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Sonex Research

Auto Ignition Systems

The fuel air mixture in the combustion chamber needs to be ignited at the correct moment to ensure efficient combustion and power generation by the engine. By clever design of the piston and correct fuel injection Sonex Research was able to design the Sonex Combustion System (SCS). It is an auto ignition system that operationally sits between the spark (9:1 ratio) and compression ignition (18:1 ratio) systems.

The Sonex Combustion System (SCS) improves the combustion of fuel in internal combustion engines (25%) and reduces soot, NOx, CO and other particulates. This technique enables engines to use an intermediate compression (12,5:1 ratio), no spark ignition and be multifuel at the same time. Aircraft engines will be able to use JET fuel and be as light as a conventional AVgas engine.

There are two developments: the Sonex Controlled Auto Ignition (SCAI) piston based technology and the Low Soot Diesel Design (LSDD) for normal compression ignited engines.

Sonex Controlled Auto Ignition, SCAI

SCAI Piston Design

Designed for four stroke engines this method is based on unthrottled, direct fuel injection and a compression ratio of 12,5:1 in combination with a patented piston design. This piston does the trick by having a number of microchambers in the piston bowl connected to the main bowl by small vents.

Slow heat release

Together with the fuel injectors these microchambers produce chemical species that are used in the next combustion event in the cylinder. This enables the engine to use sparkless ignition with moderate compression and controlable heat release for a variety of fuels.


Timing of direct fuel injection provides control of peak cylinder pressure and location, resulting in light weight engines with multifuel capability burning mogas, avgas, diesel and JET fuel while maintained gasoline peak cylinder pressures.

The result is that SCAI 'sparkless' combustion initiated by chemistry in unthrottled, medium compression DI engines reduce emissions and improve fuel economy due to the absence of a single flame front.

Follow the next link for detailed information about the Sonex Controlled Auto Ignition process.

Low Soot Diesel Design, LSDD

In the LSDD the emphasis is in placing the vent (with its high velocity jet) for maximum interaction with the soot cloud. During the power stroke, the pressure drops in the combustion chamber more rapidly than in the MCs, and highly reactive gases are expelled from the MC at high speeds into the soot cloud.

Standard compression

LSDD is at 'normal' diesel compression ratio and consists of a piston with micro-chambers (MC) with one MC/Injector Spray and one Vent/MC is located to receive a lean charge during injection. Jets from MC create high turbulence during expansion of the charge. Expansion is faster in the combustion bowl as pressure is lower.

Soot levels

In addition, the soot level remains at a reduced level when high levels of exhaust gas recirculation (EGR) are used to reduce NOx. The LSDD has been shown to be completely different and more effective than an air cell. Sonex LSDD experience with multi-cylinder turbo-charged engines shows an overall cycle soot reduction of 50% and an accompanied 10% reduction in NOx (without EGR).

Significant reductions in NOx can be achieved with common rail control of injection timing and EGR while holding the soot level at the reduced level.

It is these kinds of developments that keeps on innovating the piston engine for years to come.

Written by EAI.

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