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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