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BMW, DaimlerChrysler and Volkswagen to Team on BLUETEC for the US

7 October 2006

Automobilewoche reports that Volkswagen and its premium Audi unit, BMW and DaimlerChrysler’s Mercedes-Benz will all use the BLUETEC system developed by Mercedes-Benz and Bosch to meet US Tier 2 Bin 5 requirements for diesel light-duty vehicles starting in 2008. (Earlier post.)

Volkswagen and DaimlerChrysler have already confirmed the arrangement; BMW and DaimlerChrysler reportedly will finalize details on 12 October.

BLUETEC is a technology framework for the reduction of NOx emissions to the level required by US Tier 2/Bin 5 / California LEV II emissions requirements—the world’s most stringent. The current Tier 2 Bin 5 solution under development is a urea-SCR system that relies on the use of AdBlue—an aqueous urea solution.

Injection of the AdBlue urea solution into the pre-cleaned exhaust gas releases ammonia (NH3), causing the nitrogen oxides to be converted into nitrogen (and water) in a downstream catalytic converter.

“We may not permit that this critical technology [i.e., diesel] for the US is bad-mouthed,” explains a insider. “The German automakers with their competence in diesel must go there together,” says an AUDI manager—also because of Japanese success with hybrids [there].

October 7, 2006 in Diesel, Emissions, Vehicle Systems | Permalink | Comments (24) | TrackBack (0)

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Does anyone know if this urea alows higher cylinder temps ie. more efficient running followed by removal of these elevated NOx levels. This would minimize the mpg loss due to the more stringent emissions requirement.

Urea is injected in the exhaust gases post combustion. No effect on cyclinder temp.

Andy -

SCR systems do not require any purging and are far less susceptible to sulphur in the fuel than NOx store catalysts. They can therefore be applied to both new and legacy diesel engine designs far more easily, especially if a particulate trap must also be fitted. There is a very small negative effect on fuel economy due to the backpressure exerted by the additional monoliths of the SCR system. The urea additive is consumed at a rate of ~4% of the fuel flow and costs about half as much per gallon as diesel (Note: these numbers apply to European HDVs and may differ for LDVs on the US market). This compares well with the 4-5% increase in cost/mile observed with NOx store catalysts, which also cause increased CO2 emissions.

IMHO, it is heartening to see all three German carmakers with significant sales in the US joining forces on a SCR implementation, as the higher aggregate number of vehicles will make it easier to set up a distribution network for the urea. BMW has not sold diesels at all in the US before and, others may decide to jump on the bandwagon at a later date. US consumers will have a wider choice of vehicles featuring high fuel economy.

The distribution netowrk was one sticking point for EPA and CARB. Another was ensuring drivers could not get away with filling the urea tank with just water or nothing at all - apparently, MB's tamperproofing measures plus heating elements for reliable winter operation are now deemed satisfactory. The third concern relates to the lower engine-out exhaust temperatures of the US FTP cycle relative to the NEDC - SCR systems have a relatively high light-off temperature of ~250 deg C, below which they are basically ineffective.

An increase in engine-out NOx emissions for the sake of additional power or fuel efficiency is highly undesirable, due to elevated NVH and reduced life expectancy. If at all, the initial implementation might call for secondary post-injections in certain situations, such as cold starts and extended idling. Their sole purpose would be to ensure the SCR catalyst's light-off temperature is reached quickly and then maintained.

Future models may feature SCR (sub-)systems that are located closer to the exhaust manifold and/or washcoats with lower light-off temperatures. This would be analogous to the incremental improvements we've seen in three-way catalysts for gasoline vehicles.

Craig, engines are detuned to meet emissions. I know urea is injected as an aftertreatment. My point is if you run then combustion chamber hotter it is more efficient, lower fuel consumption her HP/HR. If urea is used then can you run a hotter chamber because you capture/convert the NOx created by higher temps, therefore increasing efficiency. I know aftertreatment dosn't change temps in the engine, just lets allows more agressive tuning!

Andy:

Combustion temperature in diesel engines is proportional to compression ratio. It could not be increased without severe effects on engine and higher HNV levels, as Rafael rightfully pointed out. No potential to higher fuel efficiency here. Also, if fuel injection events would be optimized for best fuel economy instead of low NOx, the difference in fuel efficiency will be no more then 2-3%. No real difference here also.

