Archive for August, 2013

Reacting to Diesel: The Chemistry of Diesel Combusion.

August 8, 2013

It’s been a LONG time since I’ve blogged.  Thanks to Bob for the push on getting more information up here.  I spend a lot of time answering questions on Quora.com and I’m pretty proud of some answers, so I’m including them here.

This is a response to a question about the chemistry behind diesel combustion.  There are entire university programs, companies, company divisions and books all trying to figure it out, and there’s not any single answer.  if you want more gory details I’d suggest you spend some time reading through the technology guide on Diesel Emissions Online(http://diselnet.com).  Especially this article: What Are Diesel Emissions which gives a nice overview of the main components of exhaust emissions, and this chart:

Figure 1. Primary components of Diesel emissions, (credit: What Are Diesel Emissions from dieselnet.com)

Other than the main portions which are arguably benign to an extent, the small slice of pollutant emissions is the source of many global emissions regulations.

The primary polluting emissions which are of concern for Diesel engines are Oxides of Nitrogen (NOx) and Particulate Matter (PM).  Hydrocarbons (HCs) are traditionally not an issue with Diesel engines compared to gasoline engines because HC is generally created when there is a lack of oxygen to complete the combustion process.  Since Diesel engines are a lean combustion process HCs are not as much of a concern because the HCs tend to be burned later on in combustion stroke when they combine with oxygen.  HCs still exist in Diesel, but they’re traditionally the easiest emission type to deal with.

In Figure 2 two phases of injection mixing are illustrated.  The Premixed portion occurs at the beginning of injection and the Mixing Controlled portion occurs after combustion has started and we are more interested in maintaining combustion.  Both of these phases occur within milliseconds of each other each and every time combustion occurs and you hear a “clack” of a combustion event which is typical of a diesel engine.  Higher combustion pressures effectively decrease the length of time in Premixed combustion by allowing smaller injector holes which improve atomization and rely less on in cylinder temperatures to atomize the fuel before it ignites.  This decreases the rich combustion areas because the fuel is more dispersed.  It decreases the flame quench on the walls because more of the fuel is burned before it reaches the walls.  It also decreases NOx by allowing a higher percentage of EGR because fuel is dispersed more evenly so a decrease in the amount available oxygen is not as detrimental as would be with lower pressures.

Figure 2: Fuel Mixture technologies in Diesel engines. (credit: http://dieselnet.com/tech/diesel_emiform.php)

NOx is created by localized combustion temperatures in exceedence of ~1500°C.  “Localized” temperatures are a phenomenon of the diesel combustion process which are a result of fuel burning as it is injected into the cylinder.  This creates what we call a “flame front” at the interface between fuel and oxygen where there is enough fuel and oxygen mixed for combustion to occur.  The flame from is somewhat illustrated in Figure 2 in the “Initial rapid combustion” area of the Premixed combustion phase and the white/yellow flame areas of the Mixing controlled phase.  The primary technology for dealing with this issue in cylinder is Exhaust Gas Recirculation (EGR).  EGR attempts to lower these spikes in temperature by adding enough benign gasses around the flame front to absorb some temperature before it reaches a critical NOx creating point (more info here: Exhaust Gas Recirculation from dieselnet.com).  NOx which leaves the cylinder is dealt with through Selective Catalytic Reduction (SCR) but aftertreatment is outside the scope of this question.

PM is created by a lack of oxygen to complete the burning of Hydrocarbon Chains in fuel.   PM is the least understood portion of Diesel exhaust mainly because it is such a complex, unstable and varying spectrum of structures (http://dieselnet.com/tech/dpm.php).  PM is traditionally defined in two categories: Solid Fraction (SF) and Soluble Organic Fraction (SOF).  SF is primarily Carbon while the SOF fraction is primarily Hydrocarbons.  The proportion of SF to SOF is completely dependent on the combustion process and even the size of the engine.  The SF tends to form the backbone of any particle while the SOF tends to hang onto the outside of the carbon base.  SOF may be in a range of 20% to 60% of a total particle mass.  If you know about the hydrocarbon chain, this will seem familiar since the HC chain has a backbone of carbon with a bunch of hydrogen atoms surrounding.  PM could be seen as partially reacted hydrocarbon chains which have lost their ability to quickly react due to an unorganized atomic structure.  PM which leaves the cylinder is dealt with through two aftertreatment technologies: Oxidation Catalysts which deal with the SOF fraction of fuel (as well as any HC) and Diesel particulate filters which collect any remaining portions of SF and SOF to be burned at a later time through what is called regeneration.  Aftertreatment for Diesel engines is another topic all together, so we’ll have to talk more about that later.

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