Ammonia-water cycles can produce more power than steam Rankine cycles in several applications. One of these applications is as a bottoming cycle to internal combustion engines. In the present study, ammonia-water bottoming cycle configurations for spark-ignition gas engines and compression-ignition gas diesel engines have been compared, Single-pressure Rankine cycles have been used as a basis for the comparison. Low heat source temperatures should increase the difference in power output between the ammonia-water cycle and the Rankine cycle. However, in this study, the results of the simulations show different trends. In most cases, the ammonia-water bottoming cycles with gas engines as prime movers generate more power compared to a Rankine cycle than when gas diesel engines are the prime movers. The temperature of the most important waste heat source, the exhaust gas, is approximately 100 degreesC higher for the gas engines than for the gas diesel engines. Therefore, for the gas engines, most of the waste heat available to a bottoming cycle is in the form of relatively high-temperature exhaust gas, while for the gas diesel engines more of the waste heat is in the form of relatively low-temperature heat sources. (C) 2001 Elsevier Science Ltd. All rights reserved.