Research has been undertaken into rotary compression ignition engines and the burning of diesel heavy fuel using spark ignition. The basic design parameters of the Wankel engine preclude obtaining a compression ratio higher than 15:1 or 17:1 in a practical engine, but attempts are continuously being made to produce a compression-ignition Wankel. The Rolls-Royce and Yanmar compression-ignition approach was to use a two-stage unit, with one rotor acting as compressor, while combustion takes place in the other. Conversion of a standard 294-cc-chamber spark-ignition unit to use heavy fuel was described in SAE paper 930682, by L. Louthan. SAE paper 930683, by D. Eiermann, resulted in the Wankel SuperTec line of compression-ignition rotary engines.