Isobaric piston assembly

1. A power cylinder assembly for an internal combustion engine, comprising:

a cylinder wall surrounding a cylinder bore that extends along an axis;

a piston assembly positioned in said cylinder bore and presenting a combustion surface which separates said cylinder bore into a combustion chamber on one axial side of said combustion surface and a crank case on an opposite axial side of said combustion surface and said combustion chamber having a volume;

said combustion surface being partially defined by a first upper surface of a first piece and partially defined by a second upper surface of a second piece;

a mechanical coil spring engaging said second piece and being compressed to apply a biasing force on said second piece in a direction towards said combustion chamber; and

said second piece being moveable downward relative to said first piece during operation of said power cylinder assembly in response to a pressure in said combustion chamber reaching a predetermined threshold pressure such that a pressure force acting on said second piece exceeds said biasing force of said mechanical coil spring to increase said volume of said compression chamber.

2. The power cylinder assembly as set forth in claim 1 wherein said first piece is a piston body and said second piece is a part of a plunger assembly.

3. The power cylinder assembly as set forth in claim 2 wherein said plunger assembly includes a plunger cusp which defines a portion of said combustion surface.

4. The power cylinder assembly as set forth in claim 3 wherein said piston body presents an inner wall which surrounds a through passage and wherein said plunger assembly is slidably disposed in said through passage and is sealed against said inner wall to restrict the passage of combustion gasses past said plunger assembly.

5. The power cylinder assembly as set forth in claim 4 wherein said plunger assembly further includes a sealing element which is sealed against said inner wall of said piston body.

6. The power cylinder assembly as set forth in claim 4 wherein said inner wall partially defines an oil gallery on an opposite radial side of said inner wall from said through passage.

7. The power cylinder assembly as set forth in claim 1 wherein said first upper surface includes a planar portion and a combustion bowl recessed from and integral with said planner portion.

8. A method of operating a power cylinder assembly in an internal combustion engine, comprising the steps of:

providing a power cylinder assembly which includes a cylinder bore and a piston assembly disposed in the cylinder bore, the piston assembly presenting a combustion surface which separates the cylinder bore into a combustion chamber that has a volume on one axial side of the combustion surface and a crank case on an opposite axial side of the combustion surface, the combustion surface being partially defined by a first upper surface of a first piece and partially defined by a second upper surface of a second piece, and the power cylinder assembly including a mechanical coil spring engaging the second piece and being compressed to apply a biasing force on the second piece to disposed in said second piece for providing a biasing force to said second upper surface;

during a combustion stroke, maintaining by the mechanical coil spring the second upper surface of the second piece in a fixed position relative to the first upper surface of the first piece to provide the power cylinder assembly with a compression ratio in response to pressures of combustion gasses in the combustion chamber being below a predetermined threshold pressure; and

during another combustion stroke, moving the second upper surface of the second piece downward relative to the first upper surface of the first piece to increase the volume of the combustion chamber and reduce the compression ratio of the power cylinder assembly in response to a pressure force in the combustion chamber reaching the predetermined threshold pressure to overcome the biasing force from the mechanical coil spring.

9. The method as set forth in claim 8 wherein the first piece is a piston body and the second piece is a part of a plunger assembly.

10. The method as set forth in claim 9 wherein the piston body presents an inner wall which surrounds a through passage and wherein the plunger assembly is slidably disposed in the through passage and is sealed against the inner wall to restrict the passage of combustion gasses past the plunger assembly.

11. The method as set forth in claim 10 wherein the inner wall partially defines an oil gallery on an opposite radial side of the inner wall from the through passage.

12. The method as set for in claim 10 wherein said first upper surface includes a planar portion and a combustion bowl recessed from and integral with said planner portion.