Four-stroke cycle
From Wikipedia, the free encyclopedia
| Thermodynamic cycles | |
|---|---|
|
|
| edit | |
The four-stroke cycle of an internal combustion engine is the cycle most commonly used for automotive and industrial purposes today (cars and trucks, electrical generators, etc). The Thermodynamics cycles used in internal combustion reciprocating engines are the Otto Cycle (the ideal cycle for spark-ignition engines) and the Diesel Cycle (the ideal cycle for compression-ignition engines). The Otto Cycle consists of adiabatic compression, heat addition at constant volume, adiabatic expansion and rejection of heat at constant volume. It was conceptualized by the French engineer, Alphonse Beau de Rochas in 1862 and independently, by the German engineer Nicolaus Otto in 1876[citation needed].
A rack and a toothed pinion constituted the mechanical system, which transformed the alternating motion of the piston to the circular motion of the flywheel, with jack, that is, identical to the mechanism used by Barsanti and Matteucci[citation needed]. The four-stroke cycle is more fuel efficient and clean burning than the two-stroke cycle, but requires considerably more moving parts and manufacturing expertise. (In fact, 2-cycles are not inherently inefficient or "dirty". Their simplicity of manufacture has lead them to be the preferred engine for extremely inexpensive applications. Thus, the 2-cycle engines familiar to most are designed for ease of manufacture at the cost of all other features.) Moreover, it is more easily manufactured in multi-cylinder configurations than the two-stroke, making it especially useful in high-output applications such as cars. The later-invented Wankel engine has four similar phases but is a rotary combustion engine rather than the much more usual, reciprocating engine of the four-stroke cycle.
Contents |
[edit] The Otto cycle
The Otto cycle is characterized by four strokes, or straight movements alternately, back and forth, of a piston inside a cylinder:
The cycle begins at top dead centre (TDC), when the piston is furthest away from the crankshaft. On the first stroke (intake) of the piston, a mixture of fuel and air is drawn into the cylinder through the intake (inlet) port. The intake (inlet) valve (or valves) then close(s) and the following stroke (compression) compresses the fuel-air mixture.
The air-fuel mixture is then ignited, 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, at approximately the top of the compression stroke. The resulting expansion of burning gases then forces the piston downward for the third stroke (power) and the fourth and final stroke (exhaust) evacuates the spent exhaust gases from the cylinder past the then-open exhaust valve or valves, through the exhaust port.
[edit] Valve train
The valves are typically operated by a camshaft, which is a rod with a series of projecting cams (lobes), each with a carefully calculated profile designed to push the valve open by the required degree at the right moment and to hold it open as required as the camshaft rotates. Between the valve stem and the cam is a tappet, a cam follower, which accommodates variations in the line of contact of the cam. The location of the camshaft varies, as does the quantities. Some engines have overhead cams, or even dual overhead cams, as in the illustration below, in which the camshaft(s) directly actuate(s) the valves through a tappet. This design is typically capable of higher engine speeds due to fewer moving parts in the valve train. In other engine designs, the cam shaft is placed in the crankcase and its motion transmitted by a push rod, rocker arms, and valve stems.
Starting position, intake stroke, and compression stroke.
Ignition of fuel, power stroke, and exhaust stroke.
[edit] Valve clearance adjustment
The valve clearance refers to the small gap between the valve lifter and the valve stem (or the rocker arm and the valve stem) that acts as an expansion joint in the valve train. Less expensive engines have the valve clearance set by grinding the end of the valve stem during engine assembly and is not adjustable afterwards. More expensive engines have an adjustable valve clearance although the clearance must be inspected periodically and adjusted if required. Incorrect valve clearance will adversely affect running of the engine and may result in burned valves and engine damage.
[edit] Valve clearance measurement
Valve clearance is measured when the piston is at Top Dead Centre of the compression stroke as then all the cylinder's valves are in the closed position. The valve lifter will be resting on the heel of the cam lobe.
A feeler gauge must pass through the clearance space.
If the feeler gauge will not fit in, then the clearance is too small.
If the blade of the feeler gauge fits in too loose then the clearance is too big.
The feeler gauge should fit in and out with a slight drag.
[edit] Valve clearance too wide
A too wide valve clearance will cause excessive wear to the camshaft and valve lifter contact areas, the pushrods can also bend and the engine will be noisy. Should the clearance become wide enough, valve timings will change resulting in poor performance.
[edit] Valve clearance too narrow
A narrow valve clearance will not allow for heat expansion and will result in the failure of the valve to close on its seat. The combustion chamber will not seal properly, producing poor compression and power. It is also possible that the valve can become hot enough to melt.
[edit] Adjusting valve clearance
Valve clearance adjustment must be performed to manufactures specifications. It is normal that the exhaust valve will have a larger clearance.
Adjustment is performed by either adjusting the rocker arm or by placing shims between cam follower and valve stem.
Most modern engines have hydraulic valve lifters and require no adjustment.
[edit] Port flow
The power output of the engine is dependent on the ability of the engine to allow large volume flow of both air-fuel mixture and exhaust gas through the respective valve ports, typically located in the cylinder head. Therefore a great deal of 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-consuming and expensive nature of smoothing the entire intake and exhaust track, compromises in flow for reduction in cost 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 of the cylinder diameter as possible to increase the flow into the intake and exhaust ports is one method. However, increased valve size can increase valve shrouding (the impedance of flow created by the cylinder wall.) To counteract this, valves are commonly designed to open into the middle of the cylinder (such as the Dodge 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 engines computer, optimizing engine operation at any speed or load.
[edit] Output limit
The amount of power generated by a four-stroke engine is ultimately limited by piston speed, due to material strength. Since 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.
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.
An engine where the bore dimension is larger than the stroke is commonly known as an oversquare engine, and such engines have the ability to attain higher RPM. Conversely, an engine with a bore that is smaller than its stroke is an undersquare engine. Respectively, it cannot attain as many RPM, but is liable to make more torque at lower RPM. In addition, an engine with a bore and stroke that are the same is referred to as a square engine.
[edit] See also
[edit] Bibliography
- Hardenberg, Horst O., The Middle Ages of the Internal combustion Engine, Society of Automotive Engineers (SAE), 1999
[edit] External links
- How Cars Work - Full of useful information about how cars work, including Diesel and Petrol 4 Stroke cycle!
- U.S. Patent 194047
- How Car Engines Work
- Barsanti & Matteucci official web site
- Animated Engines: Four Stroke Engine
- Free video: New Four Stroke
- The Fuel and Engine Bible - A good resource for different engine types and fuels
- 4-stroke engine cycle Free video content from CDX eTextbook
