An experimental investigation was performed on a turbocharged
spark-ignition 4-cylinder production engine fuelled with natural
gas and with two blends of natural gas and hydrogen (15% and 25% in
volume of H₂). The engine was purposely designed to give optimal
performance when running on CNG. The first part of the experimental
campaign was carried out at MBT timing under stoichiometric
conditions: load sweeps at constant engine speed and speed sweeps
at constant load were performed. Afterwards, spark advance sweeps
and relative air/fuel ratio sweeps were acquired at constant engine
speed and load. The three fuels were compared in terms of
performance (fuel conversion efficiency, brake specific fuel
consumption, brake specific energy consumption and indicated mean
effective pressure) and brake specific emissions (THC,
NOx, CO). The pressure trace was acquired in the four
cylinders to perform a cycle-by-cycle and cylinder-by-cylinder
analysis of the peak firing pressure and the indicated mean
effective pressure as well as of the main combustion
parameters.
As far as MBT timing stoichiometric operations are concerned,
the addition of hydrogen determines a reduction in the spark
advance. The higher energy content of hydrogen with respect to
methane determines lower brake specific fuel consumption values.
Conversely, similar values were found for the brake specific energy
consumption, which is intimately related to the fuel conversion
efficiency. The addition of H₂ produces an appreciable reduction in
THC which overcomes the mere reduction of HC in the blend. A small
improvement can also be found for CO emissions whereas
NOx turned out to be slightly affected by the H₂ content
in the fuel.
The air/fuel ratio in the relative air/fuel ratio sweeps was
varied in the rich and lean field up to the lean operation limit.
The addition of hydrogen determined a marked reduction of the
coefficient of variations in the lean field, thus allowing an
extension of the lean operation limit with respect to CNG
operation. The addition of hydrogen still determined a considerable
THC reduction which turned out to be even more evident as one moved
to the lean field.
Regardless of the considered fuel, a significant
cylinder-to-cylinder variation emerged, mainly to be attributed to
the cylinder spatial distribution, which in turn affects the
effectiveness of the cooling system and the uniform air/fuel
mixture distribution.