A novel leanburn concept, ‘Barrel-Stratification’ is proposed. Fuel is introduced into the cylinder through one of the intake ports of a dual-intake-valve engine of which the tumbling air motion is intensified by the sophisticated intake port design. Because the velocity component in the direction parallel to the axis of tumble is small, charge stratification realized during the intake stroke is maintained until the end of the compression stroke. By the effects of charge stratification and the turbulence enhancement by tumble, stable combustion is realized even at extremely lean conditions.
The concept was verified by flow field analysis applying a multi-color laser sheet technique and the flame structure analysis employing the blue-end image intensification realized by the interference mirror and the short delay phosphor.
The combustion process in a spark ignition engine is generally characterized as a combination of the air entrainment process, in which the flame front propagates at a speed determined by the turbulence intensity, and an eddy burning process in which the burning rate is determined by the laminar burning velocity and the turbulence length scale.
For the enhancement of air entrainment, an increase of turbulence intensity is required, while for the increase of the eddy burning rate, a reduction of turbulence scale is required.
Small scale turbulence is subject to dissipation. Therefore, to conserve the intense turbulence with a small scale until the combustion process, it is necessary to increase the kinetic energy introduced into the cylinder during the intake process, thereby conserving the kinetic energy by maintaining a large scale vortices and convert it into turbulence immediately before the combustion.
As the large scale vortex employed for this purpose, swirl, that is, the horizontally rotating air motion has been adopted. Recently it has been reported that tumble, a large scale vertical rotational air motion around the axis perpendicular to the cylinder axis, may be adopted as an alternative of the conventional swirl (1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20 and 21).
Tumble is an air motion suitable for two-intake-valve engines with a pentroof-type combustion chamber. The authors have performed studies on the generation and distortion processes of tumble and its effects on combustion (22,23). During the study, it was found that when the charge is stratified during the intake process by introducing the fuel from only one of the dual intake ports of a two-intake-valve engine, charge stratification is maintained during the compression process because the velocity component in the direction parallel to the axis of tumbling vortex is small.
By using the effects of turbulence enhancement and the charge stratification realized by tumble, it was found, significant combustion enhancement could be realized in the extremely lean conditions such as the air fuel ratio of 30.
Mitsubishi Motors Corporation named this lean combustion concept as the ‘Barrel-Stratification Method’, and developed a new engine, MVV (Mitsubishi Vertical Vortex) engine employing this concept. The authors consider that the potentiality of this method is remarkably high, because it realizes stratified charge lean combustion by efficiently using the fuel-air introduction property and in-cylinder flow characteristics inherent to the two-intake-valve engines with no additional devices such as a swirl control valve or a valve deactivation mechanism.
Comprehensive studies on this system have been performed in the authors' laboratory. Results have been reported in several papers. Effects of intake port design on tumble intensity are described in Reference (25). Characteristics of turbulence generated by tumble and its effects on combustion are reported in References (22,23). Effects of tumble and charge stratification on flame structure are described in Reference (29). Structure of the flow field after the distortion of tumble is clarified in References (27,28). Strategies for the control of mixing are proposed in Reference (30). Detailed configuration and the control strategies of the developed engine will be described in References (31,32).
In this paper, the concept of the proposed combustion system will be described using the selected experimental results. For the details, please refer to these papers.