In IC Engines There Are 2 Types of Ignition Based on the Fuel Being Used

INTRODUCTION

An ignition system is a system for igniting a fuel-air mixture. It is best known in the field of internal combustion engines but also has other applications, e.g. in oil-fired and gas-fired boilers. The earliest internal combustion engines used a flame, or a heated tube, for ignition but these were quickly replaced by systems using an electric spark.

The power required for the motion of a vehicle is obtained by controlled combustion of fuel/air mixture in the cylinder of the engine. The energy produced due to this combustion is converted and transmitted from the engine to the wheels by means of mechanical linkages. The process of combustion of fuel is initiated by the ignition system.

The ignition system has two tasks to perform. First, it must create a voltage high enough (20,000+) to arc across the gap of a spark plug, thus creating a spark strong enough to ignite the air/fuel mixture for combustion. Second, it must control the timing of that the spark so it occurs at the exact right time and send it to the correct cylinder.

In IC engines there are 2 types of ignition based on the fuel being used:

1.  spark ignition (petrol engine)

2.  compression ignition (diesel engine)

Spark ignition

In a petrol engine, the fuel and air are usually pre-mixed before compression. The pre-mixing was formerly done in a carburetor, but now (except in the smallest engines) it is done by electronically controlled fuel injection.

In this system fuel entering the engine cylinder is ignited by means of a spark. The required amount of fuel is induced into the cylinder during suction stroke. This fuel is ignited during the compression stroke by a spark produced by a spark plug. Due to the combustion of fuel large amount of heat and high pressure gases are produced which expand causing linear motion of the piston.

Based on the method by which spark is created and distributed, ignition systems are classified as:

1.  Mechanical ignition System

2.  Electronic ignition system

3.  Distributor less ignition system

Mechanical ignition system

The Mechanical Ignition System was used prior to 1975. It was mechanical and electrical and used no electronics.

The ignition system consists of a battery, ignition coil, distributor, distributor cap, rotor, plug wires and spark plugs.

Typical mechanical ignition system:

Battery

The system is powered by a lead-acid battery, which is charged by the car's electrical system using a dynamo or alternator. It provides power to the system.

Distributor

The heart of the system is the distributor. The distributor contains a rotating cam driven by the engine's drive, a set of breaker points, a condenser, a rotor and a distributor cap.

First, it is responsible for triggering the ignition coil to generate a spark at the precise instant that it is. Second, the distributor is responsible for directing that spark to the proper cylinder. The pulse arcs across the small gap between the rotor and the contact (they don't actually touch) and then continues down the spark-plug wire to the spark plug on the appropriate cylinder.

Distributor cap, rotor, plug wires

The ignition coil's secondary windings are connected to the distributor cap. The extremely high voltage from the coil's secondary -– often higher than 1000 volts—causes a spark to form across the gap of the spark plug. This, in turn, ignites the compressed air-fuel mixture within the engine.

Contact breaker

The contact breaker is operated by an engine-driven cam, and the position of the contact breaker is set so that they open (and hence generate a spark) at the exactly correct moment needed to ignite the fuel at the top of the piston's compression stroke.

Ignition coil

The ignition coil consists of two transformer windings sharing a common magnetic core—the primary and secondary windings.

a. Primary winding

The coil primary winding contains 100 to 150 turns of heavy copper wire. This wire must be insulated so that the voltage does not jump from loop to loop, shorting it out. If this happened, it could not create the primary magnetic field that is required.

b. secondary winding

The coil secondary winding circuit contains 15,000 to 30,000 turns of fine copper wire, which also must be insulated from each other. The secondary windings sit inside the loops of the primary windings. To further increase the coils magnetic field the windings are wrapped around a soft iron core. To withstand the heat of the current flow, the coil is filled with oil which helps keep it cool.

An alternating current in the primary induces alternating magnetic field in the coil's core. Because the ignition coil's secondary has far more windings than the primary, the coil is a step-up transformer which induces a much higher voltage across the secondary windings. For an ignition coil, one end of windings of both the primary and secondary are connected together. This common point is connected to the battery. The other end of the primary is connected to the points within the distributor. The other end of the secondary is connected, via the distributor cap and rotor, to the spark plugs.

Spark plug

The ignition system's sole reason for being is to service the spark plug. It must provide sufficient voltage to jump the gap at the tip of the spark plug and do it at the exact right time, reliably on the order of thousands of times per minute for each spark plug in the engine.

The modern spark plug is designed to last many thousands of miles before it requires replacement. The heat range of a spark plug dictates whether it will be hot enough to burn off any residue that collects on the tip, but not so hot that it will cause pre-ignition in the engine. Pre-ignition is caused when a spark plug is so hot, that it begins to glow and ignite the fuel-air mixture prematurely, before the spark. The gap on a spark plug is also important and must be set before the spark plug is installed in the engine. If the gap is too wide, there may not be enough voltage to jump the gap, causing a misfire. If the gap is too small, the spark may be inadequate to ignite a lean fuel-air mixture, also causing a misfire.

