ANALYSIS OF ENGINE CHARACTERISTICS AND EMISSIONS FUELED BY IN-SITU MIXING OF SMALL AMOUNT OF HYDROGEN IN COMPRESSED NATURAL GAS

The use of gaseous fuels in internal combustion engines has long been observed as a
possible method of reducing emissions while maintaining engine performance and
efficiency. Most of the research interests is focused on the use of compressed natural
gas as alternative fuel, mainly due to its wide availability, high thermal efficiency and
lower exhaust emissions compared to other hydrocarbon fuels. But compressed
natural gas has the penalty of slow burning velocity and poor lean burn ability. One
effective way to solve this problem is to mix the compressed natural gas with a fuel
that possesses the high burning velocity. Hydrogen is the best additive candidate to
natural gas due to its unique characteristics in promoting flame propagation speed,
which stabilizes the combustion process.
This research investigated the engine characteristics and emissions of a CNG-DI
engine fueled by low levels of hydrogen enrichment (lower than 10%) in CNG
utilizing an in-situ mixing system. Prior to the main experiment, two pre-experiments
were conducted to determine the best and most suitable parameters for optimization of
engine performance, combustion as well as emissions. The first experiment was to
determine the suitable injector type to be used, and it was found that the wide cone
angle injector of 70o was better for the applications. The second experiment was to
determine the suitable injection timing, and it was discovered that the earlier injection
timing was the best for this work.
In this research, the engine used was a 4-stroke single cylinder, with a swept
volume of 399.25 cc and a compression ratio of 14:1. The injection timing was set to
300o crank angle before top dead center as determined in the pre-experiment; the
engine speed from 2000 to 4000 rpm and the spark timing for all the operating
conditions were set to maximum brake torque. All the experiments were conducted at
full load and relative air-fuel ratio λ =1.0. The injection pressure was fixed at 14 bar
for all the cases.
The findings revealed that the brake torque, brake power and brake mean effective
pressure increased with the increase of hydrogen fraction at low and medium engine
speeds. The brake specific energy consumption decreased and brake thermal
efficiency increased with the increase of hydrogen percentage. In general, significant
changes have been observed with the engine characteristics at low engine speed but
the rate of increase/decrease of the parameters decreased was less significant with the
addition of higher percentages of hydrogen as well as with the increase in engine
speeds.
For all the cases, the cylinder pressure and the heat release rate increased while
the flame developement and rapid combustion duration decreased with the increase in
the amount of hydrogen in the blends. The phenomenon was more obvious at the low
engine speed, suggesting that the effect of hydrogen addition in the enhancement of
burning velocity plays more important role at relatively low cylinder air motion.
Exhaust THC, CO and CO2 concentrations decreased with the increase of
hydrogen fraction due to the increase in hydrogen to carbon ratio (H/C). However, the
variation in the NOx emissions was found to be negligible with the addition of
hydrogen.