Leading-edge variable valve lift technology for all

With global warming effects widely accepted, environmental regulations are tightening around the world. A growing number of governments are clamping down especially on smog-producing nitrogen oxide (NOx) emissions. In the European Union, Euro 5 regulations call for reduction of the NOx limit from its current 0.25 g/km to 0.18 by 2011. The following Euro 6 regulations will shrink limits to 0.08 by 2016. Even these stringent standards are considered timid compared to tougher laws in some US states.

Emissions standards have an especially big impact on the behavior of diesel-powered vehicles. Fuel-efficient diesels induce high combustion temperatures that cause these engines to produce a higher level of NOx. With half of all cars sold in Europe having diesel engines and US sales of diesel-powered cars expected to double by 2012, automakers are investigating a wide range of possible solutions to comply with upcoming standards. Unfortunately, many of these solutions reduce engine efficiency, lower power output and often involve costly exhaust treatments. With these constraints in mind, one possible way to reduce NOx emissions is to optimize in-cylinder gas management.

One of the most promising and innovative developments to overcome these drawbacks is the variable valve lift (VVL) system that accurately manages the composition of combustion chamber gases to burn fuel more effectively and reduce NOx emissions. VVL does the same job as the conventional mechanical cams it replaces controlling the sequential opening and closing (lift) of valves during each engine cycle depending on engine load requirements and the gas exchanges through the cylinder head.

Unlike mechanical cams that provide a preset "one size fits all" lift for all valves in the entire engine operation range, however, VVL provides a cleaner burning engine by using electro-hydraulic actuators linked the engine electronic control unit (ECU) to individually control the timing of each valve opening.

VVL systems on diesels engines are more effective at controlling combustion chamber temperature and regulating the compression ratio to ensure homogeneous, cleaner combustion. In this way, VVLs can drastically reduce NOx emissions while improving engine response, increasing power and torque (especially at lower rpms), and raising fuel efficiency. This sounds like an all-round excellent set of benefits with one pretty big exception: diesel engines. Unfortunately, VVLs are extremely expensive for manufacturers to develop.

Fairly straightforward for gasoline engines, diesel VVL systems present difficult engineering challenges due to the greater number of variables in a diesel combustion process and the number of auxiliary units connected to a diesel engine. Because of the lengthy development time to implement numerous new components and the related high cost, diesel VVL engines have largely been restricted to premium vehicles where the added expense could be more readily absorbed into the vehicle price.

This far-reaching innovation project at Renault has the potential to turn the auto industry upside down with a VVL development strategy in a fraction of the time and cost now required, making variable valve technology affordable for cleaner-running, more fuel-efficient mass market diesel-powered cars. The project is focused on creating a detailed computer simulation model of the entire diesel engine system, including simulating all the complex VVL controls as well.

Research is still on-going at Renault, but if it is finalized and fully deployed - and that will take a healthy investment in time and commitment - this all-inclusive "virtual engine" model will capture the development process and could readily be reused to develop numerous future vehicles. Engineers would then simply enter the different parameters for the particular car model and engine type rather than creating the entire VVL system and engine configuration from scratch. That’s the strategy, and Renault engineers on the project are working hard to make this concept a reality.

The Renault model has been created using the LMS Imagine.Lab AMESim simulation software platform to predict the performance of multi-domain intelligent systems. The software was selected for its ability to represent hydraulic, pneumatic, electrical, thermal and mechanical behavior as well as control logic in a single unified physics-based model. Engineers create the model by dragging, dropping and interconnecting simple icons in building-block fashion. This working sketch showing the relationship of all the various elements looks simple enough, but underlying it is a sophisticated representation of exactly how the entire physical system will operate in the real world. Using the LMS Imagine.Lab AMESim platform with its highly efficient solver, engineers were able to use true innovative simulation and optimization technology to develop an optimal VVL system much more quickly and effectively than would be possible with the limited number of engines and test sessions normally available for the project.

Investigating new engine technologies requires many iterations, for accurate results and thorough component-level interaction analysis. To save time and resources, the model construction and validation was outsourced to LMS International because of the company’s expertise in automotive systems and close partnership with Renault on various engine projects, including the M9R diesel engine used as a basis for the VVL project. Introduced in 2005, the 2.0-liter M9R powers a variety of cars including Renault’s Koleos, Mégane, Laguna, Espace and Renault Samsung Motors’ QM5 as well as Nissan’s Qashqai and X-Trail. Eco-friendly with NOx emissions meeting Euro 4 standards, the engine will be upgraded to an even cleaner version called M9Rb that further lowers NOx emissions to meet Euro 6 regulations.

"LMS is a long-standing partner in our engine programs. Their engineers proactively get all the data they need for the many interrelated system variables and parameters," said Yohann Petillon, Renault Air System Management project engineer. "LMS certainly has the considerable know-how and commitment it takes to model all the complexities of an advanced diesel engine."

In the first phase, LMS engineers created a complete model of the M9R. Application-specific LMS Imagine.Lab AMESim modules were used to create detailed representations of engine control, fuel injection and compression-ignition combustion. The team behind the LMS Imagine.Lab Air Path Management solution also worked closely with IFP, a world-class French research and training institute to create the latest innovative engine models.

The LMS Imagine.Lab Air Path Management solution was particularly helpful in modeling the valves, intake and exhaust ports as well as the turbocharger, low-pressure EGR loop with exhaust back-pressure control flap, and a particulate filter. Simulation output from the model under various operating conditions accurately predicted engine performance - including gas temperature and pressure, and air composition in the air path and combustion chamber. The simulation was validated by comparing data with M9R test results.

Next, three different VVL systems were modeled and evaluated in terms of emission levels, fuel efficiency and engine performance. The electro-hydraulic UniAir system was selected based on its capability to provide full coverage of every conceivable airflow possibility from zero to maximum valve lift for optimal open-close timing and "engine breathing", cycle-by-cycle according to engine speed and load. In modeling the behavior of the UniAir system, LMS engineers took advantage of LMS Imagine.Lab"s unique Hydraulic Component Design (HCD) approach to represent the specialized system hydraulics to effectively integrate the complex VVL model into the overall engine model.

LMS engineers then developed a simulation method for comparing intake and exhaust states to estimate the level of residual gas remaining in the combustion chamber - the critical burned gas ratio (BGR) that determines NOx levels. This data was then used to establish control rules for the EGR loop and VVL actuator settings for each valve, thus ensuring that NOx emissions were minimized without lowering engine performance. Likewise, control rules were developed for the turbocharger and other peripherals. Throughout this process, co-simulations were performed between AMESim and Simulink to check the control logic in the ECU and VVL system.

After LMS engineers completed the virtual engine model of the M9Rb engine, Renault engineers used the model to investigate various configurations for exhaust gas recirculation, calibrating engine control codes and evaluating engine performance in detail, even before a prototype engine was built. For example, they can study combustion chamber behavior (including temperature distributions, valve action and residual burned gases) in minute steps through the complete engine cycle.

"Having the comprehensive LMS Imagine.Lab AMESim model to work with so early in the cycle saved one year of development time," said Mr. Petillon. "LMS Imagine provided software and engineering solutions as well as decisive technical expertise that definitely helped us make a huge step forward in engine system simulation."

Thanks to the LMS Imagine.Lab AMESim model and LMS support, Renault is able to keep their development project of the M9Rb diesel on track to meet the tough Euro 6 emissions standards. More broadly, the underlying simulation model has the potential to be used to develop similar VVL-enabled diesels for future car models. In this way, LMS Imagine.Lab AMESim is helping Renault in its on-going research efforts to put the best-performing and least polluting diesel engine on the mass market.