Split-cycle diesels – with their promise of truly revolutionary engine brake efficiency – may still be more than a decade away, but Euro 7 is likely to be upon us much sooner. And, while best estimates for a timeframe for this next heavy-duty truck engine emissions legislation are the early 2020s, truck OEMs must now be working on revisions of everything from powertrains to the aftertreatment packages if they hope to meet the anticipated requirements.
So what exactly do those look like? Nothing is yet cast in concrete, but Andy Noble, head of heavy-duty vehicles at world-renowned engine developer Ricardo, foresees more than just the predicted curtailment of CO2 emissions. “We expect to see NOx limits halved from Euro 6, while NO2 is regulated for the first time, because of its known impact on human health. So NOx may be limited to 200mg/kWh for the WHSC [world harmonised stationary cycle] and 230 mg/kWh for the WHTC [world harmonised transient cycle]. Then for NO2, we expect 100 mg/kWh on WHSC and 110 mg/kWh on the WHTC.”
But that’s not all. Noble suggests that, although there is unlikely to be any change in the PM/PN (particle mass/number) restrictions at Euro 7, compared to Euro 6, it is possible that the size of particles counted will come under pressure. “We’re talking about a potential reduction from the current 23 nanometres down to 10 nanometres.”
Noble agrees that Euro 7’s likely tighter limits may well present challenges to the sensors and instrumentation community. He’s not too concerned about NOx and NO2 – they can be measured by the most advanced versions of today’s dynamometer laboratory and PEMS (portable emissions measurement systems) equipment. The problem arises with detecting and counting vanishingly small particles.
“Ten nanometres is very small, but at that size, particles are also unstable,” he explains. “And it’s not just about diesel engines at those sizes either, but also petrol and potentially natural gas engines, too – due, for example, to oil escaping past the valve guides and the turbo oil seals. So, if this new particle size regime is enforced, the entire market will be affected and the EC will be pushing the limits of what is currently feasible with existing measurement technology.”
And there could be another challenge: preventing emissions of such small soot particles will likely mean finer DPF (diesel particulate filter) pores and potentially also new substrate materials. These may, in turn, demand further engine technology alterations. “Using larger DPFs might prevent back-pressure increases,” muses Noble. “But these, too, are not without their implications, particularly in terms of penalising space and weight. Hopefully, the OEMs will learn from their Euro 6 development experiences and avoid such issues.”
What about CO2? Noble makes the very valid point that the issue for global warming is not just CO2, but all greenhouse gas (GHGs) emissions. He points to European legislation that next year will require all heavy-duty engine manufacturers to plug their dynamometer emissions data into the EC’s VECTO simulation tool to reveal trucks’ estimated operational CO2 emissions. “That should also be indicative – although no more than that – of their vehicles’ likely fuel consumption, so there will be incentives to bear down on emissions.”
However, that’s just the start. For Euro 7, the expectation is that all GHGs will need to be reported – particularly methane (CH4: 25 times CO2 global warming potential) and nitrous oxide (N2O: 298 times CO2 equivalent). “Only very small amounts of methane are produced during diesel combustion – although more for natural gas – while nitrous oxide is a by-product of the SCR [selective catalytic reduction] process,” explains Noble.
So the likelihood is that under Euro 7 legislation, VECTO may be further developed to take in dynamometer emissions data for each gas in order to reveal a total GHG picture for any truck. And that will probably be required under a range of operating conditions.
In-service testing
So far, so good. But the next stage of existing emissions legislation (Euro 6d, which will be enforced from 2019) reveals a potential for further related challenges. “Under Euro 6d, manufacturers will be required to achieve emissions limits at lower average powers, down to 10% [currently 20% at Euro 6c], to recognise typical inner city running cycles. Operating at lower powers makes the engine more fuel efficient, but it also means lower exhaust temperatures – possibly as low as 100—150C in slow urban traffic.”
Unfortunately, that has an adverse impact on the efficiency not only of the SCR for NOx conversion, but also of the DOC [diesel oxidation catalyst] for hydrocarbon conversion, both of which require in excess of 200C. Additionally, regenerating the DPF requires much higher temperatures, generally around 600C-plus at the DPF. “So manufacturers will need to turn the wick up on existing thermal management techniques to raise exhaust temperatures,” suggests Noble.
He points to exhaust and/or intake throttling, additional injectors in the exhaust and possibly VVT [variable valve timing] as per Mercedes’ OM934 and 936 5.1- and 7.7-litre engines. “On common rail systems, the OEMs could also go for late injections, although fuel washing through the bores would risk cylinder wear, so that won’t be a popular choice for heavy-duty engine manufacturers.”
Whichever combination they choose, all of these options are going to cost OEMs and operators alike, in terms of increased fuel consumed (and hence also CO2 emissions) without producing torque. That’s effectively an own-goal – but something has to give in the name of improving air quality.
And complications arising from a likely Euro 7 continue. Already at Euro 6c, engine dynamometer and on-road PEMS emissions data are collected, the latter based on a representative driver, trucks carrying 10—100% payload and a prescribed mix of urban, suburban and motorway style driving. These are required for Type Approval, but also for seven years of in-service truck operating life – with similar periodic testing of older trucks to ensure ongoing conformity.
Presently, the EC allows a maximum factor of 1.5 increase against the original dynamometer emissions measurements from new. However, for Euro 7, Noble believes that conformity factor may shrink to, say, 1.25, putting further pressure on designs for long-term engine and exhaust aftertreatment efficiency. Furthermore, PM and PN are likely to fall into the PEMS testing regime – again to ensure long-term emissions limit compliance, in this case for the DPF.
