In this study, biogas is used in a CI engine in dual fuel mode with diesel as pilot fuel. Biogas has been replicated using a mixture of methane (CH₄) and carbon dioxide (CO₂), whose flow rates can be independently varied. This helps in evaluating the effect of biogas composition on engine performance. Apart from the conventional diesel-only mode, the engine is operated in dual fuel mode with induction of (a) pure CH₄ and (b) CH₄ + CO₂ mixtures with variable CH₄:CO₂ ratios, representing biogas. Dual fuel operation is observed to have better brake thermal efficiencies compared to diesel-only mode at high loads. Though volumetric efficiency is almost identical in diesel-only and diesel–CH₄ dual modes, induction of biogas causes it to drop. Exhaust gas temperatures were higher in diesel–biogas mode, followed by diesel–methane and diesel-only modes. Dual fuel mode, especially with biogas, results in higher overall equivalence ratios. Compared to diesel-only mode, dual fuel mode has relatively high unaccounted losses at low loads, while exhaust and coolant losses are greater at high loads. Biogas composition does not significantly affect any of the above parameters except the exhaust gas temperature and air–fuel ratio, which show a slight increase with methane enrichment.

Biogas is commonly produced during the decay of organic matter. It is a mixture of methane and some non-combustible gases such as CO2 and H2S. Its viability as a renewable alternative fuel for internal combustion engines can be enhanced by methane enrichment, i.e. removal of the non-combustible constituents. One of the common techniques for using biogas in a compression ignition (CI) engine is to mix it with air in the intake manifold, induct, and compress this mixture and ignite it by injecting a small quantity of diesel or bio-diesel, which is termed as the pilot fuel. This is known as the dual fuel mode. The pilot fuel is injected close to the end of the compression stroke as in a conventional CI engine and the injected fuel quantity depends on the operating condition. An alternative approach is the Homogeneous Charged Compression Ignition (HCCI) mode. Here, a homogeneous mixture of biogas and air is inducted and compressed by the piston until it auto-ignites. While this concept combines the benefits of spark ignition (SI) and CI engines, the onset of combustion cannot be controlled directly. A detailed review of recent research pertaining to biogas purification techniques and operation of CI engines with biogas in dual fuel and HCCI modes is presented in this paper. The effects of various operating parameters on engine performance and emissions, and comparison with conventional diesel fuelled CI engines are discussed. Biogas improves combustion efficiency, NOx, and smoke emissions. However, it reduces brake thermal efficiency, volumetric efficiency, and increases HC and CO emissions. Biogas fuelling of CI engines is recommended for achieving high diesel substitution, especially under high torque operation.