The diesel's outstanding fuel economy also explains two decades of combustion engineering research to eliminate these well-known drawbacks. Concern about global warming has brought even greater impetus to these efforts, since greater fuel efficiency translates directly into reduced emissions of carbon dioxide—the principal greenhouse gas.

Resulting advances in diesel design—particularly the injection of fuel directly into the cylinders—have in recent years produced downscaled, higher-speed diesels that offer demonstrable exhaust emissions and fuel-economy gains as well as substantial drivability and noise/vibration/harshness improvements. Contrary to what many Americans think, diesels aren't dirty or noisy anymore.

That point was made clear several years ago, when Volkswagen and Audi introduced their TDI turbocharged direct- injection diesel on most of their models. "VW's TDI engine combines extremely high fuel economy (45 miles per gallon highway and 37 miles per gallon city) and very good performance," said Horst Schulte, chief technical officer for FEV Engine Technology Inc. in Auburn Hills, Mich., an engineering firm headquartered in Germany that specializes on internal-combustion engines.

Direct-injection (DI) diesels such as the TDI feature combustion chambers that receive fuel directly from the injectors and handle the entire combustion process. This design contrasts with the widely used indirect-injection (IDI) diesel engine, which has a small secondary chamber in the cylinder head connected to the main combustion chamber via a throat. IDI diesels use these prechambers or swirl chambers to start off the combustion cycle. The Diesel Research Team at Ford Motor Co.'s Research Lab in Dearborn, Mich., is developing the Diata, a small compression-ignition, direct-injection aluminum engine to power a future ultrahigh-mileage hybrid vehicle. "In IDI engines, which were the type used previously in passenger cars, fuel is injected into the swirl or prechamber in a way that enhances fuel/air mixing,"said David Boggs, a technical specialist on the Ford team. "The fuel undergoes partial combustion in that chamber, and a hot jet issues out into the main chamber."

The Robert Bosch common-rail fuel-injector system for direct-injection diesel engines will soon appear on several new European car models produced by Fiat and Mercedes-Benz

"As combustion proceeds from the swirl or prechamber, you get throttle losses and heat losses, which are the major reasons for the lower efficiency compared to DI,"Schulte said. The throttle or flow losses are generated, he added, because gases are forced from the swirl or prechamber through small bores (holes), which are necessary to improve the fuel/air mixture formation. Flow losses result. An IDI unit, he said, provides a good precondition for high heat transfer to the surrounding walls, which results in heat losses. The precombustion process also means that it takes longer to complete the whole combustion process, which reduces fuel efficiency as well.

By reducing losses associated with a divided combustion chamber, DI diesels generally offer a 15-percent efficiency improvement over IDI systems, Boggs said. "In the past, DI systems were restricted to the heavy-duty truck market. DI car engines are another example of the general industry practice of cascading new technology to smaller engine sizes."Boggs noted that the downsizing issue is important in designing smaller vehicles for reasons of packaging and weight savings.

According to Schulte, the breakthrough for engines such as the VW TDI was a fuel-injection system that provides enough injection pressure to get the particulate emissions down as well, especially at a retarded injection timing, which is required to achieve low nitrous oxide emissions. A wide speed range, generally 1,000 to 4,000 rpm, is required for passenger-car operation. It is very difficult, however, for a high-speed DI diesel to have good fuel/air mixture formation characteristics over a large speed range, he added.

"To provide sufficient mixture formation when the fuel is being injected directly into the air, it is necessary to have either a lot of air motion (swirl) or many spray holes as large engines do,"Schulte said. Modern DI diesels use high injection pressures to enhance air/fuel formation. "With increasing pressure capabilities of advanced fuel-injection systems, it is most likely that we will see more quiescent-type (low-swirl) combustion systems also on smaller DI diesel engines,"he said.

