Four-stroke engine

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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 four steps in this cycle are often informally referred to as "suck, squeeze (or squash), bang, blow."

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 and 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 crankshaft. On the first stroke (intake/induction) of the piston, as the piston descends it reduces the pressure in the cylinder, a mixture of fuel and air is forced, by at least atmospheric pressure, 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 pushes the piston downward for the third stroke (power) and in the fourth stroke (exhaust) 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, with a series of cams along its length, each designed to open a valve appropriately for the execution of intake or exhaust strokes while rotating at half crankshaft speed. A tappet between valve and cam furnishes a contact surface on which the cam slides to open the valve. The location of the camshaft varies, as does the quantity. Most engines use overhead cams, or even dual overhead cams, as in the illustration, in which cams directly actuate valves through a flat tappet. This design is typically capable of higher engine speeds because it gives the most direct and shortest inelastic path between cam and valve. 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

Valve clearance refers to the small gap between valve lifter and valve stem (or rocker arm and valve stem) that ensures that the valve completely closes, an expansion joint in the valve train. On engines that require valve adjustment, excessive clearance will cause excessive noise from the valve train "hammering" during operation. In either case, an improperly adjusted valve clearance will cause reduced engine performance, reduced engine life and excessive noise.

Most engines have the valve clearance set by grinding the end of the valve stem during engine assembly, overhead cams not needing subsequent adjustment. All engines with poppet type valves make some sort of allowance for maintaining this "expansion joint", while less sophisticated engines use solid, "non-adjustable” components which are simply ground-off at the contact points to provide the correct clearance (though the low efficiency of this design may not be practical in when the cost of labor is very high). Another method is to provide some method of manually changing the clearance with adjustable screws or shims, the implementation of which depends on and varies widely with the design of the engine. Manual valve lash adjustment is used in almost all very high-performance engines because the hydraulic adjusters used in "automatic" systems are often affected by the extreme valve train accelerations of ultra high-speed engines.

Most modern and advanced production engines use some form of automatic valve adjustment (usually hydraulic) to maintain a state known as "zero lash". In pushrod and some OHC engines this adjuster is incorporated into the tappet, lash adjuster or tip of the rocker. Many DOHC engines now employ tiny hydraulic lash adjusters in the top of the cam followers to maintain "zero lash". "Zero lash" is a desirable condition, since this allows for very quiet engine operation. Hydraulic lifters or lash adjusters also reduce required maintenance, reduce noise, help engines to perform at peak efficiency and minimize exhaust emissions by compensating for wear and expansion of various engine components. Earlier engines, mostly those with push rods and rocker arms, used adjustable tappets or hydraulic lifters to automatically compensate for valve train component and camshaft wear. Lack of valve clearance will prevent valve closure causing leakage and valve damage.

Valve clearance adjustment must be performed to manufacturer's 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 lifters and require infrequent adjustment.

Valve clearance measurement

Valve clearance is measured with the valve closed, typically at Top dead center of the compression stroke. 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 loose then the clearance is too big.

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.

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 becomes so hot that it melts.

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 closing force (normally provided by the valve spring) is inadequate to overcome the inertia of the drivetrain, and can potentially result in valve contact with the piston and causing major damage. Various countermeasures are reducing the valve train mass; for instance, using 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.

Rod length and stroke length are independent variables. Rod length is expressed as Center-to-Center (c/c) length. An engine with a particular stroke can be fitted with rods of several c/c lengths by changing the piston pin location or block deck height. A rod that is longer in relation to stroke causes the piston to dwell a longer time at top dead center and causes the piston to move toward and away from TDC more slowly. Long rod engines with a particular stroke also build suction above the piston with less force, since the piston moves away from TDC more slowly. Consequently, long rod engines tend to produce a lower port air velocity, which also reduces low speed torque. Long rods place less thrust load on the cylinder walls, thus generate less parasitic drag and result in less frictional losses as engine revolutions rise. A "short rod" engine has the opposite characteristics. “The short rod exerts more force to the crank pin at any crank angle that counts i.e.--20° ATDC to 70° ATDC” (Jere Stahl ). Short rod engines tend develop more torque at lower engine speeds with torque and horsepower falling off quickly as engine RPM rises to high levels. Long rod engines generally produce more power due to reduced engine drag, especially as engine RPM increases. Regardless of rod length for a given stroke, the average piston speed (usually expressed in Ft/sec or M/sec) remains the same. What changes as the rod length becomes shorter or longer in relation to the stroke, is the RATE of motion as the piston rises and falls in relation to the crankshaft. A long rod fitted to a given stroke generates less stress on the component parts due to the lower rate of acceleration away from and toward TDC. The average piston speed is the same; however, the peak piston speed is lower with long rods.

There is no "Ideal" rod to stroke ratio, however a ratio of about 2 to 1 seems to be the upper practical limit and 1.5 to 1 the lower limit in general practice. The Chevrolet 350 engine with a 3.48" stroke and a c/c rod has a rod/stroke ratio of 1.638 to 1. The durability and longevity of this engine seems to prove that this is a “acceptable” figure for a rod/stroke ratio number. The "small block 400" used a 3.75" stroke and a rod c/c of 5.565" for a ratio of 1.484. The SB 400 was known for torque and "running out of breath" at high engine speeds. Even with large port heads and high lift camshaft, the S/B 400 ran into a "wall" of friction when engine speeds climbed above 5000 rpm. S/B 400s we also know for wearing piston skirts and cylinder walls at a faster rate than their smaller brothers. Many people that race the S/B 400 convert the engine to 5.7 or 6.0 rods to reduce the effects of the long stroke crankshaft and lower friction within the engine. The 1967-1969 Z-28 302 engine was fitted with a 3.0" stroke crank and in some racing applications used up to a 6.0" rod, resulting in a 2 to 1 rod/stroke The 302 Chevrolet V-8 was famous for phenomenal power in the upper RPM range while it sacrificed low speed torque to gain the high RPM power and reliability.

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.

Bibliography

• Hardenberg, Horst O., The Middle Ages of the Internal combustion Engine, Society of Automotive Engineers (SAE), 1999

See also

• 4-stroke power valve system
• Atkinson cycle

External links

• Detailed Engine Animations
• How Cars Work - lots of general and specific car information, with forums
• How Four-stroke Small Engines Work
• How Car Engines Work
• Barsanti & Matteucci Foundation - "fathers of the internal combustion engine"
• Animated Engines: another explanation of the four stroke engine
• Free video: New Four Stroke
• The Fuel and Engine Bible - A good resource for comparing different engine types, fuels, and engine components
• CDX eTextbook - some videos of car components in action
• View from a camera mounted inside a four-stroke engine cylinder

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Last updated on Wednesday March 12, 2008 at 09:43:48 PDT (GMT -0700)
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