With respect to emissions from marine engines to atmosphere, discuss:
(a) Need to limit emissions of NOx, SOx, CO and CO2.
(b) What are ‘acid rain’, ‘smog’, ‘global warming’ and how it is related to the above exhaust gases emissions?
(c) Formation of NOx, Carbon monoxide, and Sulphuric acid.
(a) Emissions of NOx, SOx, CO and CO2
The oxides of Nitrogen (NOx) and Sulphur (SOx) are the “Primary pollutants” of the atmosphere.
NOx dissolve in water to form Nitrous/Nitric acids. NOx can react with O2 using ultraviolet radiation (from the sun) to form O3 (Ozone). cause smog formation and acid rain.
SOx dissolve in water to form Sulphurous/Sulphuric acids, primary causes of acid rain.
CO- Carbon monoxide (CO) results from incomplete combustion of the fuel. This gas is toxic to human life.
CO2 – Carbon dioxide is the main greenhouse gas emitted by ship which is the most damaging, estimated 55% of the greenhouse effect.
Hence emission of NOx, SOx, CO and CO2 need to be limited.
(b) Acid rain
- Acids such as sulphuric acid and nitric acid (secondary pollutants) absorbed in clouds and fall down as acid rain.
- Rain water has a PH of about 4 and it also dissolves ‘toxic’ materials in ground and becomes ‘contaminated’ ground water.
- Water is used by both plants and animals, which can damage their growth and increase in diseases, like cancer.
Smog
- NOx reacts with other chemicals in atmosphere especially in sunlight to form ground level ozone which are toxic and dangerous for both animal and plant life.
- A combination of Ozone with hydrocarbons forms photo-chemical ‘smog’. This is carcinogenic (cancer forming).
Global warming
- Fuel is a hydrocarbon and after complete combustion forms H2O and CO2, known as “Selective absorbers”
- Allow ultra-violet radiation to pass through, but absorb infra-red radiation.
- Any infra-red radiation, i.e. heat, which is generated at sea level, is prevented from escaping through atmosphere, and result in ‘global warming’.
(c) Formation of NOx
Nitrogen (79% in air) was previously considered inert in combustion reaction of an engine.
At higher temperatures and pressures of today’s highly rated engines, Nitrogen reacts with oxygen to form oxides.
N2 + O2 –Heat——>2NO
Reversible process but high temp: of gases in exhaust system prevents the reverse reaction. NO is found in exhaust gas.
Also, NO+ 1/2 O2—–>NO2
And N2 + 1/2 O2—–>N2O
NOx represents oxides of Nitrogen, such as NO, NO2 and N2O.
Formation of CO
This is due to the incomplete combustion of fuel due to
- Poor atomization
- Poor penetration/ turbulence
- Retarded fuel injection timing
- Slow turbocharger response to load change
The result is as emission of solid particulates (smoke) and carbon monoxide gas.
+ O2—-> CO + 1/2 O2
Formation of Sulphuric acid
During the combustion, Sulphur in the fuel reacts with oxygen to form Sulphur dioxides, combine with moisture in atmosphere to form Sulphuric acid.
Explain the effects of EACH of the following engine emissions on the environment and human life, stating how EACH of these emissions may be reduced:
(a) Oxides of Nitrogen
(b) Oxides of Sulphur
(c) Carbon Monoxide
(d) Unburnt Hydrocarbons
(a) Oxides of Nitrogen
Effects
- NOx dissolve in water to form Nitrous/Nitric acids.
- NOx react with O2 using ultraviolet radiation (from the sun) to form O3 (Ozone). cause smog formation and acid rain.
Reduction method
- Internal engine modifications (IEM), Water injection (DWI), Exhaust gas recirculation (EGR), Charge air humidification, Selective catalytic reduction (SCR)
- Selective catalytic reduction (SCR) reduce NOx emission by about 98%.
- Urea (as NH3 carrier) or Ammonia (NH3) is injected into exhaust stream.
- NOx reacts with the urea and ammonia to form nitrogen and water.
(b) Oxides of Sulphur
Effect
- SOx dissolve in water to form Sulphurous/Sulphuric acids, primary causes of acid rain.
Reduction method
- SOx controlled either by removing sulphur from fuel or by removing SOx from exhaust gas by scrubbing.
- A chemical reaction with sea water converts SOx into sulphates (gypsum), which already present in sea water.
- Scrubbing water pass through separation system to remove unburnt hydrocarbons and particulates before being pumped overboard.
(c) Carbon Monoxide
Effects
- Carbon monoxide is a colourless, odourless, invisible, and very toxic gas to human life.
- Even low concentrations in atmosphere may accumulate in human’s blood over a period of time with serious or fatal results.
- It is absorbed by lungs and attaches itself to red blood cells, much like oxygen.
