Cetane and Power 1

[TugboatEng]

Let's say I have a compression ignition engine and I double the cetane of the fuel it normally runs on. Is it possible that the shorter ignition delay of high cetane fuels may reduce power at lower engine speeds with a fixed injection quantity and timing?

 

[TugboatEng]

I ran two tests in a sample of the fuel that I believed would best explain a loss of power but the calorific value is actually very good compared to what I was expecting. I wish I had some diesel #2 to compare it to.

On a side note, is there any practical reason API gravity is used instead of specific gravity? It seems to unnecessarily complicate many calculations.

 

[Lou Scannon]

With higher cetane the desire to ignite faster, would be similar to advancing the spark in an SI gasoline engine, the early ignition would start applying excessive combustion pressure as the piston approaches TDC,

No. The rate of pressure rise and peak pressure in a diesel are not a direct function of the cetane number.

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

 

[TugboatEng]

I believe I have seen that HVO fuels have both s higher cetane and a slower burn rate.

 

[catserveng]

Tug,

API gravity is used by engine manufacturers for a couple reasons, mainly it's what EMA (Engine Manufacturers Association) agreed upon early on, and when doing corrections for fuel consumption and power output, API gravity directly follows temperature variation, so as temp goes up, so does the API gravity, if you look at fuel correction charts may become clearer.

Here is a link that will explain it better than I do,

https://www.engineeringtoolbox.com/api-gravity-d_1212.html

Early formulations of HVO had lower cetane numbers, and while we could calculate fuel consumption and power loss pretty close, smoke/particulate was harder to figure out. A lot of the field testing done used a Bosch smoke meter (not a really precise instrument but it did give us a benchmark), we did find in several cases that adding a cetane index improver did help reduce smoke, CO and unburned hydrocarbons and slightly increased NOx. I'm not sure but now I'd expect that suppliers of HVO would assure it meets a minimum a cetane rating.

I added a link to a copy of the CAT Marine Handbook, not sure if you have a copy. Even if your engines aren't yellow, the info is helpful for other makes as well.

Hope that helps, MikeL

 

[mfgenggear]

All I know which isn't much lol
When Californian did this back in 80's engines
Where so bad, loss of power and fuel consumption. In trying reduce emissions
Cost fuel economy and loss of power.

 

[catserveng]

I was in SoCal dealing with these issues since 1981, between CARB and the local air boards it got tougher as time went on. BACT determinations didn't always seem to have technical backing behind them, someone would make a claim and that would get pushed down to the users, and a lot of times the end results caused performance and reliability problems. Several of the "new" ideas would decrease one constituent, usually NOx, but cause drastic increases in particulate and CO. CARB finally got SWRI more involved and at least testing and evaluation got done to address some of the issues, but usually political pressure pushed most of the regulations down the users throats.

Biofuels in marine environments had other problems as well, mainly biological growth and oxidation, needing to add additional fuel additives and increased fuel delivery and storage systems maintenance adds significant costs long term for the operators.

Makes me glad I don't deal much with it anymore.

Tug, my contact at SWRI has retired and moved on, I haven't been able to find much additional info as I hoped I would.

MikeL.

 

[romke]

Unlike a gasoline cycle that does not care whether the fuel used has a higher octane number then minimally required, a diesel engine calls for a fuel with a minimum cetane number where quite a bit higher can lead to engine problems. When using fuel with a far to high cetane number the combustion proces may well start at a time that not all the fuel has been thoroughly mixed with the compressed air, what can lead to a rough running and "knocking" engine. It is also possible that, due to the fact that the to high cetane nuber can lead to start of the combustion to close to the injector tips with overheating of the injectors and local carbon buildup on the tips as a result. "More" is not always better in diesel fuel.

 

[Lou Scannon]

When using fuel with a far to high cetane number the combustion proces may well start at a time that not all the fuel has been thoroughly mixed with the compressed air, what can lead to a rough running and "knocking" engine.

That is a complete misconception. I suggest you read up on diesel combustion. Key phrases:
[ol 1]
[li]ignition delay[/li]
[li]Diffusion combustion[/li]
[/ol]


"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

 

[mfgenggear]

Main issue was retarded timing, and closed air pathways. Fuel was not an I issue then. Retarded systems.