GB2368125A

GB2368125A - Techniques for accurate determination of raw exhaust gas emissions

Info

Publication number: GB2368125A
Application number: GB0023603A
Authority: GB
Inventors: Steve Dinsdale; Greg Inman; Nicholas Collings
Original assignee: Cambustion Ltd
Current assignee: Cambustion Ltd
Priority date: 2000-09-27
Filing date: 2000-09-27
Publication date: 2002-04-24
Also published as: GB0023603D0

Abstract

Techniques involving the measurement or estimate of local water concentration in an exhaust system for the purpose of improving the estimates of real-time pollutant mass emission. The techniques involve a combination of the local water concentration together with the routinely made instantaneous exhaust pollutant concentration and instantaneous exhaust gas mass measurements.

Description

Patent Specification Techniques for accurate determination of raw exhaust gas mass emissions Description Automotive emissions legislation is placing ever tighter restrictions on vehicle pollutant production. Most legislation requires the mass of various pollutants emitted from the vehicle to be evaluated. This is typically achieved by collecting a dilute proportion of the emissions from a vehicle test cycle in a bag, which is then analysed to give the mass of each pollutant. In order to study the mechanisms responsible for the emissions, it is necessary to resolve the pollutant emissions within the test cycle. Traditional techniques for evaluation of the time-resolved mass of these emissions involve significant dilution of the vehicle exhaust flow with ambient air followed by measurement of the concentration (as a volume fraction) of each pollutant in the diluted flow. There are at least two significant problems with dilute sampling techniques: 1. The ambient air contains small quantities of the pollutants to be measured.

Although the legislation allows for this, as the performance of vehicle after treatment systems improves and the concentrations of pollutants in the vehicle exhaust approach those of the dilution air, inherent errors in the conventional techniques become unacceptably large.

2. The required specification of instruments capable of resolving the diluted emission levels with sufficient accuracy is approaching the limits of viability.

Improvement to dilution based techniques have been proposed, although they are relatively expensive. These include: a. Proportional ambient sampling [1] b. Catalytic dilution air cleaning (to remove pollutants) As an alternative to dilute measurement, taking measurement directly from the vehicle exhaust (raw sampling) alleviates both the problems listed above.

Raw gas techniques are widely used in order to preserve high frequency information for the study of transient effects. In addition, undried raw gas measurement may be necessary for components which are water-soluble (such as ammonia).

To date, raw gas based determination of mass of pollutant emissions have generally suffered from poor accuracy. This patent puts forward techniques for reducing these inaccuracies.

Mass pollutant measurements In order to determine the mass flow rate of pollutants it is necessary to simultaneously

know both the flow rate of exhaust gas and the concentration of the pollutant. i. e.

fl, (.) RMM pollutant Mass of pollutant = f mexhaust * (volume fraction of pollutant) dt 1=0 RMM exhaust

(Where RMM is Relative Molecular Mass) In general, both the exhaust mass flow and the volume fraction of the pollutant can vary rapidly over time. Therefore, in order to closely match the'bag'results, it is necessary to resolve the mass flow and the volume fraction of the pollutant accurately with respect to time.

Figure I shows a schematic diagram of an engine and exhaust system.

Total exhaust mass flow rate There are several techniques for measuring exhaust mass flow rate, these include: a. Exhaust flow meters b. Helium tracing. c. Inference from dilution flow. In a dilution system, if the total diluted flow rate is known (necessary for pollutant mass calculation), measurement of the dilution flow allows the exhaust flow to be calculated by subtraction. d. Inference from measurement of inlet mass flow rate plus fuel mass flow rate (which can be established via exhaust air fuel ratio (AFR) measurement).

Compensations for transient manifold filling and engine delays may be applied to establish exhaust flow rate more accurately from measured inlet flow In general, the measurement of exhaust mass flow is fast compared to the measurement of pollutant concentration.

Volume fraction measurement Most techniques for the measurement of common pollutants produce a volume fraction measurement (%, parts per million, ppm or parts per billion, ppb) directly. Raw (undiluted) gas measurements have the best possibility for fast and accurate time resolution of local gas concentration.

