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A study is carried out here to examine the probable causes for the seemingly contradictory results found in the present-day literature on low-heat-rejection (LHR) engines, provide plausible explanations, and indicate possible directions for future research. Almost all numerical studies predict improved thermal efficiency, increased availability in the exhaust, and reduced in-cylinder heat rejection in the case of LHR engines. The degree of improvement varies considerably from a few percentage points to several, depending on the extent of insulation and whether or not turbocompounding and Rankine bottoming cycle are included. In these simulations, care is taken not to allow the volumetric efficiency to decrease due to higher cylinder temperatures in LHR engines. In addition, air-fuel ratio and, in many instances, peak conditions are maintained constant in both the LHR and conventionally-cooled engines.