What is the engine load profile ? An answer with data analysis from yacht generators

Unlike mechanical structures, over-sizing diesel engine doesn’t lead to the safe side. It is going the other way around: oversized Diesel engines have bad combustion that leads to wear; unburned at the exhaust and loss of performance. This phenomena often occurs on diesel marine generators engines.

How can you know if your engine is oversized ?

NPS Yachting has access to the ultimate data : the “load profile”.

A simple analogy in two steps can explain this data: let’s imagine you ride a bicycling and you target a constant speed of 15km per hour

• On a flat road; you apply a normal “load” on pedals to keep 15km.h-1 on your speedometer.
• If you climb a hill, you need more energy to maintain the speed. You add “load” on pedals. When the slope increases; you have to increase the load to maintain 15km.h-1.

• If you climb a mountain; you apply your max possible load to keep 15km.h-1 on your speedometer.

Your muscles are providing power through the load applied on pedals. A marine genset is very similar to a bicycle ride at 15km per hour.

• A genset provides electrical power from the alternator at preset voltage and frequency.
• Constant frequency is obtained with constant rotation speed of the alternator.
• The alternator is directly driven by the Diesel engine.

For 50Hz; the engines generally maintain a speed of 1500 rotations per minute:

• The engine is similar to you: it provides mechanical power.
• The alternator is similar to the bicycle; except it provides electrical power.

A marine genset is not climbing hills nor mountains. So, where does the “load” come from?

Electrical devices on yacht need electrical power to run:

Power (P) = Voltage (U) x Intensity (I)

The electrical power (P) is going up and down through the Intensity (I). The Diesel engine reacts with mechanical power to maintain the rotational speed:

• It injects more Diesel to add ‘load’ on pistons when electrical load (intensity) increases.
• It injects less Diesel to decrease ‘load’ on pistons when electrical load (intensity) decreases.

That’s it about the load. Let’s continue with the “’profile” analogy :

Thanks to new technologies; you can follow your body stress during a bicycle ride. A smart watch measures your bpm and/or VO2Max. It stores and transfers data on your smartphone for post-processing on a user interface with diagrams and charts.

You can plot the time spent in different bpm ranges during your ride. It gives you a “profile” of your body stress which is linked to the load applied on pedals. You can store, access and process data on your smartphone or PC.

So; does the engine wear a smart watch ?

Yes; it is the Engine Control Unit (ECU). The ECU is gathering data from several sensors like a smartwatch. It processes and stores the data.

On the image above you can see the fuel high pressure pump on the left. It is linked to the Common Rail at bottom. Injectors on the top are fed by the common rail and wired to the ECU (top right).

The ECU computes the engine load and generates records. We can read and plot the load profile using a laptop and the corresponding software.

It? gives a true view of the yacht electrical needs: it is the amount of mechanical power used to generate electrical power.

Let’s go down the rabbit hole and check some engine load profile.

Load profile from 100kW genset with more than 10?000 running hours

• Y axis is the percentage of total running hours
• X axis is the percentage of the nominal power with 10% steps
• The columns represent the cumulative time spent at a given range in %

• The engine is never above 30% of load
• The engine spent half of its life at 20% load
• The engine spent one third of its life at 10% load

Engine manufacturers mostly agreed that generators have to run with at least 30% of load. 40% of minimal load is also a common value in literature.

This engine is oversized and will get issues:

• Glazing the liners (increase of oil consumption).
• Unburned and smoke at the exhaust.

We can repair the engine but it will not change the root cause of the problem. This yacht is a perfect candidate for a genset downsizing.

Average analysis on 15 yachts between 30 meters and 60 meters

• The gensets spent more than half of their hours below 30% of load.
• There are almost no hours above 60% of load.

Important point to keep in mind:

• Yachts are equipped with at least two gensets.
• Gensets can “add” their power thanks to synchronizing and power sharing.
• Coupling starts at a pre-defined load threshold generally around 70%.

It could explain why load is never above 70%. If the Power Management System (PMS) is starting the sharing too early: engines will never get high load. That is why load profile outcome has to be discussed with chief engineers.

A second important point: what is the upper load threshold ?

?John Deere marine genset engines rating conforms ISO 8528 prime power: Average load over a 24-hour period shall not exceed 67% of the prime rating, of which no more than 2 hours are between 100% and 110% of the prime rating.

The average load from the chart above is 30%. There is more than a 2 x factor with the ISO rule.

According to these data and our experience, we can state that several yachts have oversized genset.

What can we do?

Yachting is conservative and oversizing is also part of design DNA:

• First of all, verify the load profile prior to any major decision linked to gensets.
• Secondly, break free from the oversizing habit and go for downsizing

A good downsizing has several advantages:????????

• Higher loads is better for engine life
• Optimized investment
• Decrease Total Cost of Ownership TCO
• Decrease smoke and unburned at exhaust
• More space for access = easy maintenance

Author: Olivier Krafft - NPS Yachting Business Manager