The Fuel Pump of a two-stroke engine needs to be compatible with fuel-oil mixed fuel (typical mixing ratio 25:1 to 50:1). For example, the Mikuni DF52 diaphragm pump uses fluororubber sealing rings (with oil resistance 60% higher than that of nitrile rubber). It can maintain a pressure of 0.8-1.2 bar under the working condition where the fuel contains 2% lubricating oil, and the flow error is ≤ ±3% (JASO M342 standard). In 2022, Yamaha recalled 18,000 outboard units due to fuel leakage caused by the swelling of the nitrile sealing ring of a certain model of pump body (with a volume expansion of 12%). The repair cost for each unit reached 6,500 yuan. The improved model (part number 6E5-24410-00-00) that was changed to fluororubber had a leakage rate of zero in the 2000-hour durability test.
The corrosion resistance of the material is the key. The aluminum pump body of the Walbro WY series undergoes anodic oxidation treatment (with a film thickness of 15-25 μm), is resistant to mixed fuels with a methanol content of 30%, and after continuous operation at 40°C for 500 hours, the corrosion weight loss is only 0.02 g (the weight loss of ordinary aluminum pumps is 0.15 g). However, the conductivity of methanol fuel (conductivity of 0.1 S/m) is prone to cause electrochemical corrosion. Therefore, the outboard engine of Honda BF75 was replaced with a 316L stainless steel pump body (the cost increased by 220%), and the service life was extended from 5 years to 12 years in the salt spray test (ASTM B117).
The flow control needs to match the pulse frequency. The two-stroke engine crankcase pulse pump (such as Tohatsu M5B) relies on the intake negative pressure drive, with a pulse frequency range of 20-200 Hz. If the stiffness deviation of the pump diaphragm exceeds ±5% (such as the error of auxiliary parts reaching ±15%), it will cause a fluctuation of ±20% in the fuel supply at low speed (<2000 rpm). The risk of losing control of the air-fuel ratio increases by eight times. The 2023 study in “Small Engine Tech” shows that the flow rate of the original factory diaphragm (with a thickness of 0.3mm) is 15 mL/ pulse at a 50 Hz pulse, while that of the inferior diaphragm (with a thickness of 0.25mm) is only 9 mL/ pulse. The probability of causing high-speed oil cut-off reaches 32%.
Economic analysis shows that the price of the third-party Fuel Pump is only 30-50% of that of the original factory parts (for example, the selling price of All Balls 18-1040 is 150 yuan vs.) The original factory price is 450 yuan, but the lifespan is shortened by 70% (from 2,000 hours to 600 hours). The average annual maintenance frequency of 200 fishing vessels of a certain fishery company using auxiliary factory pumps increased from 0.2 times to 1.8 times, and the fuel efficiency decreased by 12% (the fuel consumption cost increased by 50,000 yuan per year). The original factory pump, by optimizing the spring stiffness (1.8N /mm) and the valve plate clearance (0.05mm), reduces the transportation cost per liter of fuel by 0.15 yuan and shortens the payback period to 1.2 years.
In terms of safety standards, ISO 15584 requires the outboard engine Fuel Pump to pass the random vibration test of 10-500 Hz (acceleration 8.7 Grms) and the temperature shock test from -30°C to 80°C. In 2021, a certain brand failed to pass the UL 142 certification (fire resistance test), causing 5,200 portable generators equipped with its pump bodies to catch fire, resulting in a direct loss of 23 million US dollars. The pump body that complies with the SAE J1942 standard needs to be sealed and maintained at a pressure of 2 bar for 5 minutes with a leakage of less than 1 mL. However, the leakage of inferior products can reach 15 mL, and the time for the fuel vapor concentration to reach the lower explosive limit (1.4%) is shortened by 90%.
The market trend shows that the electronically controlled two-stroke Fuel Pump (such as Olins EFP-2S) regulates the flow rate through the PID algorithm (response time 10 ms), and improves the air-fuel ratio control accuracy from ±8% of the mechanical pump to ±2% in the range of 2000-8000 rpm. Reduce carbon deposits by 45%. The initial R&D cost is as high as 12 million yuan, but the unit price can be reduced to 800 yuan after mass production. It is expected that the penetration rate in the high-end motorboat market will reach 35% by 2028. For ordinary users, choosing the mechanical Fuel Pump certified by the original factory is still the best solution in terms of cost performance.
