Four-stroke engine

A four-cycle engine is an engine which requires four strokes of the cycle. A clock is the reciprocating movement of the piston from an end point of the stroke, on the other. Therefore, the crankshaft makes one half turn during a cycle. Christian Reith man had received several patents on a four stroke engine on 26 October 1860. Regardless described in 1861 of Alphonse Beau de Rochas engineers the four-stroke cycle. Gasoline engines and diesel engines differ in the mixture formation and ignition, but can both four -stroke and two-stroke process work.

• 3.1 Advantages
• 3.2 disadvantages

Operation of a four-stroke reciprocating piston engine

The following procedure describes the four-stroke process:

1 clock, sucking

At the beginning of 1.Taktes the piston is at top dead center. 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 is drawn through the intake valve into the cylinder. For motors with internal mixture formation, such as diesel engines or gasoline direct-injection, only air is drawn. For external mixture formation, as with carbureted engines or engines with manifold injection, a mixture of air and the atomized fuel is drawn. When the piston reaches the bottom dead center, the intake valve is closed and the first cycle is completed.

2 -stroke, compression and ignition

The piston moves back towards top dead center. The mechanical work required for this comes from the rotational energy of the flywheel or in multi-cylinder engines from the stroke of another cylinder. The mixture or the air in the cylinder will be compressed to a fraction of its original volume. The height of the degree of compression is dependent on the motor construction. In gasoline engines without charging a compression ratio of about 10:1 is common in diesel engines without supercharging about 20:1. By charging it is much less. By compressing the mixture in the petrol engine to about 450 ° C and the air is heated to about 650 ° C for diesel. Shortly before reaching the top dead center, the pilot is triggered when the ignition gasoline engine and the diesel engine. The exact time is dependent on load and speed.

3rd bar, work

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

4 -stroke, ejecting

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

Valve control

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

The valves are controlled in production engines by one or more camshafts. This is driven by the crankshaft through timing belts, timing chain, spur gears or a vertical shaft. In high-performance motorcycle engines and a vertical shaft for the valve train was previously often used (for example, Norton ). The camshaft rotates at half crankshaft speed. If the camshaft down, that does not mean in the cylinder head, the hanging valves with OHV engines are actuated via pushrods and rocker arms, at SV engines with standing valves (up to the fifties ) directly via tappet. The bumpers can be omitted if the camshaft is located above ( OHC engine, overhead camshaft, currently the variant most commonly used in car engines), then the valves via rocker arms, tappets or rocker arms are controlled. With double overhead camshafts ( DOHC engine, double overhead camshaft) valves can be actuated by extremely lightweight tappets, and it results in the favorable hemispherical combustion chamber shape in the cylinder head. With two overhead camshafts can be achieved variable valve timing in a structurally simple way.

Pros and Cons

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 cycle ( ejection / suction), and only to a small extent by the dynamics of the gas column during the valve overlap. This means that fresh gas and exhaust gas over a wide speed range can be well separated from each other, which reduces fuel consumption and improves exhaust performance.
• A closed oil circuit with pressure lubrication is standard, resulting in the lubricating oil loss is very low. Only the oil serving for lubrication of the compression rings is thereby inherently lost. By producing quality modern engines tends this oil loss to zero. Although two-stroke motors can also be designed with a closed pressure circulation lubrication, but this is usually implemented only in expensive large-scale engines. In the Wankel engine, the track surface must be lubricated with oil loss.
• The thermal stress tends to be low, since only every second revolution of the crankshaft, a combustion takes place.

Disadvantages

• A lower power density of the four-stroke reciprocating engine. This is due to the idle stroke: Each cylinder provides only every second revolution a stroke and undergoing a revolution as a scavenge pump. This results in an uneven delivery of torque. However, this does not apply to 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

Today's use

Four -stroke engines now dominate the entire automobile and motorcycle. Even on mopeds with 50 cc (eg Kymco Agility, Keeway ) at 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 forms are technically and economically significant.

There are standard production engines with variants of the clock, see Miller cycle or Atkinson - cycle process. In the automotive industry balance shafts are used occasionally. Reduce the costs of their on - reciprocating piston forces of inertia.

For applications in which are light and position-independent lubricated four-stroke engines of advantage there is driven by a two- stroke mixture variants. As with other engines lubricated mixture dispensed oil tank, oil pan, the oil pump, oil retention and oil filter systems. By suitable design, fuel oil and the emission of pollutants can be reduced for four-stroke engines by oil combustion in the limits of the emission standard. Such motors are preferably used as the drive motor for portable devices (such as Stihl " 4- Mix" ).

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

Further, there are various types of valve control systems. Engines these designs use sleeves or rotary valve for the gas exchange and come with fewer moving parts than conventional controlled 4- stroke engines.

Another design is the four-stroke internal combustion engine without valves (2), works in the four measures fresh gas supply, compression, power stroke and exhaust gas discharge in a known manner. Here, however, the process is not controlled by valve or valves and the cam shaft, but rotated by the periodically working piston, which carries on its periphery and Ausströmnuten inflow grooves. The rotation via the rod -shaped armature which, with its teeth in the toothed rim situated at the underside of the piston which engages in each crankshaft revolution, and transported further by one pitch. Thereby, the piston is rotated about four divisions in each cycle. Depending on the scope of the piston, the number of teeth on the sprocket must be divisible by four. The connection of the 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 sequence of operations is shown in the accompanying animation, as it is also described in the patent disclosure document DE 10 2006 027 166.

A special design of the four-stroke engine is next to the reciprocating engine described here, the rotary piston rotary engine, made ​​at the intake, compression, power, exhaust piston during a revolution.

Trivia

All attributes used here of the place (top dead center, downward movement, bottom, single overhead camshaft ) are fixed terms and do not change when an engine " lying " or " hanging " ( upside down ) is operated.