A series of experiments was conducted on a Cooperative Fuel Research (CFR) engine operated in the homogenous charge compression ignition (HCCI) mode to study the effect of reformer gas (H2 and CO mixture) on combustion timing. Base fuels of natural gas, iso-Octane, n-Heptane and primary reference fuels were used for experiments. The engine was tested at steady state conditions. Reformer gas blending expanded the operating region of natural gas HCCI combustion toward the lean side. Keeping all other influential parameters constant, increasing reformer gas fraction in the fuel mixture advanced combustion timing and increased combustion severity. Alternatively, reformer gas addition decreased the intake heating requirement of natural gas HCCI combustion. Combustion timing alteration was attributed to changed mixture thermodynamic properties.

For low octane fuels with dual-stage ignition, reformer gas suppressed the first stage of combustion by reducing intermediate radical concentration, thus retarding the main stage of combustion. Keeping all influential parameters constant, the delayed combustion timing reduced combustion intensity and improved efficiency.

Reformer gas blending had negligible effects on engine operating parameters and HCCI combustion characteristics of high octane liquid fuels in the in the normally aspirated mode.

Changing reformer gas composition did not have strong effects on engine and combustion parameters. A mixture of 75%H2/25% CO had slightly stronger effect on combustion timing variation than that of 50%H2/50%CO.

It was found that reformer gas can be used to effectively control combustion timing of both natural gas and low octane fuels, although with different effective mechanisms in each case.