Straight si meets Euro 6 The stringency of the Euro-6
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exhaust limits that come into force at the end of the year is well documented: compared with Euro-5, the particulates limit is halved and oxides of nitrogen (NOx) cut by 80%. Truck manufacturers are fully justified in describing Euro-6 emissions as "near zero': The implication of such low limits is that the emissions target is also incredibly small, leaving no scope for manufacturing tolerances. And, as we mentioned in the first of this series running down to the Euro-6 deadline (CM 18 April), testing is no longer carried out solely on engines that are up to operating temperature. The new World Harmonised Transient Cycle test for Euro-6 engines includes an element of cold start and a lower average speed than the former European Transient Cycle, giving rise to more NOx. The need for emissions controls to work effectively at lower temperatures is a key issue in the move to Euro-6.
Finally, the requirement to prove the durability of the emission control devices, has been extended to 700,000km, up from 500,000km at Euro-5. This combination of factors makes the challenge of Euro-6 engine and after-treatment design far tougher than even the limit values suggest.
Two steps to less NOx For NOx reduction at Euro-4 and -5 we saw a choice between exhaust gas recirculation (EGR), used primarily by Scania, MAN, Isuzu and Hino; and the exhaust chemical after-treatment alternative, selective catalytic reduction (SCR), used in the main by Daf, MercedesBenz, Volvo/Renault, Iveco and Cummins Euro-6's ultra-low NOx limit means a combination of both strategies is widely regarded as the best solution, even though it entails bearing the cost, weight and complexity of two systems rather than one. EGR handles the first stage, restricting NOx generation during combustion, with SCR eliminating the rest in the exhaust.
"Using EGR as well as SCR gives us another tool in our tool box," explains Daf Trucks chief engineer Ron Borsboom, neatly summarising why nearly all truck manufacturers are tackling Euro-6 with a combination of EGR plus SCR. "We can adjust the balance of the two strategies to suit the operating conditions."
The general principle is to use EGR when the engine is starting from cold, because the SCR catalyst temperature needs to be above 200C or so before SCR can start: below 200C NOx conversion is insufficient and AdBlue crystallizes in the exhaust. However, heavy use of EGR to suppress NOx during the cold start means that combustion is inefficient, so particulate emissions are high and fuel consumption poor.
As the exhaust temperature rises the motorised EGR valve begins to close and AdBlue dosing begins, shifting the balance towards SCR. Once the SCR catalyst is at 250C to 300C it can achieve NOx conversion rates of more than 95%. This gives free rein to optimise combustion for minimum particulates and thus good fuel consumption: the engine-out NOx level is high but the SCR system can deal with that. Borsboom confirms that Daf's EGR will be doing little when Euro-6 trucks are running laden at highway speeds, only making a significant de-NOx contribution when the engine is cold or when the exhaust is cool because the engine load is light, such as in low-speed traffic.
Although most truck makers have chosen this two-stage approach, how they balance the two systems will vary. AdBlue consumption is directly related to the amount of NOx the SCR system has to remove, so with EGR handling the first stage of NOx reduction, AdBlue consumption at Euro-6 will be less than the typical 5% (of fuel consumption) at Euro-5. Most truck makers are quoting figures of 2% to 4%, depending on the EGR/ SCR split.
Of those manufacturers using EGR plus SCR, Volvo (including sister company Renault Trucks) stands out with what looks like a particularly canny solution. It uses very mild EGR, recirculating only about 10% of the exhaust gas, compared with what is probably 20% to 30% by others. Once the cold start phase is over, Volvo's Euro-6 engines will be even more highly dependent on SCR than most others. The downside is that SCR has more work to do, so AdBlue consumption is rather higher, likely to be around 6% of fuel consumption. Volvo reckons the upsides will outweigh this. First, the amount of EGR is so small that Volvo has no need to cool it. So, whereas other EGR+SCR engines use an EGR cooler and need a larger radiator to handle the additional heat rejection, Volvo's do not, saving cost and weight — an EGR cooler and its plumbing weighs around 20kg. Second, low-level EGR works fine with fairly simple turbochargers (see below), so Volvo is using a waste-gated turbo, again reducing cost and complexity.
AdBlue advocates "We have selected a technical solution that is a little different from others," says Meinrad Signer, general manager of Iveco Motorenforschung, Iveco's Swiss-based engine research and development arm. Its decision to tackle NOx using solely SCR seemed to put Iveco at odds with the rest of the industry but Scania, too, subsequently announced the option of SCR-only versions of its 9and 12.7-litre engines. Ongoing catalyst development, improving low-temperature performance, could mean other manufacturers might also offer SCR-only Euro-6 engines in future.
