The potential of a Homogeneous Charge Compression Ignition (HCCI) engine with variable compression ratio has been experimentally investigated. The experiments were carried out in a single cylinder engine, equipped with a modified cylinder head. Altering the position of a secondary piston in the cylinder head enabled a change of the compression ratio. The secondary piston was controlled by a hydraulic system, which was operated from the control room. Dual port injection systems were used, which made it possible to change the ratio of two different fuels with the engine running. By mixing iso-octane with octane number 100 and normal heptane with octane number 0, it was possible to obtain any octane rating between 0 and 100. By using an electrical heater for the inlet air, it was possible to adjust the inlet air temperature to a selected value. In this way it was possible to study the relationship between the fuel's octane number, the inlet air temperature and the compression ratio needed to get auto-ignition close to Top Dead Center (TDC). Different fuel mixture ratios of gasoline and diesel fuel have also been tested in the same manner. All tests were carried out with a constant air/fuel equivalence ratio (λ) of 3.0.The test results show that almost any liquid fuel can be used in an HCCI engine using a variable compression ratio. The indicated efficiency did not increase with increased compression ratio as much as expected. This was mainly due to a decrease in combustion efficiency with increased compression ratio. Nox emissions were overall very low, and did not increase much with increased compression ratio. With diesel fuel, smoke was generated in some cases. Emissions of unburned hydrocarbons were quite high for all cases and they increased with increased compression ratio.