DE VELO PMENT Ignition
CORONA IGNITION SYSTEM FOR HIGHLY EFFICIENT GASOLINE ENGINES Many future gasoline engines will require higher air/fuel ratios and higher mean effective pressures to further improve fuel efficiency. Federal-Mogul has taken up this challenge and has developed the Advanced Corona Ignition System (ACIS) as a new solution to reliably ignite a mix with high AFR/EGR and high MEP. During engine tests ACIS enabled a direct fuel economy improvement of up to 10 %.
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AUTHORS
DR. JOHN BURROWS, MIET, is Electronics and Analysis Manager – ACIS Ignition at the Federal-Mogul Limited in Manchester (Great Britain).
JIM LYKOWSKI is Design and Development Manager – ACIS Ignition at Federal-Mogul in Plymouth, Michigan (USA).
KRISTAPHER MIXELL, P.E., is Director Advanced Technology – Ignition at Federal-Mogul in Plymouth, Michigan (USA).
CHALLENGE TO THE IGNITION
The continuous improvement of gasoline engine efficiency has already had many impacts on engine sub-systems and components. The next step on this path will pose a challenge to the ignition. Future high-efficiency engines will require advanced lean-burn concepts and/or greater use of charge stratification to achieve even better fuel economy. In combination with higher mean effective pressures (MEPs) and more aggressive charge dilution the traditional spark plug may not suffice to provide a reliable ignition during all operating conditions of the engine. The challenge is to a technology that was developed under totally different boundary conditions. In the days of a more or less homogenous, stoichiometric charge, or mild lean-burn concept at the most, the restriction to a small ignition source was not a problem. With aggressive charge stratification, and much higher lambda (λ) this restriction is becoming a bottleneck. While the issue of higher MEPs and potentially also a slightly greater λ could still be overcome by increasing the breakdown voltage and arc duration, problems remain: If it takes longer to ignite the charge, the point of 50 %-mass fraction burnt will also be
delayed. In addition, longer arc duration, coupled with high energy, can increase plug wear. More importantly, however, nothing can be done about the location of the spark. To overcome these limitations, Federal-Mogul has developed an ignition system that is based on multiple long plasma jets instead of a single small spark, ❶. This Advanced Corona Ignition System (ACIS) enables more aggressive optimisation concepts to increase a gasoline engine’s efficiency. SYSTEM COMPONENTS AND PRINCIPLE OF OPERATION
An ACIS solution consists of two main components. The first is the two-piece igniter assembly which is mounted in the cylinder head very much like the traditional spark plug and ignition coil. The igniter assembly contains the inductor at the top and the firing tip at the bottom. The second system component is a controller which computes the trigger signal from the engine control unit and converts the 12 V DC electrical supply into the required AC voltage at a resonant frequency of around 1 MHz, which is fed forward the igniter, ❷. At this frequency the igniter emits a strong electrical field with up to 72 kV at the tips of the firing end. Originating at
❶ Corona discharges, originating at the tips of the four ACIS igniter electrodes
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DE VELO PMENT Ignition
❷ Overview of the ACIS components
the four electrodes of the igniter tip, the field extends into a large volume of the combustion chamber. The energy content of the electrical field excites the air fuel mix near the electrodes until it turns into a plasma with a high content of charged particles (ions), a process that only takes several nanoseconds as
opposed to up to 70 μs in the case of spark ignition arc breakdown. When the electron density reaches a sufficient level, multiple long streams of ionised gas extend into the combustion chamber and ignite the charge, ❸. The name corona ignition refers to these visible streams of ionised gas. The air-fuel mix
❸ Comparison between the arc-lengths of a corona system (left) and a conventional plug (right)
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is ignited in several areas at the same time, which speeds up the burn rate and results in a fast, harmonious combustion. In contrast to the spark plug which produces a single short arc, the four ion streams originate from the tip of an electrode and extend outward into the combustion chamber, ❹. Due to the low current and low heat discharge, there is no electrical erosion. Therefore the ACIS is not subject to the same level of wear as a conventional plug. Potentially ACIS can be developed to serve as a lifetime component. Already at the moment its functional principle offers a long life and longer service intervals. The biggest single ACIS benefit, however, is the vast volume which is reached by the corona. In the case of a spark plug, the electric arc is always limited to the small gap between the two electrodes. The only way to increase the energy of this ignition source is to use a higher current and to prolong the arc duration. Both measures increase the energy consumption of the ignition system and thus the parasitic electrical load. ❺ shows a comparison between the energy consumption of a spark plug and an ACIS for different on-times. The re sults were measured with a stoichiometric charge. Recent testing indicates that an ACIS application requires an on-time
