9
that the observed fraction of NO2 in NOx in the exhaust of
Engine 3 expressed as a percentage did not exceed thirteen
(Figure 6c).
DISCUSSION
Minimizing the contribution of diesel-powered mobile
equipment to concentrations of aerosols and gases plays an
important role in the efforts to reduce exposures of under-
ground miners to diesel aerosols and gases (38). The advance-
ments in the nonroad diesel engine technologies made over
the past couple decades led to substantial improvements
in combined tailpipe and blow-by emissions. As a result,
the operators got the opportunity to repower the existing
equipment and power new mobile equipment with “clean”
nonroad diesel engines that comply with the EPA Tier 4
final and superior standards [29, 47] and address some of
the health issues associated with exposure to diesel aerosols
and gases.
It is important to recognize that the technical solutions
used to lower the tailpipe emissions and meet regulatory
requirements are driven by various technical and economic
parameters (56,68) and those are often very specific to the
power class of the engine (29,47,56). The engines evaluated
in this study belong to different power classes (19 ≤ kW
37, 37 ≤ kW 56, 56 ≤ kW 130) and represent different
emission control solutions. The study was conducted with
objectives of closer examination of the tailpipe emissions
for “clean” engines and gaining additional insight into the
effectiveness and viability of those engines as a control strat-
egy for curtailment of exposure of underground miners to
diesel aerosols and criteria gases.
Based on the results of this study, the substitution of
the “traditional” engines with “clean” engines, similar to
the ones evaluated in this study (Engine 1, Engine 2, and
Engine 3), with low EC and OC mass emissions (Figure 1),
could be used by the mining industry as a strategy to
control EC and TC mass concentrations in underground
workings and potentially maintain personal exposure below
current mass-based standards (32,33,34). However, if used,
the “clean” engines that are not fitted with DPF systems,
such as Engine 1 and Engine 3, would still measurably
contribute to number concentrations of aerosols in those
workings (Figure 3 and Figure 4). Only “clean” engines
with the integrated DPF systems, similar to the one used
in Engine 2, would contribute little to both mass and num-
ber concentrations of aerosols (Figure 1 and Figure 3). The
use of advanced in-cylinder combustion controls strategies,
particularly higher injection pressures (51,52,53), resulted
in changes in size distributions of aerosols. In the case of
“clean” engines” that are not fitted with DPF systems, the
accumulation mode aerosols were characterized by measur-
ably smaller median diameters (Figure 4, Table 4) and gen-
erally contributed less to the mass concentrations of EC
than accumulation mode aerosols emitted by “traditional”
engines (Figure 1). That was the case even for Engine 1
operated at R50 test conditions where number concentra-
tions of aerosols in the accumulation mode were higher
than corresponding number concentrations of aerosols in
the accumulation mode for “traditional” engine (Table 4),
but mass concentrations of EC were substantially lower
(Figure 1). For the test and measurement conditions gener-
ated in this study, the concentrations of nucleation mode
aerosols and sub‑23 nm aerosols were found to be relatively
low when compared with concentrations of accumulation
mode aerosols (Figure 4 and Table 4). The concentrations
of nucleation mode aerosols could be potentially higher
if hot exhaust of the test engines is released in the under-
ground environment characterized by meteorological con-
ditions favoring nucleation (84).
The results of this study showed that emissions of
highly toxic NO2 could be quite an important factor
influencing selection and use of “clean” diesel engines,
particularly those fitted with catalyzed exhaust aftertreat-
ment devices, in the underground mining industry. In the
underground mines that operate engines that emit NO2
at the levels characteristic to “traditional” engines (NO2 =
0.01 – 0.20 NOx) (39,85), the NO2 exposures have been
historically maintained below action levels (e.g., one half
of the 5 ppm MSHA ceiling limit) (35,37,86) by means of
“fresh” air dilution using air quantities determined to keep
exposures to the other most critical criteria gas, namely
CO, NO, or CO2, below respective personal exposure lim-
its (40,87). However, the introduction of certain types of
catalyzed exhaust aftertreatment devices characterized by
elevated secondary NO2 emissions changed that paradigm
(39,79,88,89). In the exhaust of Engine 1 and Engine 2
evaluated in this study, the concentrations of NO2 exceeded
200 ppm (NO2 0.65 NOx) and 125 ppm (NO2 0.45
NOx), respectively. In this case, Engine 1 would be operated
in the U.S. underground metal/nonmetal mines, and the
quantities of air needed to dilute NO2 to the corresponding
permissible exposure level (PEL) should be 3.5 times higher
than those needed to dilute the next criteria gas, CO2,
to the corresponding PEL. In the similar case involving
Engine 2, the quantities of air needed to dilute NO2 should
be 1.6 times higher than those needed to dilute the next
criteria gas, NO, to the corresponding PEL. Depending on
circumstances, providing additional quantities of “fresh” air
needed to address elevated NO2 concentrations might be
technologically challenging and potentially cost-prohibitive
that the observed fraction of NO2 in NOx in the exhaust of
Engine 3 expressed as a percentage did not exceed thirteen
(Figure 6c).
