An evolution of emissions standards for engines used in new non-road vehicles and equipment is placing a burden on engineers who are having to design technologies to operate within tighter environmental parameters.
The European Union Stage V regulations, which are expected to come into force by the end of the decade, will place greater emphasis on particulate matter and NOx reductions. Significantly, the regulations will cover a much wider variety of machinery than ever before, ranging from small gardening and handheld equipment such as lawnmowers and chainsaws through to construction, agricultural and farming machinery, including dumper trucks and scissor-lifts, and even railcars, locomotives and inland waterway vessels.
Stage V implementation comes against a backdrop of increasing evidence about the damaging effects of particulate matter (PM) and NOx in diesel exhaust streams. Of pressing concern are the ultra-fine particles within PM that have been shown to cause damage to the heart and lung when inhaled. Stage V, therefore, introduces a particulate number requirement, as well as further reductions in PM mass for many engine categories. For the first time, this effectively mandates the use of diesel particulate filters (DPFs) or a selective catalytic reduction (SCR) coated DPF. Stage V will also focus on further NOx reductions, and that is expected to bring challenges in its own right.
The Stage V requirements will apply to compression ignition engines below 19kW and all engines above 560kW, all of which were previously outside legislation. This spectrum presents different challenges at either end of the scale. Problems specific to small engines can include low-temperature drive cycles.
“Smaller engine companies have been investing money to improve their engines,” says Dr John Cocker, head of technology at Eminox. “Quite a few have gone down the path of exhaust gas recirculation and SCR in preparation for more arduous legislation. But there are key challenges. One is cost. Companies cannot suddenly have products that are, say, 30% more expensive. Space is at a real premium on some of these smaller applications, too.”
Despite the broadening range covered by Stage V, experts argue that compliance need not necessarily require a revolution in exhaust aftertreatment technologies. Many of the answers have already been provided to allow engine makers to meet current Euro VI legislation. The need now is to adapt those solutions to meet new parameters, such as packaging constraints.
“Stage V should be viewed as an evolution of existing exhaust gas aftertreatment technology,” says Cocker. “The key challenge will be to optimise the aftertreatment architecture for the different applications, whilst keeping one eye on the subsystem cost. Furthermore, to make things more interesting, the OEMs do not want a huge fuel penalty as a result of a drastic increase in back pressure.”
Euro VI and proposed Stage V standards are very similar in terms of emissions limits, including the particulate number requirement. That means firms such as Eminox and Johnson Matthey are confident that they can draw upon experience of developing Euro VI systems, scaling them up and down for smaller and larger diesel engines.
Diesel engine with SCR
Thermal management
Smaller engines present a number of set problems, with wide temperature variations in the exhaust pathway requiring a planned approach to thermal management. Various options can be explored, such as close coupling a compact SCR coated DPF with the engine, or installing on the chassis a high-efficiency SCR catalyst, with low light-off temperature, and DPF complemented with advanced insulation on the entire system to retain the exhaust temperature.
Eminox, for instance, says it has a number of thermal management solutions designed either to retain exhaust gas temperature for successful operation of the exhaust aftertreatment product or to prevent the detrimental effect of radiated heat on adjacent equipment.
On larger engines, the use of high flow injectors is necessary to spray AdBlue into the SCR system to trim NOx. But, whereas a diesel engine-powered passenger car may use a few litres of AdBlue in a year, large diesel engines can use many tonnes of AdBlue. Specific attention needs to be given to tuning the spray pattern, ensuring fine atomisation for better mixing, minimising wall wetting and eliminating crystallisation of urea on the injector nozzle.
Typically larger engines are either medium speed (300 to 1,000rpm) or slow speed (less than 300rpm); slow speed units are usually two-strokes. Drive cycles for these engines are very different to those on on-road vehicles.
With both large and small engines, a key requirement is the need for compact lightweight systems that can work within the available space. “Past experience has driven us to develop novel approaches such as close proximity mixing,” says Cocker.
“We can also combine SCR and DPF technology into a single SCR on filter substrate to optimise space utilisation. Again, we draw on our experience from Euro VI exhaust aftertreatment in terms of close-coupling catalyst systems. Our investment in 3D scanning and 3D CAE modelling tools to design compact aftertreatment systems that use high-performance catalyst on small-volume substrates helps in this area. Furthermore, our computational fluid dynamics allows us to fine-tune close proximity mixing solutions.”
Balanced against these technical demands is the need to provide value for money. This is brought into perspective when considering the cost of developing a Stage V-compliant solution for a 19kW engine from scratch. “Creating it from a blank sheet of paper and apportioning development and tooling costs to the finished product would potentially price it out of the market,” says Cocker.
“An ‘evolved’ approach ensures a fast time to market, which reduces the cost and maximises the commercial opportunity. Using lessons from earlier projects culminates in a value package.” So compliance doesn’t necessarily require a revolution in design – but an evolution of existing expertise.
