ECFM-3Z: A universal combustion model for engine simulation

Stephen Ferguson, CD-adapco, London

With the likelihood that traditional Spark Ignition (SI) and Diesel Compression Ignition (CI) Internal Combustion Engines (ICE) will fail to meet future limitations on engine emissions, engine manufacturers are increasingly focusing their attention to new combustion systems such as HCCI (Homogenous Charge Compression Ignition) and CAI (Compression Auto Ignition). At present, these engine concepts look unlikely to be able to deliver the high levels of specific power typically generated by SI and DI engines. Future production engines will probably rely on a combination of combustion systems; SI and CAI for gasoline, CI and HCCI for Diesel fuel. CAI and HCCI will probably be used to reduce pollution levels at low to medium operating points, while SI and CI will be used in high load scenarios.

The advent of these new concepts has provided a significant challenge for Computational Fluid Dynamics, a numerical flow simulation technique that plays an increasingly critical role in modern engine design. CFD is used in the development of every commercial engine, and is especially useful in promoting an early assessment of the relative value of new combustion concepts. Used effectively, CFD can significantly reduce the development cost of a new engine concept.

At the heart of every CFD engine simulation is a combustion model. Unfortunately, like engines, combustion models have classically been divided into two distinct types; premixed combustion models for SI engines and nonpremixed combustion models for CI engines. This distinction has always been questionable as both SI and CI engines can exhibit both premixed and non-premixed combustion under certain operating conditions. However, with the introduction of mixed concept engines and the popularity of Gasoline Direct Injection (GDI), the strict distinction between different mixture regimes is no longer justifiable.

In order to address this challenge, CD-adapco has implemented ECFM-3Z (3 Zone Extended Coherent Flame Model) in the industry leading CFD code STAR-CD. Jointly developed by Renault and IFP (Institut Francais du Petrole), ECFM-3Z was developed to address the specific combustion challenges posed by Gasoline Direct Injection (GDI) engines. 

Unlike traditional Port Fuel Injection engines GDI engines are characterized by a high degree of charge stratification. ECFM-3Z accounts for this by splitting each computational cell into 3 zones: an unmixed air zone, a mixed fuel and air zone and an unmixed fuel zone. By overcoming the traditional single mixture regime restriction of other combustion models, ECFM-3Z provides the first general purpose approach to combustion simulation. When combined with auto-ignition and spark ignition models ECFM-3Z is valid across a wide suite of engine concepts, including GDI, HCCI and CAI.

The new model is not only able to represent accurately the combustion behaviour in new engine concepts, but also provide new modeling capabilities for the investigation of traditional SI and CI engines. Using the auto-ignition model, for example, secondary combustion phenomena such as "knock" can be more thoroughly investigated in both SI and GDI engines.

ECFM-3Z is part of a wider trend towards universal modeling, Steve MacDonald, CD-adapco founder and President explains, "CFD is moving away from simple models that address only specific problems. As a CAE solution provider, we are committed to providing more fundamental modeling approaches that allow our partners to simulate the whole system, rather than just small parts of it."

Tom Marinaccio, CD-adapco's head of consultancy is keen to recommend the ECFM-3Z model for engine simulation, "For the first time we have a universal combustion model that we can use with confidence across the complete range of ICE simulation problems."