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Posted on 01 November 2019

Advances in Ignition Systems

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Improved Gas Mileage and Lower Emissions

As fuel prices continue to rise and consumer awareness and or government regulations for lower combustion emissions increase, automobile manufacturers are exploring ignition strategies to improve the performance of their vehicles. Over the past decade, automotive ignition circuits have evolved from a purely mechanical switch used to charge the coil to engine management controlled electric switching circuits and now to full closedloop systems offering improved mileage and better performing vehicles.This article will review some of the newer ignition systems, their system and device requirements along with the benefits that these systems provide.

By Jim Gillberg, Fairchild Semiconductor

 

Let’s start by discussing what is normal for the majority of vehicles in production today. In most of the cars in production today, the ignition system is a conventional open-loop single-spark system. The module might have some features such as coil current limiting, over voltage or over temperature protection and possibly some coil primary current diagnostics and feedback. These ignition systems are comprised of circuitry designed basically for protection in the event of a condition that might be destructive to the ignition coil or the power switch. The power switch used to charge the coil is normally an Insulated Gate Bipolar Transistor (IGBT).

In addition to protection, the ignition system could have some current- sensing capability on the primary side of the ignition coil to provide diagnostic information to the engine control unit (ECU) apprising the system of the quality of the spark and therefore, the overall combustion.

Figure 1. is a diagram of the typical ignition system and Figure 2 shows the waveforms associated with this type of ignition control.

Diagram of the typical ignition system

These conventional spark-advance systems control the combustion by having detailed calibration tables that take into account, the engine’s revolutions per minute (RPM), battery voltage, coil-charging times, engine-knock sensors, and other system parameters. But these spark-advance systems are still an open-loop system controlling the engine combustion based on the expected performance.

Waveforms of the typical ignition system

An emerging ignition system known as “Ionization Sensing” is a new option on a small number of production vehicles. With ionization sensing, the secondary current of the ignition coil is monitored. The value and wave shape of this current, following the actual spark event, can be used to monitor the quality of the actual combustion of the fuel, thus allowing the engine control unit to generate the spark at the most optimum time to get the best engine performance.

Ionization Sense Ignition Systems

When the primary coil current is interrupted to fire a spark, the current in the secondary goes through three distinct stages. First, there is the actual current in the secondary caused by the magnetic coupling of the primary and secondary windings, and then there is the actual flame front of the fuel air mixture igniting. Following this phase, there is a longer phase of current that can be generated if a bias is applied to the spark plug. This current is related to the temperature in the cylinder and thus the pressure in the cylinder and can be used to determine the optimum spark timing to get the best combination of the highest fuel efficiency and lowest emissions.

To be able to detect and measure these currents flowing due to the combustion, a bias circuit needs to be developed that applies a small voltage on the ground side of the secondary of the ignition-coil windings. There are several schemes that are employed to develop this bias voltage. Some generate a voltage and apply it to the ground leg of the secondary and others develop the voltage as a result of the actual ignition spark in the secondary. Depending on the method that is used, this voltage will cause the ions that are produced during the combustion in the cylinder to flow to ground and thus the current can be detected. Below Figure 3 depicts a bias scheme and current flow of an ignition-ion sense topology, and Figure 4 is a plot showing the several stages the secondary current goes through during a combustion cycle.

Bias scheme and current flow of an ignition-ion sense topology

To achieve the best efficiency from an engine one of the key variables is the timing for the ignition of the fuel in the cylinder. As the piston moves up and down in the cylinder the engine goes through four cycles. Intake (the fuel is drawn into the cylinder) , compression (the fuel is compressed before firing), power stroke (the fuel is ignited forcing the cylinder down and providing the power from the engine) and finally exhaust where the piston forces the exhaust gases produced by the combustion out of the cylinder preparing for the next cycle. Getting the combustion to take place at the optimum moment is the goal for any ignition system. If the combustion happens too early (the piston is still in the compression mode and has not reached its highest point in the cylinder (what is known as Top Dead Center), mechanical energy is lost as the piston would be moving against the pressure wave from the fuel combustion. In the extreme, this condition is known as knock and can be damaging to the engine. On the other hand if the combustion takes place too late, with the piston already moving away from Top Dead Center, then potential work energy is lost to heat and the optimum peak pressure will not be obtained during the power stroke. Thus energy that could go into torque is lost and engine performance or mileage will deteriorate from what the engine otherwise might achieve.

The several stages the secondary current goes through during a combustion cycle

In conventional spark controllers during normal driving conditions the actual spark ignition occurs about 15 to 30 degrees before TDC (top dead center). This allows for the delay from the start of the spark until the actual combustion of the fuel and development of the pressure wave on the cylinder. The optimal timing for the actual combustion to develop the most energy out of the firing is approximately 15 degrees after TDC.

