An automobile spark ignition (SI) engine is most efficient at heavy load operating condition that comes rarely. It runs mostly at low load conditions when the fuel efficiency worsens. At regular drive conditions, VariPhase engine is at least 100% more fuel efficient than the conventional SI engines. It cuts both the operating cost and CO2 emission down to 50% or more, results in positive impact on ever depleting fossil fuel reservoirs and global warming. It can also reduce NOX emission dramatically. VariPhase SI engine is capable of using multi-fuel (Gasoline, Hydrogen fuel, CNG, LPG, Ethanol, Methanol etc.).

VariPhase engine is an improvement over prior art split cycle engine concept. VariPhase engine uses separate but paired piston-cylinder configurations of which one (the compressor unit) for the intake and compression strokes and the other (the power unit) for the expansion and exhaust strokes of the four stroke engine cycle. Compressed air is transferred from the compressor unit to the power unit where the combustion event takes place. Fuel is sprayed into the compressed air during this transfer process. VariPhase engine operates with very low residual mass fraction, which helps keeping initial charge temperature significantly low and yields high compression ratio.

Both of the compressor and power unit of VariPhase engine are provided with separate crankshafts. The crankshafts are operatively interconnected by a phase altering mechanism that, in consideration with instantaneous load requirement, alters the angular relation between the crankshafts. This produces variability in phase relation between the compressor and the power unit and makes VariPhase a variable compression ratio (VCR) engine. Prior art VCR concepts have high introduction difficulty because of their design complexity or an impracticable increase in reciprocating masses.

VariPhase engine is the first split cycle reciprocating piston engine having VCR.
VariPhase engine is also provided with an expansion chamber volume modifier that adds a compact shaped volume to initial expansion chamber. At low load, this added volume improves the expansion chamber shape assuring a suitable combustion. VariPhase engine is operable with very high Cycle Pressure-Temperature ratio along with a high over expansion cycle. It adapted variable valve timing technology to accomplish controlled blowback of intake air. This enables VariPhase SI engine to use a throttle-less intake system and thus avoiding pumping loss. These are amongst the key factors in the efficiency of VariPhase engine.

VariPhase engine can potentially replace both the conventional SI and diesel engines in the perspective of the environmental impacts and operating costs. VariPhase engine is capable of downsizing and supercharging. It can be configured from basic two-cylinder to large multi-cylinder configuration, as a result it can be configured for motorbikes to locomotives. VariPhase engine can be manufactured with known methods and technologies. The production cost of the VariPhase SI Engine is comparable to the conventional engines but the operating cost is much lower.