Can I Use Mixed Gas in My Mower?

Small Engines

• Lawn mowers use one of two types of engines, customarily with 25 horsepower or less. The smaller two-cycle engine lacks an oil filter and sump because the oil is mixed into the gasoline. The more efficient four-cycle engine produces fewer emissions because it burns only gasoline. It and has a crankcase and oil distribution system to lubricate its moving parts. Four-cycle engines are larger, heavier and run on straight gasoline.

Two-Cycle Engines

• Lawn mowers with two-cycle engines use a specific mixture of oil and gasoline which coats moving parts with a thin film of oil as the engine operates. The ratio of gasoline to oil varies among models, from 100 parts of gasoline to 1 part of oil down to 16 parts gasoline to 1 part oil. Two-stroke engine mowers are light and easy to start and expel exhaust from under the engine in front. Running a two-cycle mower on straight gasoline will eventually cause the crankshaft, connecting rod and piston to seize up for lack of lubricant.

Four-Cycle Engines

• The four-cycle engine has twice as many moving parts as a two-cycle engine. The engine expels exhaust to the side and takes more space due to the additional space needed by the crankcase. The heavier engine burns less oil, but the oil in the crankcase must be changed approximately every 50 running hours, depending on model. The engine’s higher torque makes it valuable for riding and self-propelled mowers. Using mixed gas in a four-cycle engine will eventually foul the spark plug and clog valves.

Ethanol Additives

• Fuel requirements may vary according to the age of your lawn mower, but all machines, whether two- or four-cycle, require fresh gasoline with an octane rating of over 87 to perform their best. Ethanol-gasoline mixtures have replaced regular gasoline in many areas, causing concern regarding engine wear in small engines. Studies at the University of Minnesota and University of Oklahoma found no substantial risks for 20 years when E10 (10 percent) or E20 ethanol-added gasoline was used in new machines. Similar assurances could not be made for larger percentages, such as the E85 mixture sold for auto engines and ethanol mixtures. Older engine damage generally resulted from ethanol’s solvent and hydroscopic properties, which caused the additive to tend to concentrate gummy deposits and precipitate water in gas tanks.