Lean Burn - Mitsubishi Vertical Vortex (MVV)

Mitsubishi Vertical Vortex (MVV)

In 1991, Mitsubishi developed and began producing the MVV (Mitsubishi Vertical Vortex) lean burn system first used in Mitsubishi's 1.5 L 4G15 straight-4 single-overhead-cam 1,468-cc engine. The vertical vortex engine has an idle speed of 600 rpm and a compression ratio of 9.4:1 compared with respective figures of 700 rpm and 9.2:1 for the conventional version. The lean-burn MVV engine can achieve complete combustion with an air-fuel ratio as high as 25:1, this boasts a 10-20% gain in fuel economy (on the Japanese 10-mode urban cycle) in bench tests compared with its conventional MPI powerplant of the same displacement, which means lower CO₂ emissions.

The heart of the Mitsubishi's MVV system is the linear air-fuel ratio exhaust gas oxygen sensor. Compared with standard oxygen sensors, which essentially are on-off switches set to a single air/fuel ratio, the lean oxygen sensor is more of a measurement device covering the air/fuel ratio range from about 15:1 to 26:1.

To speed up the otherwise slow combustion of lean mixtures, the MVV engine uses two intake valves and one exhaust valve per cylinder. The separate specially shaped (twin intake port design) intake ports are the same size, but only one port receives fuel from an injector. This creates two vertical vortices of identical size, strength and rotational speed within the combustion chamber during the intake stroke: one vortex of air, the other of an air/fuel mixture. The two vortices also remain independent layers throughout most of the compression stroke.

Near the end of the compression stroke, the layers collapse into uniform minute turbulences, which effectively promote lean-burn characteristics. More importantly, ignition occurs in the initial stages of breakdown of the separate layers while substantial amounts of each layer still exist. Because the spark plug is located closer to the vortex consisting of air/fuel mixture, ignition arises in an area of the pentroof-design combustion chamber where fuel density is higher. The flame then spreads through the combustion chamber via the small turbulences. This provides stable combustion even at normal ignition-energy levels, thereby realizing lean burn.

The engine computer stores optimum air fuel ratios for all engine-operating conditions - from lean (for normal operation) to richest (for heavy acceleration) and all points in between. Full-range oxygen sensors (used for the first time) provide essential information that allows the computers to properly regulate fuel delivery.