This topic might be rather questionable to most; Why would gas bubbles and diesel engines be brought up in the same context? Is it possible for something as insignificant as a tiny gas bubble to cause a failure of the entire engine?

Sometime in 2015, I received a call from a “friend” of mine who works on engine remote control systems, saying: “Mr. Ma, can you tell me, what can cause an engine to start up and operate normally while in the Ahead direction, but fail to start when operating in the Astern direction?” I was momentarily shocked and was unable to give an immediate answer to such a high difficulty question and came to understand that the engine in question was the 6S35MC-C and that the ship was currently stationed at anchorage in Chennai, India. After berthing at the dock normally, this unique problem surfaced. Thus, this “friend” of mine came over to our company and took several pneumatic valves and repair kits for the remote control system and then promptly packed up and flew off to India. You may have noticed at this point that I’ve been referring to him as a “friend” in inverted commas, this is because later on this “friend” conveniently managed to lose the invoice to these tools and has not settled this payment to this day, but that would be a topic for another day.

Fast forward more than a month later, I received a call from an unknown number, and the person on the other side sounded defeated and exhausted, with a sense of helplessness in his voice. He claimed to be the manager of a certain ship that was facing major difficulties in firing up the main engine. He lamented that they had done a whole round of maintenance, including repairs or replacements for the engine’s remote control systems, fuel pumps, fuel injectors and roller guide sleeves, even going as far as to overhaul every single cylinder to do thorough inspections, but to no avail. Even the timing charts were checked and displayed no issues. At this point, I interjected, saying, “Oh my, this ship’s been stuck for more than a month now if I’m not wrong?” He was taken aback by my comment, asking me how I knew about his ship and her condition. Thus, I explained how I took a call from a “friend” of mine a month ago about this situation. It turns out that their mainframe repair work was done by another friend of mine, and had purchased many parts and equipment from our company for this ship. This same friend had then given him my phone number. Since everyone was friends with each other, I agreed to come over to India and take a look at this problem on board the ship.

After a day of doing various paperwork to secure an Indian Visa, I boarded an overnight flight to Chennai on the next day then spent most of that day settling the various procedures I had to go through in order to get on board the ship. Once onboard, it was already 4pm in the afternoon, the ship was anchored, ready for testing, all conditions and figures looked normal. So we started the engine in the forward position and everything looked nominal, then we tried starting it in the reverse position and alas, it had failed to start.

Since all the important work to the main engine had been done before, I reasoned that the issue had to be coming from outside the main engine. I have actually had some experience in the past with bubble formation within the fuel oil causing some cylinders to not undergo normal combustion during operation, so my attention was immediately focused on the fuel oil. I noticed that the engine was, at the time, operating on Heavy fuel, and was entering the cylinders at 120 degrees Celsius. I then asked the Chief Engineer about whether or not he had tried switching to diesel fuel, and his reply was; “Yes we’ve tried, but it gave the same results.” This answer lent some doubt to my hypothesis since gas bubbles were less likely to form within a diesel fuel, given the fact that you did not have to heat up. But I did not give up on my hypothesis, and thus went straight to the fuel tank. I started draining fuel from the top of the residual valve at the fuel inlet filter, and immediately noticed some interesting findings: firstly, the fuel level rose extremely quickly. This seemed odd to me; the flow rate for such a small tube for residual fuel should not be this quick. Moreover, when shutting off the residual valve for a while (just around a minute), the fuel surface position dropped significantly. This, therefore, made me think; why would the engine give the same result when using diesel? Is it possible for the diesel to also develop gas bubbles? If that were the case, it would have come from a faulty seal in the circulating pump, which did not seem very likely. Thus, after checking with the Chief Engineer, I managed to confirm my suspicions: after changing fuel oil to diesel, they did not open the valve on the return line (which would let the heavy fuel in the pipes return to the daily tank). In this case, it would have meant that the engine would have still been consuming the heavy fuel left in the pipes; which would obviously have resulted in the same outcome.

Since it was getting late, the Superintendent intended to call it a day and run the test tomorrow. Before leaving, I insisted on testing the engine with diesel right there and then. He agreed, and after properly changing the fuel and ensuring no bubbles were observed, the engine was fired up and operated nominally in both directions. Given this promising result, we agreed to go for a test trial at sea tomorrow.

After successfully diagnosing the issue, as well as having spent the entire day working/travelling non-stop, I had an amazing sleep that night.

Early the next day, the ship started trials and functioned normally, with no issues even when going full speed. I asked the Chief engineer to switch back to the heavy fuel, and around half an hour later, the revs started going lower, and everything seemed to revert back to the troubled state, confirmed by the observation of gas bubbles showing up in the residual valve. So, where did these bubbles come from?

Following the fuel oil pipelines we came to the fuel processing unit, and to my surprise discovered that the fuel oil was at 120 degrees Celsius in both the inlet and the outlet of the fuel oil heater. While I initially suspected a temperature probe malfunction, the Chief Engineer divulged that the heating element had stopped working, and so they engineered a workaround by using a steam heating coil in the fuel tank to help heat the heavy fuel to 120 degrees Celsius. The venting line of the fuel oil system was positioned in a metal cabinet above the chimney, and at one point the entire pipeline was blocked by solidified heavy fuel. They tried to dislodge this blockage by blowing compressed air from below or even adding diesel from above but to no avail, and so, in the end, decided to insert a temporary gas release tube.

Even though the gas release tube supposedly fixed the problem, there was still has in the fuel within the cylinders of the engine, causing the revs to be severely limited. To find the origin of the gas bubbles, we decided to purge the oil in the daily tank and open the manhole cover for inspection, and discovered the fuel tank to be full of gasses, but found no leaks from the steam coil after pressure testing.

Even then, the issue became clear: the problems with bad combustion in the cylinders was due to there being air in the fuel oil, which occupied the injection space but did not contribute to combustion value. The reason for the presence of air in the fuel oil was due to the fact that they were heating up the fuel oil to 120 degrees Celsius without pressurizing the fuel. Normal heating of heavy fuel oil in the fuel tank should not exceed 100 degrees Celsius and any heating beyond that temperature requires a pressurization of the heavy fuel to 7 Bar to avoid fuel vaporization, and so this problem was identified and rectified within 2 days.

This was not the first time I have encountered such an issue involving air bubbles found in fuel oil, back in 2005 there was a Greek registered ship in China undergoing dock repairs (replacement of fuel oil pump and suction valve). After those repairs, the main engine was unable to get back up to normal revs and had to cruise on low revs from Hong Kong to Singapore. I originally thought I was not going to be able to make it back to Singapore in time to get on board this ship and take a look because I had just taken on another main engine repair job in Thailand, however, I did manage to make it back just in time due to the delayed vessel arriving as low speed the ship was travelling at. (I even recalled having to hail a cab from the airport just as I landed to go straight to the Clifford Pier to get onboard.) Back then I remembered that after letting out oil from the fuel inlet filter, the oil barrel filled up instantly. Since the gas release tube was only 8mm in diameter, the rise in the surface of the fuel oil in the barrel was completely not in line with the size of the venting valve, and after a few minutes of settling, the amount of actual fuel in the barrel did not even reach 1/3 the height of the barrel. Unfortunately, since the shipowner was short on funds, he never let me follow up on the issue after correctly identifying it.