The work output per cycle divided by the engine efficiency gives you the amount of fuel energy required per combustion event. Divide by the fuel heating value (about 42 MJ/kg for diesel) to get the fuel mass per combustion event. A CI (diesel) engine operates with an air/fuel ratio of about 20. Multiply the fuel mass by this factor and you get the air mass per cycle. The power (5 hp) is given, but you need the rated engine speed (number of cycles per second) to determine the work output per cycle.