Patent Description:
<CIT> discloses a small vehicle (scooter) of his type.

In a motorcycle powered by a gas fuel, for example, as typified by fuel cells, especially, in a scooter-type vehicle, there exists a space under a seat so as to be surrounded by a vehicle body cowl (i.e., cowling), and in the space, a gaseous fuel reactor (reacting unit), a fuel tank, fuel supply components, and other components related to the gaseous fuel reactor are placed. Such arrangement desirable in terms of protection of the components or parts of the vehicle, but if a gas whose specific gravity is lower than air is used as fuel, when the gas fuel gets released or leaked for reasons of equipment protection, a piping failure, or the like, the gas fuel is blocked by the vehicle body cowling and is not discharged through surroundings. Therefore, there causes a fear such that the gas fuel might stay in upper portion of the space under the seat (the under-seat region) by being blocked by a bottom surface of the located at the upper portion thereof.

A vehicle mounted with such components is provided with a gas fuel concentration measuring sensor, which is designed to detect any leakage of the gas fuel by placing the gas fuel concentration measuring sensor above the components related to the gaseous fuel reactor, i.e., in the under-seat region, to inform the rider of the leakage and to activate a safety device.

Incidentally, with a seat structure for a motorcycle such as disclosed in <CIT> (Patent Document <NUM>), in which a plurality of substantially dome-shaped spaces are formed under a lower surface (i.e., undersurface) of a seat base of the seat. Accordingly, if the vehicle is powered by gas fuel as described above, a leak gas fuel (this term "leak" may be readable in this disclosure as "leaking" or "leaked" gas fuel) will stay in the respective substantially dome-shaped spaces in a dispersed manner. Furthermore, since reinforcement ribs are arranged in a lattice pattern on the lower surface of the seat base, a large number of square depressions or recesses are formed on the lower surface of the seat base, and consequently, the leak gas fuel will also stay in the depressions in a manner finely dispersed in the depressions in amount.

In a structure in which a gaseous fuel reactor is mounted on a vehicle having a seat structure such as described in the Patent Document <NUM>, since the lower surface of the seat base of the seat has a complicated pattern of recess-and-protrusion (the substantially dome-shaped spaces, depressions, and the like), there is a fear, depending on an installation location of the gas fuel measuring sensor, of being difficult to detect the leak gas even if the leak gas fuel stays in the under-seat region.

In addition, there is a fear that the flow of the leak gas fuel along the lower surface of the seat base is obstructed by the reinforcement ribs arranged in a lattice pattern, making it difficult to discharge the gas fuel.

<CIT> discloses a small vehicle according to the preamble of claim <NUM>, in which a fuel cell and a fuel sensor are provided below the seat.

<CIT> discloses a motor cycle having a fuel cell for generating electric power which is used for driving the motor cycle. Also this prior art vehicle has a gas leakage sensor disposed below the seat. In this prior art vehicle, the lower surface of the seat has a plate barrier for introducing external air. A guide portion extends perpendicular to the plate barrier.

<CIT> discloses a motor cycle having a fuel cell below the seat, and a hydrogen sensor, mounted in the vicinity of hydrogen cylinders which are disposed to the rear part of the vehicle body.

The present invention has been made in view of the above circumstances and an object thereof is to provide a seat structure for a small vehicle, such as motorcycle, capable of detecting and discharging leak gas fuel staying in an under-seat region with high reliability and precision. The above and other objects can be achieved according to the present invention by providing the features of claim <NUM>.

According to the present invention, since the gas fuel sensor is disposed in the depression of the seat, the depression being formed by being surrounded at least by the seat base, if the gas fuel leaks and stays in the depression, the staying gas fuel can be led to the gas fuel sensor. Accordingly, any leakage of the gas fuel can be detected reliably by the gas fuel sensor.

In addition, since the gas fuel leaked from components related to the gaseous fuel reactor stays in the depression of the openable/closable seat, if the seat is opened at the time of gas fuel leakage, the leak (leaking or leaked) gas fuel can be reliably discharged out of the vehicle.