Automotive-market analysts at Lucas Diesel Systems in Troy, Mich., are predicting a major move in the car sector from IDI to DI technology during the next five years, mostly due to its fuel-economy advantage. In addition, various industry forecasts indicate that only 20 percent of light-duty diesel-powered vehicles will have IDI engines within five or six years. There is also the prospect of midsize family sedans producing fuel-economy figures in excess of 60 miles per gallon, according to Lucas Diesel.

At the same time, Lucas Diesel predicted, increasingly stringent emissions regulations mean that high-speed direct-injection (HSDI) car engines will have to use new fueling technologies—in particular the use of far higher fuel-injection pressures. Lucas Diesel researchers added that estimates by leading combustion engineering consultants indicate the need for pressures up to 2,000 bars.

Upcoming European exhaust emissions regulations are closely tied to reducing carbon dioxide output. This means that fuel economy, and therefore the limitation of carbon dioxide formation, may be more important in Europe than limits on nitrous oxides, which tend to be stricter in the United States. These ever-tightening U.S. emissions regulations for nitrous oxides could thwart even the new high-tech diesels, because most lean-combustion engines—those that burn fuel at high temperatures with less than the stoichiometric amount of oxygen—produce high NOx emissions. Pending U.S. standards strongly target NOx, which is linked to ozone formation.

A common-rail fuel injector includes a common pressure accumulator, a high-pressure regulator, a high-pressure supply pump, injectors, electronic solenoids, an electronic control unit, and a filter unit

In the meantime, Europe tends to be more concerned with carbon dioxide, carbon monoxide (resulting from the incomplete oxidation of carbon), and diesel particulates, which come from three sources: carbon in the fuel, the soluble organic fraction derived from fuel and lube oil, and sulfate from the oxidation of sulfur in fuel. According to a recent report from Volvo, the oil industry is starting to supply Europe and the United States with "environmentally compatible diesel fuel"for highway vehicles next year. The fuel's low sulfur content (0.05 percent) results in reduced particulate, the report said.

The differences between European and U.S. exhaust-emissions standards are leading to something of a political debate over the specific environmental goals regarding each pollutant. These choices will, in turn, determine the kind of combustion-engine system that is selected in the future, because each kind of engine design has its advantages.

This debate remains unresolved. For example, diesel engines recently won a cleaner bill of health regarding NOx emissions. Trials in France have shown that gasoline engines emit more NOx over their entire lifetime than diesel engines do. "It is a myth that the diesel produces horrific emissions,"said Jean-Francoise Cayot, deputy managing director for Lucas Diesel. "As the petrol car gets further into its service life, it emits more NOx pollution than the diesel. The whole industry recognizes this fact, but no one is making anything of it."Cayot added: "It is accepted that the diesel gives out less carbon monoxide and hydrocarbons than petrol. With exhaust-gas recirculation and the use of fuel with lower sulfur content, particulates can be cut by up to 50 percent."

The biggest recent technical advance in high-pressure injection is the common-rail design, according to Ford's Boggs. In a common-rail DI system, he said, high-pressure pumps feed into a manifold (accumulator) called a rail, which acts as something of a buffer. This arrangement gives flexibility with regard to injection pressure over the entire engine speed range. "Ford sees the future of compression-ignition direct-injection engines in the common-rail approach,"Boggs said.

Other automakers may feel the same way about common-rail technology. New cars for the European market in 1998—several models from Fiat and the Mercedes A class—will feature a common-rail DI system originally developed by Fiat Group and manufactured by Robert Bosch. Lucas Diesel has also developed a common-rail system of its own for future HSDI engines.

At the same time, Japan's Denso and parent company Toyota have produced a similar system "as a key technology to answer the challenge facing the light-duty, high-speed diesels of tomorrow,"according to Toyota literature. Common-rail systems "permit higher specific output, better fuel consumption, much reduced noise, and generally improved characteristics."Specific power and torque are increased by approximately 40 percent compared with Toyota's current IDI diesels, while specific fuel consumption—and therefore carbon dioxide emissions—is 30 percent better.

