[M.D. Holloway]

I just had a discussion last week with one of my engineers on this topic, I told him to write a white paper on the "An Overview of the Combustion ProcessIn Modern Diesel Engines and The Relationship of Pollutant Generation". Here is what he penned this weekend. Let me know what you think. If it is legit and makes sense, I will have him publish it.

Poke holes in it. I would like your feedback.

Due to increasingly strict emissions standards in the United States, diesel engine manufacturers have developed interesting methods for the control of these emissions. This is particularly true for large diesel engine manufacturers. Several strategies have been implemented and will be reviewed here.

Diesel engines now occupy a place in our society that is so prevalent and so pervasive that these engines can not be displaced over the near or long-term future. As a result, 2 basic methods of pollutant reduction have been adopted. One of these methods is called Exhaust Gas Recirculation (EGR). This method uses a small amount of the engine exhaust to achieve pollutant reduction and will be explained later. The second method uses multiple injections of fuel into the combustion chamber. The biggest proponent of this method is Caterpillar (Cat) and this method will also be explained later.

In order to understand the solutions, a fast review of the dynamics of the diesel engine is necessary. Diesel engines achieve their power by the combustion of a fuel (diesel oil or diesel) that has a very high potential for the release of energy. The basic problem with the use of diesel is achieving the conditions necessary for the fuel to burn. In the 1880’s, Dr. Diesel realized that the easiest way to achieve the necessary temperatures to cause diesel to ignite was to compress air and achieve the temperatures necessary to cause ignition. If a quantity of air at ambient temperature is compressed to 4-5% of its original volume, the temperature of the air escalates enormously. 800°F is not uncommon. At this temperature, diesel will ignite and burn quickly; where at ambient temperatures, very poor combustion takes place. Here is where the downside of diesel engines starts to take place, at the combustion temperatures achieved in diesel engines sulfur mixes with the water that is a normal by-product of hydrocarbon combustion and creates sulfuric acid along with other sulfur oxides (SOx). In addition, the nitrogen that composes 70+% of ambient air is changed to nitric oxides (NOx) and nitric acid. Particulate Matter (PM) also became an interesting topic. PM is the black smoke often associated with diesels. The origin of PM is fuel that is not completely burned in the engine. What you are looking at is small specks of carbon that did not combine with oxygen to create carbon dioxide (CO2). While CO2 is invisible, unburned carbon is clearly visible.
The clatter normally associated with diesel engines is the result of very rapid combustion. Poor fuel will create a louder clatter because the fuel is burned at a very rapid, but incomplete, rate. The cetane rating is the measurement standard for this. If the start of the burn can be delayed slightly, the better the burn will be. If the burn rate can be controlled, then the noise drops and the more complete the combustion of the fuel.

Of the 2 problems, NOx and SOx, SOx was the easiest to cure/solve/reduce. A government mandate was issued and stated that the amount of sulfur in diesel fuel was going to be substantially reduced and it was up to the engine manufacturers to resolve the issue of creating engines that would work with “Low Sulfur Diesel.” At this point, it needs to be understood that the sulfur in diesel fuel not only behaved as a pollutant, but also as a lubricant for the bits of the engine that came into contact with the fuel. This means that the high pressure fuel pump that is absolutely necessary for engine operation was no longer going to receive the necessary lubrication to operate properly under the current fuel and design regime. New designs and/or different materials were going to be necessary to have pumps that were capable of operation for the amount of time mandated by customer expectations. Please remember that diesel engines have achieved expected lifetimes measured in hundreds of thousands of miles before ANY wear related parts replacements were to be considered. Any sort of lessening of this expectation was considered unacceptable. Cost was going to be a problem, but not one that could not be overcome.

Another problem associated with reduced sulfur fuel was going to be the cost of the fuel itself. Removing the sulfur to acceptable and mandated levels, was going to involve an increased cost in the production of the fuel. This cost would be passed along to the end user and passed on again to the end customer.

Alternate diesel fuels have come under consideration over the last few years, particularly the use of vegetable oils either straight or mixed with petroleum diesel. The vegetable oil-based diesels show promise because of their inherently high cetane and virtually no sulfur content but have some interesting problems to be overcome before wide acceptance occurs. Natural gas has also been tried to fuel diesel engines and the long term results of this are not available yet but there were some very interesting problems initially with reliability.

Removing NOx was going to be a greater problem. NOx is created when the pressures and temperatures in the combustion chamber achieve a certain point. Diesel engines were more than capable of creating these types of conditions; in fact, they needed them to work with any kind of efficiency at all. The manufacturers determined that there are 2 ways that a reduction in NOx could be achieved; one is to displace some of the oxygen in the combustion chamber and deprive the nitrogen of oxygen to bond with and the other is to lower the combustion chamber temperatures sufficiently to prevent the reaction form taking place at all.

Exhaust Gas Recirculation (EGR) is the simplest method for cutting down on NOx. EGR takes exhaust gas from the exhaust manifold, passes it through a heat exchanger to lower the temperature to whatever the engine temperature is, and then sends this gas into the combustion chamber. By using exhaust gas, the chamber air mixture is slightly reduced in oxygen content and the NOx reaction does not take place. Very little machinery is added to the engine to accommodate this idea. There is also very little weight added to the engine and this translates directly into no substantial reduction in fuel mileage. Most of the major manufacturers have adopted the EGR idea. Caterpillar went in another direction entirely and that will be explained later.

This method does have certain side-effects that directly affect the engine over the long run. One of these side-effects is the introduction of soot into the engine. Soot is the result of incomplete combustion. In theory, you mix hydrocarbons with air, get it to burn, and the end products are heat, water vapor, and carbon dioxide. Soot is elemental carbon. Soot is very abrasive. Soot is nothing more than sand that is made of carbon instead of silica. Introducing soot into the combustion chamber will mean that this soot will become mixed into the engine oil and be re-circulated through the entire engine. Wear on engine parts is increased substantially. The engine manufacturers have tried to combat this by increasing the volume of the oil in the engine. Another method is the increasing size of the filters that can trap the soot. Addition of filters that are designed for soot entrapment is a good method for the removal of soot. This means that more weight is added to the truck, decreasing fuel economy. Other items to think about are increased oil and filter costs for both initial purchase and disposal because much more oil is used and at least one more filter is required. Also note that nothing is done to reduce the PM. Catalytic converters have been considered but have been rejected because of the continued presence of NOx in quantities sufficient to damage the internal materials. The use of urea injectors is being tried in Europe but is not seen as a viable system for the United States. Another side-effect was a greater strain on the engine cooling system. The engine cooling system was used as the heat dump from the exhaust gas. Now larger radiators, more coolant, and a robust heat exchanger were going to be needed to reduce the exhaust gas temperature to something that would be acceptable the air intake side of the engine and not melt things in general.