On the other end of the scale sit piezoelectric injectors. They can cycle twice as often as magnetically triggered injectors, so short duration injections can be used to generate mixture flows that are easily ignited. Plus, this injector?s circular side opening creates a cohesive cone-shaped stream flowing from the top center of the combustion chamber, and can be timed to produce a charge of varying density. Its big disadvantage is that it is larger and more expensive than magnetically operated, multiple aperture injectors.

Piezoelectric injectors, however, may prove to be invaluable in future clean diesels because the quick on-off feature gives engineers greater leeway in spray timing and density, which has a tremendous effect on the noise and pollution emitted by compression ignition engines. ?Intervals between individual fuel injections can be reduced,? says Dr. Ulrich Dohle, president Bosch Diesel Systems, ?which gives greater flexibility in meeting power, economy, and emission goals.? The advanced diesel piezoelectric injector locates several hundred thin crystals within the injector body. The movement of the piezo wafers?they expand in the presence of an electric field?is transferred to the nozzle needle in less than 0.0001 seconds, and its inline design means very little fuel return flow is needed. This reduces the delivery rate and energy required from the high-pressure fuel pump.

?The key to improving fuel consumption and reducing emissions from a diesel engine,? says Dohle, ?is to reduce internal engine emissions through optimization of the basic engine design, and improvements in the injection strategy itself.? Adding one or two pre-injections stops smoke generation just after a cold start, which also reduces combustion noise. A post-injection immediately follows the main injection to lower soot emissions, and a further injection can be added to regenerate the particulate filter. Compared to existing magnetic or piezoelectric systems, Dohle expects a common-rail system with piezoelectric inline injectors to cut internal engine emissions by as much as 20%.