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A new Light Stratified-Charge Direct Injection (LSC DI) spark ignition combustion system concept was developed at Ford. One of the new features of the LSC DI concept is to use a ‘light’ stratified-charge operation window ranging from the idle operation to low speed and low load. A dual independent variable cam timing (DiVCT) mechanism is used to increase the internal dilution for emissions control and to improve engine thermal efficiency. The LSC DI concept allows a large relaxation in the requirement for the lean after-treatment system, but still enables significant fuel economy gains over the PFI base design, delivering high technology value to the customer. In addition, the reduced stratified-charge window permits a simple, shallow piston bowl design that not only benefits engine wide-open throttle performance, but also reduces design compromises due to compression ratio, DiVCT range and piston bowl shape constraints.

Extensive simulation was carried out to investigate cam strategies for SI engine part load operation. Performance of the engine with dual independent variable cam timing (VCT) system is assessed. Over a wide range of part load operating conditions, engine performance parameters, such as fuel consumption, were expressed in forms of contour maps as a function of intake and exhaust cam timings. Based upon the simulation results, cam timings were optimized for various part load conditions. A cam strategy incorporating intake and exhaust cam retard was developed to improve fuel economy and emissions. Influences of intake and exhaust cam timing on the gas-exchange and combustion processes were also analyzed. It was shown that the fuel economy improvement by dual independent VCT is achieved primarily through reduction of pumping loss. Effects of in-cylinder charge motion and the use of differential intake cam profiles on fuel economy were examined.

Development and research of a stratified-charge direct-injection SI combustion system was carried out at Ford. This paper presents the multidimensional engine CFD modeling analysis of mixture formation and piston surface wetting under the stratified-charge conditions. The effect of various design and operating parameters including piston shape, spray cone angle, swirl ratio, injection timing, engine speed and load on charge stratification and piston-wetting due to spray impingement were studied. The results led to design changes that warranted improvement of engine functions. Of particular interest, the model was shown to be capable to predict piston surface wetting and a qualitative correlation was found between the amount of the remaining liquid fuel on the piston surface and the engine-out smoke number.

A CFD based design optimization methodology was developed and adopted to the development of a stratified-charge direct-injection spark ignition (DISI) combustion system. Two key important issues for homogeneous charge operation, volumetric efficiency and mixing homogeneity, are addressed. The intake port is optimized for improved volumetric efficiency with a CFD based numerical optimization tool. It is found that insufficient fuel-air mixing is the root cause for the low rated power of most DISI engines. The fuel-air mixing in-homogeneity is due to the interaction between intake flow and injected fuel spray. An injector mask design was proposed to alleviate such interaction, then to improve air-fuel mixture homogeneity. It was then confirmed with dynamometer testing that the optimized design improved engine output and at the same time had lower soot and CO emissions.

In-cylinder pressure traces vary significantly from cycle-to-cycle in spark-ignition (SI) engines. The variations, substantially present even when engine is stable, are magnified under certain engine operating conditions. As a result, engine torque output oscillates and engine operation becomes unstable. EGR tolerance, lean burn limit and spark retard capabilities at CSSRE (Cold Start Spark Retard and Enleanment) are mostly determined by the levels of cycle-to-cycle variations. None of the engine computer models, however, have included cyclic variations for routine industrial applications. As the application domain of engine simulation models expands into unstable engine operating conditions, the modeling of cyclic variations becomes increasingly important. In this research, reviews were conducted regarding different approaches for the simulation of cyclic variation.

The interaction between in-cylinder flow and injected fuel spray in direct ignition spark ignition (DISI) engines have been investigated. The study shows that the major effect of intake flow on the fuel spray is that the induced flow tends to make the spray collapse. This deformation of spray will prevent the fuel spray from spreading out, evaporating, and mixing well with air and also increase fuel wetting on the wall. It has been demonstrated that the intake flow effect on the fuel spray can be alleviated by increasing the masking between the two intake ports. Swirl and tumble motion effects on the fuel air mixing and wall wetting in DISI engines were also studied. A counter-rotating vortex structure is identified in the case with tumble dominating in-cylinder flow structure. This vortex structure tends to hinder fuel air mixing and increase wall wetting. It is shown that some swirl motion is helpful to improve mixing quality and decrease the wall wetting.

The effects of injector targeting and fuel volatility on transient fuel dynamics were studied with a comprehensive quasi-dimensional model and compared with experimental results from Part I of this report (1). The model includes the transient, convective vaporization of four multi-component fuel films coupled with a transient thermal warm-up model for realistic valve, port and cylinder temperatures (2, 3). Two injector targetings were analyzed, first with the fuel impacting the intake valve and in addition, the fuel impacting the port floor directly in front of the intake valve. The model demonstrates the importance of both component temperature and fuel impaction area on fuel vaporization, transient air fuel ratio (AFR) response and the amount of liquid fuel entering the cylinder. Generally, a smaller injector footprint area will lead to more liquid fuel entering the cylinder even if the spray is targeted at the back of the intake valve.

A review of flame kernel growth in SI engines and the regimes of premixed turbulent combustion showed that a misfire model based on regimes of premixed turbulent combustion was warranted[1]. The present study will further validate the misfire model and show that it has captured the dominating physics and avoided extremely complex, yet inefficient, models. Results showed that regimes of turbulent combustion could, indeed, be used for a concept-simple model to predict misfire limits in SI engines. Just as importantly, the entire regimes of premixed turbulent combustion in SI engines were also mapped out with the model.