The combustion properties of two low-volatility diesel fuels were characterized under engine-relevant conditions using a new fuel-substitution strategy that minimizes biases associated with unknown boundary conditions. The engine is operated in a homogeneous charge compression ignition mode with port fuel injection of the primary reference fuels, n-heptane and isooctane, and an upstream prevaporizer for the diesel-like heavy test fuels. The engine is first operated on the primary reference fuels (PRFs). The intake conditions are then fixed, and the heavy fuel is introduced in increasing amounts while the mass of port-injected fuel is reduced to maintain engine load and the n-heptane-to-isooctane ratio is adjusted to maintain constant combustion phasing. It is shown that this provides nearly constant in-cylinder thermodynamic and boundary conditions. The baseline condition, which uses well-characterized fuels, can be used to adjust the trapped-gas initial conditions and the heat transfer rates of a computational model, but these parameters do not change with the subsequent addition of heavy test fuel. The test fuel combustion characteristics are described in terms of the PRF number of the port-injected mixture. A simple linear blending method was found to adequately represent the heavy test fuel in terms of an effective PRF number. Testing under different operating conditions showed no significant change in the effective PRF number of the test fuels. The fuels investigated in this study were F-76, a high-sulfur-content diesel fuel, and HRD, a hydroprocessed renewable diesel fuel composed primarily of alkanes. Effective PRF numbers of 35 and -25 were found for F-76 and HRD, respectively.