Cetane numbers?

[Jerry/MT]

I understand the vague premise of what a cetane number is, vaguely similar to octane number for gas. What I don't understand is how a higher cetane number gives you more power/economy.

In a modern gas engine, higher octane can allow the computer to dial up the ignition a little bit, based on the knock sensors, and give you a little more power and gas mileage. In the old days if you had a higher octane fuel, you always dialed up the intial timing a little bit and you could feel this in the seat of your pants when you drove the car.

In a diesel engine in a tractor, there is no computer, so what does the higher cetane rating mean? Does it just give a little bit better combustion, there by giving you more power and economy?

You can't use a knock sensor (detonation sensor) on an engine that initiates combustion by detonation like a diesel. Diesels are always "knocking'.

 

[Charolais]

Around here diesel and home heating oil are the same thing.

It's the same here too, except for addatives. Our Premium diesel supplier filters and adds addatives to make it a premium fuel. We have one tractor dealer that'll tell you up front on a tractor purchase that you must use a premium fuel if you expect a warranty on their engines. Like another post said, diesel in this country is junk when compared to some other countries. Cetane is one improvement that I can use myself. There are claims that cetane added can get 2% efficiency. I can't testify to that but I will say the exhaust pipe has a different color and all my engines start better in cold weather on the higher cetane. They smoke less too.

When added to heating oil I see less soot build up in the burn chamber on the annual checkup.

 

[20 20]

I've seen it posted at some pumps around here...usually at 40.
That concerns me because that is less than recommended for many engines.

If I'm not mistaken it will say a minimum of 40, I believe it is closer to the 45???

 

[Skyco]

If I'm not mistaken it will say a minimum of 40, I believe it is closer to the 45???

But you just don't know.
I've seen some info where random tests find it as low as 38.
The stickers used to say 45 a few years ago then they changed them to 40.

 

[ModMech]

Very good question to which the answers are complex and many have been either mentioned directly or eluded to elsewhere in the replies.

Cetane is essentially the opposite of Octane. A higher Cetane rating indicates the fuel is MORE easy to ignite (called "autoignition") while a lower cetane rating is more difficult to ignite. So, within limits, higher IS better as it will provide easier cold starts, more accurate timing and a more complete burn (less soot and smoke).

Higher octane ratings indicate a greater RESISTANCE to autoignition and in most cases a slower reaction rate which actually produces lower engine output when used in an engine not designed for it.

In the USA, most diesel fuel is formulated to ASTM9751 (ASTM 975, 975-1, 9751 are often incorrectly used to mean the same standard). The fuel with which every engine manufacturer must certify their products for emissions is NOT the ASTM standard diesel but "test fuel" that is higher in quality than can be purchased bulk anywhere in the USA. The quality gap between ASTM975 and EPA test fuel was allegedly reduced by a large margin with the intorduction of ULSD in late 2006/early 2007 for 2007 USEPA "on-road" emissions. ULSD is to diesel what Unleaded gasoline is to a typical gasoline engine, I prefer to call it "Unsulfured Diesel", even though there is a small amount of sulfur remaining.

Because of the low quality of the "base stock" used in the USA for diesel (as others have correctly illuminated), it is generally true that higher cetane fuels will have lower specific energy per gallon. This is not a feature of diesel fuel, but of the cheap base stocks used for crude in the USA and the amount of "work" it takes to refine it that causes additional cetane rating reductions.

Now, there is Cetane Index and Cetane Number, they are not the same measure. As with gasoline, you have Pump (motor) and Research octane ratings, this is why you see the "R+M/2" on your gas pump. The octane rating displayed represents the AVERAGE of the Research and Motor octane ratings. There is no analogous pump rating system for diesel in the USA, which in my view is criminal. Believe it or not, different engine designs and control strategies respond to research and motor octane in different ways. Some engines perform best and with lowest emissions with high motor and low research octanes and others are the polar opposite. Diesel engines are no different in that respect.

One MAJOR difference is the cetane rating in a diesel has a material impact on emissions. Higher cetane ratings result in lower Particulate matter (PM) and often (but not always) very slightly increased Oxides of Nitrogen (NOx) emissions.

Relatively high cetane ratings result in:
-Accurate injection timing
-More complete combustion
-Lower PM emissions
-Superior cold startability
-Improved "transient" engine response
-Reduced oil contamination
-Often show a very slight positive efficiency benefit

You would be LUCKY to find diesel in most areas during the majority of the year that will actually meet ASTM975 (40 cetane min) unless it has a significant bio fraction because neat (pure) biodiesel is VERY high in cetane.

 

[dodge man]

Thanks for the response Modmech, that was pretty complete.

 

[newbury]

Here's a thread on the TDI forum.
Cetane ratings of fuel companies - TDIClub Forums

and some sample numbers from a thread:
Originally Posted by BleachedBora

BP/Amoco, 51;
Chevron, 49;
PetroCanada, 47-51
Shell, 46;
Sinclair, 46;
Sunoco Gold, 45 (often +1-5) Sunoco regular is usually 40.
Holiday Stations, 40-43
HESS, "not less than 40," average 42, sometimes "up to 45."
Husky, 40 + diesel Max additives raise another 1-3 from there (41-45 max)
Flying J, 40
Love's: 40
Pilot: 40
Valero: 40

Note a lot of sellers list it as 40, because it's at least that.

 

[ModMech]

One addendum...

The USA is nearly the only modern country not to legally mandate a minimum Cetane standard, let alone enforce industry or regulatory standards as is done in the EU, Japan and AU.

Again, you'd be lucky to locate 40 Cetane fuel at the common retail level, no matter the brand.

 

[jlrsn]

That is why I use Stanadyne Performance Formula year around in my F250 PSD and Kubota L39. Both start immediately even in below zero F weather and my F250 PSD in temperatures up to 122 degrees F when fuel is hot and thin. I've never had gelling with my F250 PSD or L39 with temps down to minus 25 degrees F.
Performance Formula - Stanadyne
Jim

 

[jcmseven]

In this case, I have noticed that Power Service claims a 4 point cetane increase for its white packaged treatment and a 4-6 for its gray packaged treatment. I assume this is at the recommended amounts introduced. If one adds, say, twice as much PS gray to cetane 40 diesel, does that get the cetane to 8-10 higher or is it still the 4-6??? Does that make any difference in the way a a diesel would perform, say from a mileage and/or power standpoint to use any of these additives?? In my area, I used to see some "premium" diesel pumps that labeled the cetane at 45, but with the expansion of ULSD I rarely can find diesel over 40 cetane anywhere around here.

John M