The decibel output of a 2-stroke push mower is generally in the range of 95 to 105 decibels, equivalent to chainsaw or motorcycle engine sound pressure level. Compare that to the 4 stroke push mower that makes a sound of approximately 85 to 95 decibels, down by 10% to 15%. For instance, experimental data in Consumer Reports test data in the United States of America in 2021 shows that for two-stroke engines, because an independent lubricating system is not present, the crankcase has to be mixed with fuel to burn it, resulting in a frequency of ignition twice that of four-stroke engines (approximately 50 times per second), and higher high-frequency vibration actually increases sound wave amplitude. Moreover, the design of the 2-stroke engine generates significantly higher pollutant emissions in its exhaust: US Environmental Protection Agency (EPA) data show that two-stroke lawn mowers emit 0.35 pounds of hydrocarbons (HC) and 10 grams of nitrogen oxides (NOx) per hour. Compared to four-stroke counterparts’ 0.25 pounds of HC and 7 grams of NOx, the concentration of pollutants is higher by 30%-40%. Consider the California Air Resources Board (CARB) 2020 regulations as an example. Two-stroke equipment has been prohibited from being sold in certain cities due to the fact that it did not pass the Stage III emission requirements.
The variation in fuel efficiency also emphasizes environmental concerns: The thermal efficiency of a 2-stroke engine is merely 18%-22%, whereas for four-stroke engines, it is 25%-30%, and the fuel waste rate is 12%-15% higher. Take the example of a lawn mower with 3.5 horsepower power. The 2-stroke engine consumes 1.2 liters of mixed oil per hour, but the 4-stroke engine consumes only 0.9 liters of ordinary gasoline per hour. Used for 50 hours a year, 15 liters of fuel can be saved and some 34 kilograms of carbon dioxide can be reduced. A study by the U.S. Department of Energy (DOE) estimates that if 10% of all U.S. households convert from two-stroke to four-stroke lawn mowers, 46,000 tons of greenhouse gases could be reduced annually.
Cost and lifetime of maintenance are also essential considerations: Due to poor lubrication, the piston ring wear rate of 2-stroke engines is 1.8 times higher than that of four-stroke engines and their mean working time is 300 to 500 hours, while the mean working time of four-stroke models is 800 to 1,000 hours. For instance, after-sales figures for home improvement store Home Depot show that 2-stroke lawn mowers have a 12% return rate, compared to only 6% for 4-stroke mowers. Consumers’ average annual maintenance expense (replacing spark plug and air filter) varies significantly: around 50 US dollars for a 2-stroke and only 30 US dollars for a 4-stroke. Market trends show that in 2022, the share of four-stroke lawn mowers sold in Australia has increased from 45% in 2018 to 67%, at least partly as a result of the EU REACH regulation prohibiting evaporation of benzene series compounds in two-stroke engines (a 0.1% threshold).
It is worth noting that the 4-Stroke push mower has the same noise and pollution control as some electric counterparts by optimizing the structure of the combustion chamber and stratified injection technology. For instance, the noise of Honda HF2622 four-stroke model is only 87 decibels, and its PM2.5 emissions are 55% less than similar two-stroke counterparts. The US Environmental Protection Agency estimates that, if the entire industry shifts to four-stroke technology, by 2030 it would avoid 1.2 million tons of unburned hydrocarbon emissions, or the carbon sink equivalent of growing 1.8 million trees. This data proves the critical role of technological iteration in sustainable development.