Cetane rating 42? What is this? - PeachParts Mercedes-Benz Forum

dpbrowne · #1 08-18-2003, 10:08 AM

Stopped at Hess the other day here in New Jersey when i saw $1.29 on the pump. Afetr a fill uyp I noticed on the pump the Cetane rating was 42. Isn't this too low. I notice some nailing at low speeds. Other stations have no ratings posted at all. Does anyone know what is the best deisel fuel to use?

#2 08-18-2003, 11:51 AM

Cetane is to diesel fuel what octane is to gasoline. It's a way of measuring its combustion factor (I think- it's beast to do a search on this site for cetane definitions). I've forgotten what it actually means.

To address the second question, I would stay only with the big name brands of gasoline/diesel fuel that are all over the country. Texaco, Shell, Sunoco, Mobil/Exxon, Gulf. Very little risk of water contamination in their fuel. I don't know what to think about Hess. We still have a few of them in New England. Same with Erving etc. I'm not too familiar with the quality of those namebrands. Basically, it's not worth screwing with the quality of fuel to save a dime or so - don't do it.

BoostnBenz · #3 08-18-2003, 02:35 PM

42 is to low, mercedes are designed to run on 45 or higher. Cetane is a number which expresses the resistance to pre-detonation. I wish I knew, every station is different, this is all I know. I tried contacting all the major companies to find just that every station uses their own blends and additives. Hess makes some great premium diesel, but it sounds like the station you went to wasn't the best, and surely wasn't premium diesel. Namebrands don't mean a thing, the traffic filling up is what lowers chances of getter water contamination. Every station varies.

Maxwell · #4 08-18-2003, 02:46 PM

you can use the low cetane stuff if you add some redline, or some vege oil

BoostnBenz · #5 08-18-2003, 03:28 PM

I use redline DFC, it is supposed to boost cetane 3-5pts along with about every other thing you'd need in an additive.

rdanz · #6 08-18-2003, 04:01 PM

Cetane rating of 42 is good most diesel stops on the east cost sell diesel that has 40 cetane which is the minimum allowed here .
Your car will run fine on 40 cetane fuel but will get better milage with a higher number. Try Power Service additive which can be bought at Wal Mart it raises cetane something like 3-5 point and is a good product.

rickjordan · #7 08-19-2003, 01:13 AM

Texaco "premium diesel" is rated at 45 cetane. I use PowerSerivce additive to help bring the cetane level up.

P.E.Haiges · #8 08-19-2003, 07:30 PM

BoosnBenz,

I think you have cetane rating confused with octane rating. There is no predetonation with Diesel fuel, providing all your injectors are OK, because the fuel isn't injected until slightly before TDC.

The higher the cetane rating, the lower the ignition temperature of the fuel which makes it easier to ignite. It is the opposite of octane rating which is resistance to preignition.

At least that's the way I understand it. If I'm misinformed, someone please explain it to me.

P E H

barkford · #9 08-19-2003, 09:19 PM

my 98 e300 has not suffered from 40 cetane diesel. No additional smoke noticeable and no hard starting. However, I do throw in a bottle of Redline about every 3rd fillup.

BoostnBenz · #10 08-19-2003, 09:34 PM

No drinking in this benz PEH.

Is nailing the only form of predetonation then?

I do agree that my defenition wasn't completely right, actually right after writing that I headed over to chevron's site to see what the exact defenition was.

Quote:

Cetane Number is a measure of how readily the fuel starts to burn (autoignites) under diesel engine conditions. A fuel with a high cetane number starts to burn shortly after it is injected into the cylinder; it has a short ignition delay period. Conversely, a fuel with a low cetane number resists autoignition and has a longer ignition delay period.

Chevron's site

Quote:

Increasing the cetane number of the fuel can decrease the amount of knock by shortening the ignition delay. Less fuel has been injected by the time combustion begins and it has had less time to mix with air. As a result, the rapid pressure rise, along with the resulting sound wave, is smaller.

So if you changed your diesel to higher cetane diesel, it will start easier and run quieter. I'd avoid 40 cetane, that is only for gravel trains and semis. That is the lowest legal amount, if your station doesn't want to put the effort into boosting that up a bit perhaps they are skipping some other additives as well.

P.E.Haiges · #11 08-20-2003, 01:24 AM

Boost,

Thats what I said.

The higher the Cetane number, the easier the fuel is to ignite. The higher the octane number, the more resistant the fuel is to self ignite with respect to temperature.

Gasoline and Diesel fuel are just the opposite inside an engine as in the air. In air, even though the octane rating of gasoline is much higher than Diesel fuel, it ignites easier because of its volatility (evaporation) since liquid fuel doesn't burn. Diesel fuel in air is more difficult to ignite because it does not evaporate easily.

But inside an engine, the Diesel fuel ignites easier because of its higher cetane rating. Gasoline resists the heat of compression because of its higher octane rating.

Cetane and octane ratings are the opposite as their numbers increase.

P E H

BoostnBenz · #12 08-20-2003, 12:18 PM

I didn't say you were wrong, I just wanted to post a word for word defenition of it and to elaborate on the effects of higher cetane. I never gave any thought to why they run quieter on higher cetane, now I know. Chevron has a pretty nice site which thoroughly discusses diesel fuels, their traits, and how they are measured.

TimFreeh · #13 08-20-2003, 12:53 PM

My .02 is that there is alot of the placebo effect in play here....

I would be willing to bet if you had a car with 3 different fuel tanks filled with 40,42 and 45 cetane fuel and had a blind selector switch that would feed fuel from each tank you would not be able to tell which fuel you were burning at any given time.

It is certainly not true that 40 cetane fuel should only be used in rock crushers. I've used it for years in all sorts of MB diesel engines - It burns just fine.

Tim

P.E.Haiges · #14 08-20-2003, 10:59 PM

Tim,

I agree with you. I have used all kinds of fuel including 10 year old fuel oil in my MB and other Diesels. I could never see any difference.

I got Diesel fuel from Agway and all they did to call it Diesel fuel was to add some green stuff to fuel oil.

I wonder what the cetane rating of peanut oil is? Or corn oil, canola oil, rapeseed oil, or used crankcase oil.?

P E H

BoostnBenz · #15 08-21-2003, 12:29 AM

I believe motor oil is in the 50-55 vicinity, as for the others I don't know.

I've only found one station which the diesel they sell is noticeably worse, I bought diesel from SpeedyQ a few times and everytime I filled up there the (cold) idle turned to crap! While it isn't a very scientific method I try different stations and see if I can find any difference in fuel mileage over several tanks, or noticeable idling differences. Maybe it is just a coincedence but BP seems to give me the best mileage, and average idle.