Petrol engines mostly. The German Luftwaffe equipped most of their airships with the Maybach C-X 6-cilinder inline petrol engine, or iterations thereof.

The British used Rolls-Royce aero engines for most of their WWI airships , most notably the Rolls Royce Hawk and Eagle. Although Sunbeam, Green and even Renault engines were used on early models.

Later Zeppelins as they grew in size, most notably the Hindenburg, used diesel engines. 4 daimler Benz 602 V16 88.5L Diesel engines in pusher configuration to be exact.The British R101 also used diesel engines: 8 cylinder Beardmore Tornado's