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Comparing Exhaust Emissions of Biogenic and Synthetic Diesel Blends

NOx comparisons of the different blends. Click to enlarge.

A research project, sponsored by the German Union for the Promotion of Oil and Protein Plants (UFOP) and Shell Research Limited, has made comparative exhaust gas studies using Shell Middle Distillate (GTL—gas-to-liquids synthetic fuel) with lubrication additives; fossil diesel fuel (DF); rapeseed oil methyl ester (RME, or biodiesel); premium diesel fuel (PDF—comprised of 60% DF; 20% RME and 20% GTL); and a blend of 95% GTL and 5% RME (B5GTL).

Overall, GTL consistently produced lower emissions than regular DF, with particularly low oxides of nitrogen emission and significantly lower mutagenicity. Biodiesel showed advantages in the hydrocarbon, carbon monoxide and particle mass emissions.

The premium diesel blend shared those low emissions, but caused—like the biodiesel—high emissions of ultra fine particles. B5GTL showed the expected combination of GTL with little shifting to RME; by contrast, the premium diesel blend obtained non-linear effects, including a lower particle mass emission but also unexpectedly high mutagenicity.

PM comparisons of the different blends. Click to enlarge.

The research was a network project of the Institute for Technology and Biosystems Engineering of the German Federal Agricultural Research Centre (FAL) in cooperation with the Steinbeis Transfer Center for Biofuels and Environmental Measurement Technology in Coburg and the Center for Occupational and Social Medicine at the University of Göttingen where the environmental impacts of different diesel fuels were examined.

The researchers conducted the preliminary test series on a 6-cylinder, 205 kW, Mercedes-Benz Euro-3 truck engine without exhaust gas after-treatment. The results—particularly the non-linear emissions and impact changes—suggest that the studies must be extended to Euro 4 vehicles with exhaust gas after-treatment in order to identify the advantages and dangers of random mixtures of fuels, to recognize the interactions, and to pass them on the engine developers.

The project is also the start of the future formulation of an engine and environmentally-tolerable biofuel that lies in between neat BTL and neat RME.




It doesn't say, but implies that the engine was not modifed for each fuel.
If that's the case, this study is useless!!


The difference between diesel fuel and biodiesel is consistent with other reports about NOx emissions... unfortunately.

It is the main reason that I have re-adjusted my position on ethanol.

Nevertheless, there are a great number of diesel-powered vehicles throughout the world and a dwindling supply of fossil fuels.

Obviously, vested interest paid for the study because they knew of such results from internal research, so they get some mileage from the study. And, studies can be attacked in various ways.

I am more inclined to wonder what could work better, which is why I found the report on ethanol injection of interest. Vested interests want us to stick with internal combustion and accept efforts like Syngas, shale oil, ANWR drilling, etc., but, as far as I know those developers have yet to find a way off our Planet.

Until evidence to the contrary, I would accept the face value of the study results as "within parameters".


Quite interesting research. Pity it was not able to compare and DME fuel also – probably because it requires modified engine. As simplest ether, it consists only from two carbon atoms divided by oxygen atom, and thus does not produce soot particles during combustion. It is produced from natural gas, practically unlimited and mostly untapped resource, and could be produced from any bioderived stock, notably biogas.

As being gas at room temperature and pressure (and liquid under moderate pressure – just like propane), it is totally environmentally safe (unlike diesel fuel, responsible for 80% of soil remediation costs and way more serious soil contaminant than gasoline) at events such as spills, pipeline ruptures, tankers and carriers accidents. Seems like ideal diesel fuel for me.


I think biodiesel's other benefits outweigh the NOx issue.


This is exactly why we should fund basic research in hydrogen. It can be produced and consumed clean.
In mean time we should fund applied research in alternative fuels. Only extreme environmentalists would believe that we could easily replace all fossil fuel derived vehicles with hydrogen in next couple of years. And only extreme non environmentalists (or oil producers what ever you want to call them) would believe that we can expand our fossil fuels usage indefinably. And we as a forward looking realists, we should support every single way that brings us closer to that dream of being energy independent and sustainable.

