All engines produce emissions. Even electric and hydrogen
vehicles produces emissions because the electricity that charges the
batteries or that produces the hydrogen fuel has to come from a power
station.
I believe that diesel engines are less environmentally damaging than at
least the petrol powered cars that most of us are currently
driving. This page explains my reasoning.
The page is split into categories of emissions. These are:-
Carbon Monoxide
Carbon Dioxode
Hydrocarbons
Particulates (smoke)
Nitrous Oxides
Supply chain issues
Carbon Monoxide
Carbon Monoxide is a poison which can and does kill. This gas is
the reason why staying in an enclosed space with a running petrol
engine is extremely dangerous. Carbon Monoxide is caused when
there is not enough oxygen to burn all of the fuel. Diesel
engines operate with an excess of air at all times except full power
because they are unthrottled. In this sense a diesel engine is an
ultra lean engine. Petrol engines on the other hand rely on a
controlled air to fuel ratio ( called a stoichiometric ratio ) and so
produce Carbon Monoxide. With more recent vehicles the Carbon
Monoxide emissions are reduced by use of a catalytic converter.
However catalytic converters have a warm up time during which Carbon
Monoxide emissions will still be high, and are easily damaged.
Due to the lean burn nature of diesel engines Carbon Monoxode emissions
are essentially non-existant.
Carbon Dioxode
Carbon Dioxide emissions are widely thought to be causing global
warming as it is a greenhouse gas. Carbon Dioxide emissions are
generally proportional to the amount of fuel consumed. As a
diesel powered vehicle will typically use 25% to 35% less fuel than a
petrol powered vehicle the Carbon Dioxide emissions are also 25% to 25%
lower. Diesel cars even compare well with electric vehicles
because the thermal efficiency ( the measure of how much energy in the
fuel produces useful mechanical power ) of the best diesel engines
matches that of power stations. For example the 110bhp Volkswagen
TDI engine is 43% efficient. Diesel fuel is denser than petrol
engines which instantly gives diesel vehicles an energy advantage for
the same volume of fuel, however more significantly diesel engines are
more efficient due to lower pumping losses ( because they are
unthrottled ) and due to more complete combustion and higher
compression ratio.
If biodiesel is used then the Carbon Dioxide impact of a diesel car
effectively become zero. This is because the diesel engine
burning the biodiesel is just releasing the Carbon Dioxide that the
plant used to make the fuel absorbed when it was growing.
Hydrocarbons
Hydrocarbon emissions are basically unburnt petrol. Incomplete
combustion manifests itself as particulate emissions in a diesel engine
rather than hydrocarbons, so hydrocarbon emission are only really an
issue for petrol engines. Hydrocarbon emissions from petrol
engines include some really nasty chemicals such as benzene which is a
known strong carcinogen ( cancer causing chemical ).
Particulates
Particulate emissions are also known as smoke. Diesel smoke is
the main problem for diesel vehicle emissions as the quantity is much
higher than that of petrol engines, and because they are clearly
visible. Somehow humans are somewhat more neurotic about
emissions that they can clearly see than those that they cannot.
Particulate emissions are caused in a diesel engine because the diesel
is injected into the engine as tiny drops of fuel. Particulate
emissions happen in a diesel engine when there is not enough oxygen to
completely burn a droplet of fuel. This typically only happens at
full power or heavy load conditions. The outside of the fuel
droplet starts to burn but then oxygen levels inside the engine drop to
the point that the inside of the droplet forms into a tiny lump of
carbon rather than being burnt. A cloud of these lumps of carbon
coming from the tailpipe then constitutes diesel smoke.
There is conflicting evidence around the health effects of
particulates. Some cities have reported higher death rates due to
heart attacks on days when particulate emissions are high. To be fair
these emissions come from many sources such as cooking and power
generation, and petrol cars as well as diesel cars, and so the true
impact of diesel smoke is harder to quantify. Such an effect
would also be totally irrelevant for healthy people as a healthy person
does not have a heart attach due to smoke levels in cities.
Some health studies have been conducted on people working in bus
maintenance depots and such workers have been found to just as heathy
as the rest of the population despite the smoky environment in which
they work.
Lastly when comparing petrol with diesel one must consider the nature
of the particles. Diesel smoke particles are typically PM10s,
which means that the particles are around 10 microns in diameter.
Petrol engines also produce particulate emissions but the particles are
typically sub micron which is why they are not visible. The
problem is that sub micron particles penetrate much deeper into human
lungs, which of cause have naturally evolved to cope with and eject
larger dust particles. It might just turn out that invisible
petrol particulate emissions could be more harmful than the larger
visible diesel particulate emissions.
There is no doubt that diesel smoke is the worst PR problem that diesel
has. This is now starting to be solved by the introduction of
particulate filters pioneered in passenger cars by Peugeot.
Particulate emissions from diesel Peugeots with particulate filters are
not only lower than petrol engines; they are so low that they cannot be
measured.
Nitrous Oxides
Nitrous Oxide emissions are caused when an engine burns not only the
fuel that it is supplied, but Nitrogen from the air itself.
Nitrogen forms 78% of normal air. Both petrol engines and diesel
engines produce Nitrous Oxides, most of which is NO or nitric
oxide. Nitrous Oxide emissions are claimed to cause acid rain and
smog. There are some differing opinions on smog emissions however
as some claim that both Hydrocarbon and Nitrous Oxide emissions are
needed in combination to form smog in cities. In fact some claim
that there is an optimum amount of Nitrous Oxide emissions for smog
formation, and once a certain concentration of Nitrous Oxides is
exceeded smog is actually reduced. As the Hydrocarbon Emissions
come almost exclusively from petrol engines one could conclude that it
is the petrol engines that are really guitly because if there were only
diesel engines then there would be Nitrous Oxides but no Hydrocarbons
and consequently no smog.
Diesel engines typically produce slightly higher emissions of Nitrous
Oxides than petrol engines when new. As both engine wear with use
the Nitrous Oxides emissions of petrol engines increase whereas those
of diesel engines remain more constant. Eventually high mileage
petrol engines produce more Nitrous Oxide emissions than a similarly
high mileage diesel engine.
Modern engines ( both petrol and diesel ) use exhaust gas recirculation
to limit both combustion temperature and the amount of clean air ( and
hence Nitrogen ) that is available to be burnt. Both petrol and
diesel engines also often use deNox catalytic converters to further
reduce emissions. Despite such measures, due to the ultra lean
burn nature of diesel engines they naturally produce more Nitrous Oxide
emissions than petrol engines though.
Supply chain issues
Because diesel engines use 25% to 35% less fuel they have a 25% to 25%
lower environmental impact on the entire supply chain of fuel.
This means 25% to 35% fewer fuel tankers on the road, fewer oil tankers
at sea, less refinement, extraction and exploration.
Diesel engines also last a lot longer than petrol engines ( often twice
as long ). This means that the car to which the engine is fitted
lasts longer and so production of motor vehicles would be less if the
entire fleet was diesel. The environmental impact of car
production must therefore also be considered.
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