The 4-stroke combustion engine

The 4-stroke internal combustion engine

Who invented the internal combustion engine

The first inventor, around 1862, was the Frenchman Alphonse Beau de Rochas. The second, around 1875, was the German doctor Nikolaus August Otto. Since none of them knew about the other's patent until engines were manufactured in both countries, there was a lawsuit. De Rochas earned a certain sum of money, but Otto was left with fame: the thermodynamic principle of the four-stroke engine is still called Otto's cycle.

The four stroke of the internal combustion engine

In any alternative motion motor, the two extreme positions between which a piston can move is called “Upper Dead Point” (PMS) and “Lower Dead Point” (PMI). In the four-stroke engine (below), each piston starts its stroke in the PMS. When you start your first downward movement, an intake valve opens at the top of the cylinder, giving way to gasoline vapor mixed with air. By the time the piston lines the PMI has sucked the precise amount of this fuel. Therefore, this first movement is called admission time.

During the second time — upwards — the intake valve is closed, while the piston compresses the fuel mixture so that it is easily ignited. Accordingly, this time is called compression time.

When the piston approaches the PMS, an electric spark ignites the compressed steam at the top of the cylinder between the spark plug electrodes. The resulting combustion, in which the temperature of the mixture can reach 2,000° C and the force up to 2 tons, pushes the piston down. It's the blast time.

By the time the piston reaches the bottom of the cylinder again, the combustion force has been exhausted. It remains only to allow combustion waste products to pass into the exhaust system, and from it into the atmosphere. At this point, a second valve, the exhaust valve, opens in the cylinder. With this, the piston, in its fourth time, or exhaust time, expels the gases through the top of the cylinder.

This is the four-time cycle theory, but in practice, the different phases are not as clearly separated as the theory suggests. For example, the engine will generate maximum energy if combustion reaches its greatest force when the piston is at the extreme point of its upward stroke (PMS). But combustion is not instantaneous, but begins in the part of the mixture that is closest to the spark plug and spreads in the form of a fan until it burns all. To allow this delay, ignition must occur a fraction of a second - or a few degrees of crankshaft rotation - before the piston reaches the PMS.

Similarly, there is a delay between the moment when a valve is opened and the moment when the fuel vapor or exhaust gas can pass through it at maximum pressure. This is why the valves are often opened a few degrees earlier (advance to opening) or closed a few degrees later (delay to closing), thereby increasing engine performance. These intervals are, of course, minimum fractions of a second, because even when idle, the piston of an ordinary car moves up or down about 1,000 times per minute.

Car builders fix the advance to the opening and the delay to the closing (which, together, are called “solapo” or “crossover” of the valves) for each type of engine, and do so on a diagram of timing of the valves. Generally, the faster an engine runs, the higher the valve crossover.

Although the piston must make four movements to complete a duty cycle, the shape of the crankshaft makes us see that each piston can only describe two times — one up and one down — for each revolution of the crankshaft itself. That is, each piston can only apply force on the crankshaft once every four stroke or two revolutions.

It is perfectly feasible to maintain the rotating inertia of the crankshaft between each explosion time by means of a flywheel or similar mechanism, and therefore it is also possible to build a four-stroke single-cylinder engine.

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