Pioneering Fuels

Technology Today® talks fuels with Dr. David W. Naegeli, an SwRI chemist interested in the energy supply, combustion problems, and the fuel sources that will drive America into the next millennium.

For more than 30 years, Dr. David W. Naegeli has carried out research on chemical lasers, combustion kinetics, fuel chemistry, alternative fuels, and engine performance and emissions. An SwRI staff scientist, Naegeli is currently working on low-emission diesel fuels, fuel tank flammability, and the thermal stability of jet fuels.

How did you become interested in fuel chemistry?

After completing undergraduate and master's degrees at North Dakota State University, I began my doctoral research at Penn State, which at that time was quite a center of fuels research. I took courses in flames and combustion, but my main interest was in kinetics and spectroscopy.

My thesis work was on alkaline metal diffusion flames, which are cool flames created in a partial vacuum by reacting substances such as carbon tetrachloride with potassium vapor. This forms a flame that appears to be very hot because it emits a relatively intense chemiluminescence. However, the flame temperature is less than 500°C compared to that of conventional fuel, such as natural gas, which is in the neighborhood of 2,000°C. The molecules in cool flames often exhibit very high temperatures in their electronic and vibrational modes, while their translational or collisional temperature is very low. This disequilibrium can lead to population inversions between the energy levels of molecules. A chemically generated population inversion is the basis for the chemical laser.

After I finished my thesis, I worked on chemical lasers at the United Technology Research Center (UTRC) in Hartford, Connecticut. Chemical lasers were sought-after because chemicals could be brought into space and made to react to create intense laser light. This was an efficient process because there was no need for a large power supply. It also had obvious potential as a weapon. The chemical lasers we developed became the basis for the Star Wars program of the 1980s.

After three years at UTRC, I joined the Guggenheim Combustion Laboratory at Princeton University, where I worked with and advised graduate students on research projects. We were trying to determine reaction rates and understand chemical mechanisms in combustion. At that time, combustion was not well understood. We were putting the building blocks together.

For almost 50 years, SwRI engineers have evaluated numerous traditional and alternative fuels and their effects on engine performance and emissions.

Why did you choose to join SwRI in 1976?

The thing that attracted me was my interest in cars. I had a couple of old Corvairs with engines I used to overhaul. I had previously consulted at the National Bureau of Standards in Washington, D.C., where I reviewed a lot of energy-related devices, such as carburetors. Every engineer wanted to make a 100 mile-per-gallon carburetor in those days --it was kind of a dream. That interest in engines and their components, and my familiarity with the work done at the Institute, convinced me to join.

What were the critical issues in fuel chemistry when you joined the staff?

There were a number of issues: finding different forms of energy, learning to modify engines to make traditional and alternative forms of energy work more efficiently, and a growing concern with the environmental impact of engine emissions.

Remember the energy crisis of the 1970s with gas lines that reached around the block? It made people wake up to the fact that the traditional energy supply is a finite resource. A lot of people thought the supply would end around 1985. Of course, that prediction wasn't even close and had the unfortunate effect of making people forget the problem. We were concerned at the time and examined all sorts of alternative fuels, such as coal. We have an almost infinite supply of coal in the ground, but what about other sources of energy?

A little later, there was more concern about fuel economy and general engine efficiency. We also started to develop smaller engines and tried to improve the combustion process in vehicles.

When exhaust emissions became a major concern, thermal reactors were developed to burn up hydrocarbons and carbon monoxide in the exhaust. These reactors were inefficient, so industry developed devices such as catalytic converters and looked at several ways to make engines run cleaner. We approached these problems using the fundamentals of combustion science. We fuel chemists believed that air quality could be improved with changes in engine design and modifications to the combustion process.

Where do we stand now? Will we have another, more serious fuel crisis?

Let's consider the next 10 to 15 years and what's happening with petroleum. There are alternative fuels, such as coal and natural gas, but they don't have the infrastructure that exists for petroleum. The majority of vehicles run on petroleum right now, and those vehicles will last for at least the next 10 years.

We've used only about half the petroleum in the world, but the other half is going to be harder to get out of the ground for delivery to the customer. Look at the production rate for any particular country --it eventually reaches a peak and then starts to decline. If you consider total world production, it too will go through a peak, projected to be between 2001 and 2005, and it will go down from there.

I see the demand for petroleum going up, while the supply remains relatively constant or slowly declines. Petroleum will certainly become more expensive. This will reverse the trend of the last 10 years or so where all our vehicles have gotten heavier, with larger, more powerful engines. Right now, America is burning fuel like there's no tomorrow.

The larger our economy gets, the worse it's going to be, because our economy is so tied to energy. It's nice that Americans can drive cars so inexpensively. They drive to work and go to the store to buy this and that. All this activity uses energy, and all the things they buy consume energy.

The Department of Energy (DOE) understands this and is trying to plan for it. One aspect of interest is in developing and encouraging the use of the diesel engine.

