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TDI vs. gasoline
- The TDi emissions
levels are among the lowest ever for Diesel powered engines. All
TDi powered Volkswagens sold in the US meet so-called "Tier
1" emission limits. The TDi is often "cleaner"
overall than gasoline powered cars. CO2 emissions are 25%
less than a conventional gasoline powered engine. CO, HC and NOx
emissions are less than previous Volkswagen Diesels. Diesel fuel
has lower evaporative emissions than gasoline. Diesel fuel also
requires less energy intensive refining than gasoline.
Diesel engines generally emit higher amounts of NOx and particles
than equivalent gasoline powered cars, even though CO and HC emissions
may be lower, and total emissions are lower due to much better
fuel consumption. The current TDI Volkswagens typically emit slightly
somewhat lower than the Tier 1 limits for NOx and particles
(around 0.052 g/mi of particulate matter [PM] and 0.82 g/mi of NOx per
EPA data), but the CO and HC emissions are far below the Tier 1 limits
and well below the emissions of the equivalent gasoline engine.
Furthermore, most of the unregulated toxic gaseous emissions tend to
be lower for diesel engines. For example, benzene (which is a known carcinogen)
is lower in diesels by nearly an order of magnitude (i.e., factor of ten)
than an equivalent gasoline engine. Diesels also tend to be significantly
lower in emissions of alkenes (e.g., ethene), carbonyls (e.g., formaldehyde),
and semivolatiles like polyaromatic hydrocarbons (PAHs, many of which are
known or suspected carcinogens).
PM has always been regulated by mass (e.g., grams per mile). However,
very recent studies show that particle number may be the more important aspect
of PM emissions. According to a "real world vehicle testing report" by University
of Minnesota renowned combustion particle scientists, new data show that PM number
emissions from modern gasoline cars may equal or exceed diesel PM levels.
It goes on to discuss gasoline PM emissions and that fact that gasoline engines
may need a particulate filter much like that of a diesel. The University of
Minnesota study showed that newer and older gasoline vehicles matched or exceeded
diesel PM number emissions at high speed/load . It appears that diesel engines equipped
with diesel particulate filters (DPFs), as many are now in Europe, will have a
significant advantage in PM emissions over gasoline engines. Other recent studies are
suggesting that gasoline PM is generally more toxic that diesel PM.
The emission levels from diesel engines tend to remain more-or-less
constant throughout the useful life of the engine, whereas gasoline
engines have many more emission-related components which deteriorate
and lead to higher and higher emissions as the engine gets older.
Volkswagen has made continuous progress on emissions through
the years, and 2000-model TDI engines emit far less than the 1996
models first available here. Further progress has been made in
Europe with new fuel-injection and emission-control technology,
but for various technical and market-related reasons, this technology
is not available here yet, but will likely be arriving within the next
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fuel is available in limited
geographical areas. The fuel is produced from a recently grown
crop of vegetation (often soybean ) and is produced without the
partial fossilization and the passage of millennia. The emission
levels of the bio-diesel is lower than the more common petroleum
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California and New York issues
- The state
of California places limits on the "fleet average" emissions of auto
manufacturers. Currently, a manufacturer can only sell a certain proportion
of "Tier 1" vehicles in relation to the number of "LEV" or low-emission
vehicles. Certain other states have copied the California legislation.
Volkswagen has stated that the withdrawal from sale of 2000-model TDI
vehicles from certain states is due to these reasons. Furthermore, California
has declared diesel exhaust to be a toxic air contaminant, although other
studies dispute this conclusion. The situation in New York is the same.
NOx and volatile organic compounds (VOCs) are involved in complex photochemical
reactions to form ground-level ozone. Recent U.S. Department of Energy-supported
studies have strongly suggested that the strict control of NOx emissions may
have the unintended consequence of making ambient ground-level ozone worse. The
California Air Resources Board (CARB) has put increasingly strict emission limits
on NOx emissions in an attempt to reduce ground-level ozone (ozone is an inhalation
health concern), for which Southern California is notorious. However, many previous
studies have shown that ozone levels are actually higher on weekends (WE) than on
weekday (WD) (WD/WE effect) when diesel truck traffic decreases relatively much more
than automobile traffic (which mostly are powered by gasoline engines). The DOE
studies have confirmed that NOx is reduced significantly more than VOC (HC) and as a
result ozone levels increase. It has been discovered that most large urban areas in
the U.S. have similar conditions in which ambient ozone levels rise with decreasing
ambient NOx levels and that NOx controls in Southern California (and other urban U.S.
locations) are generally counterproductive for reducing ambient ozone, they actually
increase ambient ozone levels. Were it not for large concurrent HC emission reductions
on weekends, weekend ozone would be even higher than it is, and the weekend/weekday ozone
difference would be even larger. DOE concludes that gasoline exhaust and gasoline vapor
account for ~80 percent of ambient NMHC (VOCs) in on-road samples and at regional air
monitoring locations suggesting that gasoline emissions are responsible for the majority
of ozone found in the SoCAB. Whether these recent findings change CARB's (or even EPA's)
ozone control strategy remain to be seen.
