System and method for controlling the flow rate of dilution air through a porous wall in an exhaust sampling system

1. An emissions test system comprising:

a dilution tunnel in which exhaust gas is mixed with dilution gas, wherein the dilution tunnel is configured to receive dilution gas flowing in an axial direction relative to the dilution tunnel and dilution gas flowing in a radial direction relative to the dilution tunnel; and

a flow control module configured to adjust the rate of the dilution gas flow in the radial direction based on at least one of a concentration of pollutant particles in the exhaust gas, a size of pollutant particles in the exhaust gas, a concentration of a gaseous emission in the exhaust gas, a type of fuel combusted by an engine producing the exhaust gas, and a rate of fuel flow to cylinders of the engine.

2. The emissions test system of claim 1 wherein the flow control module is configured to adjust the rate of the dilution gas flow in the radial direction based on the concentration of pollutant particles in the exhaust gas.

3. The emissions test system of claim 2 wherein the flow control module is configured to:

increase the rate of the dilution gas flow in the radial direction when the concentration of pollutant particles in the exhaust gas increases; and

decrease the rate of the dilution gas flow in the radial direction when the concentration of pollutant particles in the exhaust gas decreases.

4. The emissions test system of claim 1 wherein the flow control module is configured to adjust the rate of the dilution gas flow in the radial direction based on a total number of pollutant particles in the exhaust gas having a size greater than a predetermined size.

5. The emissions test system of claim 4 wherein the flow control module is configured to:

increase the rate of the dilution gas flow in the radial direction when the total number of pollutant particles in the exhaust gas increases; and

decrease the rate of the dilution gas flow in the radial direction when the total number of pollutant particles in the exhaust gas decreases.

6. The emissions test system of claim 1 wherein the flow control module is configured to adjust the rate of the dilution gas flow in the radial direction based on the type of fuel combusted by the engine.

7. The emissions test system of claim 6 wherein the flow control module is configured to:

adjust the rate of the dilution gas flow in the radial direction to a first value when a first type of fuel is combusted by the engine; and

adjust the rate of the dilution gas flow in the radial direction to a second value that is greater than the first value when a second type of fuel is combusted by the engine, wherein combustion of the second type of fuel produces more water per mass of fuel combusted relative to combustion of the first type of fuel.

8. The emissions test system of claim 1 wherein the flow control module is configured to adjust the rate of the dilution gas flow in the radial direction based on the rate of fuel flow to the cylinders.

9. The emissions test system of claim 8 wherein the flow control module is configured to:

increase the rate of the dilution gas flow in the radial direction when the rate of fuel flow to the cylinders increases; and

decrease the rate of the dilution gas flow in the radial direction when the rate of fuel flow to the cylinders decreases.

10. The emissions test system of claim 1 wherein the dilution tunnel includes an inner pipe and an outer pipe surrounding the inner pipe, the inner pipe having an inlet end configured to receive the dilution gas flowing in the axial direction, the inner pipe having pores extending through the inner pipe in the radial direction and configured to receive the dilution gas flowing in the radial direction.

11. The emissions test system of claim 10 wherein the dilution tunnel further includes an orifice plate disposed at the inlet end thereof and defining an orifice through which the dilution gas flowing in the axial direction passes.

12. The emissions test system of claim 10 further comprising a plenum configured to deliver the dilution gas flowing in the radial direction through the outer pipe.

13. The emissions test system of claim 12 wherein the plenum includes:

at least one line extending in the radial direction to the outer pipe; and

at least one valve disposed in the at least one line, wherein the flow control module is configured to adjust the position of the at least one valve to adjust the rate of dilution gas flow in the radial direction through the at least one line.

14. The emissions test system of claim 13 wherein:

the at least one line includes a plurality of lines extending in the radial direction to the outer pipe; and

the at least one valve includes a plurality of valves that are each disposed in one of the plurality of lines, wherein the flow control module is configured to adjust the positions of the plurality of valves to adjust the rates of dilution gas flow in the radial direction through the plurality of lines.

15. The emissions test system of claim 14 wherein the flow control module is configured to adjust the rate of dilution gas flow in the radial direction through each line in the plurality of lines by the same amount in response to a change in at least one of the concentration of pollutant particles in the exhaust gas, the size of pollutant particles in the exhaust gas, the concentration of the gaseous emission in the exhaust gas, the type of fuel combusted by the engine, and the rate of fuel flow to cylinders of the engine.

16. An emissions test method comprising:

delivering exhaust gas to a dilution tunnel;

delivering dilution gas to the dilution tunnel in an axial direction relative to the dilution tunnel;

delivering dilution gas to the dilution tunnel in a radial direction relative to the dilution tunnel; and

adjusting the rate of the dilution gas flow in the radial direction based on at least one of a concentration of pollutant particles in the exhaust gas, a size of pollutant particles in the exhaust gas, a concentration of a gaseous emission in the exhaust gas, a type of fuel combusted by an engine producing the exhaust gas, and a rate of fuel flow to cylinders of the engine.

17. The emissions test method of claim 16 further comprising adjusting the rate of the dilution gas flow in the radial direction based on the concentration of pollutant particles in the exhaust gas.

18. The emissions test method of claim 17 further comprising:

increasing the rate of the dilution gas flow in the radial direction when the concentration of pollutant particles in the exhaust gas increases; and

decreasing the rate of the dilution gas flow in the radial direction when the concentration of pollutant particles in the exhaust gas decreases.

19. The emissions test method of claim 16 further comprising adjusting the rate of the dilution gas flow in the radial direction based on a total number of pollutant particles in the exhaust gas having a size greater than a predetermined size.

20. The emissions test method of claim 19 further comprising:

increasing the rate of the dilution gas flow in the radial direction when the total number of pollutant particles in the exhaust gas increases; and

decreasing the rate of the dilution gas flow in the radial direction when the total number of pollutant particles in the exhaust gas decreases.

21. The emissions test method of claim 16 further comprising adjusting the rate of the dilution gas flow in the radial direction based on the type of fuel combusted by the engine.

22. The emissions test method of claim 21 further comprising:

adjusting the rate of the dilution gas flow in the radial direction to a first value when a first type of fuel is combusted by the engine; and

adjusting the rate of the dilution gas flow in the radial direction to a second value that is greater than the first value when a second type of fuel is combusted by the engine, wherein combustion of the second type of fuel produces more water per mass of fuel combusted relative to combustion of the first type of fuel.

23. The emissions test method of claim 16 further comprising adjusting the rate of the dilution gas flow in the radial direction based on the rate of fuel flow to the cylinders.

24. The emissions test method of claim 23 further comprising:

increasing the rate of the dilution gas flow in the radial direction when the rate of fuel flow to the cylinders increases; and

decreasing the rate of the dilution gas flow in the radial direction when the rate of fuel flow to the cylinders decreases.

25. The emissions test method of claim 16 further comprising:

delivering dilution gas through a plurality of lines extending in the radial direction to the dilution tunnel and spaced apart from one another in the axial direction relative to the dilution tunnel; and

adjusting the rate of dilution gas flow in the radial direction through each line in the plurality of lines by the same amount in response to a change in at least one of the concentration of pollutant particles in the exhaust gas, the size of pollutant particles in the exhaust gas, the concentration of the gaseous emission in the exhaust gas, the type of fuel combusted by the engine, and the rate of fuel flow to cylinders of the engine.