The principle of the 4-stroke push-type mower is based on four individual strokes, i.e., intake, compression, power stroke and exhaust. The crankshaft rotates 720 degrees to complete a circle of combustion, whereas the 2-stroke one only requires a 360-degree crankshaft rotation, and the sweeping and compression are completed in synchronism by the up-and-down reciprocation of the piston. Take a 160cc displacement 4 stroke push mower as an illustration. It intakes pure air in through its fuel injection mechanism in the power stroke, and features a compression ratio of 8:1. During the power stroke, the spark plug ignites the fuel mixture, and its efficiency in combustion is up to 92%. Unlike the 15%-20% direct emissions of unburned fuel as a result of oil-gas premixing in 2-stroke models, 4-stroke models experience hydrocarbon (HC) emissions reduced by 35%-40%. Experiments by the US Environmental Protection Agency (EPA) demonstrate that the emissions rate of nitrogen oxide (NOx) in 4-stroke lawn mowers is 5 grams/kilowatt-hour, while for 2-stroke units is as much as 9 grams/kilowatt-hour. This is due to the fact that lubricating oil in the latter is co-fired, raising the temperature of combustion to 600-700℃ (50-80℃ higher than for the 4-stroke engines).
In terms of fuel efficiency, the thermal efficiency of the 4 stroke push mower is approximately 28%-32%, which is significantly higher than that of the 2-stroke push mower at 20%-24%. Compared with lawn mowers of the same horsepower of 4.0 horsepower, the 4-stroke model consumes 0.8 liters of gasoline per hour, and the 2-stroke model, since oil and gas must be mixed together, consumes 1.1 liters (20% of which is lubricating oil), saving 27% in fuel costs. In 2021, research conducted in Australia revealed that consumers at a household that runs a 4-stroke engine for 50 hours each year would conserve 11.5 liters of fuel or cut the carbon dioxide level by 26 kilograms. Moreover, the separate lubrication system of the 4-stroke engine (with an oil capacity of 0.5-0.6 liters) can increase the life cycle of the engine up to 1000-1200 hours, while the 2-stroke engine is underlubricated by mixed oil with its piston ring wear rate 2.3-fold increased, as well as mean service life averaging 400-600 hours.
Noise control is another fundamental variation: 4-stroke engines, because they have a more extended combustion process (ignition rate of 25 times a second, merely 50% that of 2-stroke engines), have the ability to maintain noise in the range of 85-90 decibels, while 2-stroke models typically have more than 95 decibels. For instance, Honda GX35’s 4-stroke engine reduces the sound pressure level to 82 decibels through the use of muffler design and resonant cavities, a reduction of 13 decibels from equivalent 2-stroke engines, or an 80% reduction in sound energy. The EU’s Outdoor Equipment Noise Directive, implemented in 2019, requires the noise level of lawn mowers to be 96 decibels, which led to a sharp decline in the European market share of 2-stroke models from 38% in 2018 to 18% in 2022.
The complexity and cost of maintenance also show significant differences: The 4-stroke model has an independent oil circulation system. Change the oil every 50 hours ($10 to $15) is advised, whereas the 2-stroke machine must constantly blend fuel and lubricating oil (40:1 ratio), in which an improper ratio chance stands at 12% (based on an American Consumers Association survey), and will easily cause more carbon deposits and less power. Look at the after-sales statistics of the US retailer Lowe’s. The maintenance rate of the 2-stroke lawn mower is up to 15%, and that of the 4-stroke one is only 7%. Consumers save 40 to 60 US dollars on maintenance costs each year. Industry trends show that the global 4-stroke lawn mower market share has exceeded 72% in 2023, partly due to the fact that the California Air Resources Board (CARB) has classified 2-stroke engines as “high-pollution technology” and has a regulatory objective to eliminate their commercial application by 2030.