Whereas for cars and light commercial vehicles the emissions test is conducted on a complete vehicle with emissions results and limits in mg/km, the variety of chassis types and body styles in which a heavy-duty engine could be installed means that a more practical approach is to test the engine system (including the exhaust system) and so the emissions results are expressed in terms of power usage - mg/kWh.
As with cars, the emissions requirements for heavy-duty engines have developed over several stages, with Euro VI being introduced from January 2013. Engines are now tested over both steady-state and transient cycles and have to meet the emissions requirements on both. In addition, the Euro VI Regulation introduced in-service conformity testing performed on the vehicle with a Portable Emissions Measurement System (PEMS).
Until mid-2000, it was normally possible to meet the test cycle emissions requirements for heavy-duty engines only by developing the engine technology, without the need for exhaust gas aftertreatment, balancing the emissions of engine-out NOx and PM – as one increases, the other decreases. However, as emissions levels reduce, this ‘trade-off’ between NOx and PM on engine calibration becomes even more influential and Euro IV has thus forced the use of exhaust aftertreatment systems since 2006.
The PM-NOx trade-off
The majority of manufacturers in Europe have chosen to use this NOx-PM trade-off to minimise engine-out particulate emissions and are then using Selective Catalytic Reduction to control emissions of NOx as this method also allows improved fuel consumption (and hence reduced CO2 emissions) compared to the previous generation of engines. This technology requires a supply of urea reductant (AdBlue®) on board the vehicle and which is used at a few percent of the consumption of diesel fuel. The cost of AdBlue is substantially less than diesel fuel, and so does not compromise the fuel cost saving to the vehicle operator. The alternative to this approach is to use Exhaust Gas Recirculation (EGR) to minimise the NOx.
Diesel Particulate Filters (DPF) are now becoming more widely used on new European trucks and buses as the prime means of meeting the Euro VI emissions regulations which includes not only particulate mass but also particle number limits.
Also, many thousands of DPFs have already been retrofitted to existing vehicles – especially buses and refuse trucks, to meet local authorities’ requirements or incentive schemes to minimise urban particulate emissions. They are also available as additional original equipment from some vehicle manufacturers to meet these demands.