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

The key to diesel’s future in the United States is its ability to further reduce emissions. The
manufacturers interviewed by OTA have a number of technological innovations for the DI diesel
under development, which will reduce emissions and, in some cases, improve performance.

Variable geometry turbocharging of several types is being investigated by the industry. Current
turbochargers are well matched to piston engine requirements only over a narrow range of rpm.
New types of turbochargers includes those with pivoting inlet guide vanes, simpler variable inlet
types, so called “jet” types, and new types with “wing’ ’-shaped impellers. According to two
manufacturers interviewed, these turbochargers can extend the range of useful boost, and reduce
the low-speed drivability deficiencies of normal turbos. The increased boost can also be translated
into decreased particulate and HC emissions.

The four-valve head/central injector was already discussed with reference to the Mercedes

production IDI engine. All German manufacturers interviewed stated that this concept is even

more beneficial to a DI diesel engine and could reduce emissions by 10 percent to 15 percent.

Swirl optimization is an inherent part of the design of the new four-valve head.

Improved fuel injection is associated with higher injection pressure, electronic control of
injection rate, and the use of pilot injection. In particular, injection rate shaping and the use of
pilot injection has resulted in very significant reductions in the NOX/particulate tradeoff curve.
Pilot injection was also found to lead to very large reductions in combustion noise (up to 12
decibels at high load) in DI diesels.^63

Optimized exhaust gas recirculation (EGR) can be used principally to reduce NOX. Owing to

the very lean air-fuel ratio employed, high EGR rates (over 40 percent) are required at light loads,

and such rates have been found to reduce NOX and HC emissions simultaneously. In addition,

EGR has also been found to eliminate noisy cold start combustion, although it may increase

smoke slightly.^64

Based on manufacturers’ estimates, the total reduction in NOX emissions (at near constant

particulate emissions) possible are as follows:

l Variable geometry turbo: -3 to -5%,

l Four-valve head: -10 to -15%,

l Electronic fuel injection (FI) with pilot injection: -15 to -20%,

l Optimized EGR: -25 to -30%.

(^63) FEV Motoren Technik, “Study of Pilot Injection,” SAE paper 940674, 1994.
(^64) I. Fukutani and E. Watanabe, “Reduction of Idle Knock by EGR in a Passenger Car Diesel Engine," SAE paper 840421, 1986.