Isotopic tracing of bio-derived carbon from ethanol-in-diesel blends in the emissions of a diesel engine

The addition of oxygenates to diesel fuel reduces particulate emissions, but the mechanisms responsible for the reductions are not well understood. Measurement of particulate matter (PM), unburned hydrocarbons (HC), and carbon monoxide (CO) are routine, but determining the origin of the carbon atoms that make up these undesired emissions is difficult. The sub-attomole (<6×10⁵ atoms) sensitivity of accelerator mass spectrometry (AMS) for measuring carbon-14 (¹⁴C) allows tracing the carbon atoms from specific fuel components to soot or gaseous emissions. Radioactive materials are not required because contemporary carbon (e.g., ethanol from grain) has 1000 times more ¹⁴C than petroleum-derived fuels. The specificity of the ¹⁴C tracer and the sensitivity of AMS were exploited to investigate the relative contribution to diesel engine PM, CO, and CO₂ from ethanol and diesel fractions of blended fuels. The test engine, a 1993 Cummins B5.9 diesel rated at 175 hp 2500 rpm, was operated at steady-state conditions of 1600 rpm and 210 ft-lbs. PM was collected on quartz filters following a mini-dilution tunnel. The limited solubility of ethanol in diesel fuel required either an emulsifier (Span 85) or cosolvent (n-butanol) to prepare 10, 20, and 40% ethanol-in-diesel blends. An ignition improver, di-tert-butyl peroxide (DTBP), was added to give all blends the same autoignition properties as the baseline diesel. PM was separated into volatile and non-volatile organic fractions (VOF and NVOF) for AMS analysis. The homogeneous cosolvent blends were more effective in reducing total PM mass, but the heterogeneous emulsified blends yielded larger VOF that are easily treated by exhaust catalysts. Ethanol derived carbon tended to reside in the NVOF, especially for the cosolvent blends.