In addition, other recent studies are suggesting that carbon monoxide (CO) emissions
(most of which are from gasoline engine vehicles) are becoming more and more responsible
for generating ground-level ozone. The National Research Council's (NRC's) report confirmed
the importance of carbon monoxide in the formation of urban ozone, concluding that more than
20 percent of vehicle-related ozone pollution comes from carbon monoxide. The Council also
notes that carbon monoxide emissions will play an even larger role in ozone formation as
volatile organic compound emissions from vehicles continue to decrease. Another source
concludes that CO emissions may be responsible for as much as 35% of the ground-level ozone.
CARB (and to a slightly lesser extent EPA) focuses on reducing NOx. But atmospheric scientist
Gary Whitten of ICF Consulting notes that if the tradeoff of reducing NOx is to increase hydrocarbon
and carbon monoxide emissions, the environment would be poorly served. The reason, according to
Whitten, is that a reduction in hydrocarbon and carbon monoxide emissions has a much greater beneficial
impact on ozone formation than an equivalent reduction in NOx. Whitten concludes, "The effectiveness
of THC for reducing ozone in these simulations must be as much as 8 times better than NOx reductions
on an equal percentage of the mobile emissions basis." Since diesel engines tend to have significantly
lower emissions of CO and HC (VOCs), while generally higher emissions of NOx, one could conclude based
on these recent studies that an increasing market share of diesel-powered cars and light trucks will
have a positive impact on ground level ozone rather that the negative impact which has always been assumed.
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- The future - Tier 2- The USA will be phasing
in "Tier 2" emission limits over the next few years. These emission
limits are considerably stricter than the Tier 1 limits, and existing
TDI engines emit more than the allowable NOx and particles under the
Tier 2 limits. On the other hand, the USA will also be phasing in
requirements for truly "low sulfur" fuels, which may open up additional
options for the auto manufacturers, since sulfur contained in fuel
prevents the use of catalysts that can deal with NOx and particles
more effectively than at present. Also, the North American auto manufacturers
have a number of diesel engines of their own under development. Several
manufacturers have demonstrated the ability to meet the final Tier 2 "permanent"
bins. Diesel engines from four automakers have passed the Environmental Protection
Agency's stringent 2007 emissions regulations for cars. Toyota Motor Corp. and
Volkswagen AG are two of the automakers who have met the 2007 standards. Detroit
Diesel Corporation, a subsidiary of Daimler-Chrysler (DCX), developed a
first-generation, integrated CIDI engine and emission control system for automobile
and light-duty truck applications. This system, applied to a DCX Neon vehicle
simulating a vehicle with a 2,250-lb inertia weight, includes CLEAN Combustion©
technology in conjunction with a first-generation emissions control system.
It has achieved Federal Emission Standard Tier II Bin 3 NOx and PM emissions
levels over the transient Federal Test Procedure 75 (FTP) cycle. Combined fuel
economy (integrating FTP75 and highway fuel economy transient cycle tests) was
measured at 63 miles per gallon. This vehicle met Tier 2, bin 8 (a permanent bin)
emission limits without any emission after treatment control. FEV, a manufacturer
of fuel injection equipment, has demonstrated that with a specific combination of
emission control equipment, a light-duty diesel was able to meet California's
very strict SULEV emission standards (with near zero CO emissions). Durability
issues are still being worked on as both EPA's Tier 2 and CARB's LEV II standards
have a "full useful life" (FUL) requirement. However, Robert Bosch Corp. has
developed a particulate trap filter for diesel engines (DPF) that it says meets
the government's tough 10-year, 150,000-mile durability requirements. EmeraChem
has developed a NOx adsorber which it says retains > 95% NOx reduction efficiency
for >200,000 miles.
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- Emissions testing - In most areas with emission
testing, diesel-powered light-duty vehicles are only subjected to
an "opacity" test, which simply notes how "opaque" (i.e. black) the
exhaust is. The clean-running characteristics of the TDI engines virtually
ensures that they will pass this test, unless they are running so
badly that the owner could not help but notice. In some areas, diesel-powered
vehicles are exempted from emission testing altogether. Emissions
testing regulations vary from one area to another and change with
time. Some areas may do full testing, including testing for NOx, in
which case the condition of the EGR system, presence or absence of
aftermarket equipment such as "chips" or "boxes", and the general
condition of the engine, will all make a difference.
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All information Copyright © 1996-2002 Fred
All rights reserved. This FAQ (Frequently Asked Questions) may not
be reproduced without written permission.
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