The efficiency of an internal combustion engine is highly dependent on the peak pressure at which the engine operates. A new compound engine concept, the double compression expansion engine (DCEE), utilizes a two-stage compression and expansion cycle to reach ultrahigh efficiencies. This engine takes advantage of its high-integrity structure, which is adapted to high pressures, and the peak motored pressure reaches up to 300 bar. However, this makes the use of conventional combustion cycles, such as the Seiliger-Sabathe (mixed) or Otto (isochoric) cycles, not feasible as they involve a further pressure rise due to combustion. This study investigates the concept of isobaric combustion at relatively high peak pressures and compares this concept with traditional diesel combustion cycles in terms of efficiency and emissions. Multiple consecutive injections through a single injector are used for controlling the heat release rate profile to achieve isobaric heat addition. In this study, the intake pressure is varied to enable a comparison between the isobaric cases with different peak pressures, up to 150 bar, and the mixed cycle cases. Tests are performed at several different levels of EGR. The experiments are performed on a 12.8 L displacement 6-cylinder Volvo D13C500 engine utilizing a single cylinder with a standard 17-compression-ratio piston. In this study, the cylinder represents the high-pressure unit of the DCEE. The fuel used in all the experiments is a standard EU diesel. In each target condition, the different injection strategies are compared with the total amount of fuel kept relatively constant. The results prove that the isobaric combustion concept is feasible with a traditional injection system and can achieve gross indicated efficiencies close to or higher than those of a conventional diesel combustion cycle. Moreover, the results show that with an isobaric cycle, heat transfer losses can be reduced by over 20%. However, the exhaust energy is higher, which can eventually be recovered in the second stage of expansion. Thus, this cycle could be suitable for the DCEE concept. The CO, UHC and soot emission levels are proven to be fairly similar to those of the conventional diesel combustion. However, the NOx emissions are significantly lower for the isobaric combustion.