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The Inter-Industry Emission Control (IIEC) Program included the thermal reactor as one of the effective ways of oxidizing HC and CO in the exhaust system. However, this was accompanied by very substantial fuel economy penalties, especially in the case of small engine-low emission concept vehicles. Starting with a new concept aimed at obtaining the HC/CO oxidizing trigger temperature in the thermal reactor by modifying engine settings, the authors arrived at an economical technique of matching the thermal reactor to the engine.

A number of approaches to the development of low emissions engines have been reported in the literature. Many of these approaches, however, have reduced emissions by retarding ignition timing. This increases exhaust gas temperatures for more complete HC oxidation in the exhaust system but results in a fuel economy penalty. Previous studies by the authors indicated that NOx can be effectively reduced with judicious use of EGR. While this may result in increased HC emissions, under some conditions it also can result in improved fuel economy. The data reported in this paper demonstrate that the use of EGR is capable of reducing NOx emissions and octane number requirements while improving fuel economy. Any resultant increase in HC emissions can be controlled with an efficient HC/CO oxidation catalyst.

Careful study has been made of the aspects of unstable combustion due to heavy E.G.R. (Exhaust Gas Recirculation) while attempting to achieve low NOx emission levels. The authors have concluded that the fast burn engine allowed a wider E.G.R. tolerance, with stable combustion, at very low NOx emission levels in comparison with the conventional SI engine. This was true even under heavy E.G.R. Better fuel economy was achieved through a combination of improved cycle efficiency (using the fast burn concept) and reduced pumping loss (applying heavy E.G.R.). The Nissan NAPS-Z Fast Burn Engine was developed as a consequence of and through the use of the results mentioned above. The characteristic data of the engine (4 cyl. 1.8 ltr.) described in the paper showed a method of achieving a low NOx emission level with better fuel economy and driveability.

Earlier work has demonstrated that exhaust gas recirculation (EGR) decreases peak combustion temperature and thus reduces the concentration of nitrogen oxides (NOx) in spark ignition engine exhaust. The present authors hypothesized that NOx formation is primarily affected by the heat capacity of the combustion gases and recycled exhaust. The hypothesis was tested in an experimental program involving the admission of inert gases such as He, Ar, H2, and CO2, and water in place of EGR. In addition to confirming the validity of the original hypothesis, the test data also indicated that engine output and efficiency were significantly affected by the heat capacity of the combustion gases. The authors conclude that EGR functions by increasing the heat capacity of the working fluid, and demonstrates that the correlative changes in NOx and engine performance can be predicted from heat capacity considerations.

In the way to seek a marked reduction of NOx just by EGR which was proved to be the best answer from the past studies, this analytical study deals with combustion around the practical engine stability limit. The results show that practical engine operating stability limit with the four cylinder engine is determined by the occurrence of several percent of slow burn. Experiments applying short combustion duration (fast burn) showed that the fast burn in combination with heavy EGR improves engine stability and has a potential to achieve substantial reduction in NOx emission along with some improvement in fuel economy. This potential was also directed by mathematical model study. This fast burn concept was embodied in the newly developed Nissan Z Fast Burn Engine.