MAXIMUM
MAXIMUM
MAXIMUM
AVG. ENERGY/
VOLTAGE [V]
CURRENT [A]
POWER [W]
IGNITION EVENT [MJ]
Spark (~ 3 ms)
14
8.7
102
192
ACIS (0.5 ms)
53
2
101
160
ACIS (0.25 ms)
53
2
100
93
❺ The ACIS fuel economy benefit is achieved at a very similar energy draw per ignition event as is the case with spark ignition
❹ Close-up of the igniter tips
between 100 and 300 μs for λ = 1. The conventional spark ignition had to be on for 2000 to 3000 μs in comparison. The notable difference is explained by the physics and the high speed of the corona principle. In terms of energy consumptions both solutions are on par. For lean and diluted charges both systems require longer on-times. ENGINE INTEGRATION AND FUEL BENEFITS
Once integrated in an engine, the ACIS enables more aggressive measures to increase thermodynamic efficiency. Several boundaries, which currently apply to gasoline engines with conventional spark plug technology, can be pushed. As described below the ACIS ignition is much faster. While a specific ignition event with a plug may last 3000 μs, the same ignition event only requires 100 to 300 μs with an ACIS. This is a step change which offers a much higher degree of freedom to control ignition timing. Also, the rapid ignition and quick burning result in a better conversion of fuel energy to mechanical energy. In addition, owing to the multiple streams of ions, which extend far into the combustion chamber, the initial flame development and subsequent burn rate is faster so that the 50 %-mass fraction burnt is achieved at 5 to 8 °CA earlier than with a plug system. Also the lean limit is pushed out further with ACIS. 06I2013 Volume 74
While a spark plug permits ignition at λ = 1.5 with a very high release of energy, the ACIS achieves the same with an air-fuel ratio approaching 2. A similar improvement was found with the achievable charge dilution. Where the best spark system worked at up to 20 % during customer testing, ACIS permitted EGR levels in excess of 35 %. This par ticular benefit can be highly relevant to control the level of NOx emissions in direct injection gasoline engines. Furthermore the ACIS corona is more tolerant to high compression ratios than previous corona systems were. ACIS can help to control knock at high pressures by adjusting ignition timing. In terms of fuel consumption, the immediate effect of an ACIS application could be measured at between 5 and 10 % of fuel economy improvement. Up to 10 % reduction of fuel consumption were measured on a 1.6-l turbocharged gasoline direct injection engine. However, these results do not tell the whole story as ACIS is essentially an enabling technology, which facilitates more comprehensive gasoline engine technology paths that can, in total, result in fuel economy benefits of around 30 %. ADDED BENEFITS OF THE NEW TECHNOLOGY
ACIS is optimised for ease of implementation in high-volume applications for both current and future powertrain architectures by designing it to package within the space of a spark-ignition system. The two-piece igniter design means that the ACIS can be integrated in an engine without significant redesign of the engine or cylinder head. The igniter simply replaces the spark plug, the in ductor can either be integrated into the igniter assembly or it is installed separately where the ignition coil previously was. The ACIS controller can be located
nearby. The igniter assembly uses materials that are already proven in automotive applications to ensure durability throughout the designed service life. In particular the materials can be selected for different fuels without compromises for full time arcing requirements. As the ACIS was specifically developed for high MEP, high air-fuel ratios, high charge dilution gasoline engine strategies, the engine needs to be calibrated toward these conditions. The limit for high-load operation, which is at 25 to 30 bar with a spark plug ignition, still needs to be confirmed, as the ongoing customer testing of the ACIS is strongly focusing on part load lean and diluted charge combustion. SUMMARY AND OUTLOOK
While advanced spark plugs will continue to be a solution for millions of gasoline engines in the future, ACIS offers a new technological option for high-performance gasoline engine applications, which push beyond the boundaries of spark-ignition systems. In particular ACIS addresses lean and stratified charges, high EGR rates and high MEPs. For such high-efficiency powertrains ACIS is an enabling technology for better fuel economy and lower NOx emissions. The multiple ion streams provide a powerful ignition source which reliably ignites a mix with an air-fuel ratio of up to 2, and results in a faster burn rate plus a more consistent combustion. As the ignition process is particularly fast, ACIS also increases the degree of freedom to control ignition timing. If used as a “screw-in” replacement and combined with a suitable engine calibration ACIS itself has been measured to improve the engine’s fuel economy by up to 10 %. However, when ACIS is used as an enabler for a more comprehensive package of engine optimisation elements, it can help to facilitate fuel efficiency improvements of up to 30 %.
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