Fuel supply system for miniature engines

#1. Field of the Invention

#In general, this invention relates to a fuel delivery system for miniature internal combustion engines, more particularly for model aircraft, comprising: a fuel tank which is sealed off against atmosphere and has at least two lines connected to the engine; one line is a fuel outlet line connected to the engine's carburetor and the other line is a pressurization/venting line connected to the engine's exhaust system whereby pressure from the engine's exhaust is the primary motive force supplying fuel to the engine.

#More particularly, the exhaust pressure of each engine stroke cycle in combination with the frequency of said cycle also serve as means for metering precise amount of fuel required by the engine for maximum power and consistency.

#FIGS. 1a, 1b and 1c are a rear view, a side view and a top-down view respectively of a typical two-stroke cycle model aircraft engine & stock muffler combination which is most predominantly used in the art. FIG. 1c has a part of the engine cut away at line A-A'.

#FIG. 2 consists of three graphs from the top down respectively showing variation in horsepower, fuel consumption and torque at different engine rpm for a typical two-stroke glow ignition model aircraft engine.

#FIGS. 3a and 3b are top-down view and rear view respectively of a two-stroke miniature engine with the exhaust pressure tap fitting in place according to this invention. FIG. 3a has a part of the engine cut away at line C-C' on FIG. 3b. FIG. 3b has a part of the engine cut away at line B-B' on FIG. 3a.

#As discussed, it is highly desirable therefore to increase the exhaust-powered fuel tank pressure to a level sufficiently high to make changes in fuel head pressure in flight negligible, at the same time decreasing the muffler back pressure for more engine power and less overheating. In order to overcome such seemingly contradictory situation, it is necessary to tap onto the engine's exhaust pressure at the point as close to the exhaust port as feasible, before the high pressure exhaust gas has a chance to gain in flow velocity as well as to expand hence losing pressure drastically (according to Bernoulli principle). Referring to FIGS. 3a and 3b, there is shown a pressure tap fitting 16 with a threaded end 16a screwed into a channel 6c drilled and threaded at the upper corner of the engine's exhaust conduit 6. The channel 6c opens up directly facing the exhaust port 8 and only 2-4 milimeters away from the engine's piston 17. Preferrably, it is better to use the anterior wall 6a of the engine exhaust conduit 6 instead of the posterior wall 6b because the anterior wall is considerably cooler than the posterior wall, hence less varnish built up within the pressure tap channel 6c when fuel containing castor oil is used. Since the engine's outer casing 3a (which the exhaust conduit is a part of) is made by casting, it is necessary to make the upper corner of the anterior wall 6a of the exhaust conduit 6 sufficiently thick during the casting process in order to incorporate channel 6c by drilling afterward. A similar exhaust pressure tap is possible for four-stroke cycle engine, as shown in FIG. 4b. In this case, the pressure tap fitting 20 is screwed into a hole bore into the engine's exhaust outflow tract very close to and facing the exhaust port 19 and exhaust valve 18. Fitting 20 is different from the previous pressure tap fitting 16 in that the pressure tap fitting 20 has a tubing portion 20b extending below the threaded portion 20a in the vicinity of the exhaust port 19 in order to be even closer to the exhaust valve opening.

#Referring to FIG. 5a, there is illustrated diagramatically a complete tubing connection of all components of this fuel system. Exhaust pressure tap fitting 16 is connected to the fuel tank 27 via flexible tubing attached to the venting/pressurization tubing 23a of the fuel tank. Pressure inside the tank will then force the fuel to enter klunk 25 onto flexible tubing 24a exiting the tank through the fuel outlet/inlet tubing 23b onto another flexible tubing which is connected to carburetor fuel inlet 11 of the engine. The engine is connected to muffler 10, which consist of muffler inlet 10a, expansion chamber 10b, and tail cone 10d. This modified tail cone 10d is different from the tail cone 10c of prior art in FIG. 1c in that there now exist a number of tail pipes 21 in order to greatly reduce the muffler's back-pressure. The tail pipes 21 should preferably be generally perpendicular to the direction of flight in order to allow for aspiration of exhaust gas by the relative motion of surrounding airstream 59 according to venturi effect. The tail pipes 21 may be angled slightly rearward as illustrated by angle 22 when more aspiration is required or angled slightly forward when less aspiration is required. It is important to note that because the aspiration effect is rather weak at the speeds that model aircraft is traveling, the aspiration effect is only significant when the velocity of the exhaust gas in tail pipes 21 is low which is made possible by using tail pipes 21 of large combined cross-sectional area.

#In the relative absence of exhaust aspiration effect during operation of the engine, such as when the aircraft is standing still, the pressure inside fuel tank 27 is generally proportional to the rate of exhaust gas production of the engine which is then proportional to the fuel requirement of the engine. When the engine is running at full throttle while standing still, the amount of moving air from the propeller is inadequate for cooling. In addition to this, the relative lugging of the engine when the aircraft not moving make it more prone to detonation. The combination of inadequate cooling with high torque loading necessitate a fuel requirement considerably richer than stoichiometric fuel-air ratio in order to produce peak power and consistent operation. The fuel/air mixture is adjustable prior to take off via needle valve 7. During level flight, more cooling air is available and at the same time torque load decreases as the engine unloads. Therefore, less fuel is required even though the fuel delivery rate is the same causing the engine to run too cool, causing torque to decrease and therefore the engine cannot speeds up in order to deliver its full power potential. (Many model aircraft enthusiasts cannot resist the temptation to lean out the engine on the ground in order to gain more power in the air, often with disastrous consequence of engine quitting or overheat damage during a steep climb or a tight loop.) However, with exhaust gas aspiration at flight speed, the pressure inside muffler 10 decreases, in turn causing a decrease in pressure in fuel tank 27 and thus less fuel delivery in keeping with the lower fuel demand. Thus, the engine can always run at its optimum temperature with the correct fuel-air mixture in the air and in the ground, provided that the degree of exhaust aspiration is calibrated by varying angle 22 until maximum level flight speed is obtained even when larger size propeller is used for higher thrust at lower speed, at the same needle valve setting. Angle 22 may be adjusted referring to FIG. 9a by the use of extension tubings 57 attached to muffler tail pipes by flexible silicone rubber hose 56a. The tip of extension tubings 57 is covered by a short segment of silicone hose 56b and banded together by a thick silicone rubber band 56c in order to prevent shear movement (sliding) among the tubings thereby preventing the tubings 57 from excessive rearward flexion when exposed to high air resistance at high airspeed. Should there present bubles in fuel line, or dirt in carburetor jet spray orifice causing the mixture to become too lean, the engine will lose power and the aircraft will slow down, but the engine will not quit because at slower airspeed, more fuel pressure in relation to demands is available to counter act the restriction to fuel flow. In the conventional fuel system in the prior art as discussed earlier, even a small restriction to fuel flow usually is sufficient to cause the engine to quit. In fact, engine quitting while in flight and especially during take off is frequent enough to be a plague in model aviation, and has destroyed an untold number of model aircraft. Since the pressure in tank 27 is much higher in comparison to carburetor venturi fuel draw partial vacuum, it is the predominant motive force in fuel delivery to the engine, and allows the use of larger-bore carburetor for even more power. This type of feed-back fuel metering is adequate to ensure optimum engine operation at all speed and throttle setting even with very simple and inexpensive carburetor. In fact, the carburetor (charge-forming device) needs not be any more than a throttle body containing a throttle valve, a fuel spray jet and a needle valve at the fuel inlet for mixture adjustment.