Engine implications
Improving the CO2 picture ultimately requires increasing the engine brake thermal efficiency. And that, in turn, means going for higher peak cylinder pressures and compression ratios in order to achieve faster, cleaner combustion. “We’re certainly seeing these trending upwards,” confirms Noble. “For example, not so long ago compression ratios were typically 17:1 for larger engines. But we’re now working on 19:1 and even north of 20:1 on new engines, because those values yield improved thermodynamic efficiency.”
However, one obvious impact of both these engineering inevitabilities is a requirement for stronger cylinder heads, pistons and indeed whole crank trains. But these, in turn, imply greater engine weight and/or the use of cleverer, more expensive materials. They also demand a lot more R&D, especially where ensuring durability and longevity is concerned. So the conclusion of these immediate observations is that getting Euro 7-ready isn’t going to happen overnight.
Beyond such generalisations, though, there are several opportunities for increasing power and torque output while also cutting fuel consumption. Turbocharging is right up there, although – surprising maybe for some – not so much mechanical turbo-compounding.
The latter is well established, so the best the industry might hope for is incremental improvements at significant cost. Furthermore, although mechanical turbo-compounding undoubtedly delivers significant benefits in some parts of the operating map, it doesn’t across the board. As Noble puts it: “The benefits are in high torque and high load. But at lower loads, these reduce and can even become negative because of all the work done in rotating the extra transmission and gears.”
Instead, the future for turbocharging to achieve Euro 7 emission rates lies in hybridisation and electrification, mirroring strides already taken in the automotive sector with electronic control and electric machines. “Electric turbo-compounding, for example, is much more flexible than its traditional mechanical counterparts,” agrees Noble. “Suppliers are already developing the technologies and offering them to manufacturers for trial purposes. So, although there is nothing yet in wide-scale production for heavy-duty applications, the supplier base is out there and many are already at alpha and beta prototype stages.”
Another opportunity involves electrically-operated, electronically-controlled auxiliaries. Noble gives the example of variable-speed pumps for oil and coolant that deliver only the flows the engine needs. Some are already present in latest truck variants from the main OEMs; others are now close to production. Certainly, these are entering the mainstream and proving their positive impacts on reducing parasitic losses.
What about the roles of engine down-sizing and down-speeding in meeting Euro 7? Well that trend continues, and is likely to accelerate in the face of Euro 7 emission limits, as described. In the not too distant future, says Noble, medium haul 40-tonne artics may no longer require 12—13 litre engine prime movers. More modest 7—8 litre alternatives could well be adequate. However, larger displacement engines are likely still to reign supreme for long haul.
The fuel advantages there aren’t difficult to work out. But they’re also available from down-speeding, with Noble advising that, as a rough rule of thumb, every percentage point of down-speeding delivers an equivalent fuel economy improvement.
“That’s quite a lot,” he insists, citing R&D by Daimler and others under the EC-funded four-year CORE (CO2 Reduction) project, which reported last year. In that trial, a Daimler OM936 medium-duty 350bhp 7.7 litre engine was subject to 18% down-speeding (by 400rpm to 1,800 rpm). Peak torque had to be increased from 1,400 to 1,700Nm, but the net result was a 2% fuel improvement for a simulated mix of regional and long-distance driving.
“So work is now being done on down-sizing and down-speeding with both medium and larger heavy-duty engines,” he confirms. And he adds that reducing engine displacements holds additional promise for aftertreatment package effectiveness precisely because powertrains have to work harder. Add in improvements that can be achieved with the engine and auxiliary electrification and hybridisation technologies being adopted in cars, and the result, he says, is further assistance in meeting Euro 7.
“For example, stop-start is common in passenger cars and that could soon migrate right up the power ranges.” Likewise, 48V is already well on the way in the automotive sector and that too could be interesting for commercial vehicles, not least because of the technology’s ability to electrify parts of existing powertrains, so enabling mild hybrid approaches.
“Adding a 48V electric machine into a powertrain delivers an order of magnitude more power than a conventional alternator. Feeding it indirectly with KERS [kinetic energy recovery system] to capture braking energy then enables you to provide some kind of zero-emission driving, engine torque assist and/or reduced emissions drive for the ancillaries.”
It all helps. And, while on electric machines, all eyes are currently on Mercedes-Benz’s 26-tonne full-electric, zero-emission rigids, which are now undergoing pre-production trials across Europe (page 14). Noble and indeed most other pundits, don’t see such vehicles becoming mainstream any time soon – simply because of the price and payload penalties – but that development goes to show there are other ways of achieving Euro 7.
Talking of which, what about the future for that split-cycle diesel engine concept, which Noble describes as a game changer? “The best engines now deliver typically 45—46% brake thermal efficiency. By the mid-2020s, with the developments discussed and a bit of waste heat recovery, we might reasonably expect to break the 50% barrier.
“But our forecast is that split-cycle engines could achieve 55—60% brake thermal efficiency. That’s a very big improvement. It’s going to take a massive development effort, but we believe that’s essential if we want to keep on using internal combustion engines but make further improvements in terms of emissions.”
How massive? “The technical challenges are immense. Our work to date with the University of Sussex is showing temperatures and pressures way above what we currently deal with… For example, charge air at 750C and 100bar pressure into the combustion cylinder. Compare that to the typically 50C and 2-2.5 bar in today’s diesel engines.”
Formidable indeed.