Four-Stroke Cycle

Today Internal combustion engines in cars, trucks, motorcycles, construction machinery and many others, most commonly use a four-stroke cycle. The four strokes refer to intake, compression, combustion and exhaust strokes that occur during two crankshaft rotations per working cycle of Otto Cycle and Diesel engines.

Port flow

See Main Article Port Flow

The power output of the motor is dependent on the ability of the engine to allow a volume flow of both air-fuel mixture and exhaust gas through the respective valve ports, typically located in the cylinder head. Therefore time is spent designing this part of an engine. Factory flow specifications are generally lower than what the engine is capable of, but due to the time and expensive nature of smoothing the entire intake and exhaust track, compromises in flow for reductions in costs is often made. In order to gain power, irregularities such as casting flaws are removed and with the aid of a flow bench, the radii of valve port turns and valve seat configuration can be modified to promote high flow. This process is called porting, and can be done by hand, or via CNC machine.

There are many common design and porting strategies to increase flow. Increasing the diameter of the valves to take up as much the cylinder diameter as possible to increase the flow through the intake and exhaust ports is one method. However, increased valve size can decrease valve shrouding (the impedance of flow created by the cylinder floor.) To counter this, valves are commonly designed to open into the middle of the cylinder (such as the Chrysler Hemi or the Ford Cleveland engines with canted valves). Also, increasing valve lift, or the distance valves are opened into the cylinder or using multiple smaller valves can increase flow. With the advent of computer technology, in modern engines valves events can be controlled directly by the engine's computer, minimizing engine operation at any speed or load.

Output limit

The amount of power generated by a four-stroke engine is proportional to its speed. The speed is ultimately limited due to material strength. Since valves, pistons and connecting rods are accelerated and decelerated very quickly, the materials used must be strong enough to withstand these forces. Both physical breakage and piston ring flutter can occur, resulting in power loss or even engine destruction. Piston ring flutter occurs when the piston rings change direction so quickly that they are forced from their seat on the ring land and the cylinder walls, resulting in a loss of cylinder sealing and power as well as possible breakage of the ring. Worst is overreving (overspeed) when valves lose their forced contact with the valve train. This would occur if the valve (spring normally) closing force would be exceeded by the inertia force resulting in valve contact with the piston causing major damage. Various countermeasures are reducing the valve train mass, for instance: OHC instead of OHV, four instead of two valves (even more air/gas flow)

One important factor in engine design is the rod/stroke ratio. Rod/stroke ratio is the ratio of the length of the connecting rod to the length of the crankshaft's stroke. An increase in the rod/stroke ratio (a longer rod, shorter stroke, or both,) results in a decrease in piston speed. However, again due to strength and size concerns, there is a limit to how long a rod can be in relation to the stroke. A longer rod (and consequently, higher rod/stroke ratio,) can potentially create more power, due to the fact that with a longer connecting rod, more force from the piston is delivered tangentially to the crankshafts rotation, delivering more torque. A shorter rod/stroke ratio creates higher piston speeds, but this can be beneficial depending on other engine characteristics. Increased piston speeds can create tumble or swirl within the cylinder and reduce detonation. Increased piston speeds can also draw fuel/air mix into the cylinder more quickly through a larger intake runner, promoting good cylinder filling.

A "square engine" is an engine with a bore equal to its stroke. An engine where the bore dimension is larger than the stroke is commonly known as an oversquare engine; such engines have the ability to attain higher rotational speed since the pistons do not travel as far. Conversely, an engine with a bore that is smaller than its stroke is known as an undersquare engine; such engines cannot rotate as quickly, but are able to generate more torque at lower rotational speeds.

4strock - 4stroke atkinson cycle - 4stock valve clearance - 4strock otto cycle - 4strock valve train

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