Overview of engine & Operating principles

Overview of engine

Internal-combustion engine

In order for agricultural machinery to do their job, power is needed, and the device that generates that power is the motor. Generally speaking, a motor is a device that converts the energy of heat, electricity, water, wind, and nuclear power, into mechanical energy (power). Among motors, the devicethat converts heat energy into mechanical energy is called a heat  engine.

There are broadly two types of heat engine, according to the location where the heat energy for converting power is. There is the external combustion engine, for which fuel is burned externally to the engine, such as a steam engine, and there is the internal combustion engine, for which fuel is burned inside the engine, such as a gasoline engine or diesel engine.

Operating principles

The engine explosively burns fuel in a cylindrical container called a cylinder, creating a high temperature, high pressure gas, and the pressure from that pushes down the piston, which in turn rotates the crankshaft via a connecting rod, producing power. Also, a flywheel is mounted so that the rotary motion of the crankshaft is smooth.

The three elements necessary for combustion are combustible material (fuel), air (oxygen), and ignition source, and various mechanisms are built into the engine to let these elements into the cylinder.

In order for the reciprocating motion of the piston to convert into rotary motion of the crankshaft, and continuously rotate the engine, the actions of suction, compress, explosion, and exhaust must be performed in proper sequence and continuously. This series of strokes is called the engine cycle. A four-cycle engine refers to an engine that completes one cycle of suction through to exhaust in two rotations of the crankshaft, while a two-cycle engine refers to an engine that completes one cycle in one rotation of the
crankshaft.

Stroke

When the explosive pressure is received and the piston moves up and down in a reciprocating motion, the uppermost position of that vertical motion is referred to as top dead center, and the lower-most position is bottom dead center. The distance between the top dead center and the bottom dead center, in other words, the distance that the piston moves from the bottom dead center to the top dead center, is called the stroke.

When the piston moves from the top dead center to the bottom dead center, the crankshaft rotates 180°. When the piston does one round trip, the crankshaft rotates 360°. That is, with two strokes, the crankshaft rotates once, and with four strokes, the crankshaft rotates twice.

(1) Four-cycle engine

A four-cycle engine is an engine that performs four strokes of suction, compress, explosion (combustion), and exhaust, in one explosion cycle. In a four-cycle engine, the crankshaft rotates twice in one cycle, and the piston performs two return trips.
1) Gasoline engine

Suction stroke: When the piston begins to move downward from the top dead center, the intake valve is open, generating negative pressure inside the cylinder, and a mixture of
air and gasoline is sucked in.

Compress stroke: When the piston begins to move upward from the bottom dead center, the intake valve closes, and the air-fuel mixture that was sucked in is gradually compressed.

Explosion stroke: As the compress stroke nears its end, ignition and combustion is triggered by the electrical spark of the spark plug, causing the temperature and pressure to rise suddenly, pushing down the piston. This explosion pressure becomes the engine
output.

Exhaust stroke: When combustion has ended and the piston nears the bottom dead center, the exhaust valve opens. The piston moves upward due to the inertia of the crankshaft, pushing out the combustion gas.

Advantages

(1) Relatively compact and lightweight, yet easily obtains high horsepower.
(2) Minimal vibration and noise

Drawbacks

(1) High fuel consumption compared to a diesel engine.
(2) The structure is simple, meaning that it costs less, but it is inferior in durability
compared to a diesel engine.

(2) Two-cycle engine

In a two-cycle engine, the crankshaft rotates once and an explosion occurs on each rotation, during the cycle from suction to exhaust.

Suction and compress stroke: As the piston rises, the pressure inside the crank chamber becomes lower than the atmospheric pressure, causing the inlet to open and the air-fuel mixture is then sucked into the crank chamber. Also, while the piston is rising, the exhaust outlet and intake opening close, and the air-fuel mixture is compressed inside the cylinder.

Explosion stroke: As the compress stroke nears its end, ignition and combustion is triggered by the electrical spark of the spark plug, causing thetemperature and pressure  to rise suddenly, pushing down the piston.

Exhaust and scavenging stroke: When the piston lowers, the intake opening closes, the air-fuel mixture that was sucked into the crank chamber during the preceding stroke is
pushed out, and enters the cylinder via the scavenging passage. The air-fuel mixture that enters the cylinder pushes out the exhaust gas that was generated by the previous combustion, and the gas is exchanged.

Comparison of four-cycle and two-cycle

◆ Four-cycle

(1)There is little loss of fuel from "gas blowout", and fuel consumption is low.
(2)There is no interruption to the explosion from incomplete exhaust, so operation is smooth at low speed.
(3)The division of the work performed by each stroke is precise and carried out in an ideal manner so
there is little danger of too much heat being added to each section.
(4)The valve mechanism is complex, and there is a large number of parts, so there is a lot of mechanical noise.
(5)Because the number of explosions is small, it is necessary to increase the number of cylinders for smooth operation.

◆ Two-cycle

(1) Does not generally use valves, making it more affordable.
(2) Because the number of explosions is double that of a four-cycle engine, the two-cycle engine generates higher output for the same number of "exhaust amount" rotations.
(3) The intake and exhaust times are about half that of a four-cycle engine, therefore each tends to be insufficient.
(4) The engine may be erratic at low rotation speeds.
(5) Because there is a scavenging passage and exhaust outlet in the middle of the cylinder, the piston and cylinder can easily become overheated.
(6) It is necessary to mix the fuel and lubricant oil.