Feature 1 | GREEN SHIPPING
dependent on the number of causal parameters and output exhaust species that can be associated with each other. These methods
require further
investigation to fully establish the relationships between cause and effect. Perhaps the most refined methods for
predicting emissions can be described as first principles methods include modelling the details of the mechanical, thermodynamic and chemical processes in an engine. Te interactions between the hull, the propeller and the engine, leading to the precise conditions in the combustion space and hence the constituents of
the exhaust gas
are highly complex. Therefore, the underlying model assumptions and level of modelling sophistication and/ or supporting empirical factors in a first principles model needs to be carefully considered and a trade-off between computational power and time against the prediction accuracy of the exhaust gas emissions is required. At the highest level of sophistication it
is desirable to generate a model for engine operation that can predict emissions generation under steady-state full-load as well as when the ship and engine systems are operating in either part-load (e.g. slow steaming) or transient conditions (e.g. while manoeuvring). To investigate the possibility of such a tool, as part of the Clean North Sea Shipping Project at Newcastle University, a time based, first-principles simulation tool has been developed. Te time-based simulation approach
has been chosen for investigation because of a number of potential advantages it can provide. This approach allows transient conditions to be modelled, which is particularly important as there is a focus and need to predict exhaust gas emissions in areas of high population density, such as in port. In addition, a steady-state condition
within the model can also be achieved, in the same way as in reality, through allowing the simulation to run for sufficient time, aſter the ship-propeller- engine set points have been entered. And in fact, the simulation time required to achieve a steady-state engine performance can be artificially reduced through control of the initial conditions
42 Figure 5: Reduction potential for non-engine/combustion measures.
of the integrators within the simulation. A time-based simulation also allows
its use as a training tool, in which the user can observe the effects on engine performance, in all respects, from the time-varying output of the simulation in response to initiating changes to engine control input parameters. In this way, it provides not only a tool through which higher-level operational and design decisions can be made, but also allows ship staff the opportunity to understand and predict the outcome on engine performance of day-to-day operational choices. It also allows the precise operational
condition of the coupled hull-propeller- engine system to be taken into account and therefore, not only can it show instantaneous
performance levels,
but can also be used to guide engine maintenance or modification strategies for improved emissions, by changing air inlet temperature, valve timing, injection timing or inlet air conditions for example. The model under development is
shown schematically in Figure 5. It assumes a lumped-mass model of the engine and propulsion chain, similar to that assumed in dynamic response models for engine vibration and torsional responses. In this case a piston and crank model are coupled through their kinematic relationships, for
displacement, velocity and acceleration. A differential thermodynamic model, based on energy conservation and mass conservation, is then used to calculate the instantaneous cylinder gas pressure, and hence force, acting on the face of the piston. The total moment on the shaft is derived from the sum of the moment due to the gas pressure acting through the connecting rod, a frictional component of torque and the propeller torque, which can be specified or derived through propeller design curves. Solving the equations of motion for the piston- crank-shaft
system, in response to
gas-induced forces and total shaſt torque, allows the time-dependent motion of the piston-crank-shaſt to be resolved. To date the fuel-addition and
combustion process is modelled as a rate of heat addition per unit time between the start of injection and the completion of combustion. Heat loss is accounted for through a one-dimensional convection- conduction model and temperature- dependent instantaneous gas property, γ, is derived using published values for different elements and molecules. As yet the model does not predict NOx
or PM emissions since the simulation tool is in development and establishing that the approach can accurately model engine performance in terms of salient pressures, temperatures, power output, etc. has been the priority because it
The Naval Architect May 2012
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