Figure 5 is a graph of cylinder pressure vs. spark angle. You can see the pressure builds to a peak then diminishes. But it is not a distinct pulse. Thus the timing of the actual spark needs to account for some losses as the cylinder pressure builds before TDC thus losing some energy but should not be too late as potential energy or work provided by the combustion would be lost. Calibrating this event precisely using several open loop sensors is a real challenge. But with the ion sense ignition system the ion current stream developed due to the combustion can be monitored to actually derive the pressures over time in the cylinders and the quality of the actual combustion to optimize the spark timing. In some cases environmental conditions such as humidity and or moisture in the air can affect the combustion. This would not be a typical monitored input for the traditional engine controllers but would be accounted for in the direct control that the ion sense system could afford.

Cylinder pressure vs. spark angle

Another key advantage to the direct measurement of the combustion is that pre-ignition or knock can be detected as well as a “miss fire” or “non-firing plug”. Recent government regulations on engine controllers require the detection and signaling of engine misfires. These could be due to a fowled spark plug or a detached ignition wire. Since during a miss fire there would be no combustion wave front or following ion generated current, the lack of this ionization current can be sensed and signaled to the engine control module. All of this can be done without any additional sensors but just by using the existing spark plug in a new way. As mentioned earlier, only a few production vehicles today have this new feature but it is becoming a more interesting alternative as fuel costs and government regulations on automotive emissions increase.

Multi-spark or Multi-Charge Ignitions

Another advanced ignition system being developed and utilized on a small number of production vehicles today is the multi-spark or multicharge ignition system. The concept behind the multi-spark system is rather then just firing one spark; a series of sparks would be generated by the ignition system. With the sequence of spark events the air fuel mixture can be made leaner (less fuel) thus generating better gas mileage. Another advantage of the multi-spark approach is if there is any unburned fuel in the cylinder it may ignite on subsequent sparks and not be pushed out with the exhaust. This will generate improved efficiency and cleaner burning combustion.

This concept has been used in conjunction with a standard spark ignition by simply directing the ignition system on and off in a series of pulses to generate a stream of ignition sparks. What is different now is the use of additional electronics located in the coil on the spark plug which actually senses the primary and secondary winding currents.

Thus with a control circuit also mounted into the coil, the engine controller can set the levels for the charging and discharging of each spark insuring sufficient energy in the coil to achieve ignition. Previous generations of multi-spark ignition were primarily time based with no direct measurement of coil currents other then possibly the primary current. This new approach is more reliable to generate a strong ignition pulse and get the most out of every drop of fuel.

Figure 6a Shows an example of a multi-spark ignition and 6b the resultant wave form.

Multi-spark ignition

Multi-spark ignition wave form

The development of these newer advanced ignition systems is requiring the development of new types of electrical components. As was mentioned in the introduction, the switching element has evolved from a mechanical switch driven off the cam shaft to an electrical switching element. These first electrical switching elements only charged and discharged the ignition coil and were typically located away from the actual spark plug. As the requirements have now grown for better control, sensing and diagnostics features these electrical switching devices must be placed at the coil. As the new systems are monitoring and controlling every spark in each cylinder, this necessitates a coil for every cylinder. So the complexity of these components has increased to where it is now common place to have the IGBT (electrical switching element), the control IC and any passive devices (resistors, capacitors or diodes) all integrated into the ignition coil. The high voltage ignition wires are gone (which were expensive and prone to failure) as the high voltage signals are all included in these switch on coil devices. The coil receives the battery, control signals from the engine control units (ECU) and ground. The ECU can now both control the spark but get information returned to it on the charging rates of the coil, the quality of the combustion, the lack of a spark, conditions of the primary or secondary windings, temperature of the ignition module, etc.

Pictures the evolution from mechanical switch (points) to fully integrated igniters.

This new component is often referred to as an “igniter” including all the functions detailed above. These igniters must survive in very harsh “under-the-hood” automobile environments both physical and electrical. Ambient temperatures of 125°C are not unusual. While the igniter must dissipate over 4000 watts of peak power for each spark event. Automotive electrical transients have been well established in the industry as a result of various conditions in the vehicle. Load dump, field decay, reverse battery, high frequency noise are just a few standard transients that an igniter element must protect against. Thermal and electrical requirements for the igniter are some of the most stringent requirements for any component. Availability of igniters in the industry allows the coil manufacturers to focus on the coil magnetics and physical design, being able to add all the electronics into the coil by purchasing and integrating one completed module into the coil.

Shows examples of igniter designs from several manufacturers

All this complexity, however, must come with proven reliability at high volume manufacturing costs. Whenever automotive drive train components are considered, of prime importance is the absolute reliability of these devices. Failures of these critical components cannot be tolerated and significant effort is done to verify the performance of the devices prior to any final production use in a vehicle.

So to meet the constantly growing demand for higher fuel efficiency, lower combustion emissions and improved system diagnostics, the engine designers are employing ever more sophisticated control systems which now include the actual sensing of the quality of the combustion and the characteristics of the spark ignition itself. Some of these new systems have been in use on a small number of vehicles over the past several years but the demand for improved efficiencies will cause these systems to be incorporated into more and more of the vehicles most of us use everyday.

 

 

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