The nature and further characteristic features of the present invention will be made clearer from the following description made with reference to embodiments shown in the accompanying drawings.

Embodiments of the present invention will be described hereunder with reference to the accompanying drawings. Further, it is to be noted that in the following descriptions, terms of "front/rear," "left/right," and "up/down"(vertical) are used in an illustrated states or in a state of a driver riding on the vehicle.

First, with reference to <FIG> and <FIG> showing a left side view of a scooter-type motorcycle, provided with a body frame, components related to the fuel cell stack, and the like, to which a seat structure for a small vehicle according to a first embodiment of the present invention is applied.

A scooter-type motorcycle <NUM> which is a small vehicle, i.e., scooter-type motorcycle, according to the present embodiment uses a fuel cell stack as a gaseous fuel reactor and runs by rotating a motor <NUM> using electric power obtained by a fuel cell drive system <NUM> described later (<FIG>). As shown in <FIG>, the scooter-type motorcycle <NUM> includes a leg shield <NUM>, a screen <NUM>, and a handle bar <NUM>, which protrude in left and right sides in a front portion of the vehicle. It is further to be noted that the terms "vehicle" and "vehicle body" may be equivalently used herein.

The handle bar <NUM> is coupled integrally and rotatably to a steering shaft <NUM> pivotably supported by a head pipe <NUM> of a vehicle body frame <NUM> shown in <FIG>. A front wheel <NUM> is suspended on the steering shaft <NUM> via a pair of right and left front forks <NUM>. As the steering shaft <NUM> is pivotably supported by the head pipe <NUM> so as to be turnable left and right, the front wheel <NUM> is turned left and right when the handle bar <NUM> is operated.

As shown in <FIG>, the vehicle body frame <NUM> has the head pipe <NUM> in a front end portion thereof, and a pair of front and rear down tubes <NUM> and <NUM> extend downward from lower and upper portions of the head pipe <NUM>. The lower portion of the front down tube <NUM> bends rearward, then extends rearward, and then bends upward in approximately intermediate portion in a front-rear (longitudinal) direction of the vehicle body. Furthermore, a pair of left and right main tubes <NUM> extend rearward from an approximate center position of the front down tube <NUM> in a vertical direction. The main tubes <NUM> are connected with a lower end portion of the rear down tube <NUM> as well as with a rear end portion of the front down tube <NUM>.

As shown in <FIG>, the scooter-type motorcycle <NUM> has a double seat saddle <NUM> for two riders arranged behind the leg shield <NUM> that is located in the front portion of the vehicle.

A pair of left and right flat-plate foot boards <NUM> extends over a range from the leg shield <NUM> to the double seat saddle <NUM> to allow riders (driver and pillion passenger) sitting on the double seat saddle <NUM> to put their feet on. Further, a center tunnel cover <NUM> is provided in this range. The center tunnel cover <NUM> bulges upward between the pair of left and right foot boards <NUM>, being installed behind the leg shield <NUM> in such a way as to be continuous to the leg shield <NUM>.

Under the double seat saddle <NUM>, a rear cover <NUM> serving as a vehicle body cowl (cowling) is installed behind the center tunnel cover <NUM> in such a way as to be continuous to the center tunnel cover <NUM>. The rear cover <NUM> is designed to cover an under-seat region A located under the double seat saddle <NUM> in rear side portion of the vehicle. Specifically, the rear cover <NUM> covers the main tubes <NUM> and front down tube <NUM>, and the like of the vehicle body frame <NUM> as well as most of the components related to the fuel cell stack of the fuel cell drive system <NUM> (including a fuel cell stack <NUM>, a fuel tank <NUM>, a rechargeable battery <NUM>, a power controller <NUM>, and a motor controller <NUM> described later), which are disposed in the under-seat region A.

