Gasoline engines can operate lean to improve fuel economy, potentially reducing CO2 emissions significantly. Further, advances in combustion development for gasoline have led to the potential for reduced engine-out NOx emissions for homogeneous lean operation compared to stratified lean operation. However, NOx emissions control at the tailpipe remains a major issue, as reducing NOx emissions is a significant challenge in an excess-oxygen environment. The conventional three-way catalyst (TWC) is thus not an option for lean gasoline exhaust conditions.
At the SIA Powertrain international conference and exhibition at Versailles this week, Ricardo engineers reported on a detailed simulation study to determine the applicability of different engine operating modes (stoichiometric, lean homogeneous and lean stratified) in meeting Euro 6d Real Driving Emissions (RDE) limits for gasoline engines using lean operation zones in C-segment applications.
The simulation carried out by Ricardo incorporated a range of different combustion modes and aftertreatment solutions for NOx control over WLTP and RDE test cycles. The lean aftertreatment systems considered are combined with stoichiometric aftertreatment technology (TWC) and consist of:
- Single and twin LNT systems
- Active and passive SCR systems
These elements are also further combined into more complex systems.
The vehicle, engine and aftertreatment simulations were carried out using the Ricardo Integrated Model Based Development (IMBD) process and environment, which provides a framework for the exchange of data and information between analysis and simulation operations.
The vehicle and aftertreatment models both used a map-based simulation approach, allowing a blend of physical modeling and the direct input of system characteristic responses in the form of n-dimensional look-up tables. This reduced the time required for model parameterization and tuning and allowed very fast computational times.
The study was based on a Euro-spec C-segment vehicle, and was performed over NEDC, WLTP and RDE cycles (the last developed by Ricardo).
The Ricardo team considered two lean combustion modes: lean stratified and lean homogeneous operation. Both approaches reduce engine throttling, especially at low speed and load conditions to improve engine pumping losses and therefore fuel consumption.
Lean stratified engines inject fuel close to the spark plug and create a suitable mixture to start the combustion process. The other areas in the combustion chamber remain extremely lean.
For the lean homogeneous simulation, the engineers used a moderate lean limit (λ = 1.5), reflecting current ignition system technology. In addition, the engineers investigated the impact of applying an advanced ignition system (such as corona discharge) and extending the lean limit to λ = 2.0 (ultra-lean homogeneous operation).
The results of this work highlighted the trade-off in emissions control and fuel economy for applications with different aftertreatment technologies, as well as for combined solutions. A cost analysis was used to determine the most cost-effective solution in meeting RDE emissions limits. The research highlighted the opportunities for fuel and CO2 savings while meeting the strict new RDE regulations, through the selection of appropriate combustion and aftertreatment technology and control.
The simulation work showed that every system was able to meet Euro 6d NOx emissions limits, utilizing the ability of these engines to switch to stoichiometric operation and rely on TWC operation when needed. However, the lean-to-stoichiometric (L/S) time ratio defined the fuel consumption benefit available.
Among specific findings:
Lean stratified operation has a fuel consumption benefit over moderate lean homogeneous operation, but the ultra-lean homogeneous concept has competitive fuel consumption with stratified combustion.
LNT-based aftertreatment systems offer a cost-effective approach to achieving fuel consumption benefits in the range 8-13% for lean stratified and ultra-lean homogeneous operation.
Active SCR systems delivered marginally higher fuel consumption/CO2 benefits compared to twin LNT, but also have significantly increased costs resulting in a reduced cost to benefit ratio.
Both LNT and SCR aftertreatment systems create N2O during NOx conversion; hardware and calibration optimisation are required to reduce N2O emissions, but 20 mg/km limits appear to be feasible.
Whether used as part of a hybrid architecture or in a conventional powertrain, it is an imperative for the automotive industry to further improve the fuel efficiency of gasoline engines if future CO2 targets are to be met. Lean operation offers a route to further fuel-efficiency improvements compared with current state-of-the-art products, but this requires careful attention to aftertreatment selection and combustion control.—Ian Penny, managing director – engines business, Ricardo
E. Koehler, R. Osborne, M. Keenan, T. Down (2017) “Engine and aftertreatment strategies for lean gasoline engines to meet real driving emissions legislation” SIA Powertrain, Versailles 2017