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Sonex Controlled Auto Ignition (SCAI)



The Sonex Combustion System (SCS) improves the combustion of fuels in four-stroke, direct injected (DI) engines through design modification of the pistons to achieve chemical/turbulent enhancement of combustion.  The SCS method for four-stroke engines is based on unthrottled air induction, direct fuel injection, and a modest compression ratio of 12.5:1.  The patented SCS piston design produces, retains, and expels chemical auto-ignition aids to cause controlled compression ignition after top-dead-center of the compression stroke in response to timed direct injection of the fuel.  Significantly, all of the fuel is delivered to the piston bowl and mixed with the air during the latter portion of the compression stroke.  Ignition occurs simultaneously at a high rate throughout the combustion volume after completion of injection.  This form of the SCS, known as Sonex Controlled Auto Ignition (SCAI), operates over the full range of rpm and loadsThe multi-fuel SCAI operates at reduced peak cylinder pressure to enable lightweight engine design.


The SCAI is a low emissions, high economy combustion process enabled by Sonex pistons with micro-chambers (MCs) and vents, shown schematically in the figure below, which provide an in-cylinder method for isolating and capturing a small portion of an unthrottled air-fuel charge in each combustion cycle.  The SCAI for DI features pistons containing strategically located MCs with connecting vents, having one MC per injector spray.  The fuel and air captured in the MCs produce reactive chemical species that are carried over to cause sparkless compression ignition in the next cycle at moderate compression ratios (12.5:1).  In-cylinder fuel injection timing provides engine performance control of peak cylinder combustion pressure, thereby assuring lightweight engines.  The Sonex pistons enhance in-cylinder turbulence via the MCs and vents to promote particulate reduction during the power stroke.  The development work is pursued using a two-piece piston assembly to facilitate design refinement.  As shown in the figure, the piston body and insert contain the patented Sonex design embodiments.


Cross Section of SCAI Piston for DI Engines

showing insert containing micro-chambers and vents 


The SCAI allows ignition and combustion of low cetane heavy fuels by controlled auto-ignition at moderate compression ratios, and operates at controlled peak cylinder pressures, which should allow the design of lighter weight HFEs rather than the heavy weight required by normal diesel engines.  In addition, the SCAI operating on heavy fuels has the potential to deliver fuel economy approaching that of a diesel engine, and 25% to 30% lower fuel consumption than a gasoline engine. 

From late 2002 to November 2007 the Company worked under an agreement with the Defense Advanced Research Projects Agency (DARPA) for the development of a multi-cylinder, four-stroke, HFE combustion process for potential DoD applications.  This project was sponsored by DARPA as a technology feasibility demonstration of a means for lightweight piston engines to comply with the DoD policy directive that mandates heavy fuel for all engines.  Gasoline engines are typically 25% to 30% lighter than diesel engines; thus, fully qualified, adapted gasoline engine designs that could burn hard-to-ignite diesel and kerosene-based heavy fuels would address DoD performance, logistics and safety requirements.


The DARPA program at Sonex was aimed at adapting a lightweight, six-cylinder, normally aspirated, gasoline, spark-ignited (SI), Subaru 3.0 liter, 6-cylinder automotive engine being used in an unmanned helicopter development to operate unthrottled on kerosene-based fuel at a power level up to 400 hp.  Initial tasks demonstrated the Subaru engine has combustion chamber limitations due to the valve placement/arrangement in a pent roof head.


Sonex then adapted its 3-cylinder turbocharged diesel laboratory engine to compare a diesel-like combustion chamber to the gasoline-like combustion chamber of the first engine.  Sonex achieved the following laboratory results on this engine: (1) complete control of ignition by means of SCAI chemical ignition using fuel injection timing; (2) control of peak cylinder pressures consistent with lightweight aluminum engine design; and (3) demonstrated the advantages of the diesel-like combustion chamber at a compression ratio of 12.5:1 and turbocharging for high power output and low smoke with good fuel economy.

To develop a higher power piston, Sonex proposed to adapted a Mercedes-Benz 3.2-liter, turbocharged, in-line, 6-cylinder automotive diesel engine.  For the purposes of development work this engine provided the preferred combustion chamber that incorporates the SCAI piston with a bowl design to achieve the compression ratio of 12.5:1.  In July 2006 Sonex proposed a plan, which DARPA found acceptable, for schedule and cost risk reduction by lowering the power objective to 250 hp.  Characterization of the SCS process at the 250 hp level was expected to provide an indication that the calculations previously reported for a 400 hp HFE may be ultimately achievable in hardware.

In September 2007 Sonex successfully demonstrated to DARPA the laboratory SCAI HFE operating on JP-8 heavy fuel at power levels up to 250 hp with significant reductions in fuel consumption when compared to a gasoline engine.  In addition, exhaust emissions were reduced, validating the potential of the patented piston embodiments to manage levels of emissions in-cylinder.  The fully lean-burn SCAI process was run with full control over the entire operating range to 4,500 rpm, which is essential to a wide range of applications on any fuel.  Characterization of the SCAI near this power level provided an indication that a lightweight 400 hp HFE could be ultimately achievable.

The Sonex/DARPA agreement concluded in November 2007.  DARPA has declined to entertain proposals from Sonex for continuing SCAI HFE piston development; however, the Company is pursuing funding from other military and commercial sources.  Management believes the outcome of a focused SCAI HFE effort will enhance the utility of military UAVs, including powered parafoil-wings, as well as other applications.  The four-stroke SCS SCAI process for heavy fuel operation is based on unthrottled air flow and high rates of heat release which enable low rates of fuel consumption, particularly at less than peak power, to provide a trade-off in fuel weight relative to mission duration and/or sensor-data link payloads.

The outcomes from the DARPA project on heavy fuel also relate to the use of SCAI with gasoline.  The SCAI, which is considered a lean-burn combustion process, also has significant potential for commercial application in the automotive market for gasoline direct injected (GDI) engines to cost effectively improve fuel mileage 25% to 30%.  Sonex is seeking partners and other funding arrangements to support an SCAI-GDI program to produce compelling SCAI combustion process engine data for marketing to the automobile industry.

Sonex is seeking partners to fully develop applications of the SCAI technology.  Licensing opportunities are available.

23 Hudson Street, Annapolis, MD 21401
Tel: 410-266-5556; Fax: 410-266-5653
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