Ignition coil showing primary and secondary windings:

Cross-section of a spark plug:

Electronic ignition system

The disadvantage of the mechanical system is the use of breaker points to interrupt the low-voltage high-current through the primary winding of the coil; the points are subject to mechanical wear where they ride the cam to open and shut, as well as oxidation and burning at the contact surfaces from the constant sparking. They require regular adjustment to compensate for wear, and the opening of the contact breakers, which is responsible for spark timing, is subject to mechanical variations. In addition, the spark voltage is also dependent on contact effectiveness, and poor sparking can lead to lower engine efficiency. A mechanical contact breaker system cannot control an average ignition current of more than about 3 A while still giving a reasonable service life and this may limit the power of the spark and ultimate engine speed. In the electronic ignition system, the points and condenser were replaced by electronics. On these systems, there were several methods used to replace the points and condenser in order to trigger the coil to fire. One method used a metal wheel with teeth, usually one for each cylinder. This is called an armature. The advantage of this system, aside from the fact that it is maintenance free, is that the control module can handle much higher primary voltage than the mechanical points. Voltage can even be stepped up before sending it to the coil, so the coil can create a much hotter spark, on the order of 50,000 volts instead of 20,000 volts that is common with the mechanical systems. These systems only have a single wire from the ignition switch to the coil since a primary resistor is no longer needed.

Distributor less ignition system

The coil in this type of system works the same way as the larger, centrally-located coils. The engine control unit controls the transistors that break the ground side of the circuit, which generates the spark. This gives the ECU total control over spark timing. Systems like these have some substantial advantages. First, there is no distributor, which is an item that eventually wears out. Also, there are no high-voltage spark-plug wires, which also wear out. And finally, they allow for more precise control of the spark timing, which can improve efficiency, emissions and increase the overall power of a car.

DRAWBACKS OF CONVENTIONAL SPARK IGNITION:

1. Location of spark plug is not flexible as it requires shielding of plug from immense heat and fuel spray.

2. It is not possible to ignite inside the fuel spray.

3. It requires frequent maintenance to remove carbon deposits.

4. Leaner mixtures cannot be burned.

5. Degradation of electrodes at high pressure and temperature.

6. Flame propagation is slow.

7. Higher turbulence levels are required.

8. Economic as well as environmental considerations compel to overcome

above disadvantages and use a better system.

LASERS

A laser is a device that emits light (electromagnetic radiation) through a process of optical amplification based on the stimulated emission of photons. The term "laser" is an acronym for Light Amplification by Stimulated Emission of Radiation.

Typically, very intense flashes of light or electrical discharges pump the lasing medium and create a large collection of excited-state atoms (atoms with higher-energy electrons). It is necessary to have a large collection of atoms in the excited state for the laser to work efficiently. In general, the atoms are excited to a level that is two or three levels above the ground state. This increases the degree of population inversion. The population inversion is when the number of atoms in the excited state is more than the number in ground state. The excited electrons have energies greater than the more relaxed electrons. Just as the electron absorbed some amount of energy to reach this excited level, it can also release this energy. As the figure below illustrates, the electron can simply relax, and in turn rid itself of some energy.

This emitted energy comes in the form of photons (light energy). The photon emitted has a very specific wavelength (color) that depends on the state of the electron's energy when the photon is released. Two identical atoms with electrons in identical states will release photons with identical wavelengths. The photon that any atom releases has a certain wavelength that is dependent on the energy difference between the excited state and the ground state. If this photon should encounter another atom that has an electron in the same excited state, stimulated emission can occur. The first photon can stimulate or induce atomic emission such that the subsequent emitted photon vibrates with the same frequency and direction as the incoming photon. The other key to a laser is a pair of mirrors, one at each end of the lasing medium. Photons, with a very specific wavelength and phase, reflect off the mirrors to travel back and forth through the lasing medium. In the process, they stimulate other electrons to make the downward energy jump and can cause the emission of more photons of the same wavelength and phase. A cascade effect occurs, and soon we have propagated many, many photons of the same wavelength and phase. The mirror at one end of the laser is "half-silvered," meaning it reflects some light and lets some light through. The light that makes it through is the laser light.

The figure below illustrates the stages in laser light formation by stimulated emission:

Types of lasers

There are many different types of lasers. The laser medium can be a solid, gas, liquid or semiconductor. Lasers are commonly designated by the type of lasing material employed:

1.  Solid-state lasers: have lasing material distributed in a solid matrix

2.  Gas lasers

3.  Excimer lasers: use reactive gases, such as chlorine and fluorine, mixed with inert gases such as argon, krypton or xenon.

4.  Dye lasers: use complex organic dyes, such as rhodamine 6G, in liquid solution or suspension as lasing media.

5.  Semiconductor lasers

Properties of laser light

1. Monochromatic - photons of one wavelength. In contrast, ordinary white

Light is a combination of different wavelengths.

2. Directional- laser light is emitted as a narrow beam and in a specific

direction. This property is referred to as directionality.

3. Coherent - The light from a laser is said to be coherent. This means that

the wavelengths of the laser light are in phase.

Here are some typical lasers and their emission wavelengths:

LASER IGNITION SYSTEM IN SI ENGINES

The use of laser ignition to improve gas engine performance was initially demonstrated by J. D. Dale in 1978.

However, with very few exceptions, work in this area has for the last 20 years been limited to laboratory experimentation employing large, expensive and relatively complicated lasers and laser beam delivery systems.

More recently, researchers at GE-Jenbacher, Mitsubishi Heavy Industries, Toyota, National Energy Technology Lab and Argonne National Lab have obtained and/or built smaller high peak power laser spark plugs.

Unlike many earlier laboratory laser systems, these smaller lasers are now mounted directly onto the engine cylinder head so as to fire the laser beam directly into the chamber. This arrangement allows the laser to become a direct replacement for the traditional high voltage electrical spark-gap plug. Further reductions in laser size, price and complexity will help the laser spark plug become a commercial reality and a viable competitor to the traditional high voltage spark-gap plug.