What are two examples of coolers that will be used to remove heat from the oil?
Shell and tube coolers can be either cast bronze or cast iron, the tube plates are usually naval brass and the tubes are aluminum bronze.
The cooling media can be sea water or engine jacket water and is circulated through the tubes. The lubricating oil is directed OVER the tubes by internal baffle plates. The coolers may be single or double pass.
Plate type heat exchangers are composed of a series of titanium or stainless steel plates held in compression by two steel backing plates and separated by gaskets. The plates are corrugated to stiffen, strengthen, promote turbulent flow and increase the heat transfer area. Turbulent flow will break up the boundary layer, causing more water to come in contact with the plate surfaces. If the temperature is below 100C, nitrile rubber will be used for the gaskets. Above this temperature, a compressed asbestos fiber will be used.
Fluids will circulate by contra-flow for better efficiency.
Advantages of plate type coolers over shell and tube
- Smaller and lighter for the same performance
- No extra space required for disassembly
- Plates can be added in pairs to increase the capacity of the unit
- Damaged plates are easily removed
- Cleaning is simple and turbulent flow reduces deposits.
Disadvantages of plate type coolers over shell and tube coolers
- Leaks in plate gaskets can be difficult to trace to the affected area.
- Plate gaskets deteriorate in time and will require replacement.
- Cost
What are three different types of medium used to cool a diesel engine from the heat it generates? Where might you see each type being used?
- Fresh water: components with high levels of heat to be removed (injectors, pistons, cylinder liners).
- Oil: areas where contamination is a concern as the cooling oil will not cause serious harm if mixed with the lube oil
- Air: small diesel engines
The underside of the piston may be cooled by different methods. Briefly explain one method discussed in class on how to cool the piston of either trunk type or crosshead type engines.
Splash lubrication occurs in small trunk type engines. As the crankshaft rotates through the lube oil sump, the crank pins will hit the oil surface and throw oil up to the cylinder liner and the underside of the piston.
If you checked your oil sump level and the level was really low, what may this effect on your engine?
Loss of lubricating capacity: parts seize together, resulting in boundary level lubrication, excessive wear.
Loss of cooling capacity: increased thermal stress, creep, fatigue, thermal shock between cooled and uncooled regions.
Reduce efficiency of engine.
Sketch and describe a central cooling system suitable for a medium speed diesel engine
Purpose: to provide the necessary cooling to the various components of the main engine.
Construction: composed of 2 separate circuits- salt water and fresh water.
Salt water will be supplied from the sea chest, through a strainer and into the sea bay. Piping is fitted with isolation valves. Sea chest and bay are suitably protected from corrosion. Three sea water pumps (2 main + 1 standby) fitted in parallel with check valves to prevent back flow. Bilge ejection valve is fitted on this line. Plate-type heat exchangers with titanium plates and nitrile gaskets with contra-flow for fresh water. Coolers fitted with isolation valves, thermostats and pressure gauges. Thermostatically-controlled directional valve to divert hot salt water to the sea bay as required to maintain liquid state in sea bay.
Materials: schedule 40 galvanized iron pipe.
Fresh water is the cooling medium for the ship’s systems. Consists of a hot side and a cold side. Fresh cold water is supplied to the main engines and aux machinery through the use of an engine-driven centrifugal pump. Passes through the lube oil cooler before entering the engine. Used to cool the charge air to increase the mass of air forced into a cylinder and increases the power produced. Will cool the internal engine components (cylinder heads, pistons). Hot water is collected at the top of the engine and passes to the hot side of the system. If aux machinery does not have a method to circulate water, electrical cooling pumps are fitted to direct water to the air compressors, refrigeration compressors, and air conditioner units. The hot water will collect in a common line (hot header) to return to the plate coolers. There may be a pressure regulating valve to drop the pressure and ensure proper flow. Need pressure drop to induce flow in a closed circuit system. A thermostatic bypass valve is fitted to ensure the FW temperature is maintained above a set value. Need a vent from the highest point in the system to deaerate to the header tank.
What are the 3 main methods of diesel engine scavenging? Explain how each one accomplishes the gas exchange.
Three basic systems are in use: the crossflow, the loop, and the uniflow. All three systems have the ports angled to swirl the incoming air and direct it in the appropriate path.
In crossflow scavenging, the incoming air is directed upwards, pushing the exhaust gases before it. The exhaust gases then travel down and out of the exhaust ports.
In loop scavenging, the incoming air passes over the piston crown then rises towards the cylinder head. The exhaust gases are forced before the air passing down and out of the exhaust ports located above the inlet ports.
With uniflow scavenging, the incoming air enters at the lower end of the cylinder and leaves at the top. The outlet at the top of the cylinder may be ports (e.g. opposed piston engine) or an exhaust valve in the cylinder head.
What is the advantage of using a mechanical blower to introduce air into the engine cylinders?
- Increased horsepower per unit weight of an engine.
- Increased output of the engine (up to 50%) without materially increasing the loads on the bearing or the heat stresses on cylinders, pistons, rings, and valves.
- Supercharged engines cost less than a normally aspirated engine of the same power.
- Increased fuel economy.
Describe the purpose, materials, construction and operation of a turbocharger that may be fitted to a medium speed diesel engine.
Purpose: To provide charge air at a pressure and density higher than atmospheric using a turbine powered by exhaust gasses.
Construction: Consists of turbine, shaft, and compressor. where hot exhaust gasses power the turbine to rotate the shaft and draw air through the compressor
Turbine Blades: Ni-Cr alloy or nimonic (good resistance to creep fatigue and corrosion)
Blade roots are fir tree shaped to promote positive fixing and minimum stress concentration.
Lacing wire may be used to dampen vibration.
Turbine Casing: Cast steel and may be water cooled
Compressor impeller: Aluminium alloy or titanium, located on the rotor shaft by splines.
Air Casing: Cast iron or aluminium alloy. and shaped like a volute to convert kinetic energy to pressure energy
The shaft is supported by two bearings; one on the compressor and one on the turbo side, these may be ball roller or plain sleeve bearings.
Bearing lubrication may be through integral oil pumps (ball and roller bearings) or from the main engine oil supply or independent system (plain sleeve bearings) Oil serves to cool and lubricate the bearings.
A thrust arrangement is required to locate and hold the rotor axially in the casing and is normally located on the compressor side.
labyrinth seals are fitted to prevent leakage of exhaust gas or leakage of oil into the compressor.
Operation:
Exhaust gas enters via the gas inlet casing. the gas expands through a nozzle ring where the pressure is converted to kinetic energy. \this high velocity gas is then directed onto the turbine blades where it drives the turbine wheel and therefor the compressor at high speeds. The exhaust gas then passes though the outlet casing to the exhaust uptakes. In a medium speed diesel engine gas flow is normally axially along the axis of the rotor shaft. At idle and at startup the turbocharge may suffer from turbo lag as there is insufficient exhaust gas to adequately rotate the shaft and consequently the compressor. Turbo surge occurs when the compressor outlet pressure exceeds the scavenge trunking pressure. This may be caused by engine load changes while underway but could also indicate a more serious problem that needs investigation. A turbocharger waste gate is a pressure activated valve that controls the flow of exhaust gasses to the turbine to regulate the maximum boost pressure in the system which protects the engine and the turbocharger
