Diesel ash emissions are the non-combustible portion of diesel particulate matter
and are mainly derived from metallic lube oil additives such as Ca, Mg, and Zn as well as
engine wear metals including Fe. Diesel particulate filters (DPF) are used to filter engine
particle emissions and are routinely regenerated, oxidizing the material in the filter. After
filter regeneration, metallic ash remains. Over time, this ash can build up in the filter,
increasing the filter pressure drop and eventually plugging the filter. Ash emissions can
also be used to calculate engine lube oil consumption.
A high temperature oxidation method (HTOM) has been developed to measure
ash emissions in real-time. The HTOM uses a tube oven to oxidize the combustible
fraction of an engine exhaust sample. The remaining particles are then measured with
real-time and near real-time particle sampling techniques.
The HTOM system improvements are discussed along with validation testing and
further recommendations regarding improvements in the apparatus and procedure.
Atomized lube oil was used as a test aerosol. Special lube oil blends were formulated
to determine measurement sensitivity to specific ash constituents. Ca and Mg were
readily detectable by the HTOM. However, the HTOM was not very sensitive to Zn
based additives. Chemical equilibrium models show that Ca and Mg formed stable
oxides at oven temperatures while Zn went to a gaseous phase.
Repeatability in lube oil spray ash measurements was better than 10 %. Engine
exhaust repeats did not show as good repeatability. However engine exhaust is a much
less stable and repeatable aerosol.
Steady-state engine ash emissions were sampled from two different Diesel engines (1.9 L VW and Cummins APU) at different speeds and loads. Lube oil doped fuel was
also used in the engine tests to artificially increase oil consumption and ash emissions.
The engine lube oil consumption was predicted from known lube oil ash concentrations
and measured ash emissions. The non-doped fuel tests yielded lower than expected lube
oil consumption estimates while the doped tests showed an ash recovery of 31 %.
Transient ash measurements were demonstrated. Ash and soot measurements were
made downstream a loading DPF. The downstream soot particle surface area concentrations tracked better with the ash than soot volume concentrations. Transient
engine speed and loads were also measured by the HTOM. Ash diameter concentrations were measured in real-time showing definite spikes of ash emissions at transient load steps.