A computer-implemented method for estimating pressures in a gas engine using a real-time model-based observer, said method implemented by a pressure estimation computing device including a processor and a memory device coupled to the processor , said method comprising: receiving a design schema describing an intake manifold and a plurality of components associated with the gas engine ; segmenting the design schema into a plurality of segments defining a plurality of sections of the gas engine ; defining a fluid dynamics model associated with each of the plurality of segments; defining a plurality of interconnected 2-port elements based on the plurality of fluid dynamics models, wherein the plurality of interconnected 2-port elements forms a network along the gas engine , wherein the network includes points of intersection between interconnected 2-port elements with zero net flow and equal pressure; receiving at least one pressure measurement from at least one of a plurality of sensors associated with each of the sections of the gas engine ; estimating a plurality of pressure values at each section of the gas engine ; controlling fuel injection to at least one gas cylinder based on the estimated plurality of pressure values, further comprising: identifying a plurality of transfer functions associated with the plurality of interconnected fluid dynamics elements; and performing a model-order reduction of each of the fluid dynamics model elements by truncating the plurality of transfer functions to create a plurality of truncated transfer functions, each of the plurality of transfer functions is associated with one of the plurality of truncated transfer functions. receiving a design schema describing an intake manifold and a plurality of components associated with the gas engine ; segmenting the design schema into a plurality of segments defining a plurality of sections of the gas engine ; defining a fluid dynamics model associated with each of the plurality of segments; defining a plurality of interconnected 2-port elements based on the plurality of fluid dynamics models, wherein the plurality of interconnected 2-port elements forms a network along the gas engine , wherein the network includes points of intersection between interconnected 2-port elements with zero net flow and equal pressure; receiving at least one pressure measurement from at least one of a plurality of sensors associated with each of the sections of the gas engine ; estimating a plurality of pressure values at each section of the gas engine ; controlling fuel injection to at least one gas cylinder based on the estimated plurality of pressure values, further comprising: identifying a plurality of transfer functions associated with the plurality of interconnected fluid dynamics elements; and performing a model-order reduction of each of the fluid dynamics model elements by truncating the plurality of transfer functions to create a plurality of truncated transfer functions, each of the plurality of transfer functions is associated with one of the plurality of truncated transfer functions. identifying a plurality of transfer functions associated with the plurality of interconnected fluid dynamics elements; and performing a model-order reduction of each of the fluid dynamics model elements by truncating the plurality of transfer functions to create a plurality of truncated transfer functions, each of the plurality of transfer functions is associated with one of the plurality of truncated transfer functions. <EOS>