The three-cylinder design for MAHLE Powertrain’s downsized engine (shown here in cross section on left) generates the power of a six-cylinder engine, necessitating the need for structural and thermal optimization (right) to ensure that the engine could sustain the stresses of running at higher loads.
Stricter standards for fuel consumption and emissions are leading everyone in the automotive industry to go beyond what has been done in the past. At MAHLE Powertrain Ltd. (MPT), for instance, R&D efforts now include both extreme-downsized internal-combustion engines and range-extenders for electric vehicles. Yet, pushing the creative envelope in such new areas can bring its share of design challenges.
MPT starts every product development program with a cycle simulation to determine exactly what engine configuration and technology its customers are looking for. Moving to CAD, engineers turn to concept-level models for information on package volume, costs, and weight. Once a concept is chosen, the models are managed using product lifecycle management (PLM). For fluid studies, several 1-D tools are utilized to help avoid and reduce pressure losses in the oil and cooling systems. To guide design of the combustion chamber and related systems, CFD is employed for insight into very complex 3-D behavior.
Structural analysis of conceptual ideas is incorporated early on in the development process, using Abaqus FEA as the main workhorse for thermal and stress queries. These studies help investigate ways to reduce weight and friction of components, such as the crank train, connecting rods, bearing panel, and bearings. For preprocessing, fatigue analysis, and crank train dynamics, other tools can be coupled with Abaqus without worrying about integration, as they can all use or generate native Abaqus data.
In MPT’s downsized engine program (which started about four years ago), if fuel efficiency had been the only engineering challenge, finding solutions would have been much simpler. But car buyers everywhere refuse to give up performance, so MPT’s team was forced to find ways to deliver both horsepower and fuel efficiency. For boosting power in a small engine, direct fuel injection and turbocharging were critical add-ons. To cut fuel consumption, both weight and friction were methodically reduced wherever possible.
Since nearly every manufacturer is investing in downsized options—shrinking their engines typically about 20 to 25%—MPT decided to go to the extreme to show what is achievable. Working with Bosch-MAHLE Turbo Systems as a partner, and relying on extensive trade-off studies and design iterations, a three-cylinder (I3), heavily boosted, 50% downsized engine was developed with the same horsepower as a six-cylinder one. With power gains like this, structural FEA was key for ensuring durability of components, such as the crank train and bottom end of the engine. And thermal optimization was vitally important as well for an engine running at such high specific loads.
Even now with working prototypes of the I3 in demo vehicles on the road, the downsized concept continues to be refined, investigating a long list of additional friction-reducing technologies: a lower-friction valvetrain; improved pistons, ring packs, and bearings; a variable displacement oil pump; cooled exhaust manifolds; and enhanced boosting and intercooling. Variable valve timing, variable valve lift, and exhaust gas recirculation are also being looked at. In every case, simulation is relied upon to measure and evaluate the benefits of these technologies.
A more recent design effort has involved the development of an engine for electric vehicles that addresses the common issue of insufficient range. Range extenders (REx)—in which a small gas engine is used to recharge the battery—provide a good alternative to the traditional electric hybrid model. In these designs, extender size and thermal issues (since the engine is typically positioned directly under the passengers’ seats) are the crucial areas that our engineers are focusing on.
For the REx engine, the primary challenge has been one of balancing size and weight with durability and cost. Structural analysis has played a major role in this optimization, with simulation helping to choose cost-sensitive, lightweight-yet-durable materials for components such as the crankshaft and block. The end result is a compact, carry-on-luggage-size internal-combustion unit that can be integrated into a more typical electric vehicle.
MPT’s REx has met target performance on the testbed with a theoretical range of 400 mi (650 km) on 8.8 U.K. gal (40 L) and promises to provide an alternative to most market-ready hybrids. The I3—currently installed in two demo Volkswagen Passats for test driving—meets EU6 legislative requirements with 49 U.K.-mpg (30% savings), CO2 emission of just 135g/km, and a responsive 160 hp.
The REx took just 12 months from clean-sheet-of-paper to the building of the first prototype. For the I3, it was an even more aggressive nine months. Five years ago, prior to simulation, those times would easily have been almost double.
CAE and simulation are helping push the limits of engine technology and move toward a more energy-efficient automotive fleet—with aggressive downsizing, improved fuel efficiency, and lower CO2 emissions. In the future, standards will only get tougher, and simulation will be even more essential as engine developers work hard to stretch technology boundaries in creative and exciting ways.
Mark Stephenson, responsible for the analysis team at MAHLE Powertrain Ltd. (Northampton, Great Britain), wrote this article for AEI