Why are engineers interested in diesel engines as opposed to gasoline engines?

Diesel engines are far more efficient than gasoline engines because they are unthrottled, have higher compression levels, and are turbocharged, which significantly increases thermodynamic efficiency. The fact that the diesel engine is unthrottled is its greatest asset. Internal combustion engines breathe air into their cylinders via the intake manifold. When the pressure in the manifold is at atmospheric level or greater, the engine can breathe freely. There is a partial vacuum in the manifold of gasoline engines under most operating conditions, but the manifold pressure in a turbocharged diesel is always equal to or greater than atmospheric pressure. The gasoline engine is throttled by how much air is allowed to enter the engine, while the diesel engine is throttled by how much fuel is injected into the cylinder. When the engine does not have to work to breathe, it is more efficient.

Volkswagen currently makes a 1.8-liter diesel engine with excellent fuel economy, but it's not popular in the U.S. because it offers less performance than Americans generally prefer. Also, diesel fuel is not as readily available in some places as gasoline.

One thing you give up when you start looking for fuel economy is performance. I have a car built in the 1980s and its gear ratios are built for fuel economy. I can't race away from a stoplight in it, but I get better fuel economy.

Don't diesel engines have serious emission problems? Yes, and the problem can be tackled in two ways: through engine design and fuel chemistry.

Engine manufacturers have been working very hard to make the diesel engine clean. The industry is still, however, challenged by the problem of emissions. Diesels emit oxides of nitrogen (NO_x) emissions, just as gasoline engines do. In addition, however, they make particulate matter (PM) that is difficult to remove with current exhaust aftertreatment techniques. Gasoline engines make hydrocarbons, but these can be burned with a catalytic reactor. Particulates are much harder to eliminate. Particulate traps, basically filters, are being used in some applications to prevent PM emissions. Plasma reactors are also being considered as a possible solution. The problem with these approaches is that the equipment is bulky and consumes energy. The ideal approach is to prevent PM from forming during the combustion process. Some high-pressure fuel injection systems have been developed that greatly reduce particulate emissions in diesel engines, but they still don't meet the stringent standards that will be enforced by the government through the Environmental Protection Agency (EPA).

There have been several approaches through changes in fuel chemistry. There is a dilemma in the diesel engine. If you reduce NO_x emissions, you raise the particulates, and vice versa. DOE is interested in changing the fuel properties to reduce particulates and thereby provide more freedom to adjust the engine for low NO_x. The objective is to add oxygen and increase the hydrogen content of diesel fuel. The formation of particulate carbon during combustion decreases as the ratios of hydrogen and oxygen to carbon are increased in the fuel.

The effects of these fuel properties have recently been well demonstrated by the use of dimethyl ether in a diesel engine. When dimethyl ether was used, there were no particulate emissions. It was also possible to meet stringent standards for NOx emissions by making simple adjustments to the injection timing and exhaust gas recirculation system.

The goal is to add oxygenates to make a blended or reformulated fuel. This has already happened with gasoline in California where such oxygenates as ethanol or methyl tertiary butyl ether (MTBE) are included at about 10 percent per volume.

Regulators are thinking of doing the same thing with diesel fuels --adding oxygenates to reduce PM. We have done some fundamental work on the ignition and combustion properties of methylal and have found that, by adding 15 percent methylal to diesel fuel, we can cut PM by 30 percent, and as much as 50 percent under some conditions, without affecting engine performance.

Right now I'm working on a project for the National Renewable Energy Laboratory, doing a thorough examination of neat methylal and various methylal-diesel blended fuels. We are testing emissions and performance characteristics of the fuels in an 8.3-liter medium-sized diesel engine. If the results are encouraging, the fuels may be used in fleet tests to determine if the fuel is practical.

In a project for the National Renewable Energy Laboratory, an on-highway truck engine was configured to evaluate the emissions levels and performance of methylal and methylal-diesel blended fuels.

Didn't you recently win an award for a research paper in this area?

Yes. Our paper was about measuring the ignition delay times of dimethyl ether and methylal and then predicting them. We performed the measurements in a constant-volume combustion vessel designed to simulate the autoignition of a fuel spray in a diesel engine. Then we developed a chemical reaction mechanism and an adiabatic mixing model to predict the ignition delay times.

I believe we received the award* because the subject was timely. There is currently a lot of interest in oxygenates and the reformulation of diesel fuel.

Will there be tinkering with diesel fuels in the future?

I believe so, and DOE seems to be leaning in that direction. For another project, we are evaluating a range of oxygenates for other possible improvements. Oxygenates will increase the price of diesel fuel, just as they have in the reformulated gasoline sold in California. This project is examining the various tradeoffs --where we get the best effects on emissions and performance at the best cost, how effectively fuels ignite in the engine, and what effects on safety issues, such as fuel tank flammability, are created.

You've done a lot of work on fuel tank explosions in both vehicles and aircraft. How did you become interested in this problem?

