Operating a Compression Ignition Engine Fuelled with a Combination of a Hydrocarbon Fuel and Hydrogen

The invention concerns a method of operating a compression ignition engine () fuelled with a combination of a hydrocarbon fuel and hydrogen. The method comprises: during a cold start, fuelling the engine with the hydrocarbon fuel alone, or with the hydrocarbon fuel and a reduced proportion of hydrogen compared with warm miming conditions, and increasing the proportion of hydrogen used to fuel the engine as it warms up whilst running.