This study demonstrated the feasibility of ammonia combustion in compression-ignition diesel engines. Ammonia combustion does not produce carbon dioxide, a known greenhouse gas that contributes to global warming. Using this idea, a method was developed to introduce ammonia into the intake manifold and to inject diesel fuel or biodiesel directly into the cylinder to ignite the mixture. This dual-fuel approach was chosen because ammonia has a high resistance to autoignition. This approach was proven successful in a multicylinder, turbocharged diesel engine. The system developed required only a slight modification of the intake to implement the ammonia fuel line. The existing diesel fuel injection system remained unchanged. A liquid ammonia tank was used for fuel storage, and a high pressure relief valve regulated the ammonia flow rate. Engine combustion phasing (e.g., ignition) was controlled by diesel fuel injection. Both experiments and chemical kinetic studies were carried out for different diesel/ammonia ratios at various engine speeds and loads. Ammonia was used as an energy replacement for diesel fuel. The results showed that the peak engine torque could be achieved by using different combinations of diesel fuel and ammonia. During testing, a maximum energy replacement of 95% was measured. It should be noted that if more ammonia is added, a higher than rated power can be achieved depending on engine load conditions. This would be similar in practice to adding nitrous oxide to gasoline engines. It was also shown that CO2 emissions were reduced monotonically for the same engine torque output as the amount of the ammonia in the fuel mixture increased. Additionally, burning ammonia in engines does not necessarily increase NOx emissions despite the fuel-bound nitrogen. Lower levels of NOx emissions were obtained as long as energy substitution by ammonia did not exceed 60%. This is thought to occur because of the lower combustion temperature of ammonia. This study also showed that the engine could be operated at different load conditions by using a small quantity of diesel fuel with the appropriate amounts of ammonia to achieve desirable loads. Biodiesel was also used with ammonia at different ratios resulting in successful engine operation. Results of using biodiesel-ammonia were similar to those of using diesel fuel-ammonia.