As a big contributor to the the global emissions, the maritime sector strives to bring down its greenhouse gasses and hazardous emissions by enforcing increasing strict rules on the exhaust gasses. This gives major challenges to the maritime sector to develop greener modes of transportation. This report investigates the in-cylinder starting conditions of spark ignited gas engines, as the starting conditions have a large influence on the efficiency and NOx emissions of gas engines. The investigation is based on experiments performed on a Caterpillar G3508 gas engine. These experiments gave an insight on the starting temperature, pressure and residual mass. With the acquired knowledge on the starting conditions, we improved the induction and volumetric efficiency prediction of a state of the art 0-dimensional engine model [15]. Another contribution of this paper is the development of a way to establish the volumetric efficiency from the amount of oxygen in the exhaust gas, for a natural gas engine. We used this method to check the improvements made to the engine model. Out of the performed experiments, we concluded that the induced inlet mass temperature decreases with increasing power or mass flow. The insight found about the starting pressure for this engine is that the trapped pressure increases in relation to the manifold pressure with increasing power or mass flow. Together with an improved residual mass prediction, these insights resulted in an increase of the volumetric efficiency prediction accuracy by 2% at low powers and 10% at high power.