Tighter emission limits in the heavyduty segment are demanding new combustion processes for commercial vehicles. IAV is setting store by a combination of measures and by innovative, model-based development methods.
In the EU, the current NOx limit for heavy-duty engines in the on-road segment is 0.46 grams per kilowatt hour (Euro 6) whereas the USA prescribes 0.2 grams per bhp-hr (brake horsepower hour) (USA 2010). But it will not stop there: the California Air Resources Board (CARB) is planning to reduce the limit value by 90 percent to just 0.02 grams per bhp-hr by 2031. In Europe, the Portable Emissions Measurement System (PEMS) test procedure is tightening the requirements on nitrogen oxide emissions: They must be met across a broad payload range (between 10 and 100 percent) and also on cold starting.
New limit values for carbon dioxide emissions are posing ever greater challenges for the developers. Whereas there is currently no CO2 limit in the EU for heavy commercial vehicles, regulations are already in force in the USA, Japan and China. The EU will start to gather data (monitoring & declaration) on CO2 levels in 2019, with binding fleet limit values coming into effect from 2025 onwards.
Trade-offs between reducing NOx and CO2
In addition, there is a trade-off between reducing NOx and CO2: reducing nitrogen oxide emission by increasing exhaust gas recirculation pushes up fuel consumption and therefore also carbon dioxide emissions. The PEMS cycles give rise to a similar problem: if NOx emissions are to remain below the limit value on cold starting, it is necessary to increase the exhaust gas temperature, which also forces up fuel consumption and CO2 emission.
Although there is currently no “real driving” regulation in the non-road segment, “in-service monitoring” has already begun and will provide the basis for evaluating the methodology for future “non-road PEMS” legislation. The regulation will come into effect in 2026 at the earliest.
New approaches needed
This means new approaches are needed for developing commercial vehicle engines and their combustion processes. This includes hardware measures, such as innovative coatings on pistons and combustion chamber, as well as a reduction in wall heat losses through innovative cooling concepts, such as phase change cooling (evaporative cooling, see article on page 14). “Today, we are working with maximum cylinder pressures of 215 bar. In future, pressures of 240 to 260 bar will be conceivable”, explains Dr. Reza Rezaei, manager of the Advanced Engineering and Model-Based Development team at IAV. “Injection pressure could increase to as much as 3,500 bar which, in view of the tighter NOx limits, will make it possible to further reduce soot emissions.” The compression ratio – today at a level of approx. 17 – could be increased to 18, 19 or higher, depending on power density and peak cylinder pressure. An important aspect in optimization is thermal management which plays a crucial part in real-driving cycles. For this, IAV has presented “Slide Cam”, an internally developed concept for a switchable valve train that makes it possible to use Miller cycles in commercial vehicle engines. When necessary, a higher exhaust gas temperature can then increase NOx conversion in the SCR while keeping fuel consumption to a minimum (Advanced Thermal Management).
New piston geometries can also help to improve engine efficiency and reduce emissions. “In 2017, we presented a prototype of a heavy-duty piston that was produced using the 3-D printing method”, Rezaei says. “As a result of innovative sodium-based cooling, we can increase the temperature of the piston surface and, in this way, reduce wall heat losses.” The new piston also permits a more complex combustion chamber geometry which optimizes mixture formation and helps to reduce pollutant emissions. In the context of tighter NOx emission limits and, therefore, higher EGR rates, the innovative combustion process with enhanced mixture formation provides major potential.
Model-based development and layout
Combining the various measures in any meaningful way is a major challenge because the engine’s peak cylinder pressure, compression ratio, power density and emissions are all interrelated. To identify the best possible combination, IAV uses a process of model-based development and layout. “We can model combustion very efficiently for which we not only use our own combustion model but also our own approach for the NOx reaction kinetics”, Rezaei reports. “The models have been used very successfully since 2010 and are being improved all the time.” IAV’s experts use a particularly innovative approach for modeling soot, hydrocarbons and carbon monoxide. Their emissions are hard to simulate, which is why IAV’s experts use “hybrid” emission modeling: physical and mathematical models combined with artificial intelligence.
Optimizing the combustion process in the onroad segment is complex enough – in nonroad applications, such as cranes or tractors, it is made even more complicated by the many different application scenarios. “Engine and exhaust gas aftertreatment system are operated in completely different ranges of the engine map”, Rezaei says. “To optimize the overall system, this means we also need allembracing simulation environments and models of engine and exhaust gas aftertreatment.”
In customer projects, IAV not only uses its innovative simulation methods but also a large number of test benches. The company’s proprietary heavy-duty single-cylinder test engine is available for research and development as well as prototypes from 3-D printing which can be investigated on the test engine. “This enables us to develop customized solutions for any application and customer market that meet the different requirements of each and every customer”, Rezaei says, summarizing.