Single-cylinder Experiments for Downsizing-Oriented SI Concepts: GDI and VVL Thermodynamic Comparison

Peng

Proceedings of the Institution of Mechanical Engineers, Part D:

2011

While variable valve actuation or variable valve lift (VVL) is used increasingly in spark ignition (SI) engines to improve the volumetric efficiency or to reduce the pumping losses, it is necessary to understand the impact of variable valve lift and timing on the in-cylinder gas motions and mixing processes. In this paper, the in-cylinder flow characteristics for reduced maximum valve lifts (MVLs) were examined in a modified four-valve optical SI test engine in which the combustion system is suitable for gasoline direct-injection (GDI) operations. Three different MVLs of 6.8 mm, 4.0 mm, and 1.7 mm were tested and particle image velocimetry was employed for the measurements. It is expected that the investigation will be helpful in understanding and improving GDI combustion when a VVL system is used. The results showed that a reduced MVL could significantly enhance the in-cylinder swirl motion in all three measured horizontal planes. By comparing the swirl ratios for different MVLs, it can be found that a reduced MLV can considerably increase the swirl ratio during the intake and compression processes. In general, the swirl centre induced by a lower MVL is more stable and can stay closer to the cylinder centre. In addition, the reduction in the MVL can also increase both the high-frequency fluctuating kinetic energy and the low-frequency fluctuating kinetic energy. These may contribute to an improvement in the air-fuel mixing but also to an increase in the cycle-to-cycle variation.

Section: Reduction In the Pumping Lossesmentioning

confidence: 94%

In-cylinder air motion characteristics with variable valve lift in a spark ignition engine. Part 1: swirl flow

Peng

Proceedings of the Institution of Mechanical Engineers, Part D:

2011

Proceedings of the Institution of Mechanical Engineers, Part D:

“…As emission regulations become increasingly stringent and the cost of gasoline rises throughout the world, new engine technology which can provide improved fuel economy in vehicles is desired. A gasoline direct-injection (GDI) engine is considered a promising technology because it has the potential to provide improved fuel economy and specific power relative to a port fuel injected (PFI) gasoline engine [1]. GDI engine ultra-lean combustion is possible owing to the formation of a stratified mixture under part-load conditions, and improved specific power of stoichiometric combustion under full-load conditions.…”

Section: Introductionmentioning

confidence: 99%

Effect of a split-injection strategy on the performance of stratified lean combustion for a gasoline direct-injection engine

Park

Kim

et al. 2011

Fuel economy can be significantly improved in a gasoline direct-injection (GDI) engine owing to the stratified mixture formation strategy which guarantees stable combustion under ultra-lean air–fuel mixture conditions. In the GDI engine, it is widely accepted that the spray-guided direct-injection system, which is characterized by a close configuration between the centrally mounted injector and the spark plug, is regarded as a promising technology among the efforts to realize stratified lean combustion. With this configuration, the split injection can be an effective fuel injection scheme because of its potential to modulate the spray characteristics as well as to create a combustible mixture in the proximity of the spark plug. In this study, the split-injection strategy was assessed as a way to achieve ultra-lean operation in a GDI engine and the resulting lean-combustion characteristics were investigated in terms of the engine performance and emissions. The engine was tested at constant operating conditions (an engine speed of 3000 r/min and an indicated mean effective pressure of 0.4 MPa) with a change in the fuel injection pressure ranging from 10 MPa to 20 MPa. The excess air ratio λ varied from stoichiometry to the lean flammability limit, which turned out to be extended in the GDI approach. The engine test results show that the split injection of fuel allowed the formation of an adequately stratified mixture in lean-combustion conditions, and thus stable combustion was guaranteed. These results indicate that an excess air ratio was one of the important factors affecting the thermal efficiency and nitrogen oxide (NO x) emissions and was significantly extended to λ = 2.0 by the split-injection scheme. In addition, several fuel injection parameters such as the injection quantity, the split ratio, and the number of splits were examined with an emphasis on their impact on the fuel economy and emissions. The results demonstrate that there exists a trade-off between the NO x reduction and the efficiency, and therefore it is required to choose an appropriate injection strategy depending on the engine operating conditions in order to satisfy emission regulations.

“…Only during the late intake stage, lower valve lift will make the in-cylinder pressure be similar to the case with a throttle [8]. As demonstrated by Grimaldi et al [9], the pumping loss of a non-throttled VVL configuration could be decreased approximately by 50% compared to that of throttled conditions. In recent years, VVL has been successfully applied on several models of production engines.…”

Section: Introductionmentioning

confidence: 88%

Effects of Variable Valve Lift on In-Cylinder Air Motion

et al. 2015

An investigation into in-cylinder swirl and tumble flow characteristics with reduced maximum valve lifts (MVL) is presented. The experimental work was conducted in the modified four-valve optical spark-ignition (SI) test engine with three different MVL. Particle image velocimetry (PIV) was employed for measuring in-cylinder air motion and measurement results were analyzed for examining flow field, swirl and tumble ratio variation and fluctuating kinetic energy distribution. Results of ensemble-averaged flow fields show that reduced MVL could produce strong swirl flow velocity, then resulted in very regular swirl motion in the late stage of the intake process. The strong swirl flow can maintain very well until the late compression stage. The reduction of MVL can also increase both high-frequency and low-frequency swirl flow fluctuating kinetic energy remarkably. Regarding tumble flow, results demonstrate that lower MVLs result in more horizontal intake flow velocity vectors which can be easily detected under the valve seat area. Although the result of lower MVLs show a higher tumble ratio when the piston is close to the bottom dead centre (BDC), higher MVLs substantially produce higher tumble ratios which can be confirmed when most cylinder area lies in the measuring range.