Non-volatile particle number distributions from a single cylinder industrial diesel engine were measured at several operating conditions spanning the torque curve. The effect of increasing the air-fuel ratio by injecting compressed shop air at various boost pressures was also investigated. A bi-modal distribution separated at approximately 20 nm was observed for most operating conditions. Depending on operating condition, the engine produced between 1014 to 1015 particles per kW-hr. Energy specific particle number emissions (per kW-hr) were seen to be strongly dependent on speed and load. Minimum emissions occurred at intermediate speeds and loads. Particles below 20 nm increased with decreasing load while the opposite trend was observed for particles greater than 20 nm. Variation in total particle surface and total particle volume followed the same trends as the particles from the larger mode. The most interesting result was that external air-injection at the engine intake had almost no beneficial effect on energy specific particle number emissions, although it increased the air-fuel ratio substantially with numbers ranging from 50 to 200. The increase of particle numbers with decreasing load for sub-20 nm particles continued with external air-injection. These findings are interesting given the ubiquity of small diesels for portable/backup power, industrial and agricultural uses. The study suggests that significant PN emission reduction in these engines might require DPFs.