The Institute's fuel tank explosion work began years ago, when we were working with methyl alcohol. We found that if you stored methyl alcohol in a fuel tank by itself there could be an explosive mixture over a wide range of temperatures, from 100°F down to about 50°F. We tried to determine the temperature range inside the fuel tank at which the vapor would become explosive. At a very high temperature, the mixture is too rich to burn --it has reached the upper temperature limit of flammability. At a low enough temperature, the mixture is too lean to burn.

Fuel tank flammability is often overlooked. An example of this was in the 1980s, when there was a shortage of freeze-resistant diesel fuel in the U.S. It was a temporary situation caused by a very cold winter and the need for increased supplies for heating fuel. The diesel industry advised that the problem could be alleviated by adding gasoline to diesel fuel --just add a bit. It wouldn't hurt the fuel injectors, and the engines were expected to run fine.

This went on for about a week until people realized there was a safety problem. When gasoline is mixed with diesel, the vapor pressure changes and the flash point, which is normally about 150°F, drops to 60°F. That creates a combustible mixture that can cause an explosion. Fortunately, diesel manufacturers learned this and warned truckers before there was an accident.

There can be a flammable mixture in a tank, but you need a source of ignition before there's a problem. Ignition sources can be just about anything. Sparks can develop from an electrostatic charge just by pumping fuel into the tank. During a rainstorm, lightning can create an electric discharge by creating an electrical field across the tank. It doesn't take much energy.

The Air Force is interested in finding a good handheld detector to measure the vapors of JP-8 fuel in fuel tanks and fuel spills. Detectors can measure simple gases such as methane, pentane, and propane, but it was questionable whether they could measure JP-8 vapors.

We built a facility to create those vapors and test the detector instruments. As part of our research, we looked at the issue of fuel flammability in aircraft. We found there have been at least 26 documented fuel tank explosion accidents since 1959.

Tell us how you became interested in the role of sulfur in the thermal stability of jet fuel.

I became interested because fuel sulfur seems to be the cause of most deposit formation in the fuel systems, intake valves, and combustion chambers of engines. All hydrocarbon fuels, whether gasoline, diesel, or jet fuel, form engine deposits. Fuels always contain some oxygen, which causes auto-oxidation. In gasoline engines, deposits form on the injectors and valves. They gum things up. Many additives are on the market to --theoretically --eliminate the gum and hard deposits that form. Injectors and other parts often have to be removed for cleaning, which is an expensive process --particularly in aircraft where the parts are relatively inaccessible.

In jet engines, deposits develop because the heat exchangers and injectors are exposed to very high temperatures. Inlet air entering the gas turbine engine passes over the injector before the fuel is injected into the combustion chamber. The injectors become very hot and fuel temperatures reach nearly 500°F.

Sulfur appears to be a major culprit in making deposits. Analysis of the deposits shows sulfur and oxygen, and sometimes nitrogen, in much higher concentrations than in the fuel. In other words, the sulfur in fuel tends to concentrate in the deposit. For example, fuel might contain 0.03 percent of sulfur, while the deposit might contain 12 percent.

The purpose of our work in fuel stability has been to better understand the mechanism of deposit formation and, in particular, to explain the anomalous relationship between the rate of fuel oxidation and deposit formation. Years ago, it was assumed that fuels that auto-oxidize rapidly would cause a high rate of deposit formation. It was subsequently realized that fuels that show the greatest resistance to auto-oxidation are the ones that form the most deposits. We believe that an understanding of this anomaly will provide an important clue to the origin of deposits.

Research, so far, shows that sulfur causes fuels to resist auto-oxidation and form deposits. From this, it has been concluded that sulfur compounds oxidize and decompose to form sulfur dioxide which can oxidize further to form sulfuric acid. The sulfur compounds only cause the fuel to resist auto-oxidation when the fuel contains aromatics. It only requires a trace of sulfur, less than 10 parts per million, to cause significant inhibition of the auto-oxidation rate. The auto-oxidation of the fuel converts aromatics to benzylic hydroperoxides, which decompose to phenols when an acid catalyst such as sulfuric acid is present. Phenols are strong oxidation inhibitors that, when formed in fuel, create a resistance to auto-oxidation. Because phenols cause the fuel to resist auto-oxidation, it suggests that they also play an important role in the formation of deposits.

This is where the work stands. We have yet to explain how phenols cause deposits to form. It is this kind of enigma that makes fuel chemistry such an interesting area of research.

*The Society of Automotive Engineers presented Naegeli and coauthors Bradley L. Edgar and Robert W. Dibble (both of the University of California at Berkeley) with the Arch Collwell Merit Award for the paper, "Autoignition of Dimethyl Ether and Dimethoxy Methane Sprays at High Pressures."

Comments about this article? Contact Steve Westbrook at (210) 522-3185 or swestbrook@swri.org.

Published in the Fall 1999 issue of Technology Today, published by Southwest Research Institute. For more information, contact Maria Martinez.