The effect of compression ratios variation on maximum potential hydrogen energy share in a dual fuel engine is studied by creating a numerical model for engine combustion and emission characteristics. Hydrogen is induction through the intake port, using port fuel injector (PFI). Compression ratio is varied in steps of 14.5, 16.5 and 19.5 in all the investigations. Hydrogen energy share variation ranged from 0% to 55%. The results show a clear trade-off between maximum hydrogen energy share and compression ratio. Knock limited maximum hydrogen energy share increased from 20% to 45% as compression (CR) is decreased from 19.5 to 14.5. At all compression ratios as the energy proportion of hydrogen is increased, emissions other than NOx show an apparent decline.