Valve system

1. A valve system for an engine arrangement, the valve system comprising:

a valve chamber positioned at a junction of an inlet port, an outlet port and a bypass port, the inlet port configured for fluid communication with a flow of exhaust gas from an engine, the outlet port configured for fluid communication with an inlet of a turbine, and the bypass port configured for fluid communication with an exhaust aftertreatment device;

a rotary valve comprising a valve rotor which rotates about a valve axis within the valve chamber;

wherein the valve rotor is rotatable about the valve axis between a first position in which the valve rotor permits gas flow through the bypass port and a second position in which the valve rotor blocks gas flow through the bypass port; and

wherein the bypass port is generally trapezoidal and has a major dimension in the axial direction.

2. The valve system of claim 1 , wherein an aspect ratio of the bypass port is between around 5:1 and around 20:1.

3. The valve system of claim 1 , wherein a cross-sectional area of the bypass port is smaller than a cross-sectional area of the inlet port.

4. The valve system of claim 3 , wherein a ratio of the cross-sectional area of the bypass port to the cross-sectional area of the inlet port is between around 1:1.2 and 1:20.

5. The valve system of claim 4 , wherein the ratio is between around 1:1.2 and 1:10.

6. The valve system of claim 1 , wherein the bypass port is positioned relative to the inlet port such that an angle between the inlet port and the bypass port is acute.

7. The valve system of claim 6 , wherein the angle between the inlet port and the bypass port is between around 5° and 80°.

8. The valve system of claim 7 , wherein the angle between the inlet port and the bypass port is around 25°.

9. The valve system of claim 1 , the valve rotor having axially offset endwalls and opposing internal walls;

wherein the valve chamber comprises axially recessed endwalls relative to the inlet port, the valve rotor endwalls being received by the recessed endwalls of the valve chamber to reduce leakage between the respective endwalls in use.

10. The valve system of claim 1 , wherein the valve system is configured such that the valve system exhibits the following flow regimes in order as the valve rotor rotates from a position in which the outlet port is substantially blocked:

(i) a 100% bypass regime in which the outlet port is substantially blocked by the valve rotor;

(ii) a forced bypass regime in which the bypass port is open and the outlet port is at least partially open;

(iii) a wastegate regime in which the outlet port is open and the bypass port is at least partially open;

(iv) a 100% turbocharger regime in which the bypass port is substantially blocked and the inlet and outlet ports are open;

(v) a back pressure turbocharger only regime in which the bypass port is substantially blocked and the inlet port is at least partially blocked;

(vi) a braking regime in which the inlet port is substantially blocked; and

(vii) a poor control regime in which the bypass port is open, and the inlet and outlet ports are at least partially open.

11. The valve system of claim 1 , wherein the valve system is configured such that an angle γ subtended about the valve axis by each of the inlet port and outlet port is given by:

γ=360−2δ−β/3+2 c

where δ is an angle subtended about the valve axis between the inlet and bypass ports; c is a ratio of an angle subtended about the valve axis by the bypass port relative to the angle subtended about the valve axis by each of the inlet port and outlet port, such that an angle subtended about the valve axis by the bypass port is given by c multiplied by γ; and β is an angle subtended about the valve axis between the inlet port and the outlet port.