A swing arm <NUM> equipped with a motor <NUM> adapted to drive a rear wheel <NUM> is pivotally supported under the rear cover <NUM> so as to be vertically swingable around a pivot <NUM> of the main tubes <NUM> shown in <FIG> as a fulcrum. A reaction unit <NUM> is disposed so as to bridge between the swing arm <NUM> and main tubes <NUM>. The motor <NUM> and rear wheel <NUM> are suspended in a manner that the vertical swing thereof is dampened by the reaction unit <NUM>.

Incidentally, in <FIG>, reference numeral <NUM> denotes a side stand and reference numeral <NUM> in <FIG> denotes a center stand.

As shown in <FIG> and <FIG>, the fuel cell drive system <NUM> includes components related to the fuel cell stack such as a fuel cell stack <NUM>, a fuel tank <NUM>, a rechargeable battery (driving battery) <NUM>, a power controller <NUM>, a motor controller <NUM>, a vehicle controller <NUM> and so on.

Among the components mentioned above, the fuel tank <NUM> is surrounded and supported by the pair of left and right main tubes <NUM> and front down tube <NUM> and housed in a region from the inside of the center tunnel cover <NUM> to the lower portion of the under-seat region A in the rear cover <NUM>. Further, the rechargeable battery <NUM>, the power controller <NUM>, the motor controller <NUM> and the fuel cell stack <NUM> are also housed in the upper portion of the under-seat region A in the rear cover <NUM> in a manner of being supported by the main tubes <NUM>. In the under-seat region A, the rechargeable battery <NUM>, the power controller <NUM> and the fuel cell stack <NUM> are arranged in this order from the front side of the vehicle body, and the motor controller <NUM> is disposed, for example, on a left flank of the power controller <NUM>. Furthermore, the vehicle controller <NUM> is housed in the leg shield <NUM> in a manner of being supported by the lower portion of the front down tube <NUM>.

The fuel tank <NUM> stores a gas fuel having a specific gravity lower than that of air, such as hydrogen in a gas state, under high pressure. A main stop valve <NUM> is attached to an outlet portion of the fuel tank <NUM>, and a fuel filling port <NUM> is connected to the main stop valve <NUM> via a filler pipe <NUM>. The fuel filling port <NUM> is positioned in the center tunnel cover <NUM> mounted on the main tubes <NUM>. A high-pressure gas fuel (gaseous hydrogen) is injected through the fuel filling port <NUM>, filling the fuel tank <NUM> through the filler pipe <NUM> and the main stop valve <NUM>.

The main stop valve <NUM> is connected with a pressure-regulating valve <NUM>, which in turn is connected to the fuel cell stack <NUM> via a secondary pressure reducing valve <NUM>. The high-pressure gas fuel (gaseous hydrogen) in the fuel tank <NUM> is depressurized by the pressure-regulating valve <NUM> via the main stop valve <NUM> and then supplied to the fuel cell stack <NUM> through a secondary pressure reducing valve <NUM>. The main stop valve <NUM>, the filler pipe <NUM> and the pressure-regulating valve <NUM>, mentioned hereinbefore, are disposed at the lower portion of the under-seat region A in the rear cover <NUM> as with the fuel tank <NUM>. The secondary pressure reducing valve <NUM> is arranged in a portion from the lower portion to the upper portion of the under-seat region A.

The fuel cell stack <NUM> generates electric power through chemical reaction of the gas fuel (gaseous hydrogen) supplied from the fuel tank <NUM> and oxygen contained in air. A moist exhaust gas produced as a result of the chemical reaction is discharged through an exhaust port <NUM> (<FIG>). According to the present embodiment, the fuel cell stack <NUM> is disposed under a passenger seat portion 25B (described later) of the double seat saddle <NUM> shown in <FIG>.

The rechargeable battery <NUM> stores excess electric power produced by the fuel cell stack <NUM> or feeds the stored electric power to the motor <NUM> via the motor controller <NUM>. Further, the power controller <NUM> controls generated output of the fuel cell stack <NUM>, and stores the excess electric power produced by the fuel cell stack <NUM> in the rechargeable battery <NUM> or feeds the electric power stored in the rechargeable battery <NUM> to the motor <NUM> via the motor controller <NUM>. Furthermore, the motor controller <NUM> controls the operation of the motor <NUM>. The rechargeable battery <NUM>, the power controller <NUM> and the motor controller <NUM> are arranged under a rider seat portion 25A (described later) of the double seat saddle <NUM> shown in <FIG>.

