Basic principle of an engine
An internal combustion engine, also called an internal combustion engine or a combustor, converts the energy released during combustion into mechanical work. There are different types of engine, for example the reciprocating piston engine, the rotary piston engine (also called Wankel engine) or, as a subspecies of the reciprocating piston engine, the radial engine. It is characteristic of all that the fuel is burned within the engine. In cars there is usually a piston engine under the front hood, the others are more exotic.
As a result, we will look at how a reciprocating engine works, more precisely a four-stroke reciprocating engine. That is because its mode of operation can be divided into four bars.
Before we imagine the engine in motion, here are the most important components of the engine at rest:
Spoiler: For the wheels of the car to turn, the crankshaft must first turn. And it works that way.
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Everything Otto or what?
The term petrol engine is also mentioned again and again. It is a collective term for all reciprocating engines with spark plugs. The term goes back to Nicolaus Otto, who did not invent this type of engine, but instead invented the four-stroke system, which we explain below.
Article section: The four strokes of a four-stroke engine
The four cycles of a four-stroke engine
1st cycle: suction
When intake, the intake valves open and a mixture of fuel and air flows into the combustion chamber. The exhaust valves are closed. The piston in the cylinder moves from top dead center to bottom dead center,
Special case: With modern direct injection engines, it works a little differently. Here, only air is drawn in via the inlet valve, and the gasoline is then injected directly into the cylinder.
2nd bar: condense
When compressing, all valves are closed and the piston races back in the cylinder from bottom dead center to top dead center. The higher the compression, the more efficiently the fuel-air mixture burns in the next step. To ensure that the piston seals the space to the cylinder as far as possible and achieves high compression, it has piston rings on the edge. In order for these to rub and grind as little as possible, the cylinder wall must be wetted with lubricants.
3rd measure: work
Shortly before the piston reaches top dead center, the spark plugs ignite the compressed mixture of fuel and air. There is an explosion. Due to the pressure of the explosion, the piston races again towards the bottom dead center – it works. He transfers the force of the explosion via the connecting rod to the crankshaft, which rotates as a result. This rotation is transmitted to the car’s wheels via the gearbox.
4th bar: expelling
At the fourth stroke, the exhaust valves open and the piston pulls one last time from bottom to top dead center for this passage. The combustion exhaust gases thus reach the exhaust via the exhaust manifold. And the whole thing starts again.
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Knock in the engine – what is it?
Depending on how much pressure there is in the cylinder, the fuel can ignite even before the actual ignition, even without ignition sparks. Then a second flame front forms, which rushes against the actual flame front. With this uncontrolled combustion, often at high speeds, the engine starts knocking. To avoid this, the fuel must first be flammable at a certain temperature – in the case of petrol, this means the octane number (OZ). You can find out more about the octane number and its significance for fuel here.
Speed, crankshaft, camshaft?
During one cycle of the four cycles, the crankshaft rotates twice around itself. The engine speed, which is always displayed on the dashboard, indicates how often the crankshaft rotates per minute. At an average engine speed, the crankshaft rotates around 3,000 times a minute, at high speeds it is up to 6,000 times a minute. Formula 1 racing car engines spin up to 15,000 times a minute.
The higher the speed, the more fuel the car consumes. Because, of course, every time the four cycles are run, fuel is added to the cylinder. or in the cylinders, because there are hardly any single-cylinder engines in cars. In most engines, such as a VW Golf, four cylinders are positioned one behind the other and are called inline four cylinders. But there are also engines with two cylinders (Fiat 500) and those with up to 16 cylinders (Bugatti Chiron) in the car, these can be arranged in a V-shape, row or as a boxer engine.
Incidentally, the crankshaft not only drives the car, but also indirectly opens and closes the valves on the cylinders. It is connected to the camshaft via the timing chain or toothed belt. In such a way that the speed of the camshaft corresponds exactly to half the speed of the crankshaft – after all, the valves should only open and close once in a cycle of the four cycles.
Article Section: The Difference Between Otto And Diesel Engine
The difference between petrol and diesel engines
Otto and diesel engines differ mainly in the mixture formation and the ignition. Depending on how it works, a combustion engine fills up with super petrol (petrol engine) or diesel fuel (diesel engine).
In the petrol engine, the gasoline-air mixture is either injected in front of the cylinder and the intake valves in an intake manifold (external mixture formation) and then routed together into the cylinder. Or, as with direct injection engines, the gasoline is injected directly into the combustion chamber and only mixes with the air there. In order for the fuel in the cylinder to ignite, the gasoline engine needs support from a spark plug. That is why the gasoline engine is also called spark igniter.
Diesel without spark plug
In the diesel engine too, the fuel ignites in the cylinder during the work cycle. However, the diesel engine can do without the spark plug as spark ignition, since its diesel fuel self-ignites due to strong compression at the end of the compression stroke – the temperature of the air is then above the ignition temperature of the fuel. That is why the diesel is also called a diesel.
In contrast to the gasoline engine with external mixture formation, the diesel draws in air unthrottled over the entire speed range and only forms the fuel-air mixture in the cylinder (internal mixture formation). For this purpose, the amount of fuel is precisely matched to each speed and load condition.
The maximum compression pressure for diesel engines is significantly higher, almost twice as high as for gasoline engines. The maximum combustion pressure is also twice as high as that of gasoline engines. That is why diesel engines are more solid and heavier than gasoline engines.