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.
The Otto cycle
The Otto cycle engine was first patented by Eugenio Barsanti and Felice Matteucci in 1854 followed by a first prototype in 1860. It was also conceptualized by French engineer, Alphonse Beau de Rochas in 1862 and, independently, by the German engineer Nicolaus Otto in 1876. Its power cycle consists of adiabatic compression, heat addition at constant volume, adiabatic expansion and rejection of heat at constant volume, characterized by four strokes, or reciprocating movements of a piston in a cylinder:
intake (induction) stroke
compression stroke
power stroke
exhaust stroke
The cycle begins at top dead center (TDC), when the piston is furthest away from the axis of the crankshaft. On the intake or induction stroke of the piston, the piston descends from the top of the cylinder, reduces the pressure inside the cylinder. A mixture of fuel and air is forced (by atmospheric or greater pressure) into the cylinder through the intake (inlet) port. The intake (inlet) valve (or valves) then close(s), and the compression stroke compresses the fuel–air mixture. The air–fuel mixture is then ignited near the end of the compression stroke, usually by a spark plug (for a gasoline or Otto cycle engine) or by the heat and pressure of compression (for a Diesel cycle or compression ignition engine). The resulting pressure of burning gases pushes the piston through the power stroke. In the exhaust stroke, the piston pushes the products of combustion from the cylinder through an exhaust valve or valves.
Valve train
The valves are typically operated by a camshaft rotating at half the speed of the crankshaft. It has a series of cams along its length, each designed to open a valve during the appropriate part of an intake or exhaust stroke. A tappet between valve and cam is a contact surface on which the cam slides to open the valve. The location of camshafts vary among engines, as does the quantity. Many engines use one or more camshafts “above” a row (or each row) of cylinders, as in the illustration, in which each cam directly actuates a valve through a flat tappet. In other engine designs the camshaft is in the crankcase, in which case each cam contacts a push rod, which contacts a rocker arm which opens a valve. The overhead cam design typically allows higher engine speeds because it provides the most direct path between cam and valve.
Valve clearance measurement
Valve clearance is measured with the valve closed, typically at top dead center between the compression and power strokes. The tappet will be resting on the heel of the cam lobe. A feeler gauge must pass through the clearance space. The feeler gauge should fit in and out with a slight drag. If the feeler gauge will not fit in, then the clearance is too small. If the blade of the feeler gauge fits in too loosely, the clearance is too large.
Valve clearance too wide
A too-wide valve clearance causes excessive wear of the camshaft and valve lifter contact areas, and noise. Should the clearance become wide enough, valve timing is significantly affected, resulting in poor performance.
Valve clearance too narrow
A too-narrow valve clearance does not allow for heat expansion and results in the failure of the valve to fully close. The combustion chamber does not seal properly, resulting in poor compression, which reduces performance. The valve can also overheat and even melt. However heat expansion can have the opposite effect in overhead cam engines that use aluminium alloy cylinder heads. The coefficient of expansion of aluminium alloys are approximately twice of the steel used for the valve train and this expansion can -especially in valve trains where the cam directly operates the valve- increase the the clearance.
Port flow
The output power of an engine is dependent on the ability of intake (air–fuel mixture) and exhaust matter to move quickly through valve ports, typically located in the cylinder head. To increase an engine’s output power, irregularities in the intake and exhaust paths, such as casting flaws, can be removed and, with the aid of an air flow bench, the radii of valve port turns and valve seat configuration can be modified to reduce resistance. This process is called porting, and it can be done by hand or with a CNC machine.
Output limit
The amount of power generated by a four-stroke engine is related to its speed. The speed is ultimately limited by material strength. Valves, pistons and connecting rods (where applicable) suffer severe forces and severe acceleration, and physical breakage and piston ring flutter can occur, resulting in power loss or even engine destruction. Piston ring flutter occurs are dislodged, resulting in a loss of cylinder seal and power. If an engine spins too quickly, valves cannot close quickly enough, and this can result in contact between a valve and a piston, severely damaging the engine.
Rod/stroke ratio, an important factor in engine design, is the ratio of the length of the connecting rod to the length of the crankshaft's (or piston's) stroke. An increase in the rod/stroke ratio (a longer rod, a shorter stroke or both) results in a lower piston speed. 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 crankshaft's 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 mixture into the cylinder more quickly through a larger intake runner, promoting good cylinder filling.
