Obviousness

I must first address a critical limitation for performing an obviousness analysis under 35 U.S.C. § 103. The provided "Prior art section" in the patent text for US10697398 lists only "Prior art keywords" (fuel, carburetor, engine, dual, shaft) and a "Prior art date" (2015-06-12). It does not provide specific prior art references (e.g., patent numbers, publications) that were cited during examination or are otherwise available in the provided text for analysis.

A proper obviousness analysis requires identifying specific prior art documents and explaining how a Person Having Ordinary Skill in the Art (PHOSITA) would have been motivated to combine them to arrive at the claimed invention. Since no specific references are provided in the authoritative text, I cannot identify concrete "combinations of prior art references" as requested.

Therefore, this analysis will proceed by hypothetically discussing how a PHOSITA might have combined known elements in the field, consistent with the provided keywords and the problems the patent aims to solve, if such elements were found in relevant prior art documents existing before the priority date of 2015-06-12. This discussion is necessarily conceptual due to the absence of specific references.

Obviousness Analysis (Conceptual, due to lack of specific prior art references)

The independent claims of US10697398 generally pertain to dual fuel engines and generators, specifically focusing on mechanisms to manage liquid fuel flow (cut-off/shutoff) when switching between liquid and gaseous fuels, particularly in a batteryless pull-start context.

Relevant Areas of Prior Art (based on "Prior art keywords"):
A PHOSITA in the field of small internal combustion engines, particularly for generators, before June 12, 2015, would have been familiar with:

• Dual fuel engines/carburetors: The concept of engines operating on both liquid and gaseous fuels (like gasoline and LPG/propane) was well-known.
• Carburetors: The basic design and function of carburetors, including float bowls, throats, fuel passages, main nozzles, and idle circuits, were standard.
• Fuel control mechanisms: Various valves, solenoids, and switches for controlling fuel flow (both liquid and gaseous) were common.
• Engine starting systems: Pull-start (recoil) engines and associated electrical generation (magnetos, alternators with charge windings) were prevalent for batteryless applications like portable generators.

Hypothetical Combinations and Motivation for PHOSITA:

Given the problems identified in the patent's background (overly rich air-fuel ratio, hard starting, unstable operation, delay in switching, gum/varnish deposits from gasoline, after-fire, engine flooding), a PHOSITA would have been motivated to combine known elements to address these issues.

For Independent Claim 1: Dual Fuel Engine with Liquid Fuel Cut-off

• Core elements: Engine operable on gaseous/liquid fuel, carburetor, switch to change fuel, liquid fuel cut-off upon switching to gaseous fuel.
• Hypothetical Combination: A PHOSITA, observing existing dual-fuel engines that often experienced issues with simultaneous fuel delivery or "overly rich air-fuel ratio when both fuels are simultaneously engaged during cross-over switching" [Description, 0005], would be motivated to add a liquid fuel cut-off mechanism to a known dual-fuel engine system.
• Motivation: The motivation would be to prevent "engine flooding by stopping liquid fuel flow when starting on gaseous fuel" [Description, 0039] and to ensure cleaner, more reliable operation when running on gaseous fuel by eliminating residual liquid fuel supply. Basic fuel shut-off valves or solenoids were common in various fuel systems. Implementing such a shut-off, synchronized with a fuel selection switch (which was also known in dual-fuel systems), would be a logical step to improve functionality and address the identified problems.

