The two-stroke push mower is a light lawn mowing device with an internal combustion engine. Its major source of power is a two-stroke engine, which completes the entire cycle of intake, compression, combustion and exhaust through the reciprocating motion of the piston two times (i.e., a crankshaft revolution). Let’s take an example model. The displacement of the engine is 25cc (cubic centimeter), power rating is 1.2kW (approximately 1.6 horsepower), and fuel mixture ratio is a quantity of gasoline to two-stroke engine oil on a 50:1 volume ratio basis (500ml of gasoline and 10ml of engine oil). During operation, the fuel mixture is pre-compressed within the crankcase and subsequently discharged into the combustion chamber through the scavenging port. After ignition, it creates a burst pressure of 8-12bar (bar) that operates the piston to do work and the rate of revolution up to 7000-9000 RPM (revolutions per minute) propels the blade to shear grass stems at a linear velocity of 15-20m/s.
In comparison with four-stroke engines, the design of two-stroke push-type mowers is 40% simpler (with approximately 15 pieces, whereas more than 25 are in four-stroke machines), the weight decreases to 5-7kg (the weight of four-stroke machines is 10kg on average), but the efficiency in fuel is 15% lower (fuel consumption is approximately 0.8L/ hour, whereas four-stroke machines use 0.68L/hour). Moreover, hydrocarbon (HC) emissions are as much as 30g/kWh (8g/kWh for four-stroke engines). Two-stroke equipment, as stated by the US Environmental Protection Agency (EPA), emits 38% of non-road small engine emissions in all of the United States, of which 12% is from lawn mowers. However, its low maintenance fee (approximately 15 US dollars per annum, and 30 US dollars for the four-stroke version) and easy start-up (average start-up time through rope pull is 3 seconds) continue to delight some consumers.
Under normal use conditions, the 2-stroke push mower can be used in complicated terraries with a slope of up to ≤25°. The blade cutting width is 30-40cm, grass chip throwing distance is 1.5-2m, and the treating efficiency of thick lawns (grass height 15cm) is up to 100㎡/ hour. Let’s consider the Husqvarna 125L model as an example. It is equipped with nylon reels or steel blades, with A maximum grass chip capacity of 15L. The operating noise level is 94dB (A), which surpasses the European Union noise level of 89dB but still complies with the US OSHA limit value (95dB). The capacity of fuel is 0.5L and can operate for 40 minutes continuously under full load. It is most suited for family courtyards whose plot area is less than 500 square meters.
Regulations promote technological innovation: The 2023 California CARB regulation requires a reduction in the HC+NOx emission limit for two-stroke engines to 10g/kWh, which has prompted manufacturers to introduce stratified scavenging technology (e.g., the Mitsubishi TLE43 engine), which has led to a 45% reduction in emissions. Meanwhile, electric lawnmowers (such as EGO Power+ 56V) captured the market with minimal emissions and minimal energy consumption rates (electricity rate of $0.2 per hour to fuel rate of $0.6 per hour), and two-stroke model market share in the United States and Europe decreased from 62% in 2018 to 34% in 2023.
In terms of maintenance, the 2 stroke push mower requires the air filter to be replaced every 25 hours (at a cost of $3), the spark plugs to be replaced every 100 hours (at a cost of $5), and the fuel system cleaning cycle is 50 hours. If the mixed oil ratio is not suitable (e.g., using 40:1 rather than specifying it), it will lead to 300% additional carbon deposit, 20% less power. It can even lead to cylinder pulling failure. According to users’ surveys, 73% of repair cases are caused by inappropriate use, of which 58% are attributed to fuel problems.
Future prospects point to two-stroke push-type lawn mowers persisting to grow in emerging countries (at an annual growth rate of 5.2%), since they are affordable ($100- $200 as opposed to $300- $600 for electric machines) and easily accessible fuel. But under the strong pressure of environmental conservation, technologies such as catalytic converters (emission cut by 60%) and direct injection (fuel efficiency increase by 20%) can extend its life cycle. For instance, the model Honda HHT35S has optimized the combustion chamber design, improving the power density to 1.5kW/25cc with the Euro V emission standard compliance.