Operation of HFO engines in higher & lower loads with distillate fuel

Dear friends,
Could you please reason out why the HFO marine engines (stationary engines used for power generation) changes in to distillate fuel (diesel) at lower loads from HFO.Also why such engines are not recommended to run on diesel at higher loads(it is said, the engine couldn’t take full load with diesel).

Your comments on this are highly appreciated.

Running on HFO at low loads results in rapid fouling of the turbocharger turbine and nozzle ring.

Diesel is about 2x the cost of HFO. So if your vessel is legally allowed to burn HFO, you should switch to that fuel when not in a low load condition.

(corrected): As diesel has less energy per unit volume, if the engine used HFO during its test bed runs, and the fuel pumps were in their maximum delivery positions, then operation on diesel will not yield the same power.

Even taking this into account, if you suspect the output is low then you need to bring the engine to a power matching one of the test bed runs (usually 50% and 75%) and compare all parameters. Particular attention to be paid to Pmax, FO pump rack position, exhaust temps, T/C inlet and outlet temps, and charge air pressure.

In layman terms HFO is nasty stuff, has to be preheated to ignite from the heat of compression. On the other hand it has a lot of BTUs.
Engine designers cannot design an engine to run equally on 2 different fuels so they design to run on the nastiest high BTU fuel first as that’s the most difficult.
This information is availble in most textbooks you have. If not go to the library or avail yourself of many sources of information on the internet.
Sorry for my trite response but I see lazy students accessing gCaptain often


I was always under the impression that HFO had more energy/BTUs than diesel as well. When I was writing my response above, I looked it up to be sure and it turns out that diesel has more BTU/kg than HFO which surprised me.

Yes, I should have noted higher BTU per dollar

Thank you all for your valuable comments.

When inquired from the fore said power plant,their reasons for the particular operation pattern as follows;

1.Changing in to Diesel at lower loads from HFO
XX. jacket water temperature is reducing with the load.(then the expansion of liner diameter reduced)Hence ensure smooth contact between piston and liner surface,more lighter cleaner fuel (diesel) is used
XX. Difficult to maintain HFO viscosity as performance of heating system which is powered by exhaust gas boilers is reduced at lower loads

2.Not running at higher loads with diesel (or change in to HFO at higher loads)
(- other than to economic reasons)

XX As density of diesel is lower than HFO and as calorific value of Diesel is lower than that of HFO,larger volume of diesel is need to burn to generate an equal quantity of energy,which is generated by a considered volume of HFO.Hence injection pump is not capable of supplying adequate quantity(volume) of diesel to generate the maximum load of the engine (which is generated using HFO)

please forward your views on above facts.

By referring internet regarding this i found that due to high temperature at higher loads viscosity of Diesel may dropped causing to increase leakages at injector pumps may be the reason for not recommending diesel for higher loads.What are your opinions on that?

Your comments are really appreciated.

Try and keep your units the same when discussing this sort of thing.


This is a good point as the pumps work on a constant volume concept (for a given rack position).

Assuming 20mL of injection at a certain rack position and assuming the values in the link I previously posted, the below shows the amount of MJ per injection. Even after temperature correcting, the HFO still provides more MJ per injection.

Diesel @36C MJ/kg kg/20mL MJ injected
0.846 0.831 42.6 0.01692 0.720792
HFO @125C MJ/kg kg/20mL MJ injected
0.98 0.904 39 0.0196 0.7644