What can cause low Pmax on engine cylinder unit?

I believe that, not only once, you have noticed during engine performance check that the engine is running unbalanced with one unit running at low Pmax while others, depending on engine firing order, are trying to balance and compensate the low Pmax on that particular unit.

Power cards on Sulzer RTA engine

As you may be aware, the first instinct is to think about a faulty fuel valve on that unit and even though you might be probably right, that low Pmax can have multiple causes. The causes which can be found are as follow:

  • fuel pressure too low – this can be due defective fuel injection pump or pump fuel starvation (fuel inlet line blockage, leaking inlet pipe, cone connection over-tightening etc).
  • leaking suction/exhaust valve on four stroke engines or exhaust valve on two stroke engines – It is very important to check and compare the compression pressure Pcomp between cylinders in order to eliminate eliminate this cause and make further assumptions (the Pcomp must be nearly the same with small variations between cylinders). On the four stroke engines if there is a leaking suction valve, you can check it by comparing the inlet air temperature among units (the hotter the inlet the highest probability of a leaking valve). If there is an exhaust valve leaking (for both four and second stroke engines), this can be seen through very high exhaust temperature and low Pcomp and Pmax as the combustion takes place into the exhaust.
  • faulty fuel valve/s – as specified at the beginning this is the most common fault. In this case, the fuel injector valve need to be removed, checked, pressure tested and replaced as found necessary.
Fuel injector valves
  • wrong fuel injection timing – timing need to be checked and re-adjusted as found necessary in accordance with the engine shop trial adjustments. On the four stroke engines, usually there is a shim inserted between plunger and roller tappet of a certain thickness who’s initial dimension can be found into the engine technical file. Several times it happens that the shim found to be of a totally wrong dimension due unavailability or engineer’s in charge lack of knowledge who tries to compensate the wrong timing through fuel pump’s rack adjustment.
  • fuel pump rack adjustment – as I mentioned above this is the most common mistake found on the engines when engineers are trying to compensate the Pmax differences between cylinders without further investigations. This is done due either lack of knowledge or laziness.
  • defective fuel pump suction valve – need to be opened and inspected. Usually the spring found to be broken, thus stopping or restricting the valve stroke.
  • very poor fuel with bad ignition quality – this usually affects all cylinders.
  • piston rings blow-by – as mentioned at the beginning this can be observed if you check the Pcomp and compare it with all other units. Another sign will be high scavenge (under piston) temperature on two stroke engines or an increased sump tank pressure on the four stroke engines.
Piston scuffing and blow by marks
Cylinder liner blow by marks
  • leaking fuel injection pump safety valve – encountered few times on RTA engines. Usually the opening pressure of fuel pump safety valve is 1100 bar and found it opening at 300 bar, thus leading to fuel starvation and implicitly low Pmax.
Sulzer RTA fuel pump
  • leaking fuel injection pump spill valve – because of this pump is not able to deliver the fuel at the required pressure for proper fuel valve functioning and spray pattern.
  • wrong valve clearance
  • rail valves malfunction on common rail electronic controlled engines.

It might be few some others causes that can lead to low Pmax on the engine cylinder unit and if you encountered as such you are welcome to post it on the comments below. Any additional shared information it will be of a great help for everyone for future troubleshooting if such problem occurs.

In conclusion, in order to run the engine efficiently and to avoid engine breakdowns and damage, proper maintenance and periodical checks must be carried out as per your company and manufacturer’s maintenance plan and instructions. Periodical performance check must be carried out in order to asses and monitor the engine condition. Moreover, every engineer must be familiar with proper monitoring, how to take engine performance cards and how to interpret them as they offer valuable information about engine running condition.

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