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  #1  
Old 03-22-2008, 09:40 PM
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Lightbulb And diesel purge works because...

My theory is:

It contains Ethylhexyl Nitrate 25% by volume. This chemical speeds up the auto ignition process in the diesel engine, and increases the cetane number. The engine runs better because of the excellent burning characteristics of the substance. Why an increase in the auto-ignition rate makes for a much quieter engine, I'm not certain.

Here is an excerpt from a study performed by Sandia National laboratories:
"
The effects of an ignition-improving additive,
2-ethylhexyl nitrate, on diesel-spray evaporation,
mixing, ignition, and combustion processes
were investigated. The results indicate that the
primary effect of the additive is to increase the
radical-pool formation very early in the autoignition
period, leading to a shorter overall autoi-
nition period for a diesel spray. This effect is
greatest at lower temperature-density conditions,
corresponding to low-load and start-up conditions
in a diesel engine, and becomes negligible at the
highest temperature-density conditions examined."

Here is the link to the study.

http://www.osti.gov/bridge/product.biblio.jsp?osti_id=2401
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  #2  
Old 03-22-2008, 11:34 PM
babymog's Avatar
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Increasing the burn rate is decreasing the Cetane number, and should lead to more knock.
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  #3  
Old 03-23-2008, 12:03 AM
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Cetane from :http://en.wikipedia.org/wiki/Cetane_number
Quoted from one of the paragraphs: "Cetane number is actually a measure of a fuel's ignition delay; the time period between the start of injection and start of combustion (ignition) of the fuel. In a particular diesel engine, higher cetane fuels will have shorter ignition delay periods than lower cetane fuels. Cetane numbers are only used for the relatively light distillate diesel oils. For heavy (residual) fuel oil two other scales are used CCAI and CII."
A quote from anothe paragraph with me doing the Bolding:"Generally, diesel engines run well with a CN from 40 to 55. Fuels with higher cetane number which have shorter ignition delays provide more time for the fuel combustion process to be completed. Hence, higher speed diesels operate more effectively with higher cetane number fuels."

Cetane is a measure of the ignition delay; but higher cetane fuels have a shorter ignition delay.
I interpret this to mean shorter the ignition delay the less likely it will knock.
I interpert the sentance; "Fuels with higher cetane number which have shorter ignition delays provide more time for the fuel combustion process to be completed." to mean ignition has more total time to burn due to ignition starting sooner than with lower Cetane fuel.
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Last edited by Diesel911; 03-23-2008 at 12:16 AM.
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Old 03-23-2008, 03:53 AM
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so a higher revving diesel like ours would benefit from a higher cetane fuel, whereas a big diesel truck would like low cetaine.., right?
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  #5  
Old 03-23-2008, 06:32 AM
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Then wouldn't DP work better if you just let it idle during the purge? I was told to rev it and hold 3-4K RPM's during a purge. Wouldn't it seem, based on the above that the more time DP spends in the CC the better the cleaning action? Or would the additional heat of higher RPM's change that?
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  #6  
Old 03-23-2008, 04:34 PM
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Quote:
Originally Posted by turbobenz View Post
so a higher revving diesel like ours would benefit from a higher cetane fuel, whereas a big diesel truck would like low cetaine.., right?
The article never defines what they consider a high speed diesel engine to be. Another factor is that most "Big Rigs" have direct injection instead of pre-combustion chambers/swirl chambers. Here is another article:How Does Cetane Number Affect Engine Operation?

http://www.fuelmagic.net/Cetane%20Booster.html#Anchor-Ho-38965
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Last edited by Diesel911; 03-23-2008 at 04:40 PM.
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  #7  
Old 03-23-2008, 04:43 PM
Craig
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Quote:
Originally Posted by turbobenz View Post
so a higher revving diesel like ours would benefit from a higher cetane fuel, whereas a big diesel truck would like low cetaine.., right?
I'm not sure the term "higher speed" is meaningful. The important parameter is the mean piston speed, not the engine rpm. A low rpm engine with a longer stroke may have a similar piston speed to a higher rpm engine.
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Old 03-23-2008, 09:14 PM
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As I understand it, there is a slight delay between the start of fuel being injected into the cylinder, and when it absorbs enough heat to start combusting. The cooling effect that the evaporation of the injected fuel has on the air in the cylinder can actually be measured. Once the fuel starts combusting, the temperature and pressure rapidly escalate, speeding the start of combustion of the rest of the fuel, including that which has already been injected. The longer the delay, the more fuel is in the cylinder at the time ignition starts, and the more violent the explosion when it all burns at once. *BANG*.

If you heat the diesel until it is almost at the flash point prior to injection, you must change the timing to compensate for the decrease of delay between the start of fuel injection and the start of actual combustion.

This is also why common rail systems do a short pre-injection before injecting the rest of the load -- to let the initial amount of fuel start burning so the rest is injected into an already-ignited fuel air mixture and starts burning "immediately". By injection only a small portion of fuel early, they lessen the impact of its explostion when it starts burning and achieve a smoother, quieter running engine.

When thinking about what's going on inside our engines, don't forget that at a measly 1000RPM, the fuel must be injected, the droplets evaporate, the fuel molecules heat to combustion temperature, and the fuel start and finish combusting.. all in 0.5ms (that's 0.0005 seconds). At those durations, rate of evaporation, heat capacity, the rate of heat condution within the liqud fuel and the time for gasses to diffuse, all make a difference. Everything about a diesel engine has been designed to affect these in a positive way.
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