CHAPTER 1 - PART 2 - OPERATION OF SPARK-IGNITION AND COMPRESSION-IGNITION ENGINES

OPERATION OF THE SPARK-IGNITION (SI) ENGINE

The spark-ignition engine is an engine that uses a mixture of air and petrol vapours as fuel. This mixture is drawn into the cylinder through the intake valve.

The mixture is ignited by a spark plug that produces an electric spark, and the gases resulting from combustion exert pressure on the piston, causing it to move and perform mechanical work

The four-stroke operating cycle of the spark-ignition (SI) engine (The four operating strokes)

Stroke 1: Intake

During the intake stroke, the piston moves from TDC (top dead centre) to BDC (bottom dead centre).

The intake valve is open, and the air-fuel mixture is drawn into the cylinder. The cylinder continues to fill until the piston reaches BDC, at which point the intake valve closes.

Stroke 2: Compression

The piston moves from BDC to TDC.

The intake and exhaust valves are closed, and the air-fuel mixture is compressed. Because the process takes place so rapidly, compression occurs without heat exchange with the surroundings and is considered adiabatic.

At the end of compression, the mixture is ignited by the spark produced by the spark plug.

Stroke 3: Combustion and expansion (power stroke)

The valves remain closed, and the piston is at TDC.

The ignited mixture burns progressively, causing the gas pressure to rise. The resulting gases expand and push the piston towards BDC, performing mechanical work.

This process is called the power stroke because, during this phase, the gases produced by combustion push the piston and produce the engine's useful mechanical work.

Stroke 4: Exhaust

When the piston reaches BDC, the exhaust valve opens.

The burnt gases are expelled from the cylinder, and the piston moves from BDC to TDC.

At the end of the stroke, the exhaust valve closes, and the cycle repeats.

OPERATION OF THE COMPRESSION-IGNITION (CI) ENGINE

The compression-ignition engine is a four-stroke engine that uses diesel fuel.

Unlike the SI engine, it does not use a spark plug for ignition. Fuel is introduced into the cylinder through an injection system at high pressure.

The four-stroke operating cycle of the compression-ignition engine (CI) (The four operating strokes)

Stroke 1: Intake

During the intake stroke, the intake valve is open and the exhaust valve is closed. The piston moves from TDC to BDC, creating a partial vacuum in the cylinder.

As a result, only air is drawn into the cylinder, rather than an air-fuel mixture. In theory, the process takes place at approximately constant pressure and is considered isobaric.

At the end of the stroke, the cylinder is filled with air and the intake valve closes.

Stroke 2: Compression

Both the intake and exhaust valves are closed, and the piston moves from BDC to TDC.

The air is highly compressed, and its temperature rises significantly. The process is considered adiabatic because it takes place very rapidly, without significant heat exchange.

Stroke 3: Combustion and expansion (power stroke)

Both the intake valve and the exhaust valve are closed, and the piston is at TDC. At this point, the injection pump sprays very fine droplets of diesel fuel into the cylinder.

Due to the high temperature of the highly compressed air, the fuel auto-ignites, and combustion produces gases at high pressure and temperature. As these gases expand, the piston is pushed towards BDC, performing mechanical work, which is then transmitted through the connecting rod to the crankshaft.

Stroke 4: Exhaust

During the exhaust stroke, the intake valve is closed and the exhaust valve is open. The piston moves from BDC to TDC, expelling the gases produced by combustion.

At the beginning of this phase, the pressure in the cylinder drops rapidly, approaching atmospheric pressure.

At the end of the stroke, the exhaust valve closes, and the operating cycle repeats.

Advantages of the compression-ignition (CI) engine over the spark-ignition (SI) engine

• the fuel (diesel fuel) presents a lower risk of accidental ignition;
• fuel consumption is lower.

Disadvantages of the CI engine compared with the SI engine

• more complex construction and higher costs;
• greater engine mass;
• the need for more powerful starting systems (starter motor, battery);
• a higher noise level during operation;
• higher compression ratio.

DESIGN AND OPERATING CONCEPTS

BDC (bottom dead centre) is the position of the piston when it is furthest from the cylinder head. In engines with a conventional layout, this corresponds to the lowest point reached by the piston.

TDC (top dead centre) is the position of the piston when it is closest to the cylinder head. In engines with a conventional layout, this corresponds to the highest point reached by the piston.

The combustion chamber is the space in which combustion of the air-fuel mixture takes place.

Swept volume per cylinder is the volume swept by the piston between TDC and BDC.

Engine displacement is the sum of the swept volumes of all the engine's cylinders.

Engine speed is the number of crankshaft revolutions per minute.

The engine cycle is the complete sequence of processes that the air-fuel mixture undergoes before the initial operating conditions are restored.