Avoiding EGR cuts weight, complexity and cost. There is always a trade-off between NOx and particulates in combustion, so Iveco points out that tackling all NOx via after-treatment allows combustion to be focused on minimisation of particulates, meaning complete combustion and hence good fuel consumption. Low engine-out particulates also mean less soot in the diesel particulate filter (DPF), and no EGR means engine oil stays cleaner for longer. The drawback of the SCR-only solution is that AdBlue consumption is high — about 8% of fuel consumption, according to Iveco. Nevertheless, Iveco insists its strategy gives the lowest fluid costs — ie fuel plus AdBlue. The challenge of using only SCR is achieving a NOx conversion of more than 95%, even with the cold start factored in to the overall test cycle result. Iveco says it is using patented SCR control algorithms and novel AdBlue/exhaust gas swirl patterns for optimal mixing and atomisation.
Turbocharging Many manufacturers have introduced more sophisticated turbocharging for Euro-6, either variable geometry (VGT) or two-stage. Either can be controlled to achieve the necessary pressure gradient between exhaust and intake manifolds to drive EGR. They can also help regenerate the DPF by deliberately making the engine less efficient, forcing it to work harder, increasing exhaust temperature and burning soot trapped in the DPF.
Another trend for Euro-6 turbocharging is moving from pneumatic actuation to electric actuation for variable geometry turbochargers. This gives a swifter and more precise adjustment of flow through the turbine, refining boostpressure shaping during transient engine-load conditions. This improves driving performance but the real benefit is seen in the exhaust emissions, which are higher in transient than in steady-state running. Daf and Iveco are two of the manufacturers who have moved up to electronically controlled VGT for their Euro-6 MX and Cursor engines respectively.
Mercedes-Benz is using what it describes as an "asymmetric turbocharger" on its 12.8-litre OM 471 engine in the Actros. This is a simple but clever fixed-geometry, waste-gated design whereby exhaust gases from only three cylinders are ducted into the EGR cooler; gases from the other three cylinders go straight to the turbine, with no loss of energy. This keeps the turbine speed up, improving transient response.
Finally, both Volvo and Mercedes are also using turbo-compounding to a small extent at Euro-6; Volvo on a high-torque version of its D13K engine, Mercedes on its 15.6-litre OM 473.
Fuel injection As expected, common-rail fuel-injection is almost universal at Euro-6. The most notable exception is Volvo/ Renault's 13-litre engine, which retains unit injectors. "We found they still work well at Euro-6 so there was no reason to change," says Volvo Trucks' powertrain product manager Astrid Drewsen.
The ability to provide multiple injections for each firing cycle is a big plus for common-rail. So whereas Volvo's unit injectors provide two injections (pilot and main), as many as six are possible with common-rail systems. That said, it seems most Euro-6 engines use no more than three. Similarly, although injection pressures have increased, the typical maxima appear to be 1,800bar to 2,200bar rather than the 2,500bar or so capability that is often mentioned.
A new role for fuel-injection systems at Euro-6 is regeneration of the diesel particulate filter (DPF) in the exhaust. If exhaust temperature is within a normal operating range any soot trapped in the filter will burn, but when engine loads are light and the exhaust is cool the soot fails to burn. In this case, when pressure sensors either side of the DPF detect that backpressure has reached a predetermined value a forced regeneration is triggered.
Forced regeneration on some Euro-6 trucks is via a seventh fuel injector in the exhaust manifold. The fuel burns in the diesel oxidation catalyst (DOC), temporarily raising the temperature of the gas passing into the DPF. Others are using common-rail's ability to provide a late injection of fuel in the cylinders; it will be washed through to burn in the DOC. On its 5.1and 7.7-litre Euro-6 engines, OM 934 and 936, Mercedes is doing the job by using a novel adjustable camshaft. When required, an electromagnet at one end of the shaft opens a hydraulic valve (operated by engine oil), rotating the shaft to open exhaust valves a little earlier than usual. This reduces engine efficiency, forcing it to work harder and allowing more hot gas into the exhaust.
Closed crankcase ventilation Euro-6 rules demand blow-by exhaust gases that get past piston rings and into the crankcase are counted as part of the total exhaust emissions if they are vented to atmosphere. This has persuaded truck makers, if they didn't already, to adopt closed crankcase ventilation. Blow-by gases, a mixture of exhaust gas and an oily aerosol, are collected and filtered to remove the oil, with the gas fed to the engine's air intake and the oil returned to the sump. • • Next month we look at the exhaust after-treatment box and how Euro-6 has impacted on the rest of truck design.