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Honda Patents Plasma-Assisted Catalyst System for NOx Emissions Reduction

A sketch of an embodiment of the aftertreatment system.

A recent US patent award to Honda provides some insight into the approaches the automaker is taking to be able to meet both California LEV II LEV and EPA Tier 2 Bin 5 diesel emissions requirements in the US—thereby giving it a “50-state diesel.” The patent describes a diesel emissions aftertreatment system that combines a small plasma reactor with catalytic units to reduce NOx emissions.

NOx reduction to the level required by the regulations is the US is one of the thornier issues automakers must solve. DaimlerChrysler became the first to announce a 50-state solution when it announced its E320 BLUETEC and VISION GL320 BLUETEC earlier this year. (Earlier post.)

DaimlerChrysler is using two different technologies to bring NOx down to compliance levels. For the E320—due to be introduced later this year—the company is using a newly-developed NOx adsorber, a catalytic device that converts NOx to nitrogen.

For the larger GL320, DaimlerChrysler plans to use a urea-based injection system (using an aqueous urea solution called AdBlue, the genesis of the BLUETEC name). Both are combined with Selective Catalytic Reduction systems, which, while in principle are the same, differ in application design based on vehicle parameters and emissions targets. (DaimlerChrysler also needs to have the EPA buy into the notion of using the urea-based injection system.)

BLUETEC is a good example, however, of the current technology applied to NOx reduction.

In its patent filing, Honda acknowledges both approaches. The company also notes problems with each: that the adsorber can impose a fuel penalty due to the regeneration strategy, and the urea-injection approach requires the development of an infrastructure for another fluid.

In the proposed Honda system, the electrically-powered plasma reactor first converts oxides of nitrogen other than NO2 to NO2. In addition, and in conjunction with a reducing agent injected upstream of the reactor, it can also oxidize PM. Multiple reactors could be placed in series or in parallel, if needed.

The NO2 exhaust stream then flows to the catalyst units where it is adsorbed or reduced by alkali metals and silver.

Other companies and laboratories are exploring the use of plasma-catalyst combinations for NOx reduction.

  • Research funded by the DOE and later by ArvinMeritor led to the deployment of a plasma reformer (Plasmatron) for use with heavy-duty diesel engines. In development since the 1990s, the system reduced NOx emissions by up to 90% when used with an adsorber catalyst. It operated effectively at lower temperatures than other NOx removal systems, and it reduced the amount of fuel required for adsorber regeneration in half.

  • GM researchers have developed a plasma-assisted catalyst system (PAC) capable of reducing NOx under highly lean conditions using E-diesel or ethanol as the reductant. The system consists of a compact, energy-efficient hyperplasma reactor followed by a dual-bed catalytic reactor. They also demonstrated good NOx conversion (above 90% on average) over a wide temperature range of 200-400° C under steady-state optimum operating conditions.

  • Pacific Northwest National Laboratory also developed a two-phase approach to a plasma catalysis system, also producing reduction of NOx emissions by as much as 90%.

  • Caterpillar has looked at reformer-assisted lean NOx catalysis as well as plasma-facilitated catalysis.

  • Researchers from Ford, GM, DaimlerChrysler and PNNL collaborated on a three-phase plasma-catalyst system. (They found that with hexene as a reductant, the system reduced NOx by more than 90%; with diesel or Fischer-Tropsch reductant, however, the catalyst efficiency rapidly dropped off.)

The key to successful commercialization will be developing the right catalytic units, solving the problem of generating and maintaining the electricity required for the plasma, packaging it such as way that it works within vehicle form factors, manufacturing it cost-effectively and delivering the required low emissions.

Honda has said that it will introduce a 4-cylinder clean-diesel engine into the US market within the next three years. (Earlier post.)




The money question (literally) is:

How much is this system going to add to the cost of a vehicle? Will it be competetive with hybrids, at least?

tony chilling

Honda is still talking about the development of infrastructure for another fluid even in their own patent. A reducing fluid injected upstream of the reactor can be added.
With their Hybrid Accord not selling because people who buy hybrids want higher mpg more than better performance, Their next bad decision is a 50state Diesel? Americans simply do not like diesels and people who buy a $60,000 car do not stop at truck stops!

Frankly, my real hope is that with all their experimentation with Plasma, they actually discover Hot Fusion energy generation or at least a better TV display.

