Federal-Mogul's Kris Mixell on Advanced Corona Ignition

Conference Chairman Kristapher Mixell is the Director of Advanced Technology - Ignition (Advanced Corona Ignition System) at Federal-Mogul Corporation, USA. Automotive IQ’s Managing Editor, Will Hornick, had the opportunity to discuss ignition developments in detail and to learn more about the Corona system from Mr. Mixell.

"The biggest thing for the GTDI applications is dielectric strength. That’s what it comes down to. Ceramics need to get better to allow very high breakdown voltages to support this downsizing strategy. That’s the first major issue."

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How did you get interested in ignition systems?

Kris Mixell: Actually, my background is in base engine design and development. Prior to Federal-Mogul, I worked at Ricardo and AVL and spent some time working in combustion system design, analysis and development. Beyond flow rates and efficiency, key factors to good combustion systems are charge motion, mixture preparation and injection strategy. A tremendous amount of time and energy is spent getting the fuel and air at the right place at the right time, but at the end of the day it comes down to your ignition source. It needs to perform over a variety of conditions over the life of the engine. If you think about combustion systems as they stood, say, fifteen years ago evolving to today, the spark plug is actually quite well suited to the strategies that are out there. Placing the spark plug in the center of the chamber with good charge motion, mixture preparation and turbulence levels at time of ignition ensures very good combustion for a stoichiometric air/fuel ratio. Considering early swirl and tumble to present day GDI combustions systems, a good high energy ignition system is generally up to the challenge. Obviously there are some drive cycle points where better ignition systems could provide improvement, but overall spark plugs are very well suited to meet these needs. In terms of my interest in ignition systems, it kind of started from the base engine development side and seeing the evolution of strategies such as lean burn, high dilution and heavily boosted engines and how the full potential of these strategies could be enabled by improving ignition systems. Some of the things we see on today’s market like very high energy and multi-striking systems have enabled some nice improvements in performance and emissions but they are still fundamentally limited by a spark created in a gap.

In your opinion why are ignition systems so important in terms of fulfilling emissions targets?

K.M.: Our focus is global right now, so ignition systems must meet the demands of all different strategies. What we see coming down the pike is a move toward various lean and dilute combustion strategies combined with downsizing, downspeeding etc. Those sorts of things are driving load requirements up so you’re not talking about engines that are in the 12-14 bar BMEP range - the higher end natural aspirated engines. We are asked to provide for ignition systems that can support loads up to 30 bar BMEP. GTDI (gasoline, turbo, direct injection) is enabling engines to be downsized 20 to 30 per cent or more which increases the specific load. Those things in the short term are challenges for present ignition systems so as you increase the specific load, you’re increasing demands on the ignition system in terms of being able to provide a breakdown voltage to be able to ignite highly boosted mixtures. That’s a first step along the way.

Are you dealing with higher compression ratios with downsized engines then?

K.M.: Yes. Higher compression ratios, direct injection, and turbocharging: those are the three big things. Cylinder pressures, both peak and at time of ignition, are going up which is driving demands on ignition systems. In particular, it drives the breakdown voltage required to create an arc and ignite those mixtures; these demands are going up and up and up! What we’d like to have is a very big, fat spark which requires a large gap. But high pressure and large gap size drives the ignition requirements up even further.

Ceramics have become the limiting factor then in a sense. If you want a very large gap, say, 1.5 to 2 mm and a very high cylinder pressure for these downsized engines, the voltage requirement becomes completely unmanageable from the ceramics perspective. So now you’re not talking about breakdown voltages that are 35-40kV, you’re talking breakdown voltage requirements well in excess of 45-50 kV. Something has to give. Consequently, the gap has to be smaller to reduce the breakdown voltage. When the gap size comes down to something manageable in terms of voltage and ceramic limits, the ignition source becomes smaller and ultimately limits what strategies the ignition system can support.

In addition to downsizing, the next steps in terms of engine technology are things like lean burn, dilute combustion and stratification. These three strategies actually start requiring more ignition, a better ignition source, something that has more area or volume, and is able to affect a larger portion of the combustion chamber. Combustion comes down to probability. So the bigger the source, the more the ignition is in contact with the gases, the higher the probability. As you do things like reduce the amount of fuel in the air for lean burn, the probability for finding good ignition-favorable zones in the mixture become less and less, same thing with EGR. As you reduce the amount of fuel and/or add EGR that makes the mixture harder and harder to ignite. That, in combination with boosting, means the ignition source is being stretched in two ways. The breakdown voltages just get higher and higher so the spark gaps get smaller which makes the ignition source smaller but that is exactly the opposite way you would want if you add things like lean, dilute or stratification which wants to make the gap bigger. So you’re sort of in a Catch 22.

