Spark ignition system

IC ENGINES (ME-424) 12TH WEEK

12/4 17/4

University of Engineering and Technology Lahore, Pakistan

by

Dr. Muhammad Mahmood Aslam Bhutta

Department of Mechanical Engineering

PakistanLahore

DR. Muhammad Mahmood Aslam Bhutta Lahore,Pakistan

Objectives of Ignition System

1. Generate heat to Initiate the combustion of the air fuel mixture

2. Quantity of heat must be enough to create a self sustaining flame


Combustion in SI Engines

In an SI engine, combustion ideally consists of an exothermic subsonic flame progressing through a

premixed homogeneous air-fuel mixture.


Ignition and Flame Development Combustion is initiated by an electrical discharge across the electrodes of a sparkplug.

This high-temperature plasma discharge between the electrodes ignites the air-fuel mixture in the immediate vicinity, and the combustion reaction spreads outward from there.

Flame can generally be detected at about 6° of crank rotation after spark plug firing.

The spread of the flame front is greatly increased by induced turbulence and swirl within the cylinder.


Maximum voltage can be greater than 40,000 volts lasting for about 10-8 seconds. The total energy delivered during one discharge is generally about 30 to 50 mJ.

This gives a peak temperature in the order of 60,000 K.

A stoichiometric mixture of hydrocarbon fuel requires about 0.2 mJ (0.2 X 10-3J) of energy to ignite self-sustaining combustion. This varies to as much as 3 mJ for non-stoichiometric mixtures.

The discharge of a spark plug delivers 30 to 50 mJ of energy, most of which, however, is lost by heat transfer.


Spark Ignition Systems

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A number of systems are available, like

1. Simple magneto ignition systemsMost small engines and some larger ones uses a magneto driven by the engine crankshaft to generate the needed voltage for ignition coil

2. Battery ignition systems with coil ignition with or without transistors

In this system a battery-coil combination is used . Most automobiles use a 12-volt electrical system, including a 12-volt battery. This low voltage is multiplied many times by the coil that supplies the very high potential delivered to the spark plug.

3. Capacitor discharge systemsSome systems use a capacitor to discharge across the

spark plug electrodes at the proper time


Direct ignition Some engines have a separate high-voltage generation system for each spark plug, known as direct ignition system

The timing information is usually collected from the flywheel


Magneto Ignition SystemsA Magneto is used for current is induced in the LT winding by the changing magnetic field.

A high voltage is induced in the HT winding, it is significant only when the rate of change of the current is large in the LT winding at the instant when the contact breaker opens.



A high voltage (200—300 V) is generated in the LT windings and this energy is stored in the capacitor. Without the capacitor there would be severe arcing at the contact breaker. Once the spark has ended, the capacitor discharges.


• As engine speeds increase, the dwell period becomes shorter and spark energy will be reduced.

• Such a system can produce up to 400 sparks per second; beyond this the spark energy becomes too low and this lead to misfiring.

• For higher spark rates, twin coil/contact breaker or alternative electronic systems can be used.

CB point Ignition System Overview


In this type of system battery voltage is used to drive a charging circuit that raises the capacitor voltage to about 500 V.

At ignition, the energy stored in the capacitor is discharged through an ignition transformer, the circuit being controlled by a thyristor.

Capacitive Discharge Ignition (CDI)

The short duration (about 0.1 ms) spark is generated; the rapid discharge makes this ignition system less susceptible to spark plug fouling


Distributer


DistributorA distributor is used to distribute the

high voltage from the coil to the correct cylinder.

The central shaft is driven at half engine speed (for four-stroke cycles) and rotor arm directs the HT voltage to the appropriated sparking plug via the distributor cap.

For a four-cylinder engine there is a four-lobed cam, number of the lobes of the cam depends upon the number of cylinder connected to the distributer. It operates the contact breaker which is to interrupt the current flowing in the primary circuit of the ignition coil.


The contact breaker and capacitor are mounted on a plate that can rotate a limited amount around the cams relative to the base plate.

The position of this plate is controlled by the vacuum unit, a spring-controlled diaphragm that is connected to the inlet manifold.

The cams are on a hollow shaft that can rotate around the main shaft. The relative angular position of the two shafts is controlled by the spring loaded flyweights.


Spark PlugThe gap distance between electrodes on a modern spark plug is about 0.7 to 1.7 mm. Smaller gaps are acceptable if there is a rich air-fuel mixture or if the pressure is high (high inlet pressure by turbocharging or a high compression ratio).

If the electrode’s temperature is too hot, pre-ignition will occur.

If the electrode’s temperature is too low,carbon deposits will build up on the centralinsulator, so causing electrical breakdown.


Types of spark plugs

For older engines with worn piston rings that burn an excess of oil, hotter plugs are recommended to avoid fouling.

Modern spark plugs are made with better, more expensive materials, and have a much greater life span than those of a decade ago. Some quality spark plugs with platinum-tipped electrodes are made to last 160,000 km (100,000 miles) or more.

Spark plugs with several electrodes and two or more simultaneous sparks are now available. These give a more consistent ignition and quicker flame development.

• A cool-running engine requires a ‘hot’ or ‘soft’ sparking plug with a long heat flow path in the central electrode.• A hot-running engine (e.g. a high compression ratio engine) requires a ‘cold’ or ‘hard’ sparking plug with a much shorter heat flow path.