Engine dynamometer testing was conducted on a production automotive engine. Untreated exhaust emissions were measured over a range of hydrogen/natural gas fuel mixtures and equivalence ratios (Φ).The most important finding was that, with retarded ignition, extremely low raw NOx emissions can be attained at the same time as high brake thermal efficiency. With a 30% hydrogen mixture and Φ = 0.65, the effect of spark timing on engine efficiency is almost negligible over approximately 15° of crankshaft rotation. For an engine load of ∼400 kPa, brake thermal efficiency remained at 30%. NOx emissions can be kept below 0.05 g/kWh for bmeps up to 500 kPa and rpms above 1700, with low hydrocarbons and minimal effect on fuel economy.