There is one big problem with light-duty diesels in US. Both EPA and CARB require that emission control equipment should retain about 80% of it efficiency after 100 - 120 thousand miles 10/11 years of vehicle operation. It will be way more difficult to achieve then compliance with initial requirements.

Andy,
I follow what you are saying. Frequently, in FI diesel engines, tuners can usually gain about 20% in both HP and TQ. This is achieved through leaning out the A/F ratio, increasing boost pressure, and altering timings. In some cases, this leads to reduced fuel consumption. The side effect is usually higher NOx emissions.

Seeing that urea SCR systems adapt to the amount of exhaust gas/amount of NOx present, I would imagine that it could potentially reign in those higher emissions. The trade off is higher consumption of the urea solution.

It looks like urea has significant fossil energy inputs. Does anyone know how much petroleum-equivalent energy it takes to make a unit (volume or mass) of urea?

John:
All diesel engines are FI by definition. Leaning A/F ratio reduces power on both gasoline and diesel engines. Altering timing is not relevant to diesel engines at all. Increased boost on both gasoline and diesel engines allows to burn more fuel and hence to increase power – up to melting engine to sludge. Get into fundamentals, man.

Andrey, IU am not a diesel engineer. My father is a retired diesel engineer with many patants that show his name. I'm sorry but you are wrong. I grew up at the kitchen table on this stuff.

Guys, in a nutshell! Advancing the timing of fuel delivery both increases combustion chamber pressure and temperature which increases NOx and increases efficiency as well. Retarding the timing reduces NOx but increases unburned HC, particulate and increases fuel burn because less energy in captured on the power stroke. Air to fuel ratios in a diesel are irrelavant. Air is not controlled by a butterfly, diesels do not draw a vacuum, that is the major reason for increased efficiency over spark ignition. One interesting note my father was just telling me about urea being MIXED with the fuel in europe about 30 years ago to lower NOx. This was done on some engines(not mobile) and in labs for development work. They also used a primer charge from the injectors back then. NVH and durability aren't an issue, the engine is built strong to deal with the severe pressures not present with gasoline. Durability is improved by dealing with these pressures and NVH is a sound deadening issue not a reason the derate an engine.

Andy,
Thanks for the clarification, that was a bad call on my part about the whole A/F ratio thing seeing that diesels use compression ignition.

Dimitris,
I was wondering the same thing about the fossil fuel inputs for urea. This whole thing might be a shell game to pawn the emissions off onto other industries. However, note that their are two reactions in making urea. The first is exothermic, and the second is endothermic. Perhaps the second reaction recovers some of the enregy lost from the first?

I don't know, just speculating, though I would like to see some data on the energy inputs for urea production.

John, I think the 600MW of methane powered farm generators in California is a neat industry possibly finding a second income. They collect their farm animal waste and turn it into menthane. Sounds corny but solid waste one way, fluid the other. Manthane powered electricity from solid waste, urea providing bluetec from the fluid. Renewable energy and emissions reductions elsewhere from the animal waste. I thought the first generation of urea aftertreatment in Europe was at least partly animal urine derived. This is the type of framework I would like to see my tax dollars support. If it offsets mineral based urea and electricity it's part of the solution. There are thousands of MW's of clean renewable power waiting for political/capital will and maybe this would be a neat offshoot.

Andy,

It sounds like Caterpillar is using an approach similar to what you're apparently suggesting for improving thermal efficiency while meeting the 2010 heavy-duty emission regs. See http://www.osti.gov/fcvt/deer2005/Easley.pdf for more information.

Good news.

I also think that increasing compression ratio would be a good thing, if the urea aftertreatment allows it from an emissions standpoint.

Today diesel engines are retreating back to 16:1 compression ratio, all to chase NOx emissions. This is a tremendous waste of the potential of the diesel engine, many road going versions of which used to have 20-22:1 ratios and hence much better thermal efficiency.

Cutting back the compression ratio like the manufacturers are doing today is a backwards trend that should be reversed (I'd like to see 30:1 expansion ratio "Atkinson-ised" diesel engines). Urea could be one way of doing this.

Clett -

while there is a (mild) efficiency improvement for higher compression ratios, this applies mostly to large, heavy vehicles. In relatively lightweight passenger cars, the extra metal needed to contain the very high in-cylinder pressures makes the engine that much heavier and expensive. Increased engine weight is compounded by the beefier mounts, chassis beams, suspension, brakes etc. To preserve performance, the engine has to be made more powerful, which means its typical operating range is that much closer to inefficient idling. Larger engines also require more extensive NVH mitigation, adding yet more weight.

