In this work biogas was used in a HCCI engine with charge temperature and amount of diesel injected into the intake manifold being used to control combustion. The presence of CO2 in biogas suppresses the high heat release rates encountered with neat diesel fuelling in HCCI engines. Normally biogas use leads to a drop in thermal efficiency in both SI and CI engines. However, present results indicate that thermal efficiencies close to diesel engine values can be obtained in the HCCI mode. The NO level was less than 20 ppm and the smoke level was less than 0.1 BSU at all conditions. The best energy ratio was 50%. HC levels were very high and were lowered when the charge temperature was raised. A charge temperature of about 80-135 degrees C was needed, which can be attained though heating by exhaust gases. On the whole the HCCI mode can be a viable option to utilize biogas in a diesel engine. (C) 2009 Elsevier Ltd. All rights reserved.