The vehicle controller <NUM> is designed to control the operation of the scooter-type motorcycle <NUM> equipped with the fuel cell drive system <NUM>.

That is, when the scooter-type motorcycle <NUM> is running on a flat road at which a relatively small amount of electric power is required, the electric power generated by the fuel cell stack <NUM> is fed to the motor <NUM> from the power controller <NUM> through the motor controller <NUM> and excess electric power is stored in the rechargeable battery <NUM> from the power controller <NUM>.

On the other hand, during acceleration or during running on an uphill slope at which a relatively large amount of electric power is required for the scooter-type motorcycle <NUM> to run, the vehicle controller <NUM> feeds the electric power generated by the fuel cell stack <NUM> to the motor <NUM> from the power controller <NUM> through the motor controller <NUM> and feeds the electric power stored in the rechargeable battery <NUM> from the power controller <NUM> to the motor <NUM> via the motor controller <NUM>.

As shown in <FIG> and <FIG>, in the under-seat region A which is an enclosed space located under the double seat saddle <NUM> and surrounded by the rear cover <NUM>, if the gas fuel (gaseous hydrogen) leaks from a component related to the fuel cell stack such as the fuel cell stack <NUM> or fuel tank <NUM> of the fuel cell drive system <NUM>, as indicated by arrow α, the gas fuel rises in the under-seat region A, flows toward the rear side of the vehicle body along a seat base <NUM> which is a bottom plate of the double seat saddle <NUM>, and is discharged out of the vehicle from a portion near a rear end portion of the double seat saddle <NUM>, and any leakage of the gas fuel is detected by a gas fuel sensor <NUM>.

The gas fuel sensor <NUM> measures concentration of gas fuel (gaseous hydrogen) and detects the time (fact) when the leak gas fuel reaches or exceeds a predetermined concentration. As shown in <FIG>, the gas fuel sensor <NUM> is fixedly mounted on the fuel cell stack <NUM> using a mounting bracket <NUM> at a position under the passenger seat portion 25B near the seat base <NUM> in the under-seat region A as described in detail hereinlater. If the leakage of the gas fuel is detected by the gas fuel sensor <NUM>, the driver is informed to that effect and a safety device or the like is activated.

As shown in <FIG>, <FIG> and <FIG>, the double seat saddle <NUM> is located above the main tubes <NUM> and is made up of the rider seat portion 25A and passenger seat portion 25B formed integrally, where the rider seat portion 25A is positioned on a front side while the passenger seat portion 25B is positioned on a rear side.

Furthermore, as shown in <FIG>, the double seat saddle <NUM> is formed in such a manner as that a seat cushion <NUM> made, for example, of a urethane foam cushioning material is placed on a top surface of the seat base <NUM> which is a bottom plate formed, for example, of a synthetic resin and then a surface of the seat cushion <NUM> is covered with seat skin <NUM>. The seat base <NUM> supports loads acting on the double seat saddle <NUM>.

As shown in <FIG>, <FIG> and <FIG>, in the double seat saddle <NUM> configured as described above, a seat hinge <NUM> is installed in a front end portion of the seat base <NUM>, and a seat striker <NUM> is installed in a rear end portion of the seat base <NUM> so as to protrude therefrom. As shown in <FIG> and <FIG>, a hinge support bracket <NUM> is mounted so as to be stretched between the pair of left and right main tubes <NUM>. Since the seat hinge <NUM> is pivotably supported by the hinge support bracket <NUM> in a turnable manner, the double seat saddle <NUM> can be supported to be opened or closed by the main tubes <NUM>. When the double seat saddle <NUM> is closed, the seat striker <NUM> is restrained by a lock mechanism <NUM> mounted on the main tubes <NUM> and thereby prevents the double seat saddle <NUM> from opening unnecessarily.

