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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.

The Homogeneous Charge Compression Ignition (HCCI) is the third alternative for combustion in the reciprocating engine. Here, a homogeneous charge is used as in a spark ignited engine, but the charge is compressed to auto-ignition as in a diesel. The main difference compared with the Spark Ignition (SI) engine is the lack of flame propagation and hence the independence from turbulence. Compared with the diesel engine, HCCI has a homogeneous charge and hence no problems associated with soot and NO X formation. Earlier research on HCCI showed high efficiency and very low amounts of NO X , but HC and CO were higher than in SI mode. It was not possible to achieve high IMEP values with HCCI, the limit being 5 bar. Supercharging is one way to dramatically increase IMEP. The influence of supercharging on HCCI was therefore experimentally investigated. Three different fuels were used during the experiments: isooctane, ethanol and natural gas. Two different compression ratios were used, 17:1 and 19:1. The inlet pressure conditions were set to give 0, 1 or 2 bar of boost pressure. The highest attainable IMEP was 14 bar using natural gas as fuel at the lower compression ratio. The limit in achieving even higher IMEP was set by the high rate of combustion and a high peak pressure.Numerical calculations of the HCCI process have been performed for natural gas as fuel. The calculated ignition timings agreed well with the experimental findings. The numerical solution, is however, very sensitive to the composition of the natural gas.

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