In response to environmental and energy security concerns, the Obama administration has recently announced government regulations imposing stricter pollutant emissions mandates as well as higher fuel-efficiency standards for both automobiles and trucks.
One impact is the rising cost of aftertreatment systems in long-haul trucks to clean up pollution generated by the engine. For example, selective catalytic reduction systems, which remove nitric oxides from heavy-duty diesel-truck exhaust, can add as much as $20,000 to the price of a truck. This is about the same cost as the engine itself. Such systems furthermore require maintaining a second tank, typically containing urea, which introduces additional operating and maintenance costs for the end-user. A better option would be to design engines that solve the pollution problem in the cylinder.
To combat the growing costs, the auto industry is trying to address the issues at the source, through cleaner and more efficient engine design and control. Novel engine designs offer the promise of achieving revolutionary improvements in efficiency while maintaining low emissions through careful application of fuel kinetics.
Designs such as Reactivity Controlled Compression Ignition (RCCI) use a careful application of dual fuels to control the condition under which fuel is ignited and burned, offering substantial improvements in efficiency and emissions. Technologies such as RCCI require precise introduction of spray droplets for liquid fuel and air to be rapidly mixed in carefully controlled levels. Defining the optimal atomizer design, injection timing, and combustion chamber design is a complicated process where many variables have to be investigated.
The design problem gets even more difficult when you consider that the fuel itself has become a variable with different atomization and combustion characteristics. Using engine testing alone to achieve design optimization will result in unacceptably high development costs and time-to-market for these needed technologies. More accurate simulations of the spray and combustion phenomena inside the engine are key to achieving these goals quickly and efficiently. Engine modeling can help the industry to meet strict emissions regulations, increase fuel efficiency, and address fuel flexibility, without the use of costly aftertreatment systems.
In a recent report prepared for the U.S. Department of Energy (DOE), experts from DOE laboratories, the engine industry, and academia highlighted the critical link between the need to reduce greenhouse gas emissions and the use of advanced engine simulation, in the whitepaper: “Predictive Simulation of Combustion Engine Performance in an Evolving Fuel Environment” [Ref. 1]. This report points out that engine manufacturers need to “change from a test-first culture to an Analysis-Led Design Process” and that “a predictive simulation toolkit would accelerate the market transformation to high-efficiency, clean power sources for transportation.”
Another U.S. DOE source reported in 2006 that the single overarching need in developing clean and efficient engines was “the development of a validated, predictive, multi-scale, combustion modeling capability to optimize the design and operation of evolving fuels in advanced engines for transportation applications” [Ref. 2]. Combustion kinetics and accurate spray modeling are recognized as critical areas that require advances to support the design of clean, fuel-flexible engines that reduce greenhouse gas and pollutant emissions.
New approaches to CFD simulation
Tools and techniques used for the design of yesterday's engines are insufficient for the challenges of today's new engine designs. A consistent complaint by the industry is that combustion models in computational fluid dynamics (CFD) simulations are unable to predict quantitative results, or even accurate trends, for critical combustion processes such as ignition, flame propagation, and pollutant emissions. This problem is exacerbated by the fact that the fuels landscape continues to evolve and become more complex. Where yesterday’s engines were designed for a single fuel, today's engine specifications demand fuel flexibility, while achieving ultralow emissions.
The main problem with current combustion modeling approaches is the simplicity by which the fuel is represented. Using less accurate fuel representation in the design simulation requires calibration against extensive engine data in order to “tune” the simulation model. The tuned CFD model, however, often fails to work well under different engine operating conditions—preventing in-cylinder combustion CFD from being a truly predictive design tool that can be used by production engineers.
Faced with these problems, combustion simulation is often avoided completely and non-reacting simulations are used only as indicators that rely on established knowledge base and expertise to interpret. The impact of the lack of reliability of existing CFD approaches is that production design engineers can only make small perturbations to existing designs before they need to engage expert R&D personnel in model recalibration or significant testing expense. This limits developers' ability to innovate and to find optimal solutions to complex tradeoffs.
Better fuel models and the importance of spray physics
Attention is once again turning to improved modeling of the spray and kinetic phenomena to address the need for better simulation accuracy. Both spray dynamics and chemical kinetics have dramatic affects on reacting-flows in engines. For advanced concept engines that are based on compression-ignition strategies or dual-fuel combustion, fuel combustion kinetics becomes even more important.
For combustion kinetics, very accurate chemistry mechanisms are available and new software solution techniques allow their use in practical simulation. These techniques reduce simulation time by as much as two orders of magnitude when compared to conventional CFD, as illustrated below.
New chemistry treatments in CFD allow the use of the required accuracy for real fuel effects in reasonable run times.
As a result, complex fuel chemistry that was previously thought of as only practical for "0-D" simulations now become practical for full "3-D" engine simulations with moving pistons and valves. Better handling of chemistry with multi-component fuel representation makes predictive simulation possible within the schedule constraints of the concept phase of design. There is also less calibration necessary due to improved accuracy for a given grid.