High engine weight also changes the weight distribution between the axles, which impacts cornering behavior. This is a problem mostly for European A through C segment vehicles, in which the diesel is typically the heaviest engine option.

According to my professors, optimum *vehicle* fuel economy would actually be achieved for compression ratios of 14-15. Spark ignition engines can get close to the lower end only with monovalent CNG fuel, with E100 a close second. This requires changes not just to the engine and on-board fuel storage but also a new infrastructure for the production and distribution of these high-octane fuel grades.

Diesels can get close to the upper end only with powerful glow plugs and thermal management strategies that ensure reliable fuel ignition and acceptable emissions levels during cold starts in severe winter weather. Raising the cetane number does help but there is no need for any completely new fuel grades.

You should therefore expect the present trend toward lower compression ratios in diesel engine design to continue, even though NOx aftertreatment is now feasible.

Rafael - two questions:
1) Does the optimum ratio of 14-15:1 for the overall vehicle assume current diesel designs with their weight penalties, or is that strictly on the thermodynamic efficiency achievable without regard to weight?
2) It seems to me that the vast majority of diesel engines are still using simple cast iron blocks, with a few using aluminum blocks with cylinder liners. Many gas engines have far more advanced block designs, typically seen in sport motorcycles and high performance cars. Is there any reason these designs couldn't be applied to automotive diesels as well and save much of the current weight penalty?

It is a good thing to see that the European car manufactures are uniting under one technology to get all 50 state certifications. BTW, each of the manufactures mentioned in the article have sold diesel cars in the USA, BMW did it in the early to mid 1980s for 2 years. I hope that there is a lot more choices in Diesel vehicles for 2008.

Zach -

(a) As explained, the optimum compression range of 14-15 refers to optimum fuel economy for a light duty vehicle, not the optimum fuel efficiency for the engine alone.

For stationary applications, ship, rail and special purpose diesels, the optimum compression range is typically (well) north of 20.

(b) diesel engines tend to experience far higher mechanical stresses than gasoline engines do. The acoustic damping demands are higher for diesel engine crankcases. Heat loss to the coolant should be minimized for optimum fuel efficiency. Cast iron is superior to aluminium in these regards. The downside is the differential thermal expansion between cast iron block and aluminium piston, which has to be bridged by the piston rings.

Given recent advances in cast iron metallurgy (e.g. vermicular and sperical types, silicium inserts) as well as no-liner technology (diamond honing, laser structuring), both cost and weight of a cast iron diesel engine crankcase can actually be lower than for an adequate aluminium design - at least for high-volume, small-displacement three- and four cylinder designs.

It is incorrect to assert that aluminium and/or hybrid magnesium crankcases are always better or even just higher tech than modern cast iron technology. For example, most of VW's current-generation diesel engines for the A through C/D vehicle segments feature cast iron crankcases.

I would like to add that 14-15 compression ratio is calculated geometric compression ratio. Modern turbochargers produce higher boost pressure then couple of years ago, so firing pressure of modern diesel with 15 compression ratio is close to 10 years old engine with 18 CR.

Temperature stress in gasoline engines is higher then in diesel (hence much higher NOx generated in gasoline engines – before aftertreatment). Moreover, most gasoline engines feature cast iron cylinder liners. So temperature stress of diesel is not a big problem to aluminum engine blocks. But mechanical stress in diesel engines is much higher, especially detonation waves from fuel evaporated during ignition delay, so cast iron is better suited to contain this stress then aluminum. Top end high power turbocharged gasoline engines sometime use cast iron too, like bi-turbo 2.7 liter in-line 6 RB series from Nissan. With extreme modifications, these engines could produce power burst up to 1000hp, yet retaining decent street-driving abilities.

High CR engines too heavy for passenger cars? The VW Golf AAZ series diesel engine had 23:1 compression ratio. No probs.

How is the Golf's handling comparing the diesel versus non-diesel? Not just everyday driving but for emergency manuevers.

United they stand. Good cooperation.

My Name is Tom and I am doing a research project in NVH at the moment I am looking to contact engineers who specialize in this area preferably in the UK can anyone point me in the direction of Companies or individuals that would help

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