How does an engine work 2

The internal combustion engine - function


How does an internal combustion engine basically work?

In an internal combustion engine, a fuel-air mixture is ignited and burned in a cylinder. The pressure created by the development and temperature-related expansion of the combustion gases acts on a piston, which is displaced as a result. In reciprocating piston engines, the up and down movement of the piston (stroke) is usually converted into a rotary movement by a crank mechanism.

There are a number of different internal combustion engines, some of which use different thermodynamic cycle processes.

All combustion engines repeat the work cycle, which consists of four work steps, in a circular process:

Source: Wikipedia - Barbarossa

Ejecting and sucking in are used for gas exchange, that is, the exchange of exhaust gas (ejecting) for fresh gas (sucking in). The engine control is responsible for the technical implementation.

Compression and work serve to convert chemical energy (combustion of the fuel-air mixture) via thermal energy (heat) and potential energy (pressure) into mechanical energy (torque).

The work steps are often referred to as cycles.

However, this designation does not make sense for 2-stroke machines, as 2-stroke machines also perform all four work steps.

Here you can see the sequence of the 4 cycles on a Mitsubishi GDI cutting motor

Nice animation on the four-stroke principle


2. Compress

(= Perform service)

4. Eject

Description of the four bars

  1. in the first bar In Otto engines, a fuel-air mixture is "sucked" into the cylinder during the downward movement of the piston. In the case of intake manifold injectors, injection is usually in front of the inlet valve, in the case of direct injectors in homogeneous mode into the combustion chamber. Diesel engines only suck in air. The temperature when sucking in is around 100 ° C, the pressure around 01 - 0.3 bar.
  2. During thesecond bar the piston compresses the fuel-air mixture in the cylinder in its upward movement. In the case of direct injectors in stratified charge mode, the injection takes place only now. The final compression temperature is 350 - 500 ° C (diesel 550 - 700 ° C), the pressure rises to 10 to 18 bar (diesel 30 - 55 bar). At the end of the second cycle, ignition takes place, with spark plugs for gasoline engines and compression ignition for diesel engines.
  3. in the third bar burns the fuel-air mixture. As the temperature rises, the pressure of the mixture also rises sharply and moves the piston down in the cylinder. The maximum temperature lies between 2000 and 2500 ° C, the pressure increases to 30 to 40 bar (diesel up to 100 bar). The longitudinal movement of the piston is passed on to the crankshaft via the connecting rod and converted into a torque movement.
  4. The piston moving upwards pushes the fourth bar the burned exhaust gases from the cylinder through the exhaust into the environment. The exhaust gas temperature is 800 ° C when idling, up to 1000 ° C at full load (diesel 250 ° C LL, 600 ° C FL)

Four-stroke Otto process on a cutaway model kfztechde

further animation

Principle of the four-stroke gasoline engine (from Wikipedia - also read the note below)

(Source: UtzonBike - animation from Wikipedia)

Animation from


The movements of cycles one, two and four are carried out by the momentum that the crankshaft equipped with a flywheel has received from the work cycle. Since there is no momentum during the starting process, the crankshaft must be driven from the outside. A starting device such as a rope (chainsaw, boat engine), a pedal crank (motorcycle), a hand crank (vintage car), or a small electric motor (starter in a car) is used for this. Large engines (stationary or marine) are started by compressed air introduced directly into the cylinders.

The gas exchange between incoming fresh gases and burned exhaust gases is controlled by the camshaft. This runs with a reduction of 1: 2 coupled to the crankshaft and opens and closes the valves in the engine's cylinder head.

Comparison of the working procedures

4-stroke process: Each of the four work steps takes place during one cycle. In this case, “stroke” means a piston stroke, that is to say an upward or downward movement of the piston. During a work cycle with four cycles, the crankshaft rotates twice. The gas hub is closed, i.e. fresh gas and exhaust gas are completely separated from one another. In practice, however, there is brief contact during the so-called valve overlap.2-stroke process: Even with the 2-stroke process, all four work steps take place, but during only two piston strokes (= cycles). This is possible because some of the suction and compression (pre-compression) takes place outside the cylinder, in the crankcase under the piston or in a supercharger. The crankshaft rotates only once during a work cycle. The gas exchange is open, which means that fresh gas and exhaust gas are partially mixed.