Among the various available DI systems, Schulte said the common-rail fuel injection system provides the greatest degree of freedom. The main advantage to a common-rail system, he said, is that there is no relationship between engine speed and injector pressure. "In traditional fuel-injection systems, you can get only limited pressure at low engine speeds. In addition, high-speed engines offer reduced time for fuel/air mixture formation,"Schulte explained, so injection pressure is key to moving the combustion along at a fast pace. Common-rail systems, he added, "can generate almost 1,000 bars of pressure already at an engine speed of only 1,500 rpm, which was previously impossible."

Cam-driven injection systems, such as inline pumps, distributor pumps, unit injectors, or unit pumps, build up the injection pressure for each injection. Fuel metering and pressure buildup are therefore linked. The injection pressure results from the metered fuel quantity being pushed through the nozzle orifice by the injection piston with a velocity proportional to engine speed. In contrast to this approach, the functions of fuel metering and pressure buildup in a common-rail system are independent of accumulator injection systems.

Schulte added that diesel common-rail systems embody the same concept as gasoline-engine rails, which feature an accumulator connected with tubes to the injectors. "The basic difference between them is the injector pressure. In a common-rail diesel, you can reach 1,300 to 1,600 bars of pressure. In a gasoline rail system, the pressures are much less—3 or 4 bars."

According to Toyota engineers, a typical common-rail system comprises a supply pump, a common rail (accumulator), and injectors all joined by high-pressure piping, an electronic control unit, an electronic driver unit, and various sensors. The supply pump maintains high pressure inside the rail, and fuel is injected by opening and closing an internal electromagnetic valve in each injector. The system allows extensive freedom of control, based on sensor information. The engine computer regulates the quantity of injected fuel according to the number of engine revolutions and engine load. Injection pressure, timing, and quantity can all be varied independently and with great precision. In addition, improved electromagnetic valves have resulted in greater precision, smaller size, and lower power requirements.

Toyota engineers stressed that the compact design of their common-rail system permits the optimal placement of valves. "Size is important to narrow the angle between the paired inlet and exhaust valves, which is important in achieving good combustion in DI engines."Toyota's four-valve-per-cylinder engine design not only produces greater gas flow, which increases the potential specific power output, but also frees the geometric center of the combustion chamber to house the fuel injector optimally—exactly upright so the center of injection coincides with the center of the in-cylinder swirl motion. The resulting radial injection geometry significantly improves combustion and helps reduce exhaust emissions while it improves efficiency.

Besides variations in the quantity and start of the injection, the common rail permits the choice of pressure from 150 to 1,400 bars and the delivery of injection fuel in several portions, according to Gerhard Stumpp and Mario Ricco of Bosch's Diesel Division in Feuerbach, Germany.

In the Bosch common-rail design, a supply pump draws fuel from the tank and feeds it to the high-pressure pump (a radial piston pump). The pump is driven by the engine and delivers the fuel to the injectors in the cylinders. One part of the fuel is injected into the combustion chambers, while a smaller portion controls the injection nozzles and then flows back to the tank.

There is no specific accumulator or collection volume for the fuel in the Bosch unit, according to Stumpp and Ricco. In this system, the fuel volume between the high-pressure pump and the injectors serves as an accumulator, a trapped volume of 30 to 40 cubic centimeters. The fuel is compressible and dampens oscillations initiated by the pulsating delivery of the high-pressure pump and especially by the abrupt extraction of fuel via the injectors.

A pressure sensor measures the fuel pressure in the rail. Its signal is compared with a desired value stored in the engine computer. If the measured value and the desired value are different, an overflow orifice in the pressure regulator on the high pressure side is opened or closed. The overflow returns to the tank.

The injectors are opened and closed by the engine computer at defined times. The duration of injection, the fuel pressure in the rail, and the flow area of the injector determines the injected fuel quantity. The solenoid valves are controlled according to the accelerator position and the engine information.

