Regular engine performance test helps in evaluating the current condition of engine and optimizing engine performance. Performance test of engines will also help in identifying drop in its performance due to any deteriorating condition related to exhaust valve, fuel injector etc. If possible, the engine cylinder balance must be checked as often as possible. Measurements must be carried out at stable power and speed conditions in the load range between 75% and 85% of maximum continuous rating (MCR).
There are many factors which affect the performance of propulsion or power generation engines. The most obvious ones are the ambient conditions and the fuel quality. This means that the actual performance of the engine installed on the vessel will deviate greatly from shop trial values. Those deviations will have different characteristics depending on the design. That’s why it is very important to apply ISO correction in order to get a better comparison of data measured onboard under current conditions with shop trial data.
With regard to ambient conditions, combustion engines are among the most resilient technologies to ambient environment changes. During operation, the combustion pressures should be kept as high as possible due to fuel efficiency. The level of compression pressure (Pcomp) and maximum pressure (Pmax) depends on TC inlet temperature and barometric pressure. Nonetheless, at very high temperatures, the performance get affected. 10 °C higher inlet temp gives approximately 2 bar lower Pmax. A Pmax deviation of 3 bar corresponds to approximately 1 g/kWh or 0.5% of the specific fuel consumption.
Humidity also affects the performance, as the higher the relative humidity, the lower the temperature at which the engine performance starts to deteriorate.
As with regard to fuel quality, combustion engines can tolerate a wide variety of fuel quality and properties, but there are also some limitations and Pmax depends on the fuel quality (e.g. ignition quality and rate of heat release).
It is important to note that the difference Pmax – Pcomp, i.e. the pressure rise during combustion, must under no circumstances exceed 35 bar.
The whole idea of optimizing engine performance and balance the engine cylinders is to achieve the best possible SFOC by adjusting fuel injection as close as possible to piston top dead center (TDC). In order to achieve an optimal SFOC (specific fuel oil consumption) in g/kWh there are some adjustments that they are common for marine engines. The optimal SFOC is the maximum energy that we can “squeeze” from a gram of fuel in order to get a kWh of mechanical energy. The most common adjustments are related to fuel injection timing of the entire engine or for a particular cylinder unit if required. Fuel injection timing can be adjusted by advancing or delaying the fuel injection. This means that fuel is injected at the correct time to ensure that is starts burning just as the piston passes the top of its stroke and begins moving downwards during the power stroke. If injection takes place too soon it will result a very high cylinder pressure that will cause overload on engine bearings and possible damage. If injection is too late the fuel may still be burning when exhaust commences resulting in high exhaust gases temperature and loss of power. Correct timing depends upon the ignition quality of the fuel.
In case of marine propulsion engine, it has the FQS (Fuel quality setting) function which is the manual offset for the injection timing due to the variation in fuel ignition quality. This is a factor of the fuel and not the engine.
Modern propulsion engines are equipped with VIT (variable injection timing) function which will advance or retard fuel injection according to engine load for optimized fuel consumption and NOx emission.
Similarly, on new common rail electronic controlled engines even the exhaust valve closing and opening can be adjusted. Variable exhaust valve closing is used to control cylinder compression pressure and so keep the firing ratio (Pmax/Pcomp) within the permitted range during advanced injection. Variable exhaust opening keeps the exhaust gas pressure blow back constant by earlier valve opening at higher speed and this will help improve fuel economy and reduces deposits on the piston underside.
By taking the indicator diagrams allow the engineer to asses whether the correct timing is being achieved.
There are few general guidelines related to engine performance checks:
- maintaining Pcomp and Pmax close to shop trial model values will result in best engine performance.
- 1 bar increase in Pmax will approximately reduces SFOC by 0.30gm/kwh.
- 10 deg Celsius decrease in scavenge temperature will cause a decrease of 1-1.5 g/kwh in SFOC.
- mechanical efficiency of main engine (Ratio of Brake horse power to Indicated Power) is normally expected to be in the range of 86% to 94%. Any result out of this range needs investigation and under no circumstance it can be above 100%.
- 10 °C higher inlet temp gives approximately 2 bar lower Pmax.
- a Pmax deviation of 3 bar from average corresponds to approximately 1 g/kWh or 0.5% of the specific fuel consumption.
- correct power reporting from the torsion meter has a crucial role in the calculation of the ME SFOC, hence it is of utmost importance that vessels report the integrated average power for the reported period.
The procedure of taking engine performance is, generally, as follow:
- personal safety equipment need to be used (heat resistant gloves, face shied, face mask, helmet)
- engine must must run at the higher possible load, preferably above 70 % for proper and conclusive performance reading.
- engine must be at nominal running temperature and at a steady load. In case of main engine, engineer should liaise with navigational officer in order to ensure that there is calm sea and good weather and no rpm or vessel course alteration takes place during test period. In case of auxiliary engine, engineer should ensure that no large consumer is started during test period and generator runs at constant load during test period.
- monitoring and measurement instruments must be calibrated and in good working condition.
- if manual pressure reading must be taken, the indicator cock must be open for short period in order to remove any soot accumulation and prevent clogging of the measuring equipment.
- plug in the measuring device to the engine tachometer connector for engine rpm reading. There are different types of measuring equipment, but working principle is the same (Diesel Doctor, Leutert, Bosch etc.)
- after purging, install the measuring head on the indicator cock. Make sure that is properly and tightly installed as there is a risk of injury due possible exhaust gases burst.
- fully open indicator cock.
- press start on the measuring device and wait until the measuring equipment is getting warmed up and auto-calibrated. After that the measurement will commence, and for proper and more accurate average measurement a device set up of 10 to 20 strokes are generally preferred. The stroke number are set up into the measuring device prior performance check.
- after few seconds the measured data will appear on the display and same should be saved by pressing save.
- after measurement is saved, close the indicator cock, remove the measuring head and proceed to the next unit.
- after completion of all units some additional data need to be taken like: cylinder’s exhaust gas temperatures, temperature before and after turbochargers, piston cooling oil temperatures, scavenge air pressure and temperature, cooling water pressure and temperature, pressure drop across T/C filters, air coolers and exhaust gas economizer etc.
- all collected data must be used for ISO correction in order to get a better comparison of data measured onboard under current conditions with shop trial data.
- after checking and comparing all data, adjustments can be done if necessary and engine performance re-checked if required.
On modern new engines, there is no need to carry out manual performance checks on main engine as all engine cylinders are connected to a dedicated computer (e.g. Cylmate) and engine performance can be monitored in real time and adjusted with no delay if required. Moreover with the current internet of things (IoT) trend, the engine performance can be transmitted, visualized and altered from the company office in real time as well.
In conclusion, engine performance test is one of the most important engine checks and must be duly and diligently carried out as it helps in evaluating the current condition of engine and optimizing its performance. Although this task is specific mainly for a senior engineer, every engineer must be familiar with the above described procedure and must be able to asses, adjust and intervene if required.
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