This study looks forward to an era in which renewable hydrogen will constitute a major proportion of the supply fuel. It is very likely that the production sites will be far removed from the consumers. From this point of view, liquid ammonia as a hydrogen carrier has an advantage over hydrogen in that it has a large energy density. The long-distance transportation of liquid hydrogen to consumers consumes more energy than providing liquid ammonia, including LHV (lower heating value) loss through synthesis. Besides, considering current storage technologies, the volumetric and gravimetric storage capacity of liquid ammonia is about 2.5 times and 3 times greater than high-pressure compressed hydrogen, respectively. This is crucial for vehicles carrying large amounts of fuel needed to transport heavy freight. Ammonia can be used as a fuel for internal combustion engines. Although ammonia has a low flame velocity, hydrogen helps the ammonia mixture to burn stably. Replacing 40% of the ammonia with hydrogen, on an energy basis, enables the mixture to burn as fast as a gasoline mixture. The auto-thermal-converter that we developed is able to control the ratio of hydrogen by adjusting the air-ammonia ratio. This idea was successfully demonstrated using a single-cylinder test engine. It has been found that stable combustion can be achieved under high loads without auxiliary hydrogen. No knocking has been observed even under charged conditions thanks to the high ignition temperature. An engine system simulation indicated that ammonia has the potential to be applied as fuel for heavy-duty diesel engines. The major challenges to overcome are: (1) Achieve emission control during cold starting before the catalyst reaches its operating temperature, as well as at all engine speeds and loads. (2) Improve the combustion efficiency with less or completely without auxiliary hydrogen.