Division 9.2.2 Full flow system

Full-flow dilution; constant-volume sampling (CVS). The full flow of raw exhaust gas is diluted in a dilution tunnel. Constant flow may be maintained by maintaining the temperature and pressure at the flow meter within the limits. For non-constant flow the flow shall be measured directly to allow for proportional sampling. The system shall be designed as follows (see Figure 6.6):

1. (a)

   A tunnel with inside surfaces of stainless steel shall be used. The entire dilution tunnel shall be electrically grounded. Alternatively non-conductive materials may be used for engine categories neither subject to PM nor PN limits;

2. (b)

   The exhaust gas back-pressure shall not be artificially lowered by the dilution air inlet system. The static pressure at the location where raw exhaust gas is introduced into the tunnel shall be maintained within ± 1,2 kPa of atmospheric pressure;

3. (c)

   To support mixing the raw exhaust gas shall be introduced into the tunnel by directing it downstream along the centreline of the tunnel. A fraction of dilution air maybe introduced radially from the tunnel's inner surface to minimize exhaust gas interaction with the tunnel walls;

4. (d)

   Diluent. For PM sampling the temperature of the diluents (ambient air, synthetic air, or nitrogen as quoted in point 9.2.1) shall be maintained between 293 and 325 K (20 to 52 °C) in close proximity to the entrance into the dilution tunnel;

5. (e)

   The Reynolds number, Re, shall be at least 4 000 for the diluted exhaust gas flow, where Re is based on the inside diameter of the dilution tunnel. Re is defined in Annex VII. Verification of adequate mixing shall be performed while traversing a sampling probe across the tunnel's diameter, vertically and horizontally. If the analyzer response indicates any deviation exceeding ± 2 % of the mean measured concentration, the CVS shall be operated at a higher flow rate or a mixing plate or orifice shall be installed to improve mixing;

6. (f)

   Flow measurement preconditioning. The diluted exhaust gas may be conditioned before measuring its flow rate, as long as this conditioning takes place downstream of heated HC or PM sample probes, as follows:

   1. (i)

      Flow straighteners, pulsation dampeners, or both of these may be used;

   2. (ii)

      A filter may be used;

   3. (iii)

      A heat exchanger may be used to control the temperature upstream of any flow meter but steps shall be taken to prevent aqueous condensation;

7. (g)

   Aqueous condensation. Aqueous condensation is a function of humidity, pressure, temperature, and concentrations of other constituents such as sulphuric acid. These parameters vary as a function of engine intake-air humidity, dilution-air humidity, engine air-to-fuel ratio, and fuel composition — including the amount of hydrogen and sulphur in the fuel.

   To ensure that a flow is measured that corresponds to a measured concentration, either aqueous condensation shall be prevented between the sample probe location and the flow meter inlet in the dilution tunnel or aqueous condensation shall be allowed to occur and humidity at the flow meter inlet measured. The dilution tunnel walls or bulk stream tubing downstream of the tunnel may be heated or insulated to prevent aqueous condensation. Aqueous condensation shall be prevented throughout the dilution tunnel. Certain exhaust gas components can be diluted or eliminated by the presence of moisture;

   F1For PM sampling, the already proportional flow coming from CVS goes through secondary dilution (one or more) to achieve the requested overall dilution ratio as shown in Figure 6.7 and set out in point 9.2.3.2;

8. (h)

   The minimum overall dilution ratio shall be within the range of 5:1 to 7:1 and at least 2:1 for the primary dilution stage based on the maximum engine exhaust gas flow rate during the test cycle or test interval;

9. (i)

   The overall residence time in the system shall be between 0,5 and 5 seconds, as measured from the point of diluent introduction to the filter holder(s);

10. (j)

    The residence time in the secondary dilution system, if present, shall be at least 0,5 seconds, as measured from the point of secondary diluent introduction to the filter holder(s).

To determine the mass of the particulates, a particulate sampling system, a particulate sampling filter, a gravimetric balance, and a temperature and humidity controlled weighing chamber, are required.