IMO requirements for emissions are forcing engine manufacturers to lower emissions to almost impossible levels. The method used to reduce emissions, have created huge problems for DE propulsion.
One of the methods introduced in the marine engines is the “FUEL LIMITER”.
The fuel limiting function is basically used to hold back the fuel injection during a sudden load increase. This has always been the critical stage, because when a large load is applied to the engine the RPM will drop, which leads to an increased fuel injection , which itself leads to unburned fuel and increased particles. I’m assuming you have all seen a vessel maneuvering and blowing out black clouds from the funnel; this is what causes those clouds.
The same clouds can be observed when an older generator engine starts up, as these models always would start with the fuel rod in a 110% position, and keep it in this position until the RPM reaches the nominal RPM. During this period, the amount of fuel injected is way too high compared to the temperatures in the cylinder and the charging air pressure, resulting in large amounts of unburned fuel and black exhaust clouds.
This situation also occurs when a large load is applied to an engine running on constant speed, like a generator engine, or a variable pitch propulsion engine in fixed speed mode.
The RPM regulator has one simple task: to control the fuel rod (controlling the fuel injection) in order to keep a stable RPM.
When a sudden load increase is applied to the engine, it causes a RPM drop, which again is detected by the regulator. The regulator will then increase the fuel flow, in order to bring back the RPM to the nominal value.
This in itself is fine, but as standard regulator are “programmed” to directly increase the fuel just based on the RPM deviation from the nominal speed, this will result in a 110% injection when a large load is applied. In a diesel electric system, it is crucial that the RPM do not drop more than 4-6%, as the frequency in the system will drop accordingly. And if the Frequency drops below 56-54Hz in a 60Hz system, it will result in a blackout or propulsion loss (due to VFD trip).
The downside of keeping the frequency with a high gain regulator, is increased emissions and particles whenever the regulator is “doing its job”.
Emissions reduced by fuel limiting methods
So what remedies were made by the engine manufacturers, when the requirements for lower emissions and invisible smoke were raised during the nineties?
Yes, of course, they just implemented a “Fuel Limiter”, whose only task was to limit the amount of fuel the regulator were allowed to give compared to the charge air pressure.
This worked just fine, the only consequence was that the engine would increase the RPM much slower when it started, and would not be able to take the same load steps (load increase) as earlier.
For a propulsion engine this was perfect, and remember, the vast majority of marine engines were running as propulsion engines. The diesel electric revolution had just begun, and nobody seemed to understand the fundamental differences in driving a propulsion shaft and driving a generator. A propulsion shaft does not mind if the RPM drops when load is applied, but for a generator this leads to a frequency droop, and a certain blackout.
Reduced load step requirements for DE engines
For many years, the classification societies had strict rules for how fast a generator engine was supposed to take a load increase. The requirement was that it should be able to take the total load in two steps, without dropping the RPM below a certain level.
But when the engines (with this new sophisticated fuel limiting system) were tested, they failed the test… The new engines were not suited for generator engine!
But as this created a huge problem – and as the classification societies are “free” to please their customers - they just changed this rule, saying that engines had to “take the load in 3 load steps” instead of 2 (33 % steps instead of 50%).
Conveniently enough, now these “unsuited” engines were suddenly suited after al!!
Failure to maintain the RPM is crucial in a DE system
But how does this limited load increase create problems in a diesel electric system?
This graph shows a limited fuel injection. With the same load increase, the engine will not be able to maintain the frequency, as a result of to low amount of fuel injected. The consequence of this is a dropping RPM(Hz) resulting in a blackout.
The consequence for the propulsion engine is that the thrust is a bit delayed, which the navigators have learned to master.
BUT in a diesel electric system, the same RPM drop will create a BLACKOUT on the vessel.
The concern here is that most of the engine manufacturers do not have the full picture of a diesel electric system, they only care about their own engine and do not focus on the complete system operation at all.
Who is responsible for creating a stabile DE system?
The responsibility for a stabile DE operation has been forced on the frequency drive suppliers or the Power Management System, which are given an almost impossible task.
The DE players have still not been able to coordinate the equipment and control methods, due to the complexity of the situation where all parts of the complete system would have to work together for a smooth operation.
This is not the case today, as most of the suppliers are delivering a component, and not a complete system.
Who should be responsible for controlling the load increase required in order to maintain the low emission requirement?
The huge misunderstanding today, is that this load limitation is put in the diesel engines, where the function creates a blackout if the load is increased faster than the fuel limiting system permits and what the engine can take.
For a DE system, all engines should be adjusted to give whatever fuel is required to maintain the RPM, even if this leads to increased emissions.
The load control should be put where it belongs: ON THE CONSUMER SIDE
The problem is that there is NO cooperation between the different power consumers on the vessel. Several attempts have been made in order to control the load, but so far none is working optimal. The introduction of load control systems has created poor performances, with some approaches being directly catastrophic for the DE system, and creating blackouts if certain incidents occur.
What failures have been made, and how to correct them, will be discussed in my next blog article.
So, stay tuned, and please leave your opinion in the comment section if you do not agree or have some additional information or corrections.