It is an engine that uses as fuel a mixture of air vapors and gasoline vapors. This mixture is drawn through the intake valve into a cylinder with a piston provided with rings. The combustion of the fuel takes place with the help of a spark plug that produces a spark, the gases produced push the piston thus performing the mechanical work.
Stroke 1: intake - during the intake the piston moves from PMI (inner dead center), the cylinder is filled with fuel mixture.
When the movement of the piston begins, the intake valve opens to allow the filling of the cylinder with fuel mixture. The filling of the cylinder is done until the piston reaches PME (outer dead center) and the intake valve closes.
Stroke 2: compression - the piston moves from PME to PMI. The intake valves as well as the exhaust valves are closed and the piston moves upward, to the PMI position. The mixture is strongly compressed and ignites when the spark plug will produce an electric spark. Due to the high speed at which the compression takes place, the process takes place without heat exchange with the outside, being an adiabatic compression.
At the end of the compression, the pressure inside the cylinder reaches 7-8 daN/cm2.
Stroke 3: ignition and expansion - the exhaust valves as well as the intake valves are closed.
The piston is in the PMI position and the fuel mixture is strongly compressed and ignites with the help of the spark plug that produces an electric spark. Following the explosion, the pressure increases which takes place practically at constant level. The mixture burns progressively throughout its mass and the gases resulting from the combustion push the piston into the PME position, thus performing the mechanical work.
Stroke 4: exhaust - when the piston reaches the PME position the exhaust valve opens and the burnt gases are expelled into the atmosphere until the pressure in the cylinder becomes equal to the atmospheric one. The crankshaft (through the flywheel which accumulates kinetic energy) pushes the piston from the PME position to the PMI one, expelling the burnt gases and at PMI the valve closes.
The volume of the gases decreases as the piston moves to PMI and the cycle repeats.
PME - outer dead center: represents the position of maximum distance from the cylinder head of the piston of an internal combustion engine, a position considered towards the outside of the engine (it is the lowest point where the piston reaches). In older technical language, it was called PMI (lower dead center), a name considered improper in the case of engines with horizontally arranged cylinders and therefore was reformulated.
PMI - inner dead center: represents the position of minimum distance from the cylinder head of the piston of an internal combustion engine, a position considered towards the inside of the engine (it is the highest point where the piston reaches). In older technical language, it was called PMS (upper dead center), a name considered improper in the case of engines with horizontally arranged cylinders and therefore was reformulated.
The combustion chamber – is the place where the chemical reaction of fuel combustion takes place.
Unit displacement – represents the volume generated by the piston in its movement between the lower dead center and the upper dead center.
Engine capacity – is obtained by multiplying the number of pistons of the engine by the unit displacement.
Engine speed – represents the number of rotations performed by the crankshaft in one minute.
The operating cycle of an engine – The totality of the successive states through which the fuel mixture passes in a transformation, starting from an initial state until it returns to the initial state, is called the engine cycle.
The engine with compression ignition is a four‑stroke internal combustion engine with compression ignition that uses diesel fuel.
The place of the spark plug is taken by an injection pump that sprays fuel into the engine cylinder at high pressure.
Stroke 1: intake - the intake valves are open, the piston moves from the PMI (inner dead center) position to the PME (outer dead center) position, and air at atmospheric pressure is drawn into the cylinder. The exhaust valves are closed.
The process is an isobaric one, taking place at an approximately constant pressure.
Stroke 2: compression - the intake valves as well as the exhaust valves are closed. The piston moves from the PME position to the PMI position and the air is strongly compressed. This process is an adiabatic one, everything happening very quickly without heat exchange.
Stroke 3: ignition and expansion - the intake valves as well as the exhaust valves are closed. The piston is in the PMI position. The injection pump sprays very fine droplets of diesel into the cylinder. The temperature in the cylinder is higher than the ignition temperature of the fuel and it ignites and will burn at constant temperature. The gases resulting from the combustion will expand adiabatically, the piston will move to the PME position, thus performing the mechanical work.
Stroke 4: exhaust - the intake valves are closed, and the exhaust valves are open. The pressure in the cylinder drops suddenly until it reaches the value of the atmospheric pressure, the process taking place at constant volume, therefore isochoric. The heat is released to the outside. The piston moves from the PME position to the PMI position, expelling the combustion gases. When the piston reaches the end of the stroke, the exhaust valves and the intake valves close. The cycle repeats.
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