Internal Combustion Engine Ignition and Combustion

Ignition Delay in SI and CI Engines

Ignition Delay in SI Engines

In Spark Ignition (SI) engines, ignition delay (also known as ignition lag) is the time interval between the spark and the start of combustion (the AB phase). This delay depends on several factors, such as temperature, pressure, the air-fuel ratio, and the specific fuel structure.

Ignition Delay in CI Engines

In Compression Ignition (CI) engines, ignition delay is the time interval between the start of fuel injection and the start of combustion (the AC phase). During this period, the fuel undergoes atomization, vaporization, and mixing with air. This process consists of both physical delay and chemical delay.

Factors Affecting Ignition Delay in CI Engines

Ignition delay decreases as pressure, temperature, and the compression ratio increase. It also depends on the nature of the fuel (specifically its self-ignition temperature) and the injection pressure, where improved atomization helps reduce the delay.

Combustion Efficiency in SI Engines

Combustion efficiency in SI engines depends on various factors that affect the rate of combustion and the completeness of the air-fuel mixture burning.

• Air-fuel ratio: A proper air-fuel mixture is essential for complete combustion. A mixture that is too rich or too lean results in incomplete combustion and reduced efficiency.
• Ignition timing: Correct ignition timing ensures that maximum pressure is developed at the optimal moment in the cycle. If ignition occurs too early or too late, combustion efficiency decreases.
• Turbulence: Increased turbulence within the combustion chamber improves the mixing of air and fuel while increasing flame speed, resulting in faster and more efficient combustion.

Pre-ignition, Auto-ignition, and Fuel Additives

• Pre-ignition: This is a phenomenon where the air-fuel mixture ignites before the spark occurs. It is caused by hot spots, such as carbon deposits or an overheated spark plug, and leads to power loss and potential engine damage.
• Auto-ignition: This is the self-ignition of the air-fuel mixture without an external source once it reaches its self-ignition temperature. This can lead to abnormal combustion.
• Additives and Dopes: These are chemical substances added to fuels in small quantities to improve anti-knock properties, increase the octane number, and ensure smooth combustion.

The Battery Coil Ignition System

The Battery (Coil) Ignition System is the most widely used system in petrol engines. It utilizes electrical energy stored in a battery, increases it to a high voltage using an ignition coil, and delivers it to the spark plugs at the correct instant and in the proper firing order.

Step-by-Step Working Principle

1. CB Points Close: Current flows from the battery through the switch, ballast resistor, and primary winding to the contact breaker (CB) points and ground. A strong magnetic field builds up around the iron core of the ignition coil.
2. CB Points Open: Triggered by the distributor cam, the primary circuit is broken, causing the magnetic field to collapse rapidly. According to Faraday's law of electromagnetic induction, a very high EMF (15,000–20,000 V) is induced in the secondary winding.
3. Condenser Action: When the CB points open, the condenser charges instantly, absorbing the primary current. This prevents arcing at the points and ensures the field collapses faster, increasing the peak secondary voltage.
4. HT Current to Distributor: The 20,000 V High Tension (HT) current travels from the secondary coil through the HT lead to the central terminal of the distributor rotor arm.
5. Spark at Plug: The rotor arm directs current to the appropriate spark plug terminal (following the firing order, e.g., 1–3–4–2). The current jumps the spark plug gap (0.6–0.9 mm), creating a hot spark that ignites the mixture.
6. Ignition Timing Control: A centrifugal advance mechanism advances timing automatically as speed increases. A vacuum advance mechanism advances timing at part throttle for better fuel economy.

Limitations of Battery Ignition Systems

• Mechanical wear of CB points: The CB points are rubbed by a rotating cam, causing wear. As the gap changes, the dwell angle (the time the primary circuit is closed) changes, leading to reduced coil saturation and a weak spark. Points require adjustment or replacement every 10,000–15,000 km.
• Weak spark at high speeds: At high RPM, the points open and close rapidly. Dwell time decreases, meaning the coil does not fully saturate, causing the secondary voltage to drop and leading to misfiring.
• Voltage drop during cranking: The ballast resistor drops voltage, and the battery voltage also drops due to the starter motor load. This results in a weaker spark and difficult cold starts.
• CB point arcing: Despite the condenser, some arcing occurs at the CB points, leading to pitting, erosion, and oxidation. This alters the gap and timing over time, requiring frequent servicing.