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Emission reduction potential
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Motivation



European cities are plagued with bad air quality. Alternative fuels and alternative power trains may contribute to an improvement of the situation. This section describes their potential going into details for the most relevant emissions (PM, HC and NOx as Ozone forming gases). It is also claimed that CO is the biggest single contributor to ozone(Gary Whitthen, Ethanol's clean air impacts, latest findings). It has to be said that for P2.5 (2.5 µm) not much data exists. Measures to reduce emissions are often correlated with the environmental measurement technology. Care has to be taken not to overlook noxious emissions not listed in regulations (nano-particles) or hidden in not so noxious components (mutagenic hydro carbons).
This page is still under improvement and the findings might be adapted

Ethanol


Ethanol contributes to significantly lower emissions when used in adapted (flexible fuel) engines. Blending into gasoline/petrol and use of old vehicles will not contribute to significant improvements - it might even create problems in zones with many starting vehicles (garages, parking at events...). Under freezing conditions however PM will be decreased by the oxygen content in the fuel.
Using E20 in new vehicles the total unburnt hydrocarbons (THC) will be reduced by approximately 28% (25% on highways), CO by 21% (45%) and NOx by 34% (9%).
In old vehicles THC will be reduced only by 4% (10%), CO might be reduced by 70%(76%) but not for vehicles with close loop fuel control. This will also lead to an increase of the NOx emissions by 9% (10%). The changes are significantly different for different types of vehicles(source DEH, A testing based assessment to determine impacts of a 20% gasoline fuel blend on the Australian Passenger Vehicle Fleet source). For city areas the increase in NOx might help to reduce ozone peaks. E20 fuel vehicles show an increased Acetaldehyde emission during the warm-up phase of the drive cycle.
In total the exhaust toxics were reduced (Benzene 40%, Hexane 40%, Toluene 30%) with new vehicles, for old vehicles the reduction is for Butadiene 15%, Benzene 20% and Toluene 10%.
Particle emissions are one to two orders smaller compared to gasoline/petrol (source).
Since PM will be an issue for direct injection gasoline/petrol engines in the future, the 25% reduction is welcome. Also buses may be run on E-diesel/O2 diesel with a reduction potential of at least 20% for PM and 2% for NOx source.
more...
Low blending (E10) might contribute to significant reductions in emissions.

FT/synthetic diesel


Synfuel contributes to lower emissions in all engines, especially for cold start. Although NOx-emissions are not changed!
Using GTL diesel fuel with a higher Cetane number in unmodified vehicles shows significant reductions of HC, CO, and PM emissions (HC and CO also in the cold phase), compared to a sulphur-free European diesel fuel. GTL allows higher exhaust gas recirculation (EGR)-ratios - which may be used to reduce NOx. For neat GTL, reductions of 35% both in NOx and in soot are predicted, and with hardware adaptations this may be even more. Low blending of GTL brings big improvements in NOx and soot emissions (20% brings 43% and 50% brings 86% reduction source). For cold start CO is reduced most (70-75%), followed by HC (70%) and PM (60%) (Seyfried. VW, Environment - Friendly Synthetic Fuels from Renewables).
Experimental data from MAN showed for buses a reduction to 95% for NOx, for particles (PM) to 83%. With a more realistic cycle (ETC) PM were at 82%(V+T. 10/2005, page 391ff). This means that there is no way to reach Euro V by switching to GTL.

SVO/PPO


There is no significant positive effect for new optimized diesel engines.
But emissions might be differing depending on the fuel quality.
PPO contains no traces of carcinogens (benzene) and heavy metals, such as in conventional diesel fuels and thus those substance are supposed to be less frequent in the exhaust (burning motor oil and abrasion might contribute). This was only validated for heavy metals.
The particle size distribution for vehicles running on PPO/SVO is similar to that of fossil diesel (Ricardo, 2003).
NOx and PM10 show no validated differences.
If PPO/SVO is used too early in the starting phase, higher THC-emissions might result. The exhaust gases may have the odour of cooking oil, which however is not harmful and will not be present if an oxidation catalyst is used (source).

FAME/Biodiesel


Since the feedstock influences the fuel characteristics heavily, discrepancies in emissions are not unusual. FAME from animal fat decreases PM significantly and does not increase NOx significantly
Biodiesel used in unmodified compression ignition engines will reduce CO and possibly also HC-emission and is ideal for selective catalytic reduction (SCR) reducing NOx because of the low content of impurities.
Organic acids and/or oxygenated compounds found in biodiesel may affect the response of the flame ionization detector, thus understating the actual HC emissions (source). There is also differing data about the effect of biodiesel on particulate matter and NOx emissions. With buses and trucks a PM reduction has been proven recently (25% with B35 source).
FAME based on animal fats does increase the NOx-emissions only slightly (3%), soy bean may increase them by 15% (for B100). Also for PM, FAME based on animal fats perform better (-50%) compared to soy bean (-35%). The reduction however is more related to bigger particles. When using FAME, the PM contains more volatile materials without oxidizing cat (Kittelson, Center for diesel research, Biofuels for engines]).

Methane


Methane has significant potential to reduce emissions especially PM and also NOx.
For heavy vehicles, PM may be reduced to one third, NOx cut by half source. Detailed measurements revealed that with the smallest 8nm particles natural gas has much higher emissions compared to diesel fuelled engines with particle trap.

Hydrogen


Hydrogen has very low (but not zero) emissions.
Small NOx emissions are produced when used in ICEs due to high excess air . Practically emission free vehicles (if water vapour is no problem) with FC. If local production of hydrogen takes place, local emissions for power production have to be considered.

Hybrid power trains


Hybrid power trains have significant potential to reduce emissions for start and stop traffic. This potential may be further increased by increasing the electric driven part, which is the case for plug in hybrids or battery electric vehicles. The energy savings are 16% with diesel engines to 26% for gasoline/petrol engines, which also result in savings of noxious components. The potential depends on the size of the electric motor and is only achievable to its full extent if a significant motor size is implemented (for passenger cars 20kW).
For heavy vehicles/buses figures indicate a reduction of 30-42% for CO, 34-48% for HC and 23-35% for NOx (Eberhardt James,US DoE, Fuel and emission impacts of heavy hybrid vehicles)

DME Dimethyl Ether


If DME is used instead of liquefied petroleum gas clean propulsion systems may be combined with a regenerative fuel (if DME is produced from bio gas or wood gas). The use of an oxidation catalyst is possible and leads to very low HC and CO emissions. Test have shown that NOx emission is lower than EURO 4 (Haldor Topsoe A/S, DME as a Transportation Fuel). Because of the gaseous fuel PM is not an issue, so no particle trap is necessary.

Further reading



NREL OSTI (pdf)
Fuels in Brazil(pdf)
DEER CEM (pdf)
VTT (pdf)
UCR (pdf)



  


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