- Blood will bond with carbon monoxide 200 times faster than oxygen.❖ Levels as low as 100ppm use headaches
- 1600ppm causes death in less than 2 hours.
Reduction method
Carbon monoxide (CO) results from incomplete combustion of fuel. Reduced to a minimum by the following means.
1. Maintain fuel injection equipment in good condition.
2. Complete combustion of fuel takes place.
3. Correct and efficient scavenging of cylinder takes place.
4. Avoid overloading of cylinder units.
(d) Unburnt Hydrocarbon
Effects
- Effects of unburnt hydrocarbon emissions depend on component species; some like benzene are carcinogenic, some cause breathing difficulties and eye irritation.
Reduction method
Even with good combustion, a small percentage of the hydrocarbons emerge from combustion process.
It can be reduced to a minimum by the following means.
1. Maintain fuel injection equipment in good condition.
2. Complete combustion of the fuel takes place.
3. Correct and efficient scavenging of cylinder takes place. Avoid overloading of the cylinder units.
(a) Sketch and describe an oil mist detector of modern type using the Light scatter principle.
(b) What are the advantages of this type compared to the old obscuring type?
(c) Explain the action the chief engineer officer must take to safeguard against the risk of crankcase explosion, Should the oil mist detector become inoperative.
(a) Oil mist detector
- Individual sensors fitted at each monitoring point and sample is drawn by a suction fan through each detector.
- Light is transmitted at one end of head where sample flows through.
- Compensating receiver is directly opposite to transmitter, adjusts light intensity by feeding back a signal to transmitter.
- Measurement sensor is placed at 90˚ to transmitter, picks up scattered light produced by oil mist particles
- Result is transmitted to monitor twice per second as an analogue signal.
- At monitor, this signal is compared against a set point and an average of other readings.
- When reaches a pre-determined point, reach an alarm condition.
(b) Advantages of light scatter type oil mist detector
1. Sampling points fitted close to crankcase – no need long pipe running
2. Continuous parallel sampling – no maintenance work for selector valves.
3. Fast response time – prevent bearing failure.
(C) Safeguard against risk of crankcase explosion
- If oil mist detector becomes inoperative, inform DPA and superintendent, detect cause of failure and take corrective action
- Machinery space cannot operate under UMS conditions and keep 24-hour watch keeping
- Duty engineer take extra care
- Watch-keeping engineer check for any unusual noise, heat or smell.
- Engine is not overloaded.
- Closely monitor bearing temperature from probes and crankcase pressure.
- on a trunk type engine, rise in pressure indicate excessive blow-by from one or more units.
- If any abnormal, stop engine for detection
- Necessary spares / service engineer ordered to rectify the problem as soon as possible.
(a) Explain about constant pulse pressure system with aid of sketch.
(b) Why 2/S engine now prefer constant pressure system?
(c) List the advantages and disadvantages of the above system?
(d) In pulse system, how exhaust from one cylinder is prevented from interfering with the scavenging of another cylinder.
(a) Constant pressure system
- In this system, exhaust from all cylinders lead to a common exhaust manifold.
- Usually connected to one or more turbochargers.
- More suitable for high output engines, no need to group the exhaust pipes.
- Due to constant pressure at the turbine inlet, it operates at optimum efficiency.
- Cannot operate efficiently at part load or starting periods due to low exhaust energy
- Needs auxiliary blowers or scavenge pumps.
Pulse pressure system
- This system takes the advantages of the high pressure exhaust gas when the exhaust valve is opened.
- Using relatively small exhaust pipes a high pressure is built up during short period.
- Exhaust gas passes to turbocharger nozzles in series of intermittent pulses, or pressure waves, create high velocity to turbine.
- Exhaust gas pressure is greater than charging air pressure.
- Exhaust pipes grouped according to firing order to prevent blowing back of exhaust gas to another cylinder, in scavenging process.
(b) Why two stroke large engine are now preferring constant pressure system,
- In constant pressure system, turbine gets peak efficiency due to smooth energy transfer from steady exhaust energy.
- Compressor capacity increases and more power obtained.
- Good thermal efficiency and reduced specific fuel consumption about 5% to 7% due to better scavenging efficiency.
- Good performance at high load and simple exhaust piping (no grouping required and less disturbance)
- Opening time of exhaust valve delayed about 15 °, lengthen expansion stroke and improve efficiency
(c) Constant pressure system
Advantages
(1) Simple exhaust piping
(2) High turbine efficiency due to smooth energy transfer from steady exhaust flow and
(3) Good performance at high load and
(4) Reduction in SFOC
Disadvantages
(1) Poor efficiency at low speed and part load due to low energy available at turbine
(2) Poor response to sudden load change
(3) Require scavenging assistance for low speed and part load.
Pulse pressure system
Advantages
(1) Good performance at low speed and part load due to high energy available at turbine inlet.
