A single-cylinder direct-injection diesel engine with a displacement typical of passenger car practice was built to incorporate low-heat-rejection (LHR) components. These included a silicon-nitride piston cap, an Inconel firedeck, and stainless steel portliners, all backed by air gaps. In addition, a water-cooled engine with the same geometry was tested to provide a baseline. The LHR engine was tested with three different injector spray-tip geometries to assess the sensitivity of the results to the injection system. The LHR engine was worse than the baseline engine in most respects: it demonstrated higher fuel consumption and higher NO and HC emissions, and it rejected more heat at a given operating condition. The exhaust smoke emissions from both engines were similar, with the LHR engine having a small advantage at 2000 r/min. Destructive failure of the piston cap occurred after roughly 60 hours of operation.