Ignition Systems
The ignition system must provide an adequate voltage to initiate a discharge across the spark plug electrodes and supply sufficient energy to ignite the air-fuel mixture. This must occur for all engine operating conditions and at the appropriate time on the compression stroke.
Since 1965, new requirements for ignition systems have come up which could not be met by the conventional inductive ignition system. The introduction of new exhaust emission criteria in 1965 and the demand for improved fuel economy in 1975, has forced designers to turn to electronic for providing a system to satisfy the statutory requirements for an automobile. Legislative requirements and driver demand for better engine performance, added to the manufacturers need to offer a more sophisticated vehicle to counter a competitor’s product all show why electronic innovation in this field is taking place.
On modern engines, it is normal for the ignition system to form a subsection of an integrated management system, sharing sensors and circuits, with fuelling and (occasionally) transmission control systems. For clarity, however, it is useful to consider the ignition system separately in detail in this chapter.
16.1.
Ignition Fundamentals
16.1.1.
Functional Requirements
The ignition system supplies a spark inside the cylinder, near the end of the compression stroke, to ignite the compressed charge of air-petrol vapour. Under normal atmospheric (101 k Pa) conditions a voltage of 2 to 3 kVis required for a spark to jump across an air gap of 0.6 mm. For a spark to jump across a similar gap in an engine cylinder, having a compression ratio of 8:1 approximately 8 kV is required and for higher compression ratios and weaker mixtures, a voltage up to 20 kV may be required. Therefore, ignition system transforms the normal battery voltage of 12 V to approximately 8-20 kV and delivers it to the right cylinder, at the right time. Some ignition systems are capable of supplying up to 40 kV to the spark plugs.
The fundamental operation of most ignition systems is very similar. Conventional ignition is the precursor of the more advanced electronic systems. A coil ignition system is composed of various components and sub-assemblies. The actual design and construction of this depend mainly on the engine with which the system is associated. While designing an ignition system the most important factors to be considered include :
(i) Combustion chamber design.
(«’) Air-fuel ratio.
(Hi) Engine speed range.
(iv) Engine load.
(v) Engine combustion temperature.
(vi) Intended use.
(vii) Emission regulations.
16.1.2.
Various Types
The conventional (mechanical) and electronic ignition are the two basic types of ignition system. Programmed ignition, distributor less ignition and direct ignition can be considered as the advancement of basic systems. The basic choice for various types of ignition system is classified as follows:
| Type | Conventional | Electronic | Programmed | Distributor |
| Trigger | Mechanical | Electronic | Electronic | Electronic |
| Advance | Mechanical | Mechanical | Electronic | Electronic |
| Voltage source | Inductive | Inductive | Inductive | Inductive |
| Distribution | Mechanical | Mechanical | Mechanical | Electronic |
16.1.3.
Generation of High Tension
A number of factors are used to determine the instantaneous primary current in the inductive circuit of the ignition coil. Production of the high tension (HT) is mainly dependent on the value of the primary current. The rate of increase of primary current is important because this determines the value of the current when the circuit is ‘broken’ to produce the collapse of the magnetic field.
The instantaneous primary current can be calculated using electrical constants of the primary ignition system and the exponential equation,
where, i = instantaneous primary current R = total primary resistance L = inductance of primary winding t = time of flow of current and e = base of natural logarithms
Some typical values for conventional and electronic ignition systems are: Conventional Ignition : R = 3 – 4 Q, V = 14 V, and L = 10 mH. Electronic Ignition : R = 1 £2, V= 14 V, and L = 4 mH
In a four cylinder engine running at 3000 rpm, 6000 sparks per minute or 100 sparks per second are required (four sparks during the two revolutions to complete the four stroke cycle). Therefore each spark must be produced and used in 10 ms. Taking a typical dwell period of say 60%, the time t at 3000 rpm on a four cylinder engine is 6 ms and at 6000 rpm, t is 3 ms. Using the exponential equation above, the instantaneous current for each system is as follows :
| 3000 rpm | 6000 rpm | |
| Conventional system | 3.2 A | 2.4 A |
| Electronic system | 10.9 A | 7.3 A |
This gives an idea how the energy stored in the coil is greatly increased by the use of low resistance and low inductance ignition coils. The electronic system deals much higher current than the conventional contact breakers. The energy stored in the magnetic field of the ignition coil is calculated using the equation; E = Vi(Li ).
The spark energy is directly related to energy stored in the coil. The stored energy of the electronic system at 6000 rpm is 110 mJ, and that of the conventional system is 30 mJ. This clearly shows the advantage of electronic ignition.
16.1.4.
Ignition Advance Angle
The ideal ignition timing is dependent mainly on engine speed and engine load. With the increase in engine speed, the ignition timing is required to be advanced. It is because the cylinder charge or air-fuel mixture requires a certain time to burn, and hence it is necessary to ignite it earlier at higher engine speeds. The ignition advance angle for optimum efficiency should be sufficient to cause the maximum combustion pressure to occur about 10 degrees after TDC. A change in timing due to engine load is also required. Since the weaker mixture used in low load condition burns at a slower rate, ignition advance is necessary. A change in residual gas content and a lower charge in the cylinder cause a longer ignition delay and lower combustion rate in the mixture, thereby requiring the ignition angle to be advanced.
Spark advance is achieved in a number of ways, the simplest of which is a mechanical system consisting of centrifugal advance mechanism and a vacuum control unit. The manifold pressure is proportional to the engine load. Electronic ignition systems may also adjust the timing in relation to the temperature and mixture strength. The values of all ignition timing functions are considered and combined either mechanically or electronically to obtain the ideal ignition point. The ignition coil stores energy in the form of a magnetic field. A duel period is incorporated to ensure that the coil is charged before the ignition point.
16.1.5.
Fuel Consumption and Exhaust Emissions
The spark time is critical for maximum power and economy. The ignition timing has a considerable effect on fuel consumption, torque, driveability and exhaust emissions. Out of the three most important pollutants of exhaust emissions, hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NOx), the emissions of HC increases as timing is advanced. NOx emissions also increase with advanced timing because of the higher combustion temperature. CO changes insignificantly with timing and is mostly dependent on air-fuel ratio.
A change in timing required to improve exhaust emissions also increases fuel consumption. A larger advance is necessary with the leaner mixtures used nowadays, to compensate for the slower burning rate. This provides lower consumption and high torque but the mixture must be controlled accurately to achieve the best compromise with regard to emissions. For further details chapter 17 may be referred.
