1.  Field of the Invention
The present invention relates to an intake system for a horizontally opposed type internal combustion engine which includes a first bank and a second bank horizontally disposed opposite to each other with a crankshaft therebetween. 
2.  Description of Background Art
An intake system for a horizontally opposed type internal combustion engine according to the background art includes branch pipes of an intake manifold extending vertically as a whole and connected to intake ports opening at top surfaces of head portions of first and second banks.  Furthermore, an evaporator or a throttle body is connected to a distribution chamber provided at an upper portion of the intake manifold, and an air cleaner is connected thereto. 
However, since the intake system as described above is constructed with concentration on the upper side of the first and second banks, the overall height of the engine is increased.  This reduces the intrinsic merit of a horizontally opposed type internal combustion engine, which is intended to decrease the overall height of the engine by arranging the first and second banks horizontally. 
The present invention has been made in view of the above-mentioned point.  It is therefore an object of the present invention to provide an intake system for a horizontally opposed type internal combustion engine which can obviate the damage to the abovementioned intrinsic merit of the horizontally opposed type internal combustion engine, and can contribute to improvement in output performance of the engine. 
In order to attain the above-mentioned object, a horizontally opposed type internal combustion engine includes a first bank and a second bank horizontally disposed opposite to each other with a crankshaft therebetween.  First and second intake ports are provided at head portions of the first and second banks.  A side wall on the crankshaft side of an air cleaner disposed horizontally adjacent to and on one end side of the crankshaft is provided with first and second intake distribution ports corresponding to the first and second intake ports and arranged horizontally.  The first and second intake ports and the first and second intake distribution ports are connected respectively to each other through first and second intake pipes disposed along a top surface or a bottom surface of the engine.  Furthermore, throttle valves for controlling intake quantities in the first and second intake pipes are provided in respective upstream portions of the intake pipes. 
According to the first characteristic feature, even distribution of air into the first and second intake pipes is achieved in the air cleaner, and the distributed air is supplied individually into first and second cylinders.  As a result, interference of the intake between the cylinders is restrained, while charging efficiency can be enhanced, and an improvement in output performance of the engine can be accomplished. 
In addition, the air cleaner disposed horizontally adjacent to and on the rear end side of the crankshaft and the first and second intake pipes arranged along a top surface or bottom surface of the engine do not have a large influence on the overall height of the engine.  Therefore, it is possible to retain the intrinsic merit of the horizontally opposed type internal combustion engine such that the overall height of the engine can remain small. 
In addition to the first characteristic feature, the first and second banks include pluralities of first and second cylinders and pluralities of the first and second intake ports corresponding to the first and second cylinders.  Pluralities of the first and second intake distribution ports provided in the air cleaner corresponding to the first and second intake ports are arranged symmetrically with respect to a center line extending in the vicinity of and in parallel with the crankshaft.  The first and second intake ports nearer to the air cleaner and the first and second intake distribution ports located on the inner side and nearer to the center line are connected respectively to each other through the first and second intake pipes.  The first and second intake ports farther from the air cleaner and the first and second intake distribution ports located on the outer side and farther from the center line are connected respectively to each other through the first and second intake pipes.  Furthermore, valve shafts of the throttle valves provided in the upstream portions of the first and second intake pipes are connected coaxially with each other. 
According to the second characteristic feature, although the distances from the front-side and rear-side intake ports to the air cleaner differ in each of the banks, evening of the effective pipe lengths of the first intake pipes and evening of the effective pipe lengths of the second intake pipes can be accomplished, and inertia effects and pulsation effects of the intake can be evenly displayed in each of the intake pipes.  As a result, charging efficiency for the cylinders can be enhanced effectively, and a further improvement in output performance can be accomplished. 
Furthemore, in addition to the first characteristic feature, the first and second banks include pluralities of the first and second cylinders and pluralities of the first and second intake ports corresponding to the first and second cylinders.  Pluralities of the first and second intake distribution ports provided in the air cleaner corresponding to the first and second intake ports are arranged symmetrically with respect to the center line extending in the vicinity of and in parallel with the crankshaft.  The first and second intake ports nearer to the air cleaner and the first and second intake distribution ports located on the outer side and farther from the center line are connected respectively to each other through the first and second intake pipes.  The first and second intake ports farther from the air cleaner and the first and second intake distribution ports located on the inner side and nearer to the center line are connected respectively to each other through the first and second intake pipes.  Furthermore, the throttle valves provided in the upstream portions of the first and second intake pipes connected to the first and second outer-side intake distribution ports are arranged nearer to the air cleaner as compared to the throttle valves provided in the upstream portions of the first and second intake pipes connected to the first and second inner-side intake distribution ports. 
According also to the third characteristic feature, evening of the effective pipe lengths of the first intake pipes and evening of the effective pipe lengths of the second intake pipes can be accomplished, and inertia effects and pulsation effects of the intake in each of the intake pipes can be displayed more evenly.  As a result, charging efficiency for each of the cylinders can be enhanced effectively, and a further improvement in output performance can be contrived. 
Furthermore, in addition to the third characteristic feature, the fourth characteristic feature includes the valve shafts of the throttle valves nearer to the air cleaner connected coaxially with each other, and the valve shafts of the throttle valves farther from the air cleaner connected coaxially with each other. 
According to the fourth characteristic feature, the valve shaft of the throttle valves nearer to the air cleaner and the valve shaft of the throttle valves farther from the air cleaner can be arranged parallel with each other. 
Furthermore, in addition to the third characteristic feature, the fifth characteristic feature includes the valve shafts of the throttle valves in the first intake pipes connected coaxially with each other, and the valve shafts of the throttle valves in the sec and intake pipes connected coaxially with each other. 
According to the fifth characteristic feature, interchangeability between the valve shaft of the throttle valves nearer to the air cleaner and the valve shaft of the throttle valves farther from the air cleaner can be accomplished. 
Furthermore, in addition to any one of the first to fifth characteristic features, the sixth characteristic feature includes the first bank set off from the second bank to the opposite side of the air cleaner along the axis line of the crankshaft, and the center line set off from the axis line of the crankshaft to the side of the first bank. 
According to the sixth characteristic feature, the difference between the lengths of the first intake pipe and the second intake pipe due to offset of the first and second banks from each other can be reduced.  This contributes to evening of the effective pipe lengths of the first and second intake pipes. 
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter.  However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.