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Among the natural gases proposed for use in internal combustion engines, (a) highly compressed natural gas (CNG) or (b) methane dissolved into hydrocarbon solvents (such as propane C3H8 or butane C4H10) are generally considered practicable. Technical problems with these that remain to be solved include low energy density of CNG ((a) above) even under very high pressure as well as significant changes in fuel composition due to the changes in the internal pressure ((b)above) of the fuel tanks as fuel is drawn. Our research aims at developing pressurized gas mixture for internal combustion engines.

Use of the hydrostatic transmission (HST) in compact riding mowers has recently become more widespread due to the fact that an HST provides considerably better mowing performance than a conventional manual transmission. In addition, there are now several manufacturers that are capable of building compact and efficient HSTs. The standard power train of an HST-equipped compact riding mower consists of the engine, HST, and transaxle, laid out separately and interconnected by a V-belt. As a result, the power transmission system is highly complicated, making maintainability difficult. The recently developed model PA540A and PA420A unitized power units are compact power packages combining a vertical gasoline engine, HST, and transaxle in a single structure that can be mounted with as few as 3 bolts, allowing much greater freedom in mower design.