Advanced Automotive Technology: Visions of a Super-Efficient Family Car

design results in smoother combustion with less noise and lower NOX emissions. However, heat

transfer from the prechamber and pressure losses from the partially combusted gases as they flow

through the small passages connecting the prechamber to the main combustion chamber result in

reduced efficiency. In fact, the peak efficiency of an IDI diesel is comparable to, or only slightly

better than, that of a spark ignition engine; most of its efficiency advantage occurs at light loads.

Direct injection (DI) systems avoid the heat and flow losses from the prechamber by injecting

the fuel directly into the combustion chamber. The fuel injection system must be quite

sophisticated, as it must be capable of injecting very little fuel during the ignition delay period,

while providing highly atomized fuel and providing intensive mixing during primary combustion.

Advancements in fuel injection technology and diesel combustion chamber design has led to the

recent introduction of passenger car DI diesels by Volkswagen in their Audi and VW model lines.

Turbocharging has also been found to be particularly effective in combination with diesel

engines. Many new diesel engines, including the Volkswagen DI diesel engines, are turbocharged

and some feature intercoolers, which provide a cooler, denser charge to the engine. As a result,

the specific power of diesel engines with turbocharging now exceeds the specific power output of

naturally aspirated, two-valve per cylinder gasoline engines and approaches that of four-valve per

cylinder gasoline engines. Turbocharging and intercooling are quite costly, however, and

turbocharged engines still have some low-speed drivability deficiencies.

Four valve per cylinder technology has also been introduced by Mercedes Benz in 1994 for

several of their diesel engines. These engines have attained a specific output of 45 BHP/liter

without the use of turbocharging, levels only slightly lower than typical two-valve spark ignition

engines.^59 The four-valve engines are of the IDI type, but the central placement of the prechamber

possible in a four-valve cylinder head has resulted in improved emissions and fuel consumption

relative to a two-valve IDI engine. At full load, Mercedes claims an 8 percent reduction in specific

fuel consumption relative to a two-valve engine, but the benefit is much smaller at light loads.^60

Emissions of the new engines are also low enough to meet all U.S. standards given the current
NOX waiver. The Mercedes four-valve engine, in conjunction with California’s low sulfur, low
aromatic content diesel fuel can actually meet the LEV standards for HC, CO, and particulate.
However, NOX emissions are four times greater than applicable LEV standards. VW expects that
its turbocharged DI diesels will have emission levels similar to those of the Mercedes four-valve
IDI diesel, although the W diesel is not (yet) offered for sale in the United States but is expected
for 1996.

Data are lacking on fuel economy benefits based on the U.S. test cycles, but considerable data

exists for the European Test Cycle. The European City Cycle is significantly slower than the U.S.

city cycle, with longer idle time, and, hence, reported ECE (European Economic Commission)

city fuel economy values are 12 percent lower (on average) than U.S. FTP-based values. The

ECE 90 km/h steady-state test results in fuel economy values similar to those recorded in the U.S.

highway test, but there is no U.S. equivalent to the ECE 120 km/hr steady-state test. Official ECE