The combustion chamber pressure computed with a three-dimensional model is compared with the measured one in a rotary engine fueled with mixtures of natural gas and air. The rotary engine has a rotor displacement of 654 cm3, a compression ratio of 9.4 and uses 2 ignition sparks. The model incorporates a k-ϵ submodel for turbulence, wall function submodels for turbulent wall boundary layer transport, and a hybrid laminar/mixing controlled submodel for species conversion and energy release. Nine cases are considered that cover a wide range of engine operating conditions: rpm of 2503-5798, volumetric efficiency of 35.7-100.5% and equivalence ratio of 0.59-1.15. In all cases the computed and measured pressures agree within 12%. Except for parameters that are fixed by laminar flame information, all other model constants are the same as those used for propane and gasoline fuels in previous studies of combustion in reciprocating and rotary engines with premixed and direct-injection stratified charge engines. This and earlier studies strongly suggest that the present model can be used for predictions of pressure in firing engines for the purpose of selecting or improving engine design. It is also found that the precombustion spatial distribution of the turbulence diffusivity strongly influences the entire combustion event. Finally it is shown that the laminar conversion time becomes dominant as flames approach walls, thus slowing down their propagation rate.