A split cycle phase variable reciprocating piston spark ignition engine comprising a compressor unit having a compression chamber adapted to carry out the intake and compression strokes of a four stroke engine cycle, a power unit having an expansion chamber adapted to carry out the expansion and exhaust strokes of a four stroke engine cycle, a crossover gas passage for transferring compressed gas from the compression chamber to the expansion chamber, an expansion chamber volume modifier to provide nearly full load like combustion chamber condition at all the engine load conditions by means of modifying volume and shape of the expansion chamber, a phase altering mechanism for altering phase relation between the compressor unit and the power unit as a function of engine load variation, an electronic control unit for providing control commands for various electrically operated actuators and motors.