Furthermore, as shown in <FIG>, the seat base <NUM> of the double seat saddle <NUM> includes a lower bent portion <NUM> of the seat surrounded at least by the seat base <NUM> and intended to provide a larger space under the double seat saddle <NUM>, and a substantially dome-shaped depression <NUM> is formed in a certain portion of the seat-lower bent portion <NUM>. The seat-lower bent portion <NUM> is an upwardly recessed inner region located under the seat base <NUM> and provided with an outer edge which corresponds to a lower end portion of the double seat saddle <NUM>. According to the present embodiment, the depression <NUM> has an upper contour defined by the seat base <NUM>, and flanks defined by a defining portion <NUM>. The defining portion <NUM> is formed integrally with the seat base <NUM> by protruding downward along the vehicle body from the seat base <NUM> and extending in a seat width direction (width direction of the vehicle) while being bent substantially in an arcuate (circular-arc) shape. Gas fuel leaked from the fuel cell stack <NUM> or another component related to the fuel cell stack rises in the under-seat region A, flows toward the rear part of the vehicle along the seat base <NUM> of the double seat saddle <NUM> and stays in the depression <NUM>.

As described above, the gas fuel sensor <NUM> is mounted on the fuel cell stack <NUM>, near the seat base <NUM> of the double seat saddle <NUM> in the under-seat region A, using the mounting bracket <NUM> as shown in <FIG>, and more specifically, the gas fuel sensor <NUM> is disposed in the depression <NUM>. Furthermore, if position of an uppermost point of the defining portion <NUM> is designated as point T, at least a part of (approximately an upper half portion in the present embodiment) the gas fuel sensor <NUM> is disposed above a horizontal line U passing through point T. According to such arrangement, the leak gas fuel staying in the depression <NUM> can be detected reliably by the gas fuel sensor <NUM>. Incidentally, as shown in <FIG>, an uppermost point S of the depression <NUM> is located at a center position in the seat width direction (width direction of the vehicle body).

As shown in <FIG>, a plurality of reinforcement ribs <NUM> are provided on the lower surface of the seat base <NUM> of the double seat saddle <NUM> in a manner of protruding in a front-rear direction of the seat (longitudinal direction of the vehicle). The reinforcement ribs <NUM> are arranged in the seat width direction (width direction of the vehicle) in parallel to each other without intersecting each other.

Since the reinforcement ribs <NUM> are formed on the seat base <NUM> in the manner described above, rigidity of the seat base <NUM> can be improved without obstructing the flow of the leak gas fuel along the lower surface of the seat base <NUM>. In addition, since concave grooves <NUM> defined by the plural reinforcement ribs <NUM> are provided in continuous to a location site of the gas fuel sensor <NUM>, it is possible to guide the leak gas fuel to the gas fuel sensor <NUM> along the concave grooves <NUM>.

Furthermore, as shown in <FIG> and <FIG>, in order to ensure a riding comfort and a ventilating performance, a communication hole <NUM> is formed in the seat base <NUM> of the double seat saddle <NUM> so as to establish the communication between an upper space of the seat base <NUM> in which the seat cushion <NUM> is provided and the under-seat region A located under the seat base <NUM>. As shown in <FIG>, the communication hole <NUM> is formed at a position lower than the point T of the defining portion <NUM> as viewed from a side of the vehicle to thereby form the communication hole <NUM> at a position where the leak gas fuel does not stay.

As shown in <FIG>, edge (marginal) projections <NUM> are formed on a lower edge of the communication hole <NUM> so as to protrude downward along the vehicle body from the lower surface of the seat base <NUM>. Accordingly, the leak gas fuel flowing along the lower surface of the seat base <NUM> of the double seat saddle <NUM> is blocked by the edge projections <NUM> and thereby kept from flowing into the communication hole <NUM>.

The present invention of the configuration or structure described above provides the following advantages or effects (<NUM>) to (<NUM>).

