قسم هندسة القوى الميكانيكية
By : Mustafa Ibrahim Ali Abo elfotouh
Engines are desinged to be two strokes or four strokes of a piston that moves up and down in a cylinder. Generally, the automotive engine uses four strokes to convert chemical energy to mechanical energy through combustion of gasoline or similar hydrocarbon fuel. The heat produced is converted into mechanical work by pushing the piston down in the cylinder. A connecting rod attached to the piston transfers this energy to a rotating crankshaft
Engines having from 1 to 16 cylinders in in-line, flat, horizontally opposed, or V-type cylinder arrangements have appeared in production vehicles, progressing from simple single-cylinder engines at the beginning of the twentieth century to complex V-12 and V-16 engines by the early 1930s. Increased vehicle size and weight played a major role in this transition, requiring engines with additional displacement and cylinders to provide acceptable performance.
Piston and cylinder geometry
r=connecting rod lenght,
a= crank offset , s= piston position ,
=crank angle,Vc=clearance volume,
Bore sizes of engines ranges from 0.5m down to 0.5 cm. The ratio of bore to stroke,B/S for small engines is usually from 0.8 to 1.2.
In a reciprocating piston engine, the stroke ratio, defined by either bore/stroke ratio or stroke/bore ratio, is a term to describe the ratio between cylinder bore diameter and piston stroke. This can be used for either an internal combustion 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
Stroke/bore ratio is less common than bore/stroke ratio, but is used in some countries, such as Finland. The length of the piston stroke is divided by the diameter of the cylinder bore to give the ratio. Square, oversquare and undersquare engines
The following terms describe the naming conventions for the configurations of the various bore/stroke ratio
An engine with B=S is often called a square engine . By custom, engines that have a bore/stroke ratio of between 0.95 and 1.04 can be considered "square".
Oversquare or short-stroke engine
If stroke lenght is less than bore diameter, the engine is over square giving a bore/stroke ratio greater than 1:1 An oversquare engine allows for more and larger valves in the head of the cylinder, lower friction losses and lower crank stress Due to the increased piston- and head surface area, the heat loss increases as the bore/stroke-ratio is increased excessively. oversquare engines are often tuned to develop peak torque at a relatively high speed.
Undersquare or long-stroke engine
If stroke lenght is longer than bore diameter the engine is under square giving a ratio value of less than 1:1 At a given engine speed, a longer stroke increases engine friction and increases stress on the crankshaft. The smaller bore also reduces the area available for valves in the cylinder head, requiring them to be smaller or fewer in number. Bore And Stroke
The ratio between bore and stroke can determine the general characteristics of an engine and how it behaves. The bore and stroke of the engine decides the displacement of the engine. The larger the bore and stroke, the bigger the displacement and thus a more power engine. An engine’s bore and stroke are generally chosen to keep the piston speed to a reasonable level (around 20m/s maximum for a road car, while race engines can generally sustain 27m/s or so). A big-bore, short-stroke engine will have the piston traveling a shorter distance than one with a longer stroke but smaller bore, and this will help keep the piston speed down. In...
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