Adding Diesel to gasoline

[Odin]

Originally Posted by FourBinLabs

How can diesel fuel have a lower octane rating then regular gasoline then?

I hope I'm not pestering anyone, I really am trying to wrap my head around this.

Diesel burns a lot differently than gas does. Diesel is supposed to ignite with just compression heat thats how a diesel engine works, the fuel is sprayed in and it ignites from heat of compression gasoline has to be able to withstand the compression stroke and to be able to wait to be ignited by the spark thus you do not want it to ignite from compression there-go higher octane for gas lower for diesel

at least thats what I've been told

[FourBinLabs]

so... (high auto ignition temperature) ≠ (detonation resistance)?

[NachtRitter]

I read some of this: Gasoline FAQ - Part 3 of 4... very interesting.

What I gather out of that is the following: The octane rating is related to how "controlled" the burn is of the measured fuel. Basically, it is a measure of

the ability of the unburnt end gases to spontaneously ignite under the specified test conditions

So even though diesel fuel may have a higher auto ignition temperature, that doesn't matter... it's what the diesel fuel does once it has been ignited that matters. If there's a single ignition source (as I believe the Octane Rating test setup specifies) and the molecular structure of the fuel cannot prevent the unburnt end gases from spontaneously igniting, then it gets a lower octane rating. The rating is based on the extremes of behavior between "normal heptane" at the low end and "iso-octane" at the high end.

What Odin mentions makes sense then... once the diesel fuel ignites, it is an "uncontrolled" explosion, and the Diesel engine is designed to handle that... injecting the fuel at the top of the stroke rather than earlier in the compression cycle. On the other hand, the gasoline engine *must* have a controlled explosion, with no secondary wave fronts caused by the preliminary detonation of the unburnt gases... otherwise the engine will eventually destroy itself.

[darcane]

Originally Posted by FourBinLabs

How can diesel fuel have a lower octane rating then regular gasoline then?

I hope I'm not pestering anyone, I really am trying to wrap my head around this.

The KEY thing to remember is that a Diesel engine operates differently than gas. In a Diesel, the only substance being compressed is air. As the air is compressed, there is nothing to auto-ignite. Ignition curves and timing are meaningless to Diesels as there is no spark.

Cetane Number rates the ignition delay, and the higher the number the less delay there is. You WANT the fuel in a Diesel engine to auto-ignite, so that's why the scale is set up so high numbers indicate a fuel that is better at autoignition.

Octane Number rates the resistance to autoignition. In an Otto cycle engine (aka gasoline, or spark-ignition), you DON'T WANT the fuel to autoignite since the fuel and air are mixed before compression starts. For this reason, a higher number indicates better resistance to autoignition.

Any fuel that has a high Cetane Number MUST have a low Octane Number.
Any fuel that has a low Cetane Number MUST have a high Octane Number.

[FourBinLabs]

I appreciate all the answers I'm seeing so far. I think I may have rambled too much in my first post (apologies). I was somewhat collecting my thoughts as I was writing. My confusion here, is mainly on the differences in fuel rather than the differences of how both engine types operate.

Hypothetical: Let's say we have two imaginary engines, both are identical. These engines are built like a diesel, being that they have high compression ratios and no spark plugs. The only non-diesel aspect is how the fuel is injected. The fuel is injected on the intake stroke instead. One engine has 87 octane gasoline, the other has d1 diesel fuel.

During the compression stroke pressure builds, creating heat. Which fuel is going to ignite earlier in the compression stroke?

My instinct would be gasoline, being that it will auto ignite at 495°F. The diesel fuel, on the other hand, would have to continue to be compressed until it reach around 600°F.

[gone-ot]

...look-up the definition difference between "detonation" and "combustion"

...the gasoline "detonates" and the diesel fuel "combustes."

[FourBinLabs]

Originally Posted by Old Tele man

...look-up the definition difference between "detonation" and "combustion"

...the gasoline "detonates" and the diesel fuel "combustes."

ok, bear with me here...

Originally Posted by Wikipedia

Definition of Auto ignition: Spontaneous combustion is a type of combustion which occurs without an external ignition source.

Originally Posted by FourBinLabs

about.com defines detonation as: "Definition: An unwanted explosion of the air/fuel mixture in the combustion chamber caused by excess heat and compression, advanced timing, or an overly lean mixture."

It seems like Detonation would be considered a subclass of Auto Detonation because it is not caused by an external ignition source. As far as I can tell, detonation and auto ignition are the same, except that detonation is unwanted. Or is it a different type of burn entirely?

Originally Posted by Odin

Diesel is supposed to detonate thats how a diesel engine works

Originally Posted by NachtRitter

What Odin mentions makes sense then... once the diesel fuel ignites, it is an "uncontrolled" explosion, and the Diesel engine is designed to handle that... injecting the fuel at the top of the stroke rather than earlier in the compression cycle. On the other hand, the gasoline engine *must* have a controlled explosion, with no secondary wave fronts caused by the preliminary detonation of the unburnt gases... otherwise the engine will eventually destroy itself.

If once diesel fuel ignites, it is an uncontrolled explosion, that sounds like detonation. If when gasoline ignites, it's a controlled explosion, then does this mean that gasoline burns slower than diesel?

Old Tele man, if diesel fuel combusts rather than detonates, this would mean that it has an infinity high resistance to detonation, giving it an infinity high octane rating.

The more I think about this, the more confusing it seems.

[Odin]

diesel isn't "detonating" by definition i used the wrong word i should have said autoignition like darcane said diesel is igniting spontaneously but is a controlled burn just like gasoline diesel is just being ignited by heat of comp not spark- gas burns quicker too

one thing i think you are forgetting diesel is injected at very very very high pressure which is one of the reasons it burns the way it does (in my benz its 5500psi) it only gives off a clean and controlled burn under these pressures because its atomized into a super fine mist if you just shot it in with the intake you would not get the same results

[dcb]

preignition/autoignition is a problem with gas engines as the piston is squeezing the fuel/air on the compression stroke car.

In a diesel, the fuel is not added to the cylinder until you want it to burn, around top dead center, and it is squirted directly into the combustion chamber with the valves closed and the heat/air concentrated so it will ignite. This arrangement allows much higher compression ratios than spark ignition engines that add fuel to the airstream and compress it with the pistons before igniting it.

http://www.youtube.com/watch?v=8Bih1...eature=related

[Lazarus]

Antidotal evidence.

My son mistakenly filled up a 1999 Mazda 626 with diesel. It went about ten feet and died. We empty the tank as much as we could. Probably had about 2-4 gallons left in it. Filled it up with 93 octane. When it did start it killed every mosquito in 5 counties. Took about 6 tanks of gas to get it running right. Hard starting and smoked. Had to replace one O2 sensor in about 3 weeks whether due to diesel or mileage can't say. I would not try this but of course your mileage may very.