BAE ISU NBB

Diesel Combustion and Emissions
 

Diesel combustion is the process that occurs when a fuel blend, chosen for its ability to auto-ignite, is injected into a volume of air that has been compressed to a high temperature and pressure. When fuel is injected into the turbulent compressed air inside the engine cylinder, it does not ignite immediately. There is a time period called the ignition delay, during which the fuel must heat up, vaporize, mix with air, and undergo chemical precombustion reactions that produce the radicals necessary for spontaneous ignition. The classical notion of ignition delay calls the heating, vaporization and mixing processes the "physical delay" and the pre-reactions the "chemical delay." This can be deceptive in that all of the processes can be, and probably are, occurring simultaneously.

After sufficient time has elapsed, ignition will occur spontaneously at multiple nuclei in regions of fuel-air mixture that have fuel-air ratios close to stoichiometric.  Combustion proceeds very rapidly due to the backlog of prepared or nearly prepared fuel-air mixture formed during the ignition delay period. The rapidly rising temperatures and pressure in the cylinder accelerate the combustion in an uncontrolled manner until the backlog is depleted. This portion of the combustion process is usually called premixed combustion.  The remainder of the fuel in the spray core is still too rich to burn, so combustion slows down and is controlled by the rate at which the air is entrained and a combustible mixture formed.  This portion of the combustion process is called mixing controlled or diffusion burning.  Thus, while chemical kinetics dominates the ignition delay, the high temperatures and pressures of the post-ignition gases promote very fast reaction rates which make fuel-air mixing the rate determining process.

Some fuels will autoignite more quickly after being injected into the diesel engine than others.  The laboratory test that is used to measure this tendency is the Cetane Number Test (ASTM D 613).  This test is described in detail in the section on ignition indices.  Fuels with a high cetane number will have short ignition delays and a small amount of premixed combustion since little time is available to prepare the fuel for combustion.

Diesel engines can be divided into two basic types, direct injection and indirect injection.  A direct injection diesel engine combustion system is shown in the picture below.

Figure 1.  Direct Injection Combustion System

In the direct injection diesel engine, the fuel is sprayed into the combustion chamber directly above the piston.  The piston usually has a recess or bowl that is designed to confine the air into a region that matches the fuel spray trajectory.  This type of system relies primarily on the momentum of the fuel spray to mix the fuel and air.  A drawback of this type of system is that there is a chance that unburned fuel can contact the cylinder wall and find its way down past the piston rings into the crankcase.  When these engines are operated on fuel with low volatility, some of the fuel that is slow to vaporize may be deposited on the cylinder walls and end up diluting the lubricating oil.

For more information on fuel dilution of the lubrication oil, click here.

The second type of diesel engine is the indirect injection engine as shown in Figure 2.  In this type of engine, the fuel is injected into a separate chamber that is connected to the main chamber above the piston by a narrow passage-way.  When the piston rises toward top-dead-center, the air is forced through the connecting passage at high velocity into the small chamber, called a swirl chamber.  The high velocity air rotates at high velocity in the chamber as fuel is injected.  The diagram also shows a glow plug being inserted into the chamber, which helps with cold starting.  This type of system relies on the high velocity air swirl to mix the air so the fuel injection system can operate at lower pressures and be less expensive.  After the fuel ignites, the combusting mixture pushes back out through the passage-way where the rise in pressure does work on the piston.


Figure 2.  Indirect Injection Combustion System.

The indirect injection (IDI) engine is less efficient than the direct injection (DI) engine.  This is because the high velocity air motion in the combustion chamber causes high heat transfer rates resulting in greater loss of fuel energy.  The lower efficiency of the IDI engine has resulted in it being out-of-favor and although there are a large number of these engines currently being produced, virtually all new engine designs use direct injection technology.  An advantage of the IDI engine was that it was generally found to be more fuel tolerant.  Having the liquid fuel injected into a separate chamber kept most of the fuel away from the piston rings and lessened the likelihood of fuel contamination of the lubricating oil and fouling of the piston rings.  During the 1970s, many researchers experimented with using raw vegetable oils in diesel engines, either as pure fuels or as blends.  In those cases where good results were achieved it was generally with IDI engines.  DI engines tended to degrade slowly due to incylinder deposits and seized piston rings.

