Rp. Jarrett et al., Evaluation and correction of moisture adsorption and desorption from a tapered element oscillating microbalance, POWD TECH, 119(2-3), 2001, pp. 215-228
Of the five combustion products measured in heavy-duty vehicle emission tes
ting (carbon monoxide [CO], carbon dioxide [CO2], hydrocarbons [HC], oxides
of nitrogen [NOx], and particulate matter [PM]), PM is the only emission t
hat eludes real-time measurement. To date, filter sampling of PM over the d
uration of a test is the only accepted means of PM measurement for emission
standards certification. However, this provides no means for the researche
r to ascribe PM generation to any specific portion of a transient test, or
to correlate PM production with engine load, speed, and change of speed. Nu
merous attempts have been made to determine. real-time PM production. Conti
nuous opacity is one method, but research has shown little correlation betw
een opacity and PM. Gravimetric PM data have been measured for a range of s
teady state operating conditions for a particular engine to form a matrix o
f data used to predict real-time PM production in transient operation. This
is limited to the specific engine and the correlation is marginal. The tap
ered element oscillating microbalance (TEOM) shows the most promise in dete
rmining real-time continuous PM. Yet observations of TEOM data suggest mois
ture from combustion biases TEOM data through adsorption and desorption fro
m the TEOM filter. Moisture desorption from the TEOM filter is evident from
negative TEOM mass rate data during specific segments of transient tests.
The purpose of this study is to develop a model that predicts the moisture
effect on the TEOM filter. Compensation for the moisture effect in TEOM dat
a will yield a more accurate trace of legitimate PM. For this study, used a
nd new TEOM filters were placed in an environmental chamber and mass readin
gs were taken at five different relative humidity settings. Results showed
a strong linear correlation between moisture mass on the filter and ambient
moisture content in the air. Results also showed used filters adsorbed mor
e moisture than new, suggesting filter moisture retention is a partial func
tion of accumulated PM on the TEOM filter. A second method used to evaluate
the effects of moisture on a TEOM filter was to inject steam into the exha
ust pipe just after the engine exhaust manifold. The steam was injected at
a known mass flow rate as a step-wise function while the engine was run at
a steady state. TEOM data were collected during these tests and a model was
developed to predict the moisture adsorption and desorption on a mass rate
basis. The resulting model had a strong linear correlation with the TEOM d
ata collected during steam injection. A final approach was to apply the mod
el to a series of transient engine tests to evaluate its performance. Moist
ure mass rates from engine exhaust were determined using CO2 data and a moi
sture relation C/CO2 derived from a lean-burn combustion equation. In both
the steam injection tests and the transient tests, variables in the model w
ere determined by iteration using a forward derivative Newtonian solver. Mo
del variable values determined from both methods were similar. Subtraction
of predicted TEOM moisture mass rate from original TEOM data over the cours
e of a transient test yielded significant reduction in negative TEOM mass r
ate data. Results also suggest PM mass rates during heavy engine loading to
be less than reported by original TEOM data. These results affect the attr
ibution of PM production to transient engine operation, as well as the use
of TEOM data for inventory modeling. (C) 2001 Elsevier Science B.V. All rig
hts reserved.