A second embodiment of the present invention will be described hereunder with reference to <FIG>, in which <FIG> is a bottom view corresponding to <FIG> and showing a double seat saddle of a scooter-type motorcycle to which a seat structure for a small vehicle according to a second embodiment of the present invention is applied, and <FIG> is a view taken in a direction of arrow XV in <FIG>.

In the second embodiment, components similar to those in the first embodiment are denoted by the same reference numerals as the corresponding components, and description thereof will be simplified or omitted herein.

A double seat saddle <NUM> according to the present embodiment differs from the double seat saddle <NUM> according to the first embodiment in that a depression (recessed portion) <NUM>, which is provided on a seat base <NUM> as a bottom plate of the double seat saddle <NUM> and in which the gas fuel (gaseous hydrogen) stays, is installed near a rear end portion of the double seat saddle <NUM> and differs in that a gas fuel sensor <NUM> is fixedly attached to a lower surface of the seat base <NUM> in the depression <NUM>.

As shown in <FIG>, the lower surface of the seat base <NUM> is configured to generally slope upward toward a rear end side of the double seat saddle <NUM> while the depression <NUM> is formed by being defined by a defining portion <NUM> formed by a rear end side portion of the seat base <NUM> and rear end side portions of the seat cushion <NUM> and seat skin <NUM> of the double seat saddle <NUM>.

As shown in <FIG> and <FIG>, the gas fuel sensor <NUM> is fixedly attached to the lower surface of the rear end side portion of the seat base <NUM>, where the lower surface defines the depression <NUM> and is located in the depression <NUM>. In particular, preferably, the gas fuel sensor <NUM> is fixedly attached to that portion (i.e., a sensor installation site <NUM>) on the lower surface of the seat base <NUM> which contains the uppermost point S of the depression <NUM> located at approximate center position in the seat width direction (width direction of the vehicle) of the double seat saddle <NUM> so as to coincide with the uppermost point S in a plan view. Furthermore, at this time, the gas fuel sensor <NUM> is positioned above the horizontal line U passing through the point T which is the position of the uppermost point of the defining portion <NUM>.

As shown in <FIG>, on the lower surface of the seat base <NUM>, a plurality of the reinforcement ribs <NUM> are arranged in the width direction of the double seat saddle <NUM> so as to protrude downward along the vehicle, and the reinforcement ribs <NUM> are formed so as to avoid a region in the seat width direction under the uppermost point S of the depression <NUM>. That is, the reinforcement ribs <NUM> are not formed in that portion in the seat width direction on the lower surface of the seat base <NUM> which contains the uppermost point S of the depression <NUM> to which the gas fuel sensor <NUM> is fixedly attached. This allows the gas fuel to flow in the seat width direction in this region.

According to the configuration as described above, the present embodiment provides the following advantages or effects (<NUM>) to (<NUM>) in addition to the advantages similar to (<NUM>) to (<NUM>) achieved by the first embodiment described above.

(<NUM>) Since the depression <NUM>, in which the leak gas fuel will stay, is formed in the rear end portion of the double seat saddle <NUM>, a staying volume (hence, gas fuel staying amount) can be reduced. At the same time, the discharge of the gas fuel from the double seat saddle <NUM> is facilitated. In particular, when the depression <NUM> is provided in the rear end portion of the double seat saddle <NUM>, since the double seat saddle of a motorcycle is generally configured to be high at the rear side, it is possible to further facilitate the discharge of the gas fuel from the depression <NUM>.

(<NUM>) Since the lower surface of the seat base <NUM> of the double seat saddle <NUM> is configured to generally slope upward toward the rear end side of the double seat saddle <NUM>, the gas fuel in the under-seat region A gathers into the depression <NUM> located substantially at the rear end of the double seat saddle <NUM> instead of wandering along the lower surface of the seat base <NUM>. Therefore, since the gas fuel is not scattered and does not stay on the lower surface of the seat base <NUM>, the leak (leaking or leaked) gas fuel can be detected reliably by the gas fuel sensor <NUM> disposed in the depression <NUM>.