Rafael Seidl

Technologies that can reduce NOx in the presence of O2 are indeed very complex and expensive. The first question therefore ought to be: does the US really need NOx emissions limits so low they can only be met with such technology? Health and air quality experts will surely argue that it absolutely does and, will brook no backtracking. However, consumers/voters should consider that there is also a significant price to be paid for *not* having lean burn engine options, at the pump as well as in Iraq.

NOx emissions limits are a political choice, not a commandment from a higher power. Have EPA and CARB gone too far too fast?


With ULSD becoming available later in the year, NOx store catalysts will be the first choice of many, provided the life expectancy problem can be solved. Frequent regeneration causes thermal cycling of the washcoat. Most of the plasma assist systems seek to reduce the energy required to achieve desorption and initiate reduction, both to increase life expectancy and to reduce the fuel penalty.

Arvin Meritor's system pretreats the fuel instead, to reduce NOx production during the combustion process. That is also the objective of flameless (HCCI) combustion research for both diesels and GDI, but it is inherently limited to part load operating points.

The most cumbersome solution is SCR, because it involves a new distribution infrastructure for the AdBlue additive plus enhanced OBD functions to ensure the vehicle cannot be operated normally without it. These hurdles have been overcome in Europe but not yet in the US. Moreover, AdBlue freezes at -11 deg C, so a heater is required in the additive tank. The whole system is quite effective at reducing NOx and will become more so once the fast NOx probes required for closed-loop control become available. However, its complexity and cost effectively restricts it to commercial vehicles and luxury sedans.


I am amazed at the technological lengths we go to in order to make ICE's "less bad". If only all this intellect was being applied to EVs...

Rafael Seidl

Tony -

the vast majority Americans have no experience of modern light duty diesels with turbos, common rail injection and particulate filters. Indeed, in Europe, diesels are now considered premium engine options! For the additional up-front expense, customers get 20-30% better fuel economy and substantially more low-end torque.

Simply not liking something you don't even know is called a prejudice. Think with your head please.

As for truck stops, you are correct. However, there is no law that prevents regular filling stations in the US from offering diesel. I read somewhere that some 40% already do (varies by state I suppose). That percentage would go up very quickly if diesels werre more widely available. Besides, the most obvious application for diesels in the US would be for heavy vehicles such as pick-up trucks and delivery vans. Owners of these types of vehicles might be more willing to accept a phasing-in of diesel fuel availability than others.

Besides, stratified spray-guided GDI engines would achieve similar fuel economy (based on fuel mass, not volume!) with widely available regular gasoline. Of course, they require the same type of expensive NOx aftertreatment that diesels do if they are to meet US emissions standards.

Rafael Seidl

JN2 -

carmakers would dump the ICE in a heartbeat if someone came up with a workable alternative, including an EV setup. Why do you think so much effort is being expended on hybrids and fuel cell research? The problem is simply that nothing beats liquid hydrocarbons for energy density and rapid refuelling.

External combustion engines, e.g. Stirling or steam, are much more expensive because of the special materials required for high-temperature heat exchangers. Besides, their load response is quite poor. That hasn't stopped engineers from revisiting them regularly, if only as secondary power sources.

The notion that carmakers point blank refuse to consider alternatives to ICEs is simply false. Whenever gas prices are high or more severe emissions regs are on the horizon, every single one of them takes a second look at what might be possible.


Tony, you're still a nut-case. Diesel is available at any major intersection where there are several gas stations. Available at all toll-road stations, and of course people with $60G cars stop at truck stops! You been to one lately??? They're insane: movie theaters, spas, food courts.

Every post you spew some nonsense on how the US doesn't want diesels, yet every sales metric that now comes out shows the opposite is true. You are a silly, silly person and are deserving of a light tap on the head to bring you back to reality. :)

allen zheng

"GM researchers have developed a plasma-assisted catalyst system (PAC) capable of reducing NOx under highly lean conditions using E-diesel or ethanol as the reductant. The system consists of a compact, energy-efficient hyperplasma reactor followed by a dual-bed catalytic reactor. They also demonstrated good NOx conversion (above 90% on average) over a wide temperature range of 200-400° C under steady-state optimum operating conditions."
____Once again, GM has squandered its technology. First it was the catalytic converter, then it was hybrid technology, now this! It is not that they do not have the minds or the R&D, it is the management! They make the wrong calls with hybrid tech by going bus hybrids only, and not with cars and trucks (from light pickups to 20 tonner medium duty). They dawdled with the catalytic converter, then did not capitalize/market the fact that they were the ones to invent it. GM needs to get its management a kick in the kiester.


