Scania DI09 Exhaust back pressure
The back pressure in the exhaust system must not exceed the maximum recommended exhaust back pressure, including silencers. A higher exhaust back pressure leads to increased fuel consumption and a loss of power. The maximum recommended exhaust back pressure is 100 mbar for all engines. If the exhaust back pressure is above the maximum recommended value, this has a negative impact on engine performance. Always check the exhaust back pressure when installation is complete.
Scania DI09 Measuring exhaust back pressure
Perform the measurement on a straight section of the system approximately 1 metre downstream of the turbocharger. A simple device for measuring exhaust back pressure is a transparent plastic hose. Connect the hose to a test connection on the exhaust pipe as illustrated. Bend the hose and fill it part-way with water.
Drill a hole with 2.0 mm diameter in the exhaust pipe. It is important that the hole on the inside of the pipe has a clean, sharp edge. Residual burrs and unevenness can result in significant measurement errors.
First warm up the engine and then run it at maximum load and full load speed. At the same time, measure the level difference (a) of the water in the hose. This is a direct measurement of exhaust back pressure in mmH2O. 1,000 mmH2O is equal to 100 mbar.
Scania DI09 Type of exhaust system
Both dry and wet exhaust systems are found in marine installations. Dry exhaust systems are preferable in the case of vertical exhaust outlets. Dry exhaust pipes must be insulated to reduce the temperature and the risk of fire. For installations with exhaust outlets in the stern or at the sides more or less level with the engine exhaust connection, wet exhaust systems are recommended.
In this case, sea water is brought in and mixed with the exhaust fumes, which results in dampening of the noise level and a lower exhaust temperature. After the connection point for the sea water, the exhaust system does not need to be insulated. A very short exhaust system can be fully encased in water. If so, no silencer is needed.
Scania DI09 Wet exhaust system
The exhaust system outlet should always be positioned above the water line, even when the vessel is heavily loaded. The system must slope downwards towards the outlet. The outlet must always be lower than the water inlet in the system to prevent water from entering the engine. If the exhaust system outlet is higher than the engine
exhaust connection, the exhaust system must be designed with a water lock that prevents water from entering the engine when stationary.
If a mixing vessel is used in the wet exhaust system as a water lock and to suppress noise, the lifting height for the gas and water mixture must not be so great that the total exhaust back pressure is above 100 mbar. The mixing vessel must hold at least the amount of water that is drained from the system when the engine is stopped.
The most common type of water lock (when the exhaust connection on the engine is lower than the water line) is an upward-facing, jacketed exhaust bend at a height of at least 350 mm above the exhaust system outlet. See illustration.
If the engine is installed in such a way that the exhaust connection is located at least 350 mm above the water line, it is possible to connect a jacketed exhaust pipe with water inlet after the flexible pipe on the exhaust outlet from the engine. See illustration on next page.
The jacketed exhaust pipe is then connected to the board lead through with a rubber exhaust hose. Only corrosion-resistant hose clamps may be used for the rubber hoses.
Equip engines that have a sea water-cooled exhaust system comprising rubber hoses with a warning system that triggers an alarm in the event of high temperature. Otherwise, there is a risk of the hoses overheating if the sea water pump malfunctions or the sea water intake becomes blocked. Design the water intake to the rubber hose so that the spread of water over the surface of the hose is assured even at low engine speeds when the sea water flow is also low.
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