(<NUM>) The reinforcement ribs <NUM> are provided on the lower surface of the seat base <NUM> so as to protrude therefrom and are formed so as to avoid a region in the seat width direction under the uppermost point S of the depression <NUM>. In addition, the gas fuel sensor <NUM> is fixedly attached to that portion (i.e., a sensor installation site <NUM>) on the lower surface of the seat base <NUM> which contains the uppermost point S of the depression <NUM>. Accordingly, the gas fuel can be caused to flow in the seat width direction (width direction of the vehicle) near the uppermost point S in the depression <NUM> of the seat base <NUM>, and thus, easily flow to the gas fuel sensor <NUM>, thereby ensuring the reliable detection of the leak gas fuel by the gas fuel sensor <NUM>.

It is further to be noted that although the present invention has been described hereinabove with reference to the preferred embodiments, the present invention is not limited thereto and many other changes and modification or alternations may be made.

For example, in the double seat saddles <NUM> and <NUM>, the reinforcement ribs <NUM> and <NUM> may be provided on the top (uppermost) surfaces of the seat bases <NUM> and <NUM>. This arrangement will increase the rigidity of the seat bases <NUM> and <NUM> and improve the riding comfort of the double seat saddles <NUM> and <NUM> because of no location of any projection or recess which may cause gas fuel to stay on the lower surfaces of the seat bases <NUM> and <NUM>.

Furthermore, although the gaseous hydrogen is used as a gas fuel in the described two embodiments, another gas fuel, such as methane gas, lower in specific gravity than air may be used alternatively.

In addition, although in the described two embodiments, a vehicle which uses a fuel cell stack as a gaseous fuel reactor has been described, the present invention is also applicable to a vehicle equipped with another type of apparatus which uses gas fuel. For example, the vehicle may be equipped with an internal combustion engine which uses a gas fuel lower in specific gravity than air.

Claim 1:
A small vehicle (<NUM>) with a seat structure, comprising:
a body frame,
a seat (<NUM>, <NUM>) located above and supported by the body frame to be opened or closed;
a body cowl (<NUM>) covering an under-seat region (A) located under the seat;
a fuel tank (<NUM>) disposed in the under-seat region, supported by the body frame, and adapted to store a gas fuel lower in specific gravity than air,
a gaseous fuel reactor (<NUM>) disposed in the under-seat region to cause the gas fuel to undergo a chemical reaction; and
a gas fuel sensor (<NUM>) disposed in the under-seat region to detect gas fuel leaked from components related to the gaseous fuel reactor (<NUM>) including the gaseous fuel reactor and the fuel tank,
characterized in that the seat (<NUM>) includes an under-seat curved bent portion (<NUM>) surrounded at least by a seat base (<NUM>, <NUM>) which is a bottom plate of the seat (<NUM>) so as to provide a larger space in the under-seat region, if an uppermost point of a defining portion (<NUM>) is designated as a point T, the defining portion (<NUM>) protruding downward along a vehicle body from the seat base (<NUM>, <NUM>) to define a substantially dome-shaped depression (<NUM>, <NUM>) in a part of the under-seat curved bent portion, the gas fuel sensor (<NUM>) is located in the depression (<NUM>, <NUM>) and at least a part of the gas fuel sensor (<NUM>) is arranged above a horizontal line passing through the point T, a plurality of reinforcement ribs (<NUM>, <NUM>) are disposed to a lower surface of the seat base (<NUM>, <NUM>) so as to extend downward therefrom, the reinforcement ribs (<NUM>, <NUM>) extend in parallel to each other without intersecting each other substantially in a front-rear direction of the seat (<NUM>, <NUM>) to thereby define concave grooves (<NUM>) to the lower surface of the seat base (<NUM>, <NUM>) so as to open downward , and the concave grooves defined by the reinforcement ribs extend substantially in the front-rear direction, the concave grooves (<NUM>) defined by the plural reinforcement ribs (<NUM>) are arranged in continuous to a location site of the gas fuel sensor (<NUM>).