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DIESEL ENGINE EMISSIONS
potential risk of increasing maintenance back pressure imposed on the engine. To but also that for the increased auxiliary
costs, makes operators hesitate to reduce back pressure, the filter must be equipment such as heat exchangers,
introduce these systems. NOx-reduction regenerated by burning off the soot and cooling fans and additional piping.
by SCR (Selective Catalytic Reduction) soluble organic fraction of the trapped For DMUs, a potential loss of
converters requires urea to be used as an particulates. Particulate filters are passenger capacity is to be anticipated,
additional operational fluid. In this manufactured either with a catalytic together with functional problems. If the
application, urea consumption is coating or without a catalyst. The effect of position where after-treatment device is
approximately 5% of the fuel a catalyst is to lower the temperature at located (e.g. on the roof) is far away from
consumption. Vehicles have to be which the captured carbon particles will the engine (often underfloor), this will
equipped with a second tank for the urea, burn off. Without a catalytic coating, it is result in undesirable cooling of the
which must be protected from freezing necessary to raise the temperature of the exhaust gases before they reach the
(urea freezes at -11°C but starts to filter to 600°C to burn off the carbon. reactor, leading to very poor efficiency.
crystallise at around +5°C); urea is very Due to the relatively low exhaust
aggressive so storage tanks and fuel lines temperatures of engines in railway The legislative dilemma
have to be made from stainless steel. In operation, it seems certain that some The service life of railway vehicles,
addition, urea is not chemically stable, form of active regeneration, either whether locomotives or multiple-units, is
which necessitates additional logistics to by electrically heating the filter or by much longer than in the road sector. As a
avoid excessive storage periods. burning additional fuel in the exhaust, result, rail operators tend to replace diesel
Another potential disadvantage of will be necessary for regeneration. engines halfway through the service life of
SCR devices is their poor efficiency when Of these two options, the more efficient, the vehicle.
used in railway applications. Idling times in terms of energy consumption, is An additional problem caused by the
are long relative to working cycles. The burning fuel in the exhaust stream, but – Directive 26/2004 is the requirement that
resulting low exhaust gas temperatures in general – regeneration means extra such replacement engines have to comply
mean that an SCR device will only energy consumption. with current emission limits. At present, it
function properly for 15% of the total is still fairly straightforward to replace an
useful lifetime of the engine. Side effects, older engine by a new generation engine.
such as ammonia slip, will cause
‘‘Adding EGR to diesel
This is likely to change with the
additional problems.
reduces the specific
introduction of IIIB limits. The limits will
A little-noted aspect of using urea necessitate additional space for after-
heat ratio of the
selective catalytic reduction to control of treatment systems and bigger cooling
NOx emissions from diesel engines is its
combustion gases in the
equipment which will not fit into the
effect on CO2 emissions. Optimal selective power stroke’’ vehicle. Fuel costs contribute to almost
catalytic reduction operation requires 90% of the Life Cycle Costs of the engine;
approximately one mole of ammonia for the remaining 7% are maintenance costs.
every mole of nitrogen oxides reduced. Application in railway service It is quite evident that new-for-old engine
In terms of mass, for a reduction of As railway vehicles have stringent replacement will be paid off quickly purely
1,000 kilograms of NOx, 479 kilograms limitation on mass (an 18t axle load for through fuel savings.
of CO2 are released. secondary lines) as well as on fitment Future IIIB compliant engines are
Last, but not least, the application of space, the introduction of external after- unlikely to be more fuel efficient than
SCR devices would generate a noticeable treatment devices such as SCR and/or DPF UIC II or IIIA compliant engines. At some
increase in costs for the railways. An is of major concern for manufacturers and stage in a vehicle’s life, maintenance
ATOC (Association of Train Operating operators. Realistic assumptions show a requirements would demand the
Companies) study has revealed that in weight increase of 3 to 4 tonnes requiring replacement of the installed engine by a
Great Britain, approximately 100 urea extra space of about 2m3 for a 4-axle new type. If additional devices like DPF
filling stations would be needed. This main line locomotive with a 2000 kW and SCR have to be fitted, an increase in
would result in an annual capital cost of engine and about 0,5-1 tonne for railcars service costs would be anticipated without
between €15 to 18 million on the when adding SCR+DPF. a potential gain in fuel saving. In this case,
operators’ bills. Costs of similar The latest European rolling stock replacement will not take place even if it is
magnitude for other Railways are to manufacturers’ investigations indicate that technically feasible. If a new engine gives
be envisaged. voluminous additional equipment will emission benefits but at the same time
have to be installed in rail vehicles in only results in operational disadvantages,
Particulates order that they meet the new 2004/26/EC the owner is more likely to extend the life
Capturing particulates in a DPF device Stage IIIB emission limits. Not only does of the existing engine until the end of the
results in a progressive plugging of the the space requirement of the after- useful life of the locomotive.
filter, with a consequent increase in the treatment devices need to be considered,
74 EUROPEAN RAILWAY REVIEW
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ISSUE 3
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2008
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