Conventional four-stroke engines have certain disadvantages as they are complicated and difficult to manufacture and assemble because there are numerous parts as compared to two-stroke engines. The additional parts, for example include, valve trains consisting of intake and exhaust valves, followers in the case of push tube trains for transmitting motion from cam lobes to rockers, just rockers in the case of overhead cam and belt or chain drives for overhead cam types. Also included are cam gear or pulley as the case may be, valve springs and retainers, camshafts, and cam covers in some cases. Also, the method of assembling the main components varies depending on how the cylinder, crankcase, crankcase cover, piston rod and crankshaft assemblies are made. Also, in the conventional hand held four-stroke engines, the oil is either recirculated in a wet type lubrication or pre-mixed with fuel for mist lubrication.
It is known in the prior art that four-stroke engines have cylinder blocks (with or without a separate cylinder head) and crankcases as the case may be with or without crankcase covers. For example, cylinders manufactured by MTD Southwest has a cylinder head integral with the cylinder and has a separate crankcase which has main bearings to support the crankshaft and a separate volute attached to the crankcase. The volute also has bosses for the ignition module. Another example is a Honda engine which has a cylinder block including a cylinder, where the upper half of the crankcase is integral with the cylinder block and a lower half of the crankcase which, when assembled together, support the main bearings. In this case, there is no separate crankcase cover and the belt drive for the overhead valve system is a wet type, where the upper and lower half of the crankcases together form a reservoir for the lubricating oil and the belt is completely enclosed. The enclosure is integral with the upper half of the crankcase. A similar design is used for a push tube type of valve train. Reference may be made to U.S. Pat. Nos. 6,539,904, 6,672,273, 6,427,672, 6,508,224, 6,705,263 (belt drive), and U.S. Pat. No. 6,021,766 (push tube).
Some Honda full crank engines have the crankcases split at an angle to the crankshaft as disclosed in U.S. Pat. Nos. 6,250,273 and 6,644,290. The front half of the crankcase is integral with the cylinder block and has bearing boss to support the front half of the crankshaft and the rear half of the crankcase has another bearing boss to support the outboard side of the crankshaft. The cam gear or the pulley for transmitting the motion to the overhead valves is in the outboard side.
Another example of engines with push tubes are disclosed in U.S. Pat. Nos. 6,213,079, 7,243,632, and 6,119,648. Some engines use gears to transmit rotation from crankshaft to the overhead camshaft, which is running at half the crankshaft speed as disclosed in U.S. Pat. No. 6,152,098. In most cases where the engine has a two piece block, the top or front half and lower or outboard half of the crankcase, the valve train is on the outboard side.
In the case of upper and lower halves of crankcases (or left and right halves as in Kioritz U.S. Pat. No. 6,119,648, the disadvantages are that the upper and lower halves are first assembled together and then the bearing bores are machined. They are taken apart for the final assembly. They are not interchangeable. A sealing gasket is used to seal the two halves. As such, the cost of such a system is higher than the one proposed in the design disclosed herein.
Prior art disclosed in U.S. Pat. No. 2,287,508 refers to a wet lubrication, where the oil is pumped into various sections of the engine and oil is recirculated. The lubricating pump is not described clearly, but it is driven by the camshaft. In the handheld trimmer sold by Mitsubishi model TL 26, the oil pump is driven by the crankshaft on the outboard side, but it is two-stroke engine and 100% of the air-fuel mixture enters the crankcase chamber.
Thus, engine designers are constantly trying to design engines that have less parts, are simpler, and less expensive to manufacture.
Disadvantage with conventional four stroke engines for hand held application is that the oil in the crankcase may seep into the combustion chamber and cylinder head when the equipment is turned upside down.
In some hand held engines, such as trimmers and blowers, the location and size of the fuel tank is constrained by the size of the engine and the shape and size of the crankcase cover. The constraint is more when the fuel is propane gas. Propane gas are typically made out of metal and preferably of cylindrical shape. Therefore the height of the whole engine is significantly higher when propane tank is mounted either on the top or bottom of the engine. The embodiments disclosed here provide many advantages over the prior arts.
In some electronic fuel injection system, the fuel is gasoline or diesel. However, in small hand-held engines, electronic fuel injection system is complex as it requires a separate fuel pump, either integral with the throttle body or a separate system. In either case, the cost of the system becomes expensive because of the additional parts.
The embodiment described here does not require a pump, as the fuel is already at a pressure in the LPG fuel tank or even in a compressed natural gas tank. The advantage described in this embodiment is that the throttle body can be integral with the pressure regulator and a metering chamber if necessary. It is also possible to have a pressure regulator only either integral with the throttle body or a separate pressure regulator commonly used with LPG tanks.