The use of fossil fuels and their impact on humans and nature is becoming a bigger concern worldwide. An interesting concept to lower polluting emissions coming from maritime engines is the combined use of a dual-fuel engine and an underwater exhaust system. In the dual-fuel engine natural gas is used as the main fuel type. It is seen as a cleaner alternative to the traditional fossil fuels that are being used. In order to ignite the air-gas mixture in the cylinder a small amount of diesel fuel (pilot fuel) is used, therefore making it a dual-fuel engine. Some advantages of using an underwater exhaust system are no direct emissions into the atmosphere, increased space on decks and reduced noise from the exhaust. However the downside is that such a system causes back-pressure effects which can deteriorate engine performance. So far research on these back-pressure effects on a dual-fuel type engine is lacking, therefore this thesis focuses on the static back-pressure effects on a dual-fuel engine.
For this purpose experiments were performed on a constant pressure turbocharged, 4-stroke dual-fuel engine. Engine performance and emissions were recorded for load points along the propeller and generator curve for three cases of back-pressure. The back-pressure was controlled by a butterfly valve in the exhaust pipe of the engine. Due to the fact that this butterfly valve could only be controlled manually, the back-pressure during the experiments could not be kept at a constant level. Also the engine experienced an absolute back-pressure of 1.15 bar with the valve fully open which was already quite high. And in the natural gas that was used a substantial amount of hydrogen was present. An existing diesel engine model (DE-B) that was developed at the TU Delft was then adapted to turn it into a dual-fuel engine model. It is a mean value first principle (MVFP) model. This dual-fuel MVFP model was then matched to the tested engine by using the experimental data. After the matching the model was used to test levels of back-pressure that could not be reached experimentally, due to engine limits, to see the effect on engine performance at these levels. The results were then analysed to see what the critical engine parameters are for the limits of the engine.
The experimental results showed that at low power more pilot fuel is needed to combust the gaseous fuel. It was also clear that there was a changing combustion efficiency for the gaseous fuel along the propeller curve. For the part load conditions the combustion efficiency was lower than at low or high power. For the generator curve the total efficiency was more constant, but always lower than the efficiency of the propeller curve. The results also showed that with increasing back-pressure the fuel consumption decreased for both the pilot and gaseous fuel, with the exception of the gaseous fuel flow at part load for the propeller curve. The recorded emissions showed lower levels of O2, CO and unburned hydrocarbons and higher levels of CO2 and NOx with increased back-pressure. Looking at the emissions in combination with the fuel flows, they showed signs of improved combustion efficiency with increased back-pressure. Increasing the back-pressure caused lower pressures and temperatures at the inlet side of the engine and higher pressures and temperatures at the outlet side. So for the thermal loading of the engine the outlet side is the critical one. The tested dual-fuel engine had a waste-gate installed. The model runs with higher levels of back-pressure showed that the pressure on the inlet side could become so low, that the pressure at which this waste-gate becomes active is no longer reached. This causes high pressures and temperatures before the turbine and high temperatures of the exhaust valve. The model also showed that at high levels of back-pressure the flow through the turbine becomes too low and not enough air is being put into the engine, so the air-excess ratio becomes too low, making the air-excess ratio also an important parameter for the engine limits. Finally it was shown how the dual-fuel MVFP model can be used to define acceptable limits of back-pressure for the air-excess ratio, exhaust valve and outlet receiver temperature.
The main recommendation for further research is to investigate the combustion efficiency of the gaseous fuel. Both the fact that at part load the efficiency was at it worst and the lower fuel consumption when back-pressure was increased need to be looked at. This will also help with improving the current dual-fuel MVFP model. Next to that it would be interesting to see what happens with different types of gaseous fuel and how the engine performs under a fluctuating back-pressure as induced by waves.