DISCUSSION
Minimizing the contribution of diesel-powered mobile
equipment to concentrations of aerosols and gases plays an
important role in the efforts to reduce exposures of under-
ground miners to diesel aerosols and gases (38). The advance-
ments in the nonroad diesel engine technologies made over
the past couple decades led to substantial improvements
in combined tailpipe and blow-by emissions. As a result,
the operators got the opportunity to repower the existing
equipment and power new mobile equipment with “clean”
nonroad diesel engines that comply with the EPA Tier 4
final and superior standards [29, 47] and address some of
the health issues associated with exposure to diesel aerosols
and gases.
It is important to recognize that the technical solutions
used to lower the tailpipe emissions and meet regulatory
requirements are driven by various technical and economic
parameters (56,68) and those are often very specific to the
power class of the engine (29,47,56). The engines evaluated
in this study belong to different power classes (19 ≤ kW
37, 37 ≤ kW 56, 56 ≤ kW 130) and represent different
emission control solutions. The study was conducted with
objectives of closer examination of the tailpipe emissions
for “clean” engines and gaining additional insight into the
effectiveness and viability of those engines as a control strat-
egy for curtailment of exposure of underground miners to
diesel aerosols and criteria gases.
Based on the results of this study, the substitution of
the “traditional” engines with “clean” engines, similar to
the ones evaluated in this study (Engine 1, Engine 2, and
Engine 3), with low EC and OC mass emissions (Figure 1),
could be used by the mining industry as a strategy to
control EC and TC mass concentrations in underground
workings and potentially maintain personal exposure below
current mass-based standards (32,33,34). However, if used,
the “clean” engines that are not fitted with DPF systems,
such as Engine 1 and Engine 3, would still measurably
contribute to number concentrations of aerosols in those
workings (Figure 3 and Figure 4). Only “clean” engines
with the integrated DPF systems, similar to the one used
in Engine 2, would contribute little to both mass and num-
ber concentrations of aerosols (Figure 1 and Figure 3). The
use of advanced in-cylinder combustion controls strategies,
particularly higher injection pressures (51,52,53), resulted
in changes in size distributions of aerosols. In the case of
“clean” engines” that are not fitted with DPF systems, the
accumulation mode aerosols were characterized by measur-
ably smaller median diameters (Figure 4, Table 4) and gen-
erally contributed less to the mass concentrations of EC
than accumulation mode aerosols emitted by “traditional”
engines (Figure 1). That was the case even for Engine 1
operated at R50 test conditions where number concentra-
tions of aerosols in the accumulation mode were higher
than corresponding number concentrations of aerosols in
the accumulation mode for “traditional” engine (Table 4),
but mass concentrations of EC were substantially lower
(Figure 1). For the test and measurement conditions gener-
ated in this study, the concentrations of nucleation mode
aerosols and sub‑23 nm aerosols were found to be relatively
low when compared with concentrations of accumulation
mode aerosols (Figure 4 and Table 4). The concentrations
of nucleation mode aerosols could be potentially higher
if hot exhaust of the test engines is released in the under-
ground environment characterized by meteorological con-
ditions favoring nucleation (84).
The results of this study showed that emissions of
highly toxic NO2 could be quite an important factor
influencing selection and use of “clean” diesel engines,
particularly those fitted with catalyzed exhaust aftertreat-
ment devices, in the underground mining industry. In the
underground mines that operate engines that emit NO2
at the levels characteristic to “traditional” engines (NO2 =
0.01 – 0.20 NOx) (39,85), the NO2 exposures have been
historically maintained below action levels (e.g., one half
of the 5 ppm MSHA ceiling limit) (35,37,86) by means of
“fresh” air dilution using air quantities determined to keep
exposures to the other most critical criteria gas, namely
CO, NO, or CO2, below respective personal exposure lim-
its (40,87). However, the introduction of certain types of
catalyzed exhaust aftertreatment devices characterized by
elevated secondary NO2 emissions changed that paradigm
(39,79,88,89). In the exhaust of Engine 1 and Engine 2
evaluated in this study, the concentrations of NO2 exceeded
200 ppm (NO2 0.65 NOx) and 125 ppm (NO2 0.45
NOx), respectively. In this case, Engine 1 would be operated
in the U.S. underground metal/nonmetal mines, and the
quantities of air needed to dilute NO2 to the corresponding
permissible exposure level (PEL) should be 3.5 times higher
than those needed to dilute the next criteria gas, CO2,
to the corresponding PEL. In the similar case involving
Engine 2, the quantities of air needed to dilute NO2 should
be 1.6 times higher than those needed to dilute the next
criteria gas, NO, to the corresponding PEL. Depending on
circumstances, providing additional quantities of “fresh” air
needed to address elevated NO2 concentrations might be
technologically challenging and potentially cost-prohibitive