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.

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.

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.

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?

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.

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.

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.

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.

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?

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.

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

United they stand. Good cooperation.