According to Schulte, the problem with high-pressure systems is that they require highly precise manufacturing. "Very high tolerances are needed to handle the higher forces, which leads to an expensive fuel system."Current advanced fuel-injection system such as common rails can account for 30 to 40 percent of the total engine cost, he noted.

"The electronic management in the newer fuel-injection systems is time-based control systems—injection timing can therefore be very flexible and high precision,"Schulte said. "Previous injection systems relied on mechanical components whose geometry determined the injection event."

"One benefit of electronic controls is that you've got an electronic solenoid on the injector nozzle, which means you can do pilot injection—a kind of injection rate shaping,"said Ford's Boggs. "Instead of straight on/off nozzle control as in mechanical injection systems, the newer electronic systems allows you to open the injector nozzle gradually and do some dithering.

"With an on/off-type system,"he added, "all you can do is slam the nozzle open and introduce a large amount of fuel all at once."This practice introduces a lot of fuel into the combustion chamber during ignition delay—it takes a certain amount of time to vaporize the fuel, then mix it with the air to get the right fuel/air equivalence rate. When the burning first starts, there is too much fuel available, Boggs explained, which raises noise levels (knock) and NOx emissions. Without pilot injection, DI engines suffer from high noise levels caused by the rapid rise in cylinder pressure that occurs when combustion takes place, according to Toyota engineers.

Ford's Diata small diesel prototype, which was designed to power the automaker's P2000 hybrid vehicle, uses a common-rail direct-injection system developed by FEV Engine Technology

In the pilot injection technique, a small quantity (1 to 2 cubic millimeters per stroke) is injected before the main injection. A typical injection period is about 300 microseconds. For this purpose, the pilot quantity has to be controlled precisely and must take place at the right time interval before the main injection. Too small and too early raises the noise; too large increases the particulate emissions. In the best case, the quantity decreases with increasing engine speed and its interval—in crank angles—to the main injection increases with rising engine speed.

"If you can slowly introduce the fuel and just burn a little at the start,"Boggs said, "you can then have the rest of the fuel come in after the flame has begun. This tends to make NOx emissions to go way down."He said that a lot of development still needs to be done on the pilot injection technology; for example, an infinitely variable nozzle is theoretically desirable for even finer control.

It is also possible to implement postinjection processes for still-to-be-developed de-NOx catalysts that will be essential for future exhaust-gas-reduction technologies, and achieve optimum drivability with the aid of feed-forward and feedback through the control system. (Siemens, for example, is developing a "selective-catalytic-reduction"de-NOx exhaust-treatment unit based on urea.) In postinjection processes, fuel can be injected during the expansion stroke and can serve as a reducing agent for a future de-NOx catalyst. Injection sequences that include both pre- (pilot) and postinjection sprays will be needed to reduce NOx emissions enough to meet the upcoming Euro-3 and US-98 exhaust regulations.

According to Lucas Diesel, the challenges to the success of the common-rail systems include the refinement of pilot injection, which is necessary—even in transient conditions—to reduce acceleration noise; new actuators with shorter response times to allow an acceptable time between the pilot and main injections; new processes for smaller tolerances and the calibration of injectors; and a high-pressure pump with variable flow control to reduce power consumption.

"There are other alternatives to the common-rail design,"Schulte said. "The race is not yet decided."One contender is the high-pressure radial piston pump, which both Lucas Diesel and Bosch produce. This type of pump, which can attain pressures of 1,500 bars, has two radially positioned pistons that are arranged in opposition to each other so they can "squeeze"the fuel out at high pressures as they approach each other.

Inside the pump, the pump shaft rotates together with two radial distributor plungers. These plungers are supported by roller shoes on the fixed cam ring, with cam elevations. When the rollers pass over the cams, both pistons are forced inward simultaneously, pressurizing the fuel if the solenoid valve is closed. Modern radial piston pumps are electronically controlled with solenoids determining which cylinder gets what quantity of fuel at what time. General Motors introduced an Opel DI e