The energy loss of this plasma system could be partially recouped by putting the plasma reactor ahead of a turbo. Since turbos get thier boost from thermal load, the increased temperature would help spool the turbo. Then put the Catalytic converter after the turbo.

fyi CO2

"NOx emissions limits are a political choice, not a commandment from a higher power. Have EPA and CARB gone too far too fast?"

Rafael, you have iron lungs? EPA gone too far too fast?
You must live on the side of the Atlantic...


NOx emission limits in europe are a lot higher,

NOx are a political choice


Mitsubishi, in support of their GDI engines, developed a catalyst designed to attack NOx specifically 10 years ago. Problem is that only the gasoline in California comes close enough to the low sulfur levels required to keep it from becoming useless quite quickly. This allows their engine to run with a 40:1 a/f ratio in part load operation without worries of extremely high NOx emissions.

Reduce sulfur content in gasoline further than the 2007 goals and you open the door for catalysts specialized for NOx reduction to use lean burn engine technology (like GDI with very lean air fuel ratios).


I agree, lower NoX emissions requirements are an impractical step in the US, they should have reqired lower sulfur in the fuel FIRST.


I think this means that Honda's diesel will have to be hybrid because of this patent states that is requires electricity for the plasma.

Whats in a name


> Every post you spew some nonsense on how the US doesn't
> want diesels, yet every sales metric that now comes out
> shows the opposite is true.

OK; prove it. Sure, some Americans want Diesel engines; some people would want carburated cars if they were available. I can assure that at least in California, you will not find much support for making our air quality worse so that we can have these vehicles.

I also have recent experience with European Diesels, and I am not impressed. They aren't terrible cars, but they are loud (at least outside the cabin), they do smell, and I was absolutely amazed to see modern cars which puffed smoke on tip-in. I haven't seen this in over 20 years. Hybrids are a better option by far.


Umn, ok:

Smell??? My 2005 Golf TDI smells like nothing. Even if I start it in the garage, there is only the slightest hint of exhaust smell. Smoke??? Something wrong with the specific car if it was smoking and less than a few years old. None of the VW TDIs 03+ that I have dealt with (Toureg included) smell or smoke, even under hard acceleration.

John W.

Rafael: you say car manufacturers would dump the ice in a heartbeat if they had a chance. Perhaps some of the newer startup companies we see popping up, but as for the big makers, I highly doubt it. There is waay too much invested and involved in their current setups to "dump" the ice like it's a hot stone. And there are many other "interests" in the current setup. Think of the millions that are employed by the current auto industry. There are millions and millions of relatively complex automobiles built by our citizens all over the world.

Then there is the multi-billion dollar parts industry. Their security (and also the whole service industry, broadly speaking) is ensured largely by the ice, or rather, how they all seem to breakdown so regularly after the warranty period ends (and sooner, esp with domestics). Lose the I.C.E. for, say, an EV, and half of North America is out of work! The powers that be would never allow that to happen. At least, it will take a lot of time and pressure: it won’t happen immediately. No big maker is going to drop the ice overnight. There are many changes that could have happened already that don't because of vested interests and politics, sadly. Maybe the climate is different somewhat in Europe.


John W.:

Rafael also says:

carmakers would dump the ICE in a heartbeat if someone came up with a workable alternative, including an EV setup. Why do you think so much effort is being expended on hybrids and fuel cell research? The problem is simply that nothing beats liquid hydrocarbons for energy density and rapid refuelling.

If there were no engineering problems to EVs, your arguments might have some merit, John. But since we haven't solved some huge problems in over 100 years of the automobile, I doubt there's a Grand Conspiracy at work here. We wouldn't be putting those millions out of work overnight, either. There would be more than enough time to retrain mechanics.


"...I can assure that at least in California, you will not find much support for making our air quality worse so that we can have these vehicles...."

If diesels are so bad for air quality, then why does air quality (specifically smog) not only not improve in California, but is often WORSE on weekends when diesel truck traffic decreases by as much as 80%?

I would argue that reducing the number of GASOLINE vehicles on the roads (especially the older high-emitter) and replacing them with clean diesel cars would have a far greater impact on improving air quality since VOCs and CO control the "ozone production efficiency" of NOx. I agree with Raphael and a few other posters that there's certainly no hurry to reduce NOx emissions, like current regulations mandate.


I think Mr. "whats in a name" was just one of the other known diesel haters on this site going for a troll. Pathetic.