Those are the limitations. In order to meet fuel economy and emissions demands you can employ downsizing add EGR and/or lean to complement to help bring down NOx emissions. Those two strategies can be at odds with one another when it comes down to what you really need from an ignition source. If you’re only concentrating on a highly optimized CO2 strategy, it is conceivable that an aggressive downsizing strategy could be employed, you could stay with a conventional emission system, manage spark gap and make better ceramics and everybody would be happy. But you still need to worry about other parts of the emission strategy such as reduction of NOx. It is likely that additional strategies need to be employed which include some lean and/or dilution, meaning that you will need a bigger ignition source with the ability to tolerate various levels of turbulence. Those two needs can be opposites.

There are several systems on the market today. Looking at the state-of-the-art today, multi-striking is one solution. Evidence shows that with mild-to-moderate levels of dilution and stratification, multi-striking has delivered mild levels of improvement. Although it does have some benefit, it’s fundamentally limited due to sparking a 1mm gap or smaller gap. High energy also faces limitations. There are trade-offs between gap, breakdown voltage and duration to balance ignition needs with service life. Additionally, as you change mixtures by adding/reducing fuel and adding EGR you make them harder and harder to ignite.

There’s less oxygen in the recycled air coming through?

K.M.: In lean burn there’s less fuel and on the EGR side there is more exhaust gas which reduces the amount of oxygen in the total mixture. Suffice it to say whenever you reduce the amount of fuel or add EGR it makes the mixture more difficult to ignite and slows the burn speed. Then you have to start doing things that are thermodynamically inefficient like running very, very early ignition timing, which offsets some of the benefit of running EGR or lean in the first place. This shows the need to speed up the burn rates to be able to realize the thermodynamic benefits that you’d hoped to have achieved by employing these strategies in the first place. So again, it all points back to needing a better ignition source.

You have gone over the pros and cons of some of the major systems in the market, are there any others worth mentioning?

K.M.: There are other types like laser. That’s another one which, in the past five years, has gotten some press. Laser is an interesting concept because its breakdown voltage or the energy required isn’t the same as a spark plug. In principle you don’t have some of the requirements/dependency in terms of pressure and so on. You can put the ignition source anywhere in the chamber which is kind of cool. On the other side of it you’re still limited to a very small pin point ignition source. Some research shows that this can be mitigated but there are some limitations with that. Also some of the costs and complexities associated with it make it an interesting proposal, but in terms of our evaluation of laser, we saw it as interesting with some potential but in terms of commercial feasibility we saw it as something much further down the road.

Is it due to the accuracy of current technology?

K.M.: There are a variety of issues associated with laser but I wouldn’t even say it’s an issue of being accurate. That part of the science is pretty well understood as is the energy requirement but it’s giving something small enough and durable enough to package in an automotive environment at the cost requirements that drive automotive ignition systems. There are some other practical issues that everybody seems to be talking about. For instance the optical access to the combustion chamber: how do you keep it clean? We decided not to pursue it and look more at our corona ignition.

Why do you think the Corona is the best compromise or the best system currently available?

K.M.: What interested us about corona when we evaluated the basic technology was a system that ticked all the boxes in terms of function within the combustion chamber. You have something with a very large spatial ignition source. It would support things like lean, dilute and stratified combustion. It also has sufficient energy levels to be able to support very high loads to enable downsizing and downspeeding. It was one of the first ignition systems that we came across that fundamentally changed the approach in terms of how combustion was initiated via the ignition source. If you think about high energy, it’s just a spark plug with more heat. You think of multi-striking, same sort of thing with different parameters. All of these things are fundamentally limited by the spark in the gap. So from our perspective, this corona ignition source was really quite interesting. You could see its effect in pressure curves and see some of the benefit right away. Then we looked at it working backwards from the needs of combustion to how we could implement it. We discovered that from the geometric standpoint, materials, the power levels it required to drive the system, these would be quite compatible with what engine developers are willing to accept today. The shapes, sizes and materials meet functional and capability requirements. For instance, our approach is that we use copper core center electrodes and alumina ceramic which are basically spark plug technology but are more than able to meet the demands of systems today and tomorrow.