with: OpenOffice Draw

on: 10/24/2004

by: A. Schierwagen


  • 2-stroke engines have a higher power density because they do work with every revolution of the crankshaft.
  • 2-stroke engines can be built much easier and cheaper because, unlike four-stroke reciprocating engines, they do not require valve control. It is necessary because the inlet and outlet openings for fresh and exhaust gas have to be opened or closed with every second revolution of the crankshaft. In 2-stroke engines, the piston can take on this task, as opening and closing take place with every revolution of the crankshaft.
  • Without valve control, lower inertia forces occur in 2-stroke engines, which is why higher speeds are possible. This additionally increases the power density.
  • 2-stroke engines of conventional design have a higher specific consumption and poorer exhaust gas values ​​because they lose part of the fuel-air mixture unburned by being flushed over. Over-flushing occurs when fresh gas mixes with the exhaust gas and is expelled. This can be prevented by direct injection of the fuel (such as in a two-stroke diesel engine).
  • 2-stroke engines no longer have the same performance as today's 4-stroke engines because, unlike the 4-stroke engines, they have not been further developed, but have been replaced by the 4-stroke engines because of their high consumption and poor exhaust emissions .

Two-stroke engines are mainly used where the price of the engine (simple construction) and the high power density have priority over fuel consumption and environmental protection. This applies above all to engines with a small displacement: mopeds, mopeds, Trabants, chainsaws, model making, motorcycle racing and large marine engines, which are also built as two-stroke diesel engines.

Wherever legislators and consumer interests pay attention to environmental protection and fuel consumption, four-stroke engines have prevailed.


Exotic types of engines

Rotary piston engine

The Rotary engine is a Rotary piston engine, named after Felix Wankel. In a rotary engine, an arched-triangular piston rotates in an oval-disk-shaped housing in an only slightly oscillating movement. The constant movement in the same direction of rotation results in a very smooth motor run.

The rotary piston engine is very compact and does not require any valve control. Apart from the different types of movement, the principle of power generation corresponds to that of the gasoline engine. The four strokes are not carried out during an upward and downward movement of a piston, but rather during the rotary movement in a disk. Just as there are several cylinders in the reciprocating engine, several disks can also be combined in the Wankel engine.

for animation

You can find much more information at


Stelzer engine

The Stelzer engine, named after its inventor Frank Stelzer, is a two-stroke free piston engine. In the Stelzer engine, only the piston is moved during the entire work process. Its different piston diameters open and close various openings in the housing and thus control the gas exchange at the same time.


Spherical piston engine

The basic concept of the spherical piston engine is a rotary piston engine which, as a special feature, realizes both the intake control and the size of the combustion chamber by means of a tumbling movement.

Two-stroke twin piston engine (Puch)

The Puch two-stroke twin-piston engine was a direct-current flushed two-stroke engine, in which two pistons in directly adjacent cylinders share the combustion chamber and act on a crankshaft throat via a split connecting rod. Because of its technical characteristics and its modest fuel consumption, it was used in motorcycles. The Puch works in Graz manufactured engines of this type from 1923 to 1970. Source: Wikipedia

short video about the double piston engine (Deutsches Museum)

5-stroke engine, Miller cycle, Atkinson cycle, articulated connecting rod are other interesting engine variants.

more about the combustion engine

* This article is based on the article 'Internal Combustion Engine' from Wikipedia and is licensed under the GNU Free Documentation License.

more about the engine: | PS | Engine repairs | Control |

Some historical background

125 years of Daimler's gasoline engine

Gottlieb Wilhelm Daimler developed the first high-speed petrol engine in 1883, after which he received the patent on April 3, 1885, and also built the first motor vehicle with a combustion engine in 1885, i.e. 125 years ago.

He was born on March 17, 1834 in Schorndorf and died on March 6, 1900 in Cannstatt near Stuttgart. He was a German engineer, designer and industrialist.

His goal has always been the development of small, fast-running internal combustion engines that should be usable everywhere and drive vehicles of all kinds on land and on water. In 1883 he registered a single-cylinder four-stroke engine with the patent office together with Maybach, who was employed by him, which, unlike the stationary gas-powered four-stroke engines that had been used until then, was powered by the combustion of gasoline. The combustion took place after the gasoline had been gasified in the float carburetor and via glow tube ignition of the gasoline-air mixture that was precompressed in the cylinder. The single-cylinder engine made of cast iron and brass, completed in 1885, weighed only 60 kilograms, had a displacement of 264 cm³ and developed less than 1 hp at 650 revolutions per minute. On April 3, 1885, Daimler received Imperial Patent No. 43926 for its power machine, which went down in technical history as a grandfather clock.

Another invention by Daimler and Maybach was the riding car constructed in 1885, the first motorcycle with a gasoline engine. This was followed by the installation of the gasoline engine in a boat and thus the invention of the motor boat. In October 1886, Daimler built his engine into a carriage, making him the inventor of the four-wheeled motor vehicle.

Daimler's and Maybach's engine (as well as that of Diesel) is one of the inventions that changed the world forever.

Source: Wikipedia user: Enslin

Johannes Wiesinger

edited: 04/26/2019
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