In support of an ongoing U.S. Navy alternative fuel evaluation program, the combustion characteristics of two very different alternative diesel fuels were evaluated in a direct-injection marine diesel engine across a variety of speeds and loads. The fuels were an algal-based hydrotreated renewable diesel fuel (HRD) with cetane number of similar to 75 and a synthetic paraffinic kerosene (SPK) with cetane number of similar to 25. These fuels were experimentally tested as blends with conventional petroleum-based military diesel fuel (designated F-76) with cetane approximate to 46, giving a cetane number range from 25 to 75. Start of injection (SOT) was characterized using a strain gauge to determine actuation of the mechanical unit injector; SOI remained essentially unchanged for changes in fuel type. As expected based on cetane number, ignition delay (IGD) increased with greater amounts of SPK fuel and decreased for greater amounts of HRD fuel in the test blend. Energy release analysis showed that longer IGD led consistently to slightly advanced combustion phasing, as indicated by the location of 50% mass fraction burned, decreased overall combustion duration, and greater maximum rate of pressure rise due to greater fuel-air premixing. Fuel consumption was 0-5% higher for these alternative fuels. Ignition delay was modeled using a detailed primary reference fuel mechanism tuned to match the measured cetane number of each neat and blended fuel. The modeled chemistry was able to capture relative changes in the experimentally observed IGD, suggesting that the measured differences in physical properties, which will affect spray development, do not contribute as significantly to differences in IGD. The results suggest that typical higher cetane alternative fuels, such as HRD, have no deleterious effects from the perspective of combustion characteristics. Processes that yield lower cetane alternative fuels, such as SPK, while still achieving satisfactory performance, begin to show signs of problems through delayed combustion, increased rates of pressure rise, and higher peak pressures, which induce higher mechanical stress and combustion noise.