(2) Good response to sudden load change due to good T/C acceleration.
(3) Not require scavenging assistance.
Disadvantages
(1) Complex exhaust piping system/groupings
(2) If no proper grouping of exhaust pipes, exhaust blow back to low pressure scavenging unit
(d)
- To prevent a back-flow of exhaust from one cylinder into another, necessary to subdivide exhaust into a number of manifolds, and each connected to a separate inlet nozzle at the turbine.
- Pipes arranged in small diameter to boost up pressure pulse and in short straight length to prevent energy losses.
- Up to three cylinders connected to each manifold without interference depending on their firing order.
- Number of exhaust branches depends upon firing order, number of cylinders and turbo charger design.
With reference to main propulsion engine turbochargers:
(a) State how in-service performance checks are undertaken for EACH of the followings:
(1) The gas side (2) The compressor (3) The suction filter (4) The after cooler
(b) Describe the action to be taken to allow the main engine to operate in the event of a turbocharger failing such that it may not be used.
(a) (1) The gas side
- Turbocharger gas side efficiency assessed for given power setting by T/C speed
- Exhaust gas temperature differential across the turbocharger, indication of amount of energy taken from exhaust gas to drive turbine
- Fouling on nozzle ring and blades lead to a decrease in energy conversion.
(2) The compressor
- Given turbocharger speed at particular power setting, compressor performance decrease from scavenge pressure decrease
- Usually caused by fouling of compressor wheel/ diffuser.
(3) The suction filter
- Dirty suction filter will result in similar symptoms to those given in (2) above.
- Manometer fitted across the suction filter show an increase in differential pressure.
(4) The after cooler
- Aftercooler efficiency drop detected by scavenge air temperature increase after cooler
- Fouling on air side, differential pressure increase across air cooler show on manometer.
- Fouling on water side, differential pressure not increase across air cooler
- Different temperature reduce between cooling water inlet and outlet.
- All above faults could result in turbocharger surging
(b) This will depend on the type of the engine, turbocharging system, & number of blowers.
For a 4 stroke pulse turbocharged engine fitted with a blower/at either end
1. Remove damaged rotor, air casing, and filter/silencer.
2. Blank gas casing, exhaust gas can pass through the casing
3. Blank off air outlet to engine inlet manifold.
4. Operate the engine with single blower
5. Reduce engine load as per maker’s instruction for efficient combustion.
For a 2 stroke crosshead engine with a single constant pressure turbocharger
1. Remove rotor and blank gas casing exhaust gas pass through casing or alternatively rotor is locked in position
2. Blanked off gas inlet and outlet, exhaust gas is routed through an emergency bypass pipe.
3. On MAN-B&W engines to improve air flow to auxiliary blower, remove compensator (bellows) between compressor outlet and air cooler inlet
4. On Sulzer RTA engines, doors on scavenge space are opened to improve air flow to auxiliary blower.
5. Reduce engine load as per maker’s instruction for efficient combustion.
(a) How can you examine the working principles of turbocharger?
(b) Why are lubrication of turbochargers and cooling of scavenge air required?
(a)
(1) Differential pressure meter (manometer) across the T/C suction air filter check for cleanliness of filter
(2) Differential pressure meter (manometer) across the air side of the air cooler check for cleanliness of the air cooler’s air
(3) Air cooler performance monitored by temperature difference across air side and water side of the cooler.
(4) Scavenge air pressure monitored by a pressure sensor in scavenge air trunk.
(5) Pressure sensors fitted in manifolds and after T/C to monitor exhaust gas pressure.
(6) Temperature Sensors monitor each cylinder exhaust gas temperature and T/C inlet and outlet exhaust gas temperatures. Energy used to drive compressor temperature differential to determine. Difference between compressor and turbine energy indication of turbocharger efficiency.
(7) Tachometers to indicate turbocharger speed. used to assess turbocharger performance.
(b)
Lubrication of Turbocharger
- Turbocharger’s rotor rotates at high speed, due to friction large amounts of heat are generated in bearings
- Friction caused by fluid friction as experienced in sleeve bearings, or rolling friction as experienced in ball and roller bearings.
- Lubricant supplied serves both as a coolant and a lubricant.
- In sleeve bearing, generally larger amounts of lubricant are required in sleeve bearings.
Cooling of Scavenge Air
- During compression of air at turbo-blower, increase temperature by some 40˚C with a corresponding reduction in density.
- Air coolers fitted on turbo-charged engines to cool the air after turbo-blower.
- Cooling air reduces both temperature and volume, and increases density.
- Increase scavenge efficiency and entered large amount of air into engine.
- More fuel burned and increase in engine power.
- Colder air also cools the internal parts of the cylinder more effectively during scavenge period.
- Engine to maintain at safe working temperature and lower compression temperature reduces stress on piston rings, piston and liner.
