The True Cost of a Small Petrol Engine as a Workshop Prime Mover: Fuel per Shaft Kilowatt-Hour, Lifetime Maintenance and the Load Factor Below Which Sharing Wins

A 196 cc petrol engine rated at 4.85 kW peak and 3.50 kW continuous is costed as a workshop prime mover over a stated service life of 1500 hours. Fixed costs — purchase, oil, plug, air filter, an expected repair budget and maintenance labour — total 0.55 US dollars per engine hour regardless of load. Fuel dominates at low load because specific consumption worsens sharply: at a load factor of 1.00 the engine uses 0.564 litres per delivered shaft kilowatt-hour and costs 0.749 dollars per kilowatt-hour in total, while at a load factor of 0.25 it uses 1.275 litres per kilowatt-hour and costs 1.967 dollars per kilowatt-hour, and at 0.10 it costs 3.826 dollars per kilowatt-hour. Over a life spent at a load factor of 0.50 the engine delivers 2625.0 kWh, burns 2395.5 litres of petrol and costs 3340.50 dollars in total. A shared engine, with fixed costs split among three users and scheduled at a load factor of 0.60, delivers energy at 0.955 dollars per kilowatt-hour; a single-user engine matches that figure only at a load factor of about 0.74 or above. Below that load factor, sharing wins on these numbers. Human power provides a further boundary: at demands of 75 W or less, the engine costs 9.59 dollars per kilowatt-hour, above even a food-only human cost of 4.00 dollars per kilowatt-hour, although human power does not compete at all if the worker's time is valued in full, at 160.00 dollars per kilowatt-hour. The low purchase price of a small engine conceals a cost per unit of work that depends, above all, on how hard the engine is actually loaded.