The powertrain technology warfare that rages within the portals of the world’s major car companies shows no sign of abating. Increasingly focused on fuel economy and emissions, it is driving a new wave of research as the gasoline engine—with hybrid technology as its ally—fights the diesel, and diesel also lines up with battery and electric-motor technology to potentially make huge strides in both fuel economy and emissions, albeit at high unit cost.
At the center of much of this intense struggle is technology provider Ricardo. The company continues to optimize the gasoline and diesel engine and the hybridization of both, but it is also assessing some novel solutions. And it is considering how legislation will affect individual global markets and emerging consumer tastes, to identify a range of technologies that the company needs to develop and have ready.
Development of engines that burn alternative fuels is crucial, as is adaptive calibration for truly flex-fuel applications. But conventional gasoline and diesel engines continue to have enormous potential, believes the company.
Across much of Europe, the diesel engine now powers about 50% of new cars, from upper premium sedans to sports cars, SUVs, and small family models.
Although the fuel efficiency of the diesel compares very favorably with hybrid-gasoline powertrains for many real-world duty cycles, without carrying a significant capital cost penalty, development challenges persist. These differ between geographical regions; in the U.S., diesel must retain its fuel-economy advantage while meeting demanding Federal Tier 2 emissions regulations.
For diesel to be viable in the U.S. automotive market long term, its associated technology must be developed beyond the current Tier 2 Bin 5 fleet average requirement, toward Tier 2 Bin 2 levels. This fuel-neutral standard represents a particularly significant hurdle for the diesel in terms of NOx, imposing a limit of approximately one-tenth that of the equivalent Euro 5 within the EU. Conversely, the challenge for the diesel in Europe is one of further increasing fuel economy at low cost, and helping to meet decreasing fleet average CO2 emissions requirements.
“The complex tradeoffs for future diesel products in terms of NOx, fuel economy, and cost will be very different between North America and Europe,” explained Ricardo's Diesel Project Director, Matt Beasley. “But the range of candidate technologies is essentially the same. Our approach in addressing these apparently conflicting requirements is therefore to develop and demonstrate a range of technologies which can deliver the very strict NOx limits of U.S. Tier 2 Bin 2, while investigating the many ways in which these same technologies can be mixed to deliver the competitively priced, low-CO2 products that the European market will demand.”
First stage in the current diesel research effort being undertaken by Ricardo has been to demonstrate the lowest possible engine-out NOx emissions that can be achieved using latest-available engine technology. In September 2007, Ricardo announced that it had produced a diesel engine offering a competitive power rating of 65 kW/L (87 hp/L) but with engine-out NOx levels within U.S. Tier 2 Bin 5 limits, which are about one-sixth those of Euro 5.
The engine used advanced air-handling systems including two-stage series-sequential turbocharging and advanced exhaust-gas recirculation (EGR), together with closed-loop cylinder-pressure-based engine controls. “Having shown the feasibility of Bin 5 engine-out NOx levels, we are now focusing our efforts on the demonstration of aftertreatment options capable of delivering Bin 2 vehicle-exhaust emissions,” explained Beasley. “Our research demonstrator vehicle will use a lean-NOx trap in addition to the diesel oxidation catalyst and diesel particulate filter systems installed for the Bin 5 development.
“This is just one system configuration,” continued Beasley. “The ultimate objective of our research is to understand the complex tradeoffs involved in the integrated application of advanced diesel engine and aftertreatment technologies. In this way, we hope to be able to deliver cost-optimized solutions tailored for the specific needs of each market and vehicle segment.”
In general terms, the reduction of cost and emissions is the focus for diesel engine development, while the focus for gasoline is mainly about the need to improve fuel economy.
Ricardo’s Director of Gasoline Engines, Steve Sapsford, underlines the importance of next-generation systems that will leverage technology synergies between direct injection and either advanced valvetrain or boosting technologies. “We see fewer synergies between complex valvetrains and boosting technologies, as these both seek to minimize pumping losses.”
Ricardo already has a considerable research portfolio of experience in second-generation direct injection (DI) boosted gasoline demonstrator engines. In the U.S., it recently completed a stoichiometric demonstrator codenamed DI-Boost in collaboration with Bosch. Based on a Cadillac CTS-V vehicle, the 3.6-L V6 DI-Boost engine delivered 536 N·m (395 lb·ft) and 293 kW (393 hp). Prior to this, the company demonstrated a lean-boosted DI concept based on a midsize European passenger car with what it terms “aggressive downsizing.”
“Our experience of boosted DI indicates that we can expect in the region of a 10% fuel-economy improvement with a boosted stoichiometric direct-injection gasoline engine in comparison with its naturally aspirated counterpart,” said Sapsford. “However, if we operate the engine with charge dilution at the majority of operating conditions—using either excess air or EGR—the fuel-economy benefits can be in the region of double this figure.”
The downside of running lean is the need for NOx aftertreatment and the associated increase in system cost, he said. So Ricardo is now researching a DI concept based on high levels of boosted EGR (in place of excess air), which holds the prospect of lean-boosted DI levels of fuel economy without the requirement for lean NOx aftertreatment.
Second-generation advanced gasoline engines based on synergies between valvetrain concepts and DI combustion are equally diverse. In addition to direct-injection engines with cam phasing, cam-profile switching, and variable valve-lift systems, this family of advanced gasoline engines offers the potential to implement controlled auto-ignition (CAI) or homogenous-charge compression-ignition (HCCI) combustion to dramatically reduce NOx emissions and improve part-load fuel consumption, said Sapsford. “The control challenge in enabling the engine to switch smoothly between spark-ignited combustion and CAI is significant but achievable,” he said, “and the NOx emissions and fuel-economy benefits for a naturally aspirated engine can be attractive.”
Beyond second-generation gasoline engines are novel combustion systems such as the Ricardo 2/4SIGHT concept engine. It incorporates a direct-injection gasoline combustion system in which the design of intake and exhaust ports, combined with appropriate changes in fuel injection, ignition, and valve timing, enables operation in both two-stroke and four-stroke modes.
An advanced valvetrain and sophisticated control system manage driver demands, coordinate operation of the boost and charge air system, and control the valves and fuel-injection equipment at an individual cylinder level, so that smooth transitions between two- and four-stroke operation without torque interruption should be possible, according to the company. It is now engaged in a collaborative research program that aims to assess the potential of the concept on a testbed engine.
The demonstrator engine is based on a 2.0-L V6 that is intended to deliver levels of performance and driveability typically associated with 3.0- to 4.0-L V8 gasoline engines. Its camless valvetrain provides the flexibility to evaluate valve switching strategies. The multi-cylinder 2/4SIGHT engine, which is being tested at the University of Brighton, has run successfully in four-stroke mode and two-stroke mode with high specific torque. Simulation work has indicated the potential for a 30% fuel-consumption improvement.