In terms of our infrastructure, we are already heavily invested in these technologies because Federal-Mogul has both the technology and capacity of Champion Spark Plug and the recently acquired BERU in the OE market segment. If you think about what the system requires, it requires a coil driver, which we presently make in Carpi, Italy, as well as capacity to build an igniter very similar to a spark plug. We have substantial knowledge in terms of ceramics processing and metalworking on the spark plug side. As we envisioned it, a corona ignition system (ours is called the Advanced Corona Ignition System or ACIS) is a system that could meet the needs of the consumer. It was a system that we could build, at least from the igniter side, with the materials and technologies that we’re presently quite confident with and that we’re very, very good at. Because of those things we thought corona had the potential to be able to offer benefit to the customer with proven durability, proven materials and because we’re not talking about using a lot of exotic materials or yet-to-be-invented processing, it has potential to be cost-benefit competitive.

The way our plug igniter assembly is designed, it basically fits into existing engine architectures. The biggest change we had to make was drive electronics which is something the system requires. It’s an add-on. There are lots of things on the market today that tell us that we can do that, too.

Is there an aftermarket for this technology?

K.M.: There are some possibilities for after-market applications. It’s not the principal strategy at the moment. The reason being that in order to fully take advantage of ACIS, you really need to use it in a way where you can reap maximum benefits. Things that are being developed like downsizing, downspeeding, lean, dilute and stratification are the sorts of things that aren’t really on the market today and really benefit from the improved capabilities of the system.

As I was saying at the beginning, today’s systems and their evolutions work quite well with combustion systems that are on the market today. There’s some improvement to be had but it’s not like you’re going to pull a switch and see huge improvements. There are some drivability improvements and some other things associated. The after-market side is something that we could explore down the road but at the moment it’s not a principal part of our development.

In general, what do you see as being the limiting factor for the ignition industry moving forward?

K.M.: The biggest thing for the GTDI applications is dielectric strength. That’s what it comes down to. Ceramics need to get better to allow very high breakdown voltages to support this downsizing strategy. That’s the first major issue. It’s a compromise between breakdown and a large ignition source. If you could have infinite breakdown capability what you’d like to do is run bigger and bigger gaps to get the ignition source larger to be able to support these other strategies which are coming along. So I see ceramics as a key word for ignition where it stands today from the spark ignition standpoint.

Precious metals are important because as the breakdown voltages get higher you need to have this balance between high voltage and having reasonable service lives. So the higher the voltage and the higher the energy, the plug’s service life becomes a requirement, particularly in high-load applications.

The volatility in that particular market in terms of cost is also a factor I imagine.

K.M.: Yes, that’s right. As you know, today many of the higher-end plugs that run much higher load cycles are going to platinum and iridium and so on… using fine wires. Here’s an example: One of the things that’s quite interesting about a spark plug is that you produce a lot of heat in the spark gap whenever you create the arc. That energy really goes to two places. It goes to heating up the mixture and then having the mixture combust. The second place that it goes is to heating up the gap and heating the electrodes. The way that’s combatted obviously is having harder and harder materials that are more durable to resist this sort of heat. The other thing to consider too is the amount of mass that’s there at the point of ignition. So it means a lot of manufacturers are going to finer and finer wires that will reduce the amount of heat that’s pulled out of the initial part of the combustion process.

Look at spark plugs ten years ago or fifteen years ago: If you look at the electrode in a majority of cars you can see it’s a pretty fat electrode on both sides. What you see now if you look at the higher-end applications looks like a little pencil lead. A lot of the higher-end spark plugs are going to electrodes which are 0.6 or 0.7mm diameter which is a very fine wire and then there’s a precious metal pad on the other side, it helps to keep the gap from growing. So, there’s a lot of optimization that goes on in terms of making those wires small to keep the mass down, to keep the heat going into the mixtures as opposed to being soaked up by the plug. These things are very, very important. Again, from a corona ignition standpoint, because of the way we approach combustion, a lot of the combustion initiation occurs away from the actual ignition source itself. We aren’t as sensitive to a lot of these issues that are forcing people toward fine wire in a conventional spark ignition system. Those are the big limiting factors.

In terms of fuel efficiency, I’ve heard anything from three to five up to even 9% potential gain if you have the right ignition system. Are those numbers realistic?

K.M.: I would say it depends on the strategy because the way ignition systems and combustion systems work is they are highly synergistic. If you said, "Okay I’m going to put this system on to an engine today and run it on the same strategy and just adjust the spark timing to take advantage of the faster burn with the conventional strategy," then there’s some benefit even though today’s ignition system of plug placed with high energy and so on, works really well. There are still parts in the speed and load range where combustion can be improved.

