Glycerol is a biorenewable co-product of biodiesel manufacturing which continues to receive interest as a diesel fuel additive for transportation, heat and power applications. Its unique chemical properties necessitate emulsification which leads to complex combustion features, specifically particulate matter emissions, of interest to a broad scientific community. This study examines the combustion and emissions of glycerol emulsion fuels in a four cylinder, high-speed, indirect-injection engine during ISO D8178 D2 testing protocols. Three glycerol emulsion fuels are prepared at 10, 20 and 30% by weight into a 50/50 mixture of biodiesel and low-sulfur diesel. The emulsion fuels meet US EPA emissions limits for the engine class, but with increased fuel consumption due to low energy density. Particulate matter is shifted from chain-like to cluster-like particles with increasing glycerol concentration. Primary particle size is unaffected by glycerol concentration with nominal particle diameter of 25 nm. Numeration of primary particles in agglomerated material is reduced with increasing glycerol concentration and is accompanied by increased semi-volatile organic matter fraction. Direct particle imaging using TEM suggests that gravimetric, optical and condensation counters methods do not sufficiently characterize or quantitate particulate matter emissions resulting from glycerol emulsion combustion.