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Four-stroke
The four-stroke internal combustion engine is the cycle most commonly used for automotive and industrial purposes today (cars and trucks, generators, etc).One of the thermodynamics cycle used in internal combustion engine is Otto Cycle.It consist 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. 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. 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.
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 center, when the piston is at its uppermost point. On the first downward 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 upward 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 of 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 upward stroke (exhaust) evacuates the spent exhaust gases from the cylinder past the then-open exhaust valve or valves, through the exhaust port.
Valve timing
In its original configuration, the four-stroke engine relies entirely on the piston's motion to draw in fuel and air (Naturally Aspirated Engine), and to force out the exhaust gasses. As the piston descends on the intake (inlet) stroke, the increasing volume within the cylinder causes a partial vacuum which draws in the air/fuel mixture. This relies on atmospheric pressure. The intake valve then closes, the piston ascends, and the mixture is compressed and ignited, causing the piston to descend again. As the exhaust valve opens, the piston ascends once more and forces the exhaust gases out. This was the technique used in early four-stroke engines. It was soon discovered, however, that at rotational speeds approaching 100 revolutions per minute (RPM) or greater, the exhaust gasses could not change direction quickly enough to exit past the exhaust valve by the piston's motion alone.
At high rotational speeds, consistent flow through the intake and exhaust ports is maintained by allowing the intake and exhaust valves to be open simultaneously at top dead center (known as valve overlap). The momentum of the exhausting gas maintains the outward flow and creates a suction effect on the cylinder known as scavenging, helping to draw the intake charge into the cylinder. In order to retain efficiency, however, the exhaust valve must be closed soon enough so that too much fuel/air mixture from the intake port is not drawn into the engine's exhaust, wasting fuel. In a high-power situation such as racing, where high engine speeds and forced induction are common, this wasted fuel charge can serve to cool the exhaust valve and prevent detonation.
After ignition of the fuel/air charge, as the piston approaches bottom dead center, combustion slows. Just before the charge is finished burning, the exhaust valve is opened at approximately twenty degrees of crankshaft rotation before bottom dead center. This allows the still-expanding gasses inside the cylinder to push out through the exhaust port, starting exhaust flow and giving the exhaust flow momentum. Though a small amount of force is lost through the exhaust port that could be driving the piston, the force that the piston must exert on the gasses to exhaust them from the cylinder is reduced, resulting in increased efficiency.
Exhaust systems in many situations are a compromise between cost of production, optimum flow, low emissions, and low noise levels. Also, exhaust gas must be kept away from the air that the engine's driver or pilot or operator breathes. Restrictions in an exhaust system, including emissions equipment, mufflers, and simple exhaust tubing can restrict proper exhaust flow. In multi-cylinder applications, in which many cylinders share a common exhaust pipe, pressure waves created by cylinders exhausting gas can impede flow of exhaust from other cylinders. Since this prevents exhaust gas from exiting the cylinder, the overlap of the intake valve can result in reversion, when exhaust gas enters the intake port. The internal pressure problems due to a multi-cylinder engine sharing a common intake plenum can be overcome by using a carburetor or injector for each cylinder.
Accomplishing maximum volumetric efficiency for a given engine is not a formulaic process. Variables such as flow rates , overlap, valve lift, porting specifications and the location of valve events create a large set of variables. Different intake and exhaust equipment is tested at different speeds and loads, and the end result is usually a compromise between power, emissions, and cost, except in situations where maximum power is desired regardless of cost or emissions (such as racing.) The new volumetric efficiency and valve run are in animations
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 above, 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.
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 how the engine runs.
If the valve clearance is too wide the engine will be noisy and can also cause undue wear to the camshaft and valve lifter contact areas. The push rods can also be bent. If the clearance becomes wide enough valve timing will be changed and the result will be poor engine power. If the valve clearance is too narrow it can cause problems.
A narrow valve clearance will not allow for heat expansion and will result in the failure of the valve to close on its seat. This results in the failure of the combustion chamber to seal, and thus poor compression and power. The valve will also become quite hot, and the valve can melt.
Some valve clearances are adjusted when the engine is cold, others when the engine is hot. You need to check what the manufacturer recommends. Some engines also have different clearance specifications on the exhaust and intake valves. If this is the case the exhaust valve will normally have the larger of the two clearances.
Valve clearance is measured when the engine is at Top Dead Centre (TDC). At Top Dead Centre, both valves are in the closed position. The valve lifter will be resting on the heel of the cam lobe.
Overhead engines adjust the valve clearance with the adjustable rocker arm. Remove the valve cover, select a feeler gauge in accordance with the specification and try and slide the feeler gauge into 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. The valve clearance is adjusted by turning the rocker arm pivot nut in the centre of the rocker arm. Loosen the lock nut and turn the adjustment nut, then when you have the clearance set, tighten the lock nut once again.
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.
Output limit
The amount of power output 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.
Bibliography
- Hardenberg, Horst O., The Middle Ages of the Internal combustion Engine, Society of Automotive Engineers (SAE), 1999
External links
- How Cars Work - Full of useful information about how cars work, including Diesel and Petrol 4 Stroke cycle!
- U.S. Patent 194,047
- PV & TS diagram of OTTO Cycle
- Animated Engines: Four Stroke Engine
- The Fuel and Engine Bible - A good resource for different engine types and fuels
