Split-cycle air-hybrid engine with expander deactivation

1. A split-cycle air-hybrid engine comprising:

a crankshaft rotatable about a crankshaft axis;

a compression piston slidably received within a compression cylinder and operatively connected to the crankshaft such that the compression piston reciprocates through an intake stroke and a compression stroke during a single rotation of the crankshaft;

an expansion piston slidably received within an expansion cylinder and operatively connected to the crankshaft such that the expansion piston reciprocates through an expansion stroke and an exhaust stroke during a single rotation of the crankshaft;

an exhaust valve selectively controlling gas flow out of the expansion cylinder;

a crossover passage interconnecting the compression and expansion cylinders, the crossover passage including a crossover compression (XovrC) valve and a crossover expansion (XovrE) valve defining a pressure chamber therebetween;

an air reservoir operatively connected to the crossover passage and selectively operable to store compressed air from the compression cylinder and to deliver compressed air to the expansion cylinder; and

an air reservoir valve selectively controlling air flow into and out of the air reservoir;

the engine being operable in an Air Compressor (AC) mode, wherein, in the AC mode, the XovrE valve is kept closed during an entire rotation of the crankshaft, and the exhaust valve is kept open for at least 240 CA degrees of said entire rotation of the crankshaft; and

wherein, in the AC mode, the exhaust valve closing position and the exhaust valve opening position are symmetrical, within plus or minus 10 CA degrees, about the top dead center position of the expansion piston.

2. The split-cycle air-hybrid engine of claim 1 , wherein, in the AC mode, the exhaust valve is kept open for at least 270 CA degrees of said entire rotation of the crankshaft.

3. The split-cycle air-hybrid engine of claim 1 , wherein, in the AC mode, the exhaust valve is kept open for at least 300 CA degrees of said entire rotation of the crankshaft.

4. The split-cycle air-hybrid engine of claim 1 , wherein, in the AC mode, a residual compression ratio at an exhaust valve closing position is 20 to 1 or less.

5. The split-cycle air-hybrid engine of claim 1 , wherein, in the AC mode, a residual compression ratio at an exhaust valve closing position is 10 to 1 or less.

6. The split-cycle air-hybrid engine of claim 1 , wherein, in the AC mode, the exhaust valve closing position and exhaust valve opening position are symmetrical, within plus or minus 5 CA degrees, about the top dead center position of the expansion piston.

7. The split-cycle air-hybrid engine of claim 1 , wherein, in the AC mode, the exhaust valve closing position and exhaust valve opening position are symmetrical, within plus or minus 2 CA degrees, about the top dead center position of the expansion piston.

8. The split-cycle air-hybrid engine of claim 1 , wherein, in the AC mode, the compression piston draws in and compresses intake air which is stored in the air reservoir.

9. The split-cycle air-hybrid engine of claim 1 , wherein, in the AC mode, the air reservoir valve is opened when air pressure in the crossover passage is higher than air pressure in the air reservoir.

10. A split-cycle air-hybrid engine comprising:

a crankshaft rotatable about a crankshaft axis;

a compression piston slidably received within a compression cylinder and operatively connected to the crankshaft such that the compression piston reciprocates through an intake stroke and a compression stroke during a single rotation of the crankshaft;

an expansion piston slidably received within an expansion cylinder and operatively connected to the crankshaft such that the expansion piston reciprocates through an expansion stroke and an exhaust stroke during a single rotation of the crankshaft;

an exhaust valve selectively controlling gas flow out of the expansion cylinder and into an exhaust port;

a crossover passage interconnecting the compression and expansion cylinders, the crossover passage including a crossover compression (XovrC) valve and a crossover expansion (XovrE) valve defining a pressure chamber therebetween;

an air reservoir operatively connected to the crossover passage and selectively operable to store compressed air from the compression cylinder and to deliver compressed air to the expansion cylinder; and

an air reservoir valve selectively controlling air flow into and out of the air reservoir;

the engine being operable in an Air Compressor (AC) mode, wherein, in the AC mode, the XovrE valve is kept closed during an entire rotation of the crankshaft, and the exhaust valve is opened at a position at which pressure in the expansion cylinder is approximately equal to pressure in the exhaust port.

11. A method of operating a split-cycle air-hybrid engine including:

a crankshaft rotatable about a crankshaft axis;

a compression piston slidably received within a compression cylinder and operatively connected to the crankshaft such that the compression piston reciprocates through an intake stroke and a compression stroke during a single rotation of the crankshaft;

an expansion piston slidably received within an expansion cylinder and operatively connected to the crankshaft such that the expansion piston reciprocates through an expansion stroke and an exhaust stroke during a single rotation of the crankshaft;

an exhaust valve selectively controlling gas flow out of the expansion cylinder,

a crossover passage interconnecting the compression and expansion cylinders, the crossover passage including a crossover compression (XovrC) valve and a crossover expansion (XovrE) valve defining a pressure chamber therebetween;

an air reservoir operatively connected to the crossover passage and selectively operable to store compressed air from the compression cylinder and to deliver compressed air to the expansion cylinder; and

an air reservoir valve selectively controlling airflow into and out of the air reservoir;

the engine being operable in an Air Compressor (AC) mode;

the method including the steps of:

keeping the XovrE valve closed during an entire rotation of the crankshaft;

keeping the exhaust valve open during at least 240 CA degrees of said entire rotation of the crankshaft; and

keeping the exhaust valve closing position and the exhaust valve opening position symmetrical, within plus or minus 10 CA degrees, about the top dead center position of the expansion piston;

whereby the expansion cylinder is deactivated to reduce pumping work performed by the expansion piston on air in the expansion cylinder.

12. The method of claim 11 , keeping the exhaust valve closing position and the exhaust valve opening position symmetrical, within plus or minus 5 CA degrees, about the top dead center position of the expansion piston.

13. The method of claim 11 , further including the steps of drawing intake air into the compression cylinder, compressing the intake air, and storing the compressed air in the air reservoir.

14. The method of claim 11 , further including the step of opening the air reservoir valve when air pressure in the crossover passage is higher than air pressure in the air reservoir.