Diesel Emissions

Why do we worry about them?
The main reason we worry about the emissions from diesel engines is because the EPA regulates them.  Table 1 shows the allowable values for each of the 4 pollutants that the EPA currently regulates.

Table 1. EPA Heavy Duty Diesel Emissions Standards

          *PM standard for buses dropped to 0.05 in 1996 with 0.07 in-use.

Table 2 shows emissions data for five different engines that are typical of current in-use on-highway diesel engines.  It can be seen that these engines have no trouble meeting the regulation levels for CO and HC.  However, NOx and particulate are usually very close to the regulated values.  Furthermore, measures taken to reduce NOx tend to increase particulate and measures to reduce particulate usually increase NOx.

Table 2. Typical Emissions Levels of Current Engines

The other reason why we worry about diesel emissions is that these substances are toxic and can contribute to health problems.

Carbon monoxide - Is colorless, tasteless, and odorless.  I+ Does not irritate the skin or mucous membranes.  CO binds to the hemoglobin in the blood with 210 times the strength of oxygen.  As contact with CO increases, more and more hemoglobin is bound to CO and the oxygen carrying capacity of the blood decreases resulting in headaches, dizziness, unconsciousness and ultimately death.

Unburned Hydrocarbons - Some constituents in the unburned hydrocarbon material are strong irritants of mucous membranes such as the eyes and throat.  This is particularly true of aldehydes.  Unburned hydrocarbons participate (along with NO_x and sunlight) in smog-formation reactions that produce ozone and other irritants.

Oxides of Nitrogen - The primary oxide of nitrogen in engine exhaust is nitric oxide, NO.  However, after being exhausted, the NO reacts photochemically with sunlight to produce nitrogen dioxide, NO₂.  In diesel exhaust, 10-30% of the NO_x may be emitted directly as NO₂.  NO is not a strong irritant, but it binds to hemoglobin in the same manner as CO.  NO₂ is a strong irritant of mucous membranes and it also binds to hemoglobin.  NO binds with 1000 times the strength of CO and NO₂ binds with about 1/3 the strength of NO.NO₂ is major participant in smog formation reactions.

Particulates - There is considerable current controversy about the hazard associated with diesel particulate.  Traditionally, the hazard has been associated with the hydrocarbon portion of the particulate.  This fraction, commonly referred to as the soluble organic fraction or volatile fraction, contains high molecular weight polynuclear aromatic hydrocarbons that are known carcinogens.  The usual argument is that soot particles are inhaled and deposit deep in the lungs where the carcinogens have access to sensitive tissue.  More recent data suggests that non-hydrocarbon-containing inert particles in the same size range as diesel particulate may cause similar health problems.

Typical particulate data for diesel fuel, biodiesel, and B20 are shown in Table 3.  While the total particulate matter is the quantity regulated by the EPA, the data presented here show the various components that make up the particulate, the volatile organic fraction, the sulfate and bound water, and the soot.

Table 3.  Particulate Matter Composition

The volatile organic fraction is the portion of the particulate matter that originates from unburned and partially burned fuel and lubricating oil.  It is determined by placing a particulate sample in a vacuum oven and baking until all the volatile material has been removed.  A similar quantity is the soluble organic fraction (SOF), which is determined using a chemical solvent to remove the hydrocarbon fraction.  The sulfate fraction consists of sulfuric acid and related sulfates that originate from the sulfur in the fuel.  These compounds are usually hygroscopic so they contain an amount of bound water that may be greater than the weight of sulfate itself.  With low sulfur diesel fuels (< 0.05%) the sulfate fraction is usually low.  The final fraction is the soot.  The soot consists mainly of carbon with small amounts of hydrogen.

Table 3 shows that biodiesel causes substantial reductions in the soot fraction of the particulate.  The reduction varied between 61 and 71%.  However, this is offset by an increase in the VOF that varied between 4.5 and 43%.  The soot reduction is still not completely understood but is most likely due to the presence of the oxygen in the fuel decreasing the formation of soot in the high-temperature, fuel-rich portions of the fuel spray.  The mechanism for the increase in VOF is also not understood but is probably associated with the low volatility of the biodiesel allowing some of the fuel to survive the combustion process as small droplets.