Investigating the Impact of Compression Ratio on Hydrogen Combustion and Emission An Experimental Analysis

Recent development of hydrogen (H₂) internal combustion engine (ICE) technologies has demonstrated that they produce zero CO₂ and ultra-low NO_X emissions without aftertreatment. Therefore, H₂ ICE is well suited as a sustainable zero-carbon thermal power unit towards the net-zero target in the future.

Increasing the compression ratio (CR) in IC engines can improve the indicated thermal efficiency (ITE). However, in spark ignition (SI) engines using gasoline, the CR is typically limited to around 10:1 due to knocking combustion. Hydrogen, with its higher autoignition temperature and octane number, can be used with a higher CR. This study aims to experimentally assess hydrogen as a direct replacement for gasoline in highly boosted single-cylinder SI engines with different CRs by swapping engine pistons. Two sets of experimental testing have been conducted at various engine speeds. Starting from a lambda (λ) sweep test involved leaning out the combustion to reach the engine stability-operating limits. The second test involved conducting a load sweep test on every CR at various loads to get the engine’s maximum in-cylinder pressure limits.

The results indicate that, under the synergistic restriction of the engine strength and the lean-burn limitation, there is a crucial trade-off between peak and maximum engine power with various CR. Specifically, a higher compression ratio (i.e. CR=12.39) resulted in a 5% increase in ITE compared to CR=9.27. However, the lower compression ratio increased the maximum engine torque by 3.50 bar of indicated mean effective Pressure (IMEP). In addition, although hydrogen can remain stable combustion across a broad range of lambda and operating loads, the NO_X emissions increased with CR, due to the higher combustion temperature. These findings provide valuable insight into hydrogen engine applications and improve the understanding of SI hydrogen engine’s performance and development.