Four-stroke engine

A four-stroke engine is an internal combustion engine , which for the circuit process requires four clocks. A clock is the reciprocating movement of the piston from one end point of the stroke . secondly The crankshaft therefore performs during a cycle by half a turn. Christian Reithmann had on 26 October 1860 received several patents on a four stroke engine. Regardless, in 1861, described the engineer Alphonse Beau de Rochas , the four-stroke cycle. gasoline engines and diesel engines differ in the mixture formation and ignition , but can both four-stroke and also in the two-cycle process work.

1 Clock, sucking

At the beginning of the piston is at the top 1.Taktes dead . The exhaust valve is closed and the inlet valve. The piston moves in the direction of the crankshaft . During the downward movement of the piston, a gas mixture or air through the intake valve into the cylinder sucked. For motors with internal mixture formation , such as diesel engines or gasoline direct injection engines will only sucked air. When external mixture formation , as with carburetor engines or engines with port injection, a mixture of air and atomized fuel is drawn. When the piston reaches the bottom dead center, the intake valve is closed and the first stroke is completed.
2 Stroke, compression and ignition

The piston moves back towards top dead center. The required mechanical work comes from the rotational energy of the flywheel or in multi-cylinder engines of the power stroke of another cylinder. The mixture or the air in the cylinder will now be compressed to a fraction of its original volume. The height of the degree of compression depends on the engine type. In gasoline engines without charging one is compression ratio of about 10:1 common in naturally aspirated diesel engines over 20:1. With charging it is much less. Due to the compression, the mixture with the petrol to about 450 ° C and the air is heated at about 650 ° C on diesel. Just before the top dead center when the gasoline engine is ignition and the diesel engine triggered the pilot. The exact time depends on load and speed.
3 Clock, work

After top dead center – the diesel engine still follows the main injection – burns the mixture charge more independently. The temperature in the burning gas mixture of a gasoline engine is 2200-2500 ° C [1] and the pressure up to 120 bar . The diesel engine is bar from 1800 to 2500 ° C and 160 [1] . The piston moves in the direction of bottom dead center, the combustion gas performs mechanical work on the piston, and cools down. Just before the bottom dead center is the petrol version of residual pressure of about 4 bar and the diesel just under 3 bar. The exhaust valve begins to open.

4 Stroke, ejecting

When the piston bottom dead center again, leaves with the upward movement of the piston, the gas is pushed out of the cylinder. At the end of the exhaust stroke, it is the so-called valve overlap . The intake valve is opened before the piston reaches the top dead center and before the exhaust valve has closed. The exhaust valve only closes shortly after the piston has reached the top dead center.

valve timing

Per cylinder, there is at least one intake and one exhaust valve, but also 3 or 4 valves per cylinder are common, sometimes 5 ( Audi ) or even 8-valve ( Honda NR ). 4-valve engines to achieve, because of faster gas exchange, higher speed and thus more power than two-valve engine. So-called 16V engines are mostly 4-cylinder engines with 4 valves per cylinder.

The valves are in series by one or more engines camshafts controlled. This is from the crankshaft through timing belts , timing chain , spur gears , or a vertical shaft driven. With high-performance motorcycle engines and was the valvetrain formerly often a vertical shaft used (eg Norton). The camshaft rotates at half crankshaft speed. If the camshaft down, that is not in the cylinder head, the hanging valves are in OHV engines on bumpers and rocker actuated at SV engines with standing valves (up to the fifties) directly via tappet. The bumpers can be omitted when the camshaft is located above ( OHC engine, overhead camshaft, currently in the car -engine most commonly used version), then the valves via rocker arms are tappets and rocker arms control. With double overhead camshafts ( DOHC engine, double overhead camshaft), the valves are operated by very light tappets, and the favorable results in the form of hemispherical combustion chamber cylinder head . With two overhead camshafts can be a structurally simple way, a variable valve control to achieve.

Advantages and disadvantages

Advantages and disadvantages of the four-stroke engine over the two-stroke engine
Benefits

The gas exchange takes place largely by volume displacement in the fourth and first stroke (Exhaust / suction), and only to a small extent by the dynamics of the gas column during the valve overlap. This fresh gas and exhaust gas over a wide speed range are well separated, which reduces fuel consumption and improves exhaust emissions.
A closed oil circuit with forced lubrication is standard, resulting in the loss of lubricating oil is very low. Only the oil which is used for lubrication of the compression rings needs be lost due to the principle. By the production quality of modern engines this oil loss is nearly zero. Two-stroke engines can be designed but also with a closed pressure lubrication, but this is usually only implemented in complex large engines. The Wankel engine, the track surface to be lubricated with oil loss.
The thermal stress tends to be lower, since only at every other revolution of the crankshaft, combustion takes place.

Disadvantages

A lower power density of the four-stroke reciprocating engine. This is because the idle stroke: Each cylinder provides only at every second rotation a working cycle, and runs as a revolution wash pump. This results in an uneven delivery of torque. That is not true of the Wankel engine.
Four-stroke engines have a mechanically more complex structure than two-stroke engines. The expense is due to the use of controlled valves and the necessary pressure lubrication.
Higher production costs
More space for the same power consumption and more weight. Particularly important for two-wheelers.

Today’s use

Four-stroke engines now dominate the entire automobile and motorcycle. Even on mopeds with 50 cc (eg Kymco Agility , Keeway ), Lawn mowers and other small appliances they occur, such as the Honda GX25 engine, down to a size of 25 cc.

Variants

Some of the principle of the four-stroke engine varying shapes are technically and economically significant.

It will mass produce engines with variants of timing, see Miller-cycle or Atkinson-cycle process . In automobile occasionally balance shafts used. They reduce the costs of their on-reciprocating piston inertial forces.

For applications where lightweight and independent of position lubricated four-stroke engines are beneficial, there are with two-stroke mixture powered variants. As with other mixed lubricated motors account for oil tank, oil pan , oil pump , oil retention systems and oil filter . By suitable design, fuel and oil, the leaves pollutant emissions from oil combustion in the limits of the emission standard for reducing four-stroke engines. Such motors are preferably used to power portable power tools (eg Stihl “4-mix”).

Certain special types of four-stroke engines do not have camshaft. The valves are actuated pneumatically, hydraulically or electrically. This type of valve train has been established yet in the series engine. But the development of an electromagnetic valve actuator is driven since the late 1990s.

There are also various types of slider controls . Motors in this design, use sleeve or rotary valve for the gas exchange and can get by with fewer moving parts than conventional controlled 4-stroke engines. [2]

Another design is the four-stroke internal combustion engine with no valves, the four bars in the supply of fresh gas, compression, stroke work and flue gases in a known manner. The process is not, however, by slider or camshaft and valves controlled, but by periodically rotated working piston, which carries at its periphery and Ausströmnuten inflow grooves. The rotation takes place over the anchor-shaped connecting rod which, with its teeth in the bottom of the piston located on the ring gear , wherein each crankshaft revolution and engages by one pitch further promoted. Thereby is rotated at each cycle of the piston by four subdivisions. Depending on the scope of the piston, the number of teeth on the ring gear must be divisible by four. The connection between connecting rod and piston is a floating device such as a ball or a spherical bearing. The piston length must be longer than the piston stroke. The complete operating sequence is shown in the attached animation, as described in the Patent Disclosure DE 10 2006 027 166th [3]

A special design of the four-stroke engine is next to the one described here reciprocating engine of the rotary piston Wankel engine , done at the intake, compression, power and eject during a piston rotation.

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