Carl has it exactly right. We need to get prevent all of the older cars from being used as daily drivers. The states which have smoggy cities should long ago have declared that all older vehicles (say 10 years or older) as "antiques." These vehicles would be limited to approx. 3000 miles a year without a hefty (say $1/mile) penalty at re-registration for any extra miles. Exceptions might be made via a standard emissions test. As was said above, it is a political decision to limit affordable clean diesels. It should also be a political decision to reduce the greater harm eminating from older vehicles. Commercially registered trucks should be required to be retrofit with the latest after-treatment without a doubt, even at significant cost. They are the worst offenders, and are a bad example for the business comunity. These measures will do far more to affect our health than will prohibitively raising the cost beyond that of a Euro 5 diesel. The reg's must fit the ability of the public to reasonable afford the technology. When reasonable 50 state diesel reactor/catalysts comes along, they will be adopted.

Whats in a name

I'll fully admit that my hatred for Diesel passed the rational point awhile ago, but I am more than willing to look again as improvements are made.

As Chingy notes, individual cars do not smell (trucks, on the other hand do). However, the combined contribution of massive numbers of Diesels of all types on European city centers most definitely makes them smell, and leads to choking air quality during rush hours. They also, most definitely, let out a puff of smoke each time the accelerator is pressed. Try playing 'guess the Diesel' in traffic some time. Do the most modern Diesel engines with particulate filters do this? Probably not, but then again neither have gasoline engines. For 30 years. I won't miss watching a fleet of old Diesels age.

As to the new Diesels, they are definitely better. However, even with the extraordinary effort being put into their emissions systems they will in most cases only meet the lowest emissions bins. I don't see this as the heady rush of a technology who's time has come.

As to the 'weekend effect' and regional emissions management, this problem is a tough nut to crack. I don't see more pollution as the best solution, though, even if that type of pollution may possibly provide short-term gains. The main problems here are the long time constant of fleet turnover and the asymmetrical contribution of a small number of bad cars. Speaking of the LA basin, in another 10 years we will see the effects of the massive decreases in gasoline vehicle emissions started in the mid 90's. Hopefully, a few years after that the reductions being made to the Diesel truck fleet today will start to take effect and lead to further improvements.

Rafael Seidl

What's in a name -

glad to see that you are keeping an open mind. Diesels have pros and cons, just as gasoline engines and hybrids do. There is no magic bullet here.

Historically, California has taken the lead in tightening emissions regulations, and for good reason: LA's unique combination of topography, wind direction and abundant sunshine is highly conducive to smog formation. Politicians in for other states have chosen to adopt CARB regulations lest they be accused of neglecting a public health issue. The EPA is to some extent in competition with CARB for the same reason. Regulators in many other countries believe US regs are the gold standard that they should aspire to.

The result is that the highly unusual situation of LA has strongly influenced emissions legislation all over the world, almost regardless of variations in regional climate. As long as gas was cheap, terrorism unrelated to oil and climate change just a hypothesis, this did not matter much. Now that the situation has changed, emissions regs and their impact on consumer choice et. al. ought to be a legitimate subject of debate. It's about hitting the right balance for the 21st century.


All right, Whats...., I can deal with that. But, I would like your comment now that I indeed "proved it" in relation to the US consumer market for light duty vehicles. Just in case the one instant and certain example I gave is not enough, try these, in order:,2941,0-64-68975-1-1-text-1-0-0-0-0-0-0-0-0-0,00.html

Then this one as a follow up:

Whats in a name


First, I don't see press releases (or transcriptions of press releases) as proof of anything. This is like asking the parent of a kindergartener which kid in the class is smartest.

Second, I think there is definitely better evidence of Diesel demand than this. 3000 units sold out of 6000 units shipped? That's not exactly a groundswell. Doubling their 2005 expectation by selling 10000 units? Their expectation was made before Katrina, etc., and was pretty low.

Third, this is not the best test case. The Liberty is a Diesel, but it does not use best-in-class technology. It is quite audible, does smoke, and isn't particularly powerful even for a Diesel. I was surprised when it came out that DC didn't at least use one of their own, much better Diesel engines.

I think that the VW Diesel article you linked to earlier is a better example. I don't doubt there are plenty of people who want a Diesel--the question is how many want to increase pollution to do it. As I said; there are plenty of people who would want a new carburated car burning leaded fuel if they were available. That doesn't mean we should start selling them again.

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