March 2006 Articles & Features

Launch Tech

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Eye On Electronics

By Mike Dale | March 2006

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New high-tech diesels and diesel fuels will challenge everything you know about diesel-powered vehicles, and provide new service opportunities, as well.

This year roughly a third of all cars sold in Europe will be powered by diesel engines. In countries like Belgium and France , the figure is close to half. While sales in the U.S. have been low, changing government regulations and new engine technology will soon open the doors to increased sales of diesel-powered vehicles here. Electronic controls will make possible new high-tech diesels that will change the way Americans view these machines.

For Europeans, the advantage of 25% better fuel economy with fuel that costs half as much as gasoline is a strong incentive. The London Taxi Cab Association reports that diesel taxis there average around 27 mpg in the city and as much as 34 on the highway. The same group says that with good maintenance, the engines can last 500,000 miles or more. The improved torque characteristics that stem from the high compression ratios is also cited as a plus in stop & go driving.

What this could mean for the U.S. market was discussed recently by Chrysler Group President and CEO Dieter Zetsche: “If Americans chose diesel passenger cars at the same rate as car buyers in Western Europe , some 3 million liters of crude oil would be saved annually. In addition, there would be an 8-million-ton reduction in CO2 emissions.”

For the U.S. market, though, the list of the diesel's disadvantages is rather long. Primary among them is the issue of pollution. Diesel pollution comes in three forms�particulate matter, oxides of nitrogen (NOX) and sulfur. Particulate matter (soot) comes in the form of fine granules (less than 1 micron in size) made mostly of carbon and carbon compounds. Some 40 different chemicals have been identified in diesel exhaust. The soot from diesel exhaust is recognized by the EPA and the World Health Organization as a cancer-causing material. The California Air Resources Board (CARB) estimates that 71% of the airborne cancer risk in Southern California is caused by the 2% of the vehicles in that area that operate on diesel fuel. Other health issues, such as increased asthma problems and lung disease, are also of concern.

Combustion in the diesel process involves lean mixtures and very high compression ratios. Typical diesel combustion ratios are between 20 and 24:1. Pressures inside the combustion chamber can be 40 to 50 times higher than in a gasoline-fueled engine. The combination of excess oxygen, high pressures and high temperatures form a natural breeding ground for NOX.

The real problem is the third pollutant�sulfur. Diesel fuel can be and most often is made from lower quality oil stocks, which are typically contaminated with sulfur. Current American
fuels contain about 500 ppm of sulfur. The fuels used in China have 10 times that much. The sulfur content poisons catalytic converters and particulate traps much like tetraethyl lead does to the converters on gasoline engines. The sulfur also exits with the exhaust, adding to the formation of smog and acid rain.

The three technologies Chrysler talks about as the core of its high-tech diesel strategy include electronic engine management, particulate filters for trapping and removing soot and use of selective catalytic reduction (SCR). Both the particulate emissions strategies and the SCR process require low-sulfur fuel and electronic controls to be effective.

Significant progress has already been made on reducing sulfur content, and more is on the way. Since the late 1970s, when the first diesel passenger cars were built in the U.S. , there has been a reduction of 90% in soot and NOX emissions. In June of 2006, the 500-ppm limit on sulfur content in U.S. diesel fuels will be reduced to 15 ppm. Low-sulfur diesel fuel will make practical a number of new technologies that will further improve the emissions performance of these engines.

Particulate traps can remove up to 90% of the soot diesel engines produce. The trap is a ceramic filter within the exhaust system that captures the particles before they enter the atmosphere. The captured particles are then burned off during vehicle operation. To do this, advanced electronic control systems, catalytic coatings and fuel-borne catalysts (FBCs) are used. FBCs include metals such as cerium, iron and platinum. The materials are dosed into the fuel under engine management control to help control not only particulates but hydrocarbons and polluting gases, as well.

Regeneration or purging of the trap must be done on a programmed basis to keep the filter from plugging up with soot. After the purge cycle is complete, an ash or residue remains that eventually must be cleaned out during routine service.

There are two competing processes for reducing oxides of nitrogen. The NOX storage converter is very efficient so long as its operating temperature remains in a very narrow range and input levels of sulfur are kept low. When the storage catalyst is in the regeneration or purge mode, a very complex electronic control strategy is needed to make sure the temperatures are kept within safe limits. During the actual purge cycle, the engine is actually made to run rich, and fuel economy is affected.

The SCR process involves introducing into the exhaust stream a gaseous or liquid reducing agent that is absorbed onto a catalyst. The heart of the SCR process is in the chemical formula of what it actually does:

4NO+4NH3+O2 4N2+6H2O

For us nonchemists, what this means is that when four molecules of NOx are brought together with four molecules of ammonia in the presence of the catalytic material, four molecules of pure nitrogen and six of water are produced. What was NOx now becomes air and water after the conversion process. One outfit called Clean Diesel Technologies has patented a system that can reduce NOX by 70% to 90% with urea injection so long as the catalytic converter is maintained between 320° and 500°C.

This is not the same catalytic converter as is used for gasoline engines. Specialty catalytic converters use either of two technologies: Vanadium-based chemistry works the best but is not popular with CARB because of the metals used. Catalysts with a Zeolite washcoat can be used but are less effective. In either case, the operating temperature range is very narrow, requiring precise electronic engine management control.

The conversion of NOX into molecular nitrogen requires careful control of the amount and the location of the ammonia injected. Since the amount needed depends on the amount of NOX present, and since that varies with engine operating circumstances, a software-dosing algorithm is needed. The engine management system monitors engine operation to determine the likely production of NOX. The amount of ammonia required is then injected into the converter.

A paper issued by the Mesa Research Institute points out that if too much ammonia is injected, it will pass through the catalyst and into the atmosphere. Since ammonia is a pollutant in its own right, this process, called ammonia slip, must be prevented. Ammonia slip is most likely to occur when the SCR temperature is too low for the conversion to occur or when the electronic controller sees a transient engine condition to which it cannot react fast enough. A better way to control the injection amount is to have a feedback device that monitors the amount of ammonia present in the exhaust as it passes through the converter.

To be an effective sensor, an ammonia detector has to be able to measure very small quantities of ammonia in an exhaust stream that should ideally have little or no ammonia in it. Much like an O2 sensor, an ammonia sensor must be able to handle the high temperatures of the converter and be able to react fast enough to keep the overall process under control.

There are a half-dozen different detection methods for ammonia. What they all have in common is that they work only within a narrow temperature range. The most practical technology for automobiles is a diffusion process where a substrate material is exposed to the exhaust gas stream. As the ammonia diffuses onto the matrix, it changes the conductivity of the material. This can then be used to change an output voltage signal in response to the ammonia concentration level. The key element here is that it's a closed-loop system that feeds the result back to the engine management system.

Through the use of turbos, electronic controls, newly formulated fuels and pollution technology, the stink, rattle, lack of power and smoke will no longer be the hallmarks of a diesel-fueled vehicle. There will be new service opportunities in both maintenance and repair for those who update their knowledge of diesel technology.