diesel engine-like low-speed torque. Appropriate gearing and driving skill is required for performance use.

Willys also used mostly undersquare engines, in fact the L134 and F134 engines, with their fairly small 79.4 millimetres (3.13 in) bore and 111.1 millimetres (4.37 in) stroke, are probably the most undersquare engines ever built (for Jeeps).

The Dodge Power Wagon, among other vehicles, used a straight-six Chrysler Flathead engine of 230 cubic inches (3.8 litre) with a bore of 83 millimetres (3.27 in) and a stroke of 117 millimetres (4.61 in) - yielding a substantially under-square stroke ratio of 0.70.

Undersquare engines tend to be less common than oversquare, but this form of engine is still used in some applications. For example, a modern 8.4 litre Valmet 645 inline-6 tractor diesel engine is a longstroke/undersquare engine, but has an output of over 224 kilowatts (305 PS; 300 bhp) with turbocharger and intercooler.

Numerous Volkswagen Group petrol engines are undersquare.

The popular Mazda Miata also uses an undersquare engine.

References

Wikimedia Commons has media related to: Internal combustion engines

External links