Production of the artillery tractors began on 3 December 1914. These engines were later used for the first British tanks ever built, the prototypes "Little Willie" and "Mother" and later in the production Mark I tank.[81] One major difficulty for the tanks was the fine oil haze above their Daimler engines which the enemy quickly learned meant tanks were operating nearby if out of sight.[82] The early tanks weighed up to 28 tons.[82] They were all Daimler powered. The Mark IV tank - the first major improvement in design - had uprated engines delivering 125 hp; these engines had aluminium pistons and are believed[note 2] to have been designed by W. O. Bentley while he was working on the Bentley BR1 rotary engine in Coventry.[82]