Combustion and emissions characteristics of a compression-ignition engine using dual ammonia-diesel fuel

Abstract

<p>This study investigated the performance of a compression-ignition engine using a dual-fuel approach with ammonia and diesel fuel. With the world's increasing need for alternative energy and clean emissions, ammonia stands out as a viable candidate since its combustion does not produce the known greenhouse gas, carbon dioxide. Ammonia is one of the world's most synthesized chemicals and its infrastructure is well established. Ammonia can be regarded as a hydrogen carrier and used as a fuel. However, ammonia is highly resistant to autoignition and readily vaporizes under atmospheric conditions. Therefore, this study introduced ammonia vapor into the engine intake manifold and used the existing diesel injection system to inject diesel fuel or biodiesel to initiate combustion in the cylinder. The test engine was a four-cylinder, turbocharged diesel engine with slight modifications to the intake manifold for ammonia induction.</p> <p>An ammonia fueling system was developed in order to control and measure the amount of ammonia allowed into the engine. Dynamometer tests were performed to measure the engine power, fuel consumption, in-cylinder pressure histories, and exhaust emissions.</p> <p>Engine test results showed that ammonia could be burned in a compression-igntion engine using the present dual-fuel approach. Various combinations of amounts of ammonia and diesel fuel were successfully tested using two major engine operation schemes. One scheme was to use different combinations of ammonia and diesel fuel to achieve a constant peak torque equal to that of 100% diesel only combustion. And the other was to use a minimum quantity of diesel fuel and vary the amount of ammonia to achieve variable engine loads.</p> <p>Under constant peak torque operation, in order to achieve favorable fuel efficiency, the preferred operation range was to use 40 ~ 60% energy provided by diesel fuel in conjunction with 60 ~ 40% energy supplied by ammonia. Exhaust carbon monoxide and hydrocarbon emissions using the dual-fuel approach were generally higher than that of using pure diesel fuel, while nitrogen oxides (NOx) emissions varied with different fueling combinations. NOx emissions could be reduced using the dual-fuel operation if ammonia accounted for less than 40% of the total fuel energy due to the lower combustion temperature resulting in lower thermal NOx. If ammonia accounted for the majority of the fuel energy, NOx emissions increased significantly due to fuel-bound nitrogen. On the other hand, soot emissions could be reduced significantly if a significant amount of ammonia was used due to the lack of carbon present in the combination of fuels. Despite the overall high ammonia conversion efficiency (nearly 100%), exhaust ammonia emissions ranged from 1,000 to 3,000 ppmV and further after-treatment will be required due to health concerns.</p> <p>The variable engine load operation resulted in relatively poor fuel efficiency due to the lack of diesel energy to initiate effective combustion. Exhaust ammonia emissions ranged between 4,000 and 12,000 ppmV under the conditions studied.</p> <p>In-cylinder pressure history was analyzed to obtain the heat release rate data and combustion phasing. Results indicated that ignition delay increased with increasing amounts of ammonia due to its high resistance to autoignition. As a result, the peak cylinder pressure decreased because of the lower combustion temperature of ammonia. Engine testing using combinations of ammonia and biodiesel (B100) were also performed and results had the same trends as using the ammonia-diesel approach. It is recommended that further combustion optimization using direct liquid ammonia injection be performed to increase combustion efficiency and reduce exhaust ammonia emissions.</p>