If you want a better ignition system like ours, what we’ve seen is that if the combustion system and the ignition system from a base engine are pretty well sorted out there’s some benefit but it’s not huge. If you can find parts of the speed/load ranges that also happen to be part of the fuel economy drive cycle the benefits are good. So in other words if we fix a problem where the ignition system is sub-optimized with the components that are there, we can help fix that by employing ACIS with a conventional strategy. But if you go to something like lean burn, that’s where the spark plug can support lean combustion and then there’s some incremental amount where we can take that over and above lean combustion. Because they’ve adopted this strategy we can go further yet; same thing with EGR.

Benefit is highly dependent upon the strategy and the way it’s utilized. So if you have a perfectly optimized spark system and you put our system in, there’s some benefit in terms of faster burn and some efficiency gains, but typically they’re not huge. They’re on the low end of the scale that you mentioned. If you happen to have an engine that’s running a conventional strategy where the ignition system is sub-optimized, there might be some problem points that they’ve had to compromise on and there is a little bit of benefit there as well. It helps to extend that range into the mid-range of the values you are talking about.

If you start talking about ignition systems that are truly a limiting factor, there’s some more to be had. i.e. If you start talking about going to lean burn. Typically the accepted value for a spark ignition system is around a lambda of about 1.5, about a 21 air to fuel ratio, give or take - with our system in place.

Compared to conventional systems, that’s pretty lean.

K.M.: Yes, it’s pretty lean. But if you look at what we’ve been able to accomplish at customer testing internally, we’ve been able to take that lean limit from 1.5 to about 1.8 and in some cases 1.9. So you’re talking about a substantial extension of the lean limit. Exactly where that lean limit lies and how they implement that strategy depends upon the benefit. The same holds true for EGR. Typically the accepted limit for EGR is around, I think, between 15 and 20 percent, which is a lot of exhaust gas to be putting back to the engine. We’ve seen EGR range extension approaching 30 to 35 per cent. So again, how the engine manufacturer implements the ability to run leaner, run more EGR, run a combination of the two plus implementing downsizing strategy is really what gives the final benefit. So it’s not really fair for us to say, "Yes, we can give you exactly X". A very long-winded answer! It comes down to, "how will you employ it and how will that strategy work relative to the drive cycle?" And that’s really where you’ll see it. There’s a symbiotic relationship between the ignition system and the combustion system. Those two need to work together very well and that takes development.

One of the things that we’ve taken on as part of this project is really working with a lot of customers, understanding the whole range of geometry, piston configuration, plug placement, injection strategy, bore sizes, fuel types, so we’re really getting a complete view in terms of how these things work together. Particularly, I would say over the past year we’ve made a lot of advances. I would say our system has been really picking up in terms of its ability to perform. It’s an exciting time for us and I always say that I probably have one of the best jobs in the company because I get to work on a new product that is truly ground-breaking. I’ve got a great team and the customers we deal with are extremely interesting and highly professional. It’s really good being able to work in a collaborative environment with a customer who’s really happy to tell us what they want, tell us what they need and for us to be able to match, technically, what they want in words. That’s really cool. The company has been highly supportive of the project. Federal-Mogul is committed to creating innovative products; trying to go out and meet customer needs. It’s a fantastic opportunity and like I said, it’s really cool.

Would a 48 volt power supply have an impact on what you guys do? I know the German OEMs are serious about moving in that direction.

K.M.: For our system in particular? We would see a benefit for 48 volts. That has to do with the specifics of our system. That might make a small difference for traditional ignition. In general, maybe a little bit. We would see a benefit mostly because it would reduce the complexity of the electronics that we’re presently building. So that’s where I can speak to you specifically. One more comment in terms of road blocks with respect to our system. We’re at the point now where the performance is really coming along nicely; we’re meeting the targets that a few of our customers have set up for us. I’d say the biggest thing right now is really moving along with durability. We’ve been moving along step-by-step, we get a little more durability and then we add the performance and then durability goes away. Then we get the durability back, we add more performance, the durability goes away again. That’s where we’ve been struggling for the last 2 or 3 years.

A step forward, half a step back and so on.

K.M.: Exactly, particularly from a few of our customer’s, we’re at the point now in terms of what they’re expecting to see in terms of lean limit extension and in terms of EGR extension. We’re at the point where we’re doing some really detailed performance development work. It’s exciting. But the key thing now is getting the durability right.

Thank you very much for the interview. I appreciate the detail you went into.