For Independent Claim 2: Batteryless Dual Fuel Generator with Fuel Shutoff

• Core elements: Batteryless pull-start engine, alternator, carburetor with float bowl/fuel passage, fuel shutoff attached to carburetor to close liquid fuel passage upon selection of gaseous fuel.
• Hypothetical Combination: Building upon the motivation to prevent liquid fuel issues in dual-fuel systems, a PHOSITA would consider applying a liquid fuel cut-off to a batteryless generator. The challenge in a batteryless system is powering such a cut-off if it's electrical (like a solenoid). Prior art likely included various solenoid-operated fuel valves. If a solenoid valve was chosen, a PHOSITA would recognize the need for power. The patent states that "Fuel cut solenoid 74 is preferably powered by a magneto 76, alternator, engine flywheel with a charge winding, or other electrical power generator having a charge winding or coil 78" [Description, 0042]. Tying such a solenoid to the existing electrical generation system (magneto/alternator for spark/charging) of a pull-start engine would be a straightforward engineering solution, even if requiring "Additional turns can be added to a charging coil... to increase the output voltage" [Description, 0045] or "solenoid windings... can be modified to operate at lower voltages" [Description, 0046] to function at low RPMs during starting.
• Motivation: The motivation is to extend the benefits of a liquid fuel cut-off (preventing flooding, improving starting on gaseous fuel, reducing emissions/after-fire [Description, 0064]) to portable, batteryless generators, which often operate in situations where clean, reliable starts on alternative fuels are desirable. Using the engine's inherent power generation (magneto/alternator) for this control would be a common design choice to maintain the "batteryless" characteristic.

For Independent Claim 3: Carburetor with Non-Linear Fuel Shutoff

• Core elements: Carburetor with float bowl, throat, fuel passage; fuel shutoff with first end in carburetor (actuates to close fuel passage), second end external (to actuate first end); fuel shutoff actuates free from linear motion.
• Hypothetical Combination: Prior art would include various types of valves (e.g., gate valves, ball valves, rotary valves) for controlling fluid flow. While many fuel shut-offs operate linearly (e.g., plunger-type solenoids), a PHOSITA designing a compact and reliable seal within a carburetor's float bowl would recognize the advantages of rotary motion. The patent explicitly states that a "rotating valve such as manual fuel shutoff 110, reduces the likelihood that fuel will leak from carburetor 62" [Description, 0092].
• Motivation: The motivation to implement a non-linear (e.g., rotary) fuel shutoff, as described by a "valve tip 180 (e.g., oval or stadium shape) rotates parallel to inlet 174 of fuel passage 164" [Description, 0075, 0076, 0079], would be to achieve improved sealing and reduced leakage compared to linear plunger-type valves, particularly when sealing against a flat surface within the float bowl. Rotary valves were well-known in fluid control, and adapting one to a carburetor's fuel passage to achieve a robust seal without linear movement (which can cause wear on O-rings and lead to leaks over time) would be a design choice driven by reliability and manufacturing considerations. Using "polyoxymethylene (POM) or other thermoplastic material" for the valve tip [Description, 0108] for sealing is also a known material selection for such applications.

For Independent Claim 4: Method of Assembling a Dual Fuel Engine

• Core elements: Providing an engine, attaching a carburetor (with throat, float bowl, fuel passage), coupling gaseous fuel source to throat, liquid fuel source to float bowl, coupling a switch to change fuel, attaching liquid fuel cut-off to carburetor to close fuel passage upon switching from liquid to gaseous fuel.
• Hypothetical Combination: This claim describes a method that essentially formalizes the assembly of the systems described in Claims 1 and 2. The steps involved (attaching carburetors, connecting fuel lines, installing switches, and integrating fuel cut-offs) would be standard assembly practices for any engine or generator manufacturer. The specific step of "attaching a liquid fuel cut-off to the carburetor to close the fuel passage upon actuation of the switch from liquid fuel to gaseous fuel" [Description, 0021] directly corresponds to the functional features discussed above.
• Motivation: The motivation for such an assembly method would be to build a dual-fuel engine that overcomes the operational difficulties associated with simpler dual-fuel systems lacking precise liquid fuel control during transition. The method steps reflect the engineering solutions a PHOSITA would arrive at to create a reliable and user-friendly dual-fuel system, addressing the problems of fuel mixing, hard starting, and emissions.

In summary, while specific prior art documents are unavailable for this analysis, the general concepts of dual-fuel engines, carburetors, various fuel control mechanisms, and batteryless engine power generation were well-established before the priority date of US10697398. A PHOSITA, motivated to solve known problems of inefficient fuel switching, engine flooding, and emissions in dual-fuel systems, particularly in batteryless applications, would have a logical basis to combine these existing technological elements to arrive at the claimed inventions. The specific implementation of a rotating fuel cut-off would be a design choice driven by known advantages of rotary valves for sealing and reliability.