A method for determining the total mass flow through an engine (10) comprises
measuring (32) the fuel flow to the engine (10) and measuring the
concentration of carbon dioxide in the gases exhausted (22) from the engine
(10). The concentration of carbon dioxide is measured by detecting the infrared
radiation absorption spectrum of carbon dioxide using a laser (38) and a
detector (46). The method comprises determining (48) the air to fuel
ratio in the engine (10) from the fuel flow and the concentration of carbon
dioxide. The method comprises determining (48) the air mass flow through
the engine (10) from the air to fuel ratio and calculating (48) the
total air and fuel mass flow through the engine (10) by adding the air mass
flow through the engine (10) to the fuel mass flow to the engine (10).
