fuel efficiency (energy efficiency)

Fuel efficiency expresses the useful work obtained per unit of fuel consumed — typically distance per kilogram (km/kg) for vehicles or kilowatt-hours per kilogram (kWh/kg) for stationary generators. For compressed biogas (CBG) used in CNG-certified vehicles, fuel efficiency is essentially identical to fossil CNG because IS 16087:2016 specifies a minimum 90% methane content, giving CBG the same calorific value (around 49 MJ/kg) and the same combustion characteristics inside the engine. Engine ECUs cannot distinguish the two.

Indian operational data from Indian Oil and BPCL fleet trials shows passenger cars achieving 18-22 km/kg on CBG, light commercial vehicles 14-16 km/kg, and heavy buses 3.5-4.5 km/kg — within 2-3% of the same vehicles running on fossil CNG. The marginal differences come from minor variations in CO2 content (allowed up to 4% under IS 16087:2016) and trace inerts, not from methane itself. Engines do not require remapping, sensor recalibration, or component changes; the OEM warranty is preserved.

Several factors shape real-world fuel economy beyond fuel quality. Driving cycle (city stop-go versus highway cruise) typically swings consumption by 25-35%. Cylinder pressure at fill matters — a fully filled 200-bar cascade delivers full range, but cascade decay during high-demand periods can shorten the perceived range. Ambient temperature also affects fill density: in Indian summer above 40 deg C, the same nominal fill pressure delivers 8-12% less mass than in winter.

Economically, CBG fuel efficiency translates directly into operating cost. At the SATAT-fixed CBG retail price (Rs 65-72/kg at the pump after taxes) and 20 km/kg, the running cost is around Rs 3.50/km — roughly 50% below diesel and 60% below petrol at current Indian retail prices. This cost-per-kilometre advantage, combined with engine compatibility, is the central commercial argument for fleet conversion to CBG under the SATAT framework.