Abstract :
An evaluation was performed on the efficiency and emissions from an engine fuelled with compressed natural gas (CNG) and a mixture of natural gas and hydrogen, respectively. The mixtures of CNG and hydrogen were named HCNG.
Two different mixtures were investigated; pure CNG, and 29 vol% H2 in CNG (named HCNG). For each mixture, four engine speeds were tested under full load conditions. For each mixture and each engine speed four different excess air ratios (λ) were chosen.
The lean limit for the pure CNG tested in this engine is approximately λ=1.8, and the lean limit for the HCNG is even leaner, i.e. at about λ=2.0. This is as expected, and in accordance with the literature.
Considering the total emissions of unburned hydrocarbons and nitrogen oxides, it is easily seen that the positive effect of adding hydrogen to the CNG increases as a function of lambda.
By adding hydrogen to CNG, an increase in power will result in the λ investigated. The efficiency for HCNG is greater than for CNG for the same λ, and the difference in ηth,brake between HCNG and CNG for the same λ increases with increasing excess air ratio.