Most raw gas systems extract a sample directly from the exhaust and remove any water content before measurement. This technique is accurate for dry gas analysis and in combination with a dry exhaust gas flow (which can be obtained from the inlet air and fuel flows, together with the equation for fuel properties and combustion) gives an accurate instantaneous pollutant mass flow. The maximum bandwidth of dry gas analysers is somewhat limited and therefore, the time-resolution of the pollutant mass emissions is also limited In order to achieve reliable fast measurement of exhaust pollutants, a raw, undried sample can be taken. In this case, allowance for the variable water content in the raw sample must be made.

Sources of error in raw gas mass emission determination Condensation effects (on mass flow rate) A potential source of error in the raw gas approach is that the instantaneous mass flow of gas is not generally constant throughout the engine and exhaust system. There are substantial variations in instantaneous exhaust mass flow at locations through the exhaust system immediately after the vehicle is started. The primary reason for this is the condensation of water, especially in after-treatment systems such as catalytic converters.

During this period of operation, after-treatment systems (catalysts, NOx traps etc.) are not generally effective and tailpipe pollutant levels are high. Therefore measurement errors during this period are particularly significant.

An internal combustion engine exhaust contains significant quantities of water. For example, in a spark ignition gasoline engine the exhaust gas contains around 14% water by mass at stoichiometry. When the engine is started this water rapidly condenses onto the initially cold surfaces of the exhaust system. The nature of catalytic converters means that they have large surface areas available for this process to take place and will initially remove a large fraction of the water content from the exhaust. Thus the mass flow entering the catalyst is greater than that leaving it. The condensing surfaces are rapidly heated by the condensation process to the dew point of the exhaust gas (around 55 C at stoichiometry). When all surfaces reach the dew point condensation stops, and as the surfaces continue to increase in temperature the process is reversed and the water reevaporates into the exhaust stream. At this point the mass flow leaving the catalyst will be greater than that entering (although this process generally occurs more slowly than the initial condensation and the corresponding instantaneous effect on the exhaust gas mass flow is smaller). Once all surfaces are above the saturation temperature they will become dry and the mass flow through the system will again be uniform.

The water content of the exhaust gas can be measured locally and a correction can therefore be made to the total mass flow rate at the engine exhaust port in order to allow for condensation effects. This technique can give an estimate of the local mass flow rate at arbitrary points in the system.

It is, however, possible to estimate the water content of the exhaust by measuring its temperature, to remove the need for direct measurement of water content. The exhaust gas (exiting, for example, from the catalyst) will be approximately saturated during the condensation and evaporation regimes. When a gas is saturated, a measurement of its temperature can be used to indicate the water content [2].

Cross-sensitivity effects (on concentration measurements) In addition to uncertainty in raw gas mass flow due to unknown water content, some techniques for measuring specific exhaust components have sensitivities to other substances which are present in exhaust gas, including water. These sensitivities lead to errors in the measurement of the intended emission. Such cross-sensitivities are repeatable and generally characterised by instrument suppliers. An example of this is chemi- luminescent nitrogen oxide (NO) detection which can have cross-sensitivity to the

concentration of water and carbon dioxide (CO2). Currently, workers use estimates of the concentration of these interfering exhaust components without taking account of the effect of water condensation and re-evaporation.

By means of the techniques described above (i. e. direct measurement of water concentration or calculation of this from a temperature measurement) together with the known cross-sensitivities, the emission measurement errors due to water may be more accurately corrected for. In addition, the corrected concentrations of other species (eg CO2) along with the instrument cross-sensitivities allow more accurate corrections for these interferences to be made.

References to Figure 1 1 Exhaust tailpipe 2 Post catalyst measurement location 3 Temperature measurement locations 4 Catalyst 5 Pre catalyst measurement location 6 Air to fuel ratio measurement location 7 Engine 8 Air flow meter 9 Engine air inlet

Claims

Claims 1. A technique to estimate the exhaust gas mass flow at a measurement location using the total engine mass flow and allowing for condensation and evaporation in the exhaust and after-treatment system by using an estimation of water concentration in the exhaust.
2. A technique to estimate the exhaust gas mass flow at a measurement location using the total engine mass flow and allowing for condensation and evaporation in the exhaust and after-treatment system by using a measurement of water concentration in the exhaust.
3. A technique to estimate the water concentration in the exhaust gas from measurements of the exhaust gas temperature and knowledge of the condensation and evaporation process for the purpose of improving the estimate of exhaust gas mass flow at a measurement location.
4. A technique according to claim 3 whereby the estimated water concentration at a measurement location is used to correct the measurement of exhaust gas components utilising a known or estimated sensitivity of the instruments to water concentration.