… hydrogen requires very low ignition energy therefore utilization of hydrogen in IC engines can potentially cause preignition and backfire. Use of hydrogen also leads to low power output and constraints the operating load range of the engine because of very low density of hydrogen, which in-turn reduces the volumetric efficiency of the engine significantly. Therefore use of 100% hydrogen as a total replacement of gasoline in a spark ignition (SI) engine is rather challenging and difficult.

On the other hand, natural gas is being used as an alternate fuel to conventional gasoline for last few decades. It is utilized either in the form of compressed natural gas (CNG) or liquefied natural gas (LNG). Natural gas resources have been discovered in various forms world-wide. There is tremendous interest in using natural gas on a large scale worldwide because it is relatively cleaner fuel due to its highest H/C ratio (4:1) amongst all hydrocarbon fuels. However natural gas suffers from poor lean-burn capabilities, low flame speed and poor idle stability, which makes CNG engine relatively low efficiency engine due to its longer combustion duration and less agility. Engine’s lean operation could be extended by increasing H₂ fraction in the test fuel and also by increasing intake manifold pressure. … addition of hydrogen to CNG can significantly improve combustion characteristics of CNG by increasing its lean limits and flame burning velocity.

—Verma et al.

The study focused on the behavior of engines using HCNG blends, in which the H/C ratio was varied. The H/C ratios for the blends tested were:

• CNG: 4.0
• 10HCNG: 4.22
• 20HCNG: 4.50
• 30HCNG: 4.85
• 40HCNG: 5.33

They also tested the engine using 100% hydrogen. The researchers used a port fuel injected (3 bar), single cylinder, .948-liter research engine with a compression ratio of 11:1.

Overall, the 20 HCNG blend (H/C ratio of 4.5) showed the best overall performance. Other findings included:

• The lean-burn limit of the test fuels was directly proportional to the H/C ratio of the test fuels, and was highest for neat hydrogen amongst all test fuels.

• For a fixed relative air-fuel ratio (RAFR), thermal efficiency increased with increasing H/C ratio, except for hydrogen, where it decreased because of lower volumetric energy density of the combustible charge.

• P_max increased with increasing H/C ratio of the test fuels and CAP_max advanced with increasing engine load.

• Heat release rate was highest for hydrogen; hydrogen also showed the lowest ignition delay.

• Exhaust gas temperatures increased with decreasing H/C ratio of the test fuel.

• NO_x emissions were highest for 10HCNG (H/C ratio of 4.22) up to 55 N·m torque. However, the NO_x value was highest for 20HCNG at full load.

• Brake thermal efficiency (BTE) was superior for 20HCNG.

• HC and CO emissions were negligible for the entire engine load range for all test fuels.

Resources

• Gaurav Verma, Rajesh Kumar Prasad, Rashmi A. Agarwal, Siddhant Jain, Avinash Kumar Agarwal (2016) “Experimental investigations of combustion, performance and emission characteristics of a hydrogen enriched natural gas fuelled prototype spark ignition engine,” Fuel, Volume 178, Pages 209-217 doi: 10.1016/j.fuel.2016.03.022