Concept: The Ultra-Efficient, Two-Stroke Bonner Engine

13 November 2006

Phantom view of the Bonner engine. Click to enlarge.

In a poster session at the upcoming 25th Army Science Conference, to be held 27-30 November 2006 in Orlando, the Army Research Laboratory’s Vehicle Technology Directorate is presenting an overview of three unconventional engine concepts being developed under its sponsorship.

The three engines are: a low-power, high energy-density Nutating engine targeted at unmanned air and ground vehicles, auxiliary power units, and generator applications; a turbine concept that uses semi-closed engine cycle synergistically coupled with a vapor-absorption refrigeration system; and the Bonner Engine, a new, two-stroke combustion engine concept in which two cylinders reciprocate in a 90° X-configuration, each between two semi-fixed, movable pistons.

...The Bonner Engine is unmatched in its ability to operate at maximum efficiency over its entire RPM range, and in its ability to deliver constant power at any desired altitude. It is expected to be the most fuel-efficient intermittent combustion engine ever conceived. The Bonner Engine will greatly reduce fuel consumption for power generation applications, and will allow both air and ground vehicles to achieve significantly increased range and/or payload.

—P. L. Meitner, Army Research Laboratory

Cross-section of the Bonner Engine.

The pistons contain the fuel nozzles—and, in a gasoline application, the spark plugs. Each moving cylinder incorporates the intake and exhaust ports/valves and is open on its ends. A novel, dual-offset crankshaft which generates zero side loads on the cylinders and pistons enables the Bonner’s geometric configuration. (See diagram at right.)

Each reciprocating stroke is stopped and countered (at dead center) by compression and ignition—not by the crankshaft—and all pressure on the crank is power-producing.

Features of the Bonner Engine include:

• Pre-Compression. A pre-compression chamber surrounds each end of the moving cylinder, and provides an initial boost to the incoming charge. The pre-compression chamber can be configured to provide a constant boost pressure, regardless of altitude or air density.

• Variable Compression and Induction. Each semi-fixed piston has a small range of motion (actuated hydraulically) which is used to continuously vary the overall compression ratio with engine speed. At low speed (idle) the piston is in its lowest (innermost) position, resulting in the highest overall compression ratio. At higher speeds the piston is moved progressively outwards, giving lower overall compression ratios.

  This mechanism also allows for continuous variation of air inlet size, timing and duration with engine speed.

• Exhaust Energy Recovery. Exhaust passes through a rotary valve which, at the appropriate time, routes it to a chamber above the moving cylinder, helping to push the cylinder downward during its power stroke. During the moving cylinder’s upward motion the rotary exhaust valve vents the exhaust overboard.

Unlike conventional 2-stroke engines, the Bonner Engine utilizes separate fuel and oil supplies, and conventional piston rings provide complete isolation between the lubricating oil and the fuel/air flow, resulting in a cleaner burn.

Since the engine’s combustion chamber intake and exhaust are located at opposite ends of the moving cylinder, the incoming charge does not have to change direction between inflow and outflow, thus enabling uni-flow scavenging. This, plus the totally controllable exhaust back pressure, combined with the variable intake charge timing assures that scavenging losses will be negligible.

Because the Bonner Engine uses conventional piston configurations, all knowledge in existing data bases can be utilized to optimize the burning characteristics (for heavy or gasoline fuel), thus minimizing engine emissions. Also, the unique, two stage compression process of the Bonner Engine allows for the best possible mixing of fuel and air. This generates a very even combustion event, and offers an excellent chance of achieving Homogeneous Charge Compression Ignition (HCCI), the holy grail of piston engine research.

Phase I development of the Bonner engine has been completed, and the developers are working on a 600cc prototype engine under a current Phase II contract.

(A hat-tip to Larry!)

Resources:

• Overcoming Present-Day Powerplant Limitations via Unconventional Engine Configurations