Rafael Seidl

Significant quantities of NOx are produced above local temperatures of 2100K. The rate of production rises sharply with increasing temperature and the length of time spent at these elevated levels. It is COMPLETELY independent of the heat source! Whereever you have a flame front, you encounter temperature excurions to 2500K or higher.

That said, NOx production in compression ignition engines is reduced by fuels with higher cetane number because they combust as they are injected (low ignition delay). However, within the tolerance band of diesel fuel specifications, there is only so much that can be done. In Europe, the minimum cetane number is 51. In countries where diesel is popular, values of 53 to 56 are often encountered. Neat DME has 55-60, xL up to 99, biodiesel ~45.

By contrast, US refineries rely heavily on hydrocracking to meet gasoline demand. The middle destillates of the hydrocracked fluid, known as light cycle oil (LCO), are the primary constituents of US diesel fuel. The minimum cetane number is just 40 (45 in CA), resulting in slightly higher NOx and noise production.

One interesting option emerging out of research is what AVL calls "alternative combustion", feasible for part load in the low-to-mid RPM range. In this scenario, there is only single injection into a highly premixed (= swirling) charge of air and cooled externally recirculated exhaust gas (40-50% EGR rate). The exhaust gas retards ignition such that the injection event (4-5 deg crankshaft) is over before significant heat release begins. Therefore, no flame front is observed and NOx production is largely suppressed. PM production is also largely avoided as there are few zones of very rich mixture. The concept will likely be introduced in European series production passenger cars with large displacement diesel engines within the next 2-3 years.

Mark A

I agree with Andrey, that DME is the ideal diesel fuel. But the changeover would be too great, overall. Maybe certain diesel locomotives can make the plunge first, into DME (is there a Union Pacific engineer here?). Creating more supply for other transportation.

It is also interesting to see that the biodiesel did not make a better showing than it di. Did Willie Nelson see this? Yes the tests would have showed better results with the engine optimized for each fuel, but in real world conditions, who would take apart and modify their engines with each tankful? Yes the test was fair. It was not meant as a competition, just a evaluation.


Is this a fair assessment of biodiesel? I was under the impression that there is a huge difference in biodiesel fuel quality because of the lack of standards?

richard schumacher

One defect in the study is that they did not determine the sulfur content of the RME. The GTL and DF used were quite low in sulfur, <5 ppm and 3 ppm respectively, which accounts for much of their low PM emissions. Perhaps RME modified to reduce the sulfur content would also have low PM emissions.

richard schumacher

Oh, crap, please disregard that last. Still it would be useful to know the sulfur content of the RME.


RME has a 0% sulfur content, one of the reasons it has the lowest PM (.03) emissions. RME does have higher NOx, but, keep in mind it is CO neutral, none of the others are. A 50/50 GTL/RME with an NOx after-treatment would result in very low all around emissions.


Pick your poison.

Certainly, if cost effective it is worthwhile to use a biodiesel blend to reduce green house gases. The study supported such effort.

On the other hand, a reduction in NOx emissions is critical, so RME would seem to be unsatisfactory as a complete substitute.

So, how do diesel fuel producers and fleet managers in the U.S. best respond?


On the other hand, a reduction in NOx emissions is critical, so RME would seem to be unsatisfactory as a complete substitute.

So, how do diesel fuel producers and fleet managers in the U.S. best respond?

Gasoline engines put out lots of NOx -- they deal with it by treating the exhaust in a catalytic converter. However, sulfur precludes serious exhaust treatment of diesels. The use of no/low sulfur fuels like bio and ULSD makes possible treatment of diesel exhaust. The technology is different for diesel though, so it will likely take a couple of years to shake out.

allen zheng

Use a blend of ethanol and one of these liquid fuels to reduce NOx and particulates.

Hugh Laue

Agree with last post (add ethanol). Also water-in-diesel emulsion bus fleets are operating quite successfully in USA and Europe, I hear, giving reduced NOx and particulates. However, ethanol addition easier to implement.

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