Answer EKM Question 44

Q44. A diesel generator when fitted in a machinery space which is periodically unmanned may be equipped with monitoring alarms of the exhaust temperatures. Discuss the relative merits of 

A. Individual cylinder maximum temperature alarms; 

B. Individual cylinder maximum and minimum temperature alarms; 

C. Individual cylinder maximum temperature alarm and an alarm for any two cylinders exhaust temperatures deviating more than 35°C. 

Explain how arrangement can be provided for.

Answer: A. Merits of Individual cylinder maximum temperature alarms. 

Individual max temperature alarms are essential for monitoring of the running generator. 

Any alarm is an Indication of a malfunction within respective unit or on the sensing element itself. similarly high temperature alarm for a particular unit of a generator will give an indication of a malfunction in that particular unit. Reasons for the high temperature alarms are highlighted below. 

• Leaking exhaust valve: A leaky exhaust valve will cause blow-by of exhaust gas during combustion, thus causing an abnormal rise in exhaust gas temperature, causing an alarm. Alarm aids in spotting blow-by on time will otherwise, further damage the valve due to wire drawing effect. ultimately resulting in valve failure. The unit will stop firing due to lack of compression and overload the other units, engine will run unbalanced, raises further problems like bearing failure, breakage of components etc. 
• Faulty fuel Injector: A faulty fuel Injector will drip and cause afterburning, which in the absence of an alarm can burn the exhaust valve and smoky exhaust. 
• Improper or changed tappets: Improper tappet settings can cause the exhaust valve to open earlier allowing blow by of combustion gases. 
• Fuel injection tinting retarded: If the timing of fuel injection is retarded after burning will take place leading to burning of the valve. Although problems related to timing occur gradually and can be Identified with good watch keeping procedures, the high temperature alarm is another tool to identify the problem. 
• Fuel pump rack settings: if the fuel pump rack settings are changed or if the rack is stuck. The unit may run overloaded and if allowed to continue running will damage the engine and its components. 
• All the above conditions can lead to major breakdown and also loss of engine. A high temperature alarm will give an early indication and thus allow for preventive maintenance. It will also give the status of the temperature sensor, as there will be a false alarm in case of sensor failure. 

B. Individual cylinder maximum and minimum temperature alarms. A low exhaust temperature or minimum temperature alarm also indicates malfunctioning in a particular unit. The reason for a low temperature alarm may be due to the unit not firing. For a generator running on full load one unit not firing will have the same consequences as an engine running with a unit cut off. The engine will run unbalanced, the other units will take the load of this unit leading to overloading of the bearings, breakage of components etc. 

Some of the reasons unit not firing could be:

• Exhaust valve breaking: If the exhaust valve of a particular cylinder breaks, it will fall into the cylinder, this piston will hit the valve and there will be loud knocking sound but during urns period the only way to get an indication will be due to a low temperature alarm which will occur due to the unit not firing. 
• No fuel Injection: If the delivery valve of the fuel pump for that unit starts leaking, no fuel will be delivered. Similarly if the high pressure pipe breaks, fuel delivery to the injector will cease or reduce. If the bolt connecting the fuel rack to the pinion shears off the fuel delivery may remain constant irrespective of load change. This problem could lead to either a high temperature alarm or a low temperature alarm depending on load change. 

C. Advance in fuel Injection timing: 

• If the timing is advanced, fuel will be injected earlier causing improper combustion. Although the problems related to fuel injection timing occur gradually, and can be identified with good watch keeping procedures. A low temperature alarm also helps in giving indication. 
• In addition to the above reasons the alarms can also give an indication for common problems, for e.g. a fouled air confer or turbocharger will cause rise in exhaust temperatures thus triggering the high temperature alarms for multiple units. 
• Individual cylinder maximum temperature alarm and an alarm for any two cylinders exhaust temperature deviating more than 35° C. 
• Due to wide load range, between 35 - 85 % depending on the requirement, exhaust temperature will also have a large range during operation. The exhaust temperature alarms will be at a set paint below/above the normal operating temperature. 
• If the generator is operating at low load and for any reason there is a malfunction which causes the exhaust temperature to rise in a particular unit, the alarm may occur after a long time or it may not get activated at all, if the generator continues to run at low load as the exhaust temperature might not go high enough to activate the alarm. 
• The same thing can happen if a generator is operating at high load and due to the some malfunction the exhaust temperature starts to reduce. In this case the low temperature alarm will either come after a long time or it might not get activated at all. 
• In both the above cases if the alarm does  not occur or it comes late the engine will continue in run in this state causing further damage to the components and the engine itself. An exhaust gas deviation alarm is normally set to go off if the temperature difference between 2 or more units is 35° C or whatever the deviation set point is. 

Explain how arrangement can be provided for. 

A diesel engine monitor monitors the exhaust gas temperature of each cylinder. Each cylinder is connected to a thermocouple, The monitor scans the analog signal from the thermocouple which is proportional to the exhaust temperature of the respective cylinder. This signal is digitized by a microcomputer after It has been stored. The microcomputer averages the last temperature stored from each cylinder to form a total moving average for the engine which moves as each engine cylinder exhaust temperature is rescanned. The computer also checks whether the reading is due to an open thermocouple. The computer then compares the latest reading to the previous reading to check if it has moved more than 100°. It then compares the reading with the high temperature set point and low temperature set point. It also compares the reading with the moving average to check for deviation.