Future internal combustion engines demand higher efficiency but progression towards this is limited by the phenomenon called knock. A possible solution for reaching high efficiency will be to improve the anti-knock quality of the fuels by blending high-octane fuel with a low-octane fuel. In this study, the non-linear blending effect by blending oxygenated/non-oxygenated fuels of high octane number with low octane fuels were studied in three different combustion modes: Spark ignition (SI), Homogeneous Charge Compression ignition (HCCI) and Compression Ignition (CI). For SI combustion, RON and MON was used for the fuel rating, for HCCI combustion, Lund Chevron HCCI fuel number and for rich combustion conditions, Derived Cetane Number (DCN) was used to understand the fuel auto-ignition behavior. A Cooperative Fuel Research (CFR) engine was used for SI and HCCI mode whereas Ignition Quality Tester (IQT) was used for CI mode. The non-linear blending behavior was described using the concept of blending octane number. Five octane additives including ethanol, methanol, 1-butanol, toluene and iso-octane were used in this study, of which ethanol and methanol gave the strongest octane enhancement effect whereas iso-octane resulted in the weakest octane enhancement. The base fuel composition and octane number also had an important role in the blending behavior of the fuels. The non-linear blending of fuels highlighted that some of the blended fuels behaved similarly in both SI and HCCI combustion mode, therefore the study was further extended to understand the pre-spark heat release or Low temperature heat release (LTHR) in both the combustion modes. Knock occurs in SI due to end-gas auto-ignition and for HCCI, the combustion is controlled by auto-igniting of the complete charge inside the cylinder. Therefore fundamentally the combustion process in the end gas region of SI and HCCI combustion modes is controlled by auto-ignition. In this respect, HCCI combustion was used as an alternative path to understand the end-gas auto-ignition in SI engine using the standard CFR engine. Pre-spark heat release or low temperature reactions were detected in both the combustion modes.