Technical Papers
Techniques for Low NOx Combustion on Medium Speed Diesel Engine*
Seita Akimoto**, Shigeru Yoshikawa**, Tohru Yonezawa**, Kanji Kanemitsu***
Techniques for low NOx combustion without degrading the specific fuel consumption were developed for the medium speed diesel engine. First concept for low NOx is the reduction of high temperature combustion zone. Owing to this concept. the high-temperature combustion flame zone is reduced by the injection direction near to the piston surface. Second concept for low NOx is the reduction of the flame temperature owing to the quick mixing of the high-temperature combustion zone and the low-temperature air portion. It is necessary to obtain the air motion which has the quick mixing of the combustion zone and air portion. The air motion adopted in this study was the intake swirl induced by the intake valve with shroud. The medium speed engine with a bore of 280 mm was used in the application of the new techniques for reducing NOx emission. The engine based on the low NOx techniques has the decrease in NOx emission by 40% and almost the same specific fuel consumption compared with the base engine.
1. Introduction
As for the NOx regulation in the medium speed engine, stringent regulations have been enforced not only by the Air Pollution Control Law (NOx＜950 ppm) but also by the local governments in major cities, the gas engines small in NOx emission and the diesel engine high in efficiency and low in fuel consumption are installed according to the NOx regulatory value, and these engines are now classified domestically. In addition, the NOx regulation is also internationally enforced to marine engines by IMO (International Maritime Organization). The regulated value is never be stringent, and it is feared that degradation of the fuel consumption in the trade-off relationship with NOx may cause the increase in the operational cost of marine engines. In all case, the low NOx technology to prevent the fuel consumption from being degraded for both engines for land use and marine use becomes necessary and essential.
In this study, two concepts (1) on the low NOx combustion, i.e., (1) reduction of the combustion zone of λ=1 and (2) promotion of the mixing of the combustion flame zone and air portion, are developed on the medium speed engine. The low NOx technology is proposed for the specific technology to realize these concepts taking into consideration the difference in the combustion phenomenon of both the compact high speed engine and the medium speed engine, and its effectiveness is demonstrated using the single cylinder engine for study. As a result, the NOx emission can be reduced by approximately 40% with same fuel consumption, and the result is reported below.
2. Low NOx Technology
2.1 Low NOx Concept
The combustion phenomenon in the diesel engine to dominate the performance (output, fuel consumption, exhaust gas) of the diesel engine, the so-called diesel combustion, follows the process of the fuel injection, fuel spray formation and formation of the fuel-air mixture in combustion chamber, ignition, and combustion. Fig. 1 shows the combustion process of the medium size diesel engine using the combustion photographs and the models on the temperature in each zone, i.e., in (1) the initial stage of combustion, (2) the middle stage of combustion, and (3) the final stage of combustion.
Firstly, in the initial stage (1) of combustion, the fuel is injected into the compressed air, and naturally ignited at a position where the fuel and the air is mixed in an optimum condition, that is, self-ignition. The mixed condition of the fuel and air is a condition of the excess air ratio λ=1, which means that the fuel is mixed with air in an optimum condition. This condition of combustion is the stoichiometric combustion of fuel-air mixture, and the combustion temperature is very high, and the NOx emission is high.
To solve the problems, in the diesel engine, it is firstly necessary to reduce the zone in which the stoichiometric combustion of fuel-air mixture (λ=1) is performed, that is, to reduce the part of the stoichiometric combustion of fuel-air mixture (λ=1).
Secondly, in the middle stage of combustion (2), the part of the stoichiometric combustion of fuel-air mixture (λ=1) at high temperature is burned in a relatively slow manner while mixed with air, and the flame grows. Since the temperature of unburnt air is low, the combustion temperature is gradually dropped by the mixing of the combustion portion with the air of low temperature.
* Translated from Journal of MESJ Vol. 34, No. 2 (Manuscript received Oct. 24, 1998)
** Yanmar Diesel Engine Co., Ltd. (Oyamazaki, Kyoto)
*** Petroleum Energy Center (Minato-ku, Tokyo)
