Motor vehicle equipped with engine noise emission preventing device

A motor vehicle comprising a power unit including an internal combustion engine, a radiator and a transmission, a sound insulating cover located below the power unit, and connecting plate members which securely connect the sound insulating cover to the body of the motor vehicle, thereby forming a noise insulating duct in which the engine is located, so that the engine noise is effectively prevented from being directly emitted outside the vehicle.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to an improved in a motor vehicle to reduce the 
noise level thereof, and more particularly to a noise emission preventing 
device for use in an automotive vehicle, by which the engine noise is 
effectively prevented from being emitted outside the vehicle. 
2. Description of the Prior Art 
In a technique of preventing engine noise emission from a motor vehicle, 
there has been conventionally used an enclosure type engine noise 
insulating device wherein the engine is enclosed substantially in a sealed 
condition by means of a noise insulating plate. Such a conventional 
insulating device, though it has a considerable insulating effect, is 
defective in that the structure is too complicated, requires a large 
number of parts and is inferior with regard to ease and frequency of 
maintenance. There is also a problem with regard to the internal thermal 
load with respect to the cooling of the engine, which requires a separate 
means for reducing the thermal load. Further, as the noise insulating 
plate is supported by the engine body, the vibration (particularly the low 
frequency component) of the engine body is transmitted to the noise 
insulating plate, even though an elastic support manner is employed. The 
noise insulating plate will therefore become a second noise source and 
consequently weaken the noise insulating effect. Accordingly, such a 
conventional device does not meet desired requirements and thus is 
difficult to be put to practical use. 
SUMMARY OF THE INVENTION 
The present invention overcomes the drawbacks encountered in the 
conventional motor vehicle equipped with an engine noise insulating 
device, by forming an engine noise insulating duct in which the internal 
combustion engine is disposed in order to prevent the engine noise from 
being directly emitted outside the vehicle. 
It is the main object of the present invention to provide an improved motor 
vehicle in which the engine noise is effectively prevented from being 
emitted outside the vehicle, solving the problems raised in a conventional 
engine noise emission preventing device of a motor vehicle. 
Another object of the present invention is to provide a motor vehicle 
provided with an improved engine noise emission preventing device which is 
excellent in preventing engine noise from being emitted both in the 
lateral and fore-and-aft directions of the vehicle. 
A still another object of the present invention to provide a motor vehicle 
provided with an improved engine noise emission preventing device whose 
constituent members are not directly contact with an engine body, and 
accordingly, the vibration of the engine cannot be transmitted to the 
device, thereby preventing constituent members of the preventing device 
from becoming a second noise source. 
A further object of the present invention is to provide an automotive 
vehicle provided with an improved engine noise emission preventing device 
which does not degrade the cooling effect to the engine since cooling air 
flows smoothly through the engine noise emission preventing device in 
which the engine lies. 
A still further object of the present invention to provide an automotive 
vehicle provided with an improved engine noise preventing device which is 
simple in construction and accordingly easy in assembly and maintenance 
thereof.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to FIGS. 1 to 5 inclusive of the drawings, there is shown a 
preferred embodiment of a motor vehicle (no numeral) in accordance with 
the present invention. The motor vehicle of this case is an automotive 
truck of the cab-over-engine type wherein a vehicle cabin C or driver's 
compartment lies over an internal combustion engine 10. The engine 1 is as 
usual elastically supported by the frame 12 or chassis of vehicle body 14. 
The frame 12 includes two rigid elongate members 12a, 12b which extend 
oppositely and in parallel with the longitudinal axis (not shown) of the 
vehicle. Accordingly, the elongate members 12a, 12b extend from the front 
portion or panel F of the vehicle toward the rear portion R of the 
vehicle. As will be understood, the engine 10 forms part of a power unit 
16 for generating power to operate the vehicle. A radiator 18 for cooling 
engine coolant is located slightly forward of the engine 10 and fluidly 
connected to the engine though not shown. The engine 10 is provided with 
an exhaust manifold 20 which is followed by an exhaust pipe 22 to 
discharge exhaust gases from the engine into ambient air. A transmission 
24 is located rearward of the engine 10 and so mechanically connected to 
the engine that the main shaft (not shown) of the transmission 24 is 
aligned with the crankshaft (not shown) of the engine 10. The vehicle 
cabin C is defined on a floor portion 26 or lower wall member forming part 
of the vehicle body 14. The floor portion 26 extents to the front panel F. 
As shown, the engine 10 is located below the floor portion 26. The 
reference character W.sub.f represents one of front wheels of the vehicle. 
Two support or connecting plate members 28a, 28b are securely connected to 
the elongate members 12a, 12b, respectively, by means of welding indicated 
by marks of , or bolts (not shown). The support plate members 28a, 28b 
have the function of providing sound insulation to prevent the engine 
noises from being emitted outside thereof. 
A sound insulating cover 30 is formed with two side sections S.sub.1 and 
S.sub.2 which are opposite to each other and extend in parallel with the 
elongate members 12a, 12b. Each of sections S.sub.1 and S.sub.2 is 
provided with a flange portion (no numeral) which is secured to the flange 
portion (no numeral) of each of the two support plate members 28a and 28b 
by means of bolts 32 and nuts 34, as clearly viewed in FIG. 4 which shows 
an example of the connection between the support plate member 28a, 28b and 
the sound insulating cover 30. It is to be noted that a sealing member 36 
such as a packing is interposed between the two flange portions of the 
support plate member 28a or 28b and the flange portion of the side section 
S.sub.1 or S.sub.2 of the sound insulating cover 30. The sound insulating 
cover 30 extends from the vehicle front portion F adjacent the front end 
of the vehicle to the rear portion of the vehicle adjacent the rear end 
24a of the transmission 24. As viewed in the drawings, a noise insulating 
duct D is formed by the floor portion 26, the frame 12, the support plate 
members 28a, 28b and the sound insulating cover 30 in order to enclose 
therein the engine 10. The noise insulating duct D is formed with a 
forward opening A which is located at the vehicle front portion F and a 
rearward opening B which is located adjacent the rear end 24a of the 
transmission 24 to be opened toward the vehicle rear portion R. 
As shown in FIG. 6, the lower wall member 26 includes generally vertical 
opposite side wall sections 26a, 26b which defines therebetween a 
tunnel-like space (no numeral) in cooperation with a upper wall section 
26c. The radiator 18 is securely disposed within the tunnel-like space in 
such a manner that the flat surface thereof is generally parallel with the 
outer surface of the vehicle front portion F so that the air flow or 
vehicle cruising wind can effectively strike against the flat surface of 
the radiator 18. The sound insulating cover 30 extends away from the 
radiator 18 in the vehicle forward direction by a length L great enough to 
be effective for reducing the power unit noise emitted from the front 
portion F of the vehicle. In this case, the sound insulating cover 30 
extents and reaches a cross member 12c of the frame 12. Preferably, the 
length L of the forward extending part 30a of the noise insulating cover 
30 is not less than the value of the side wall section (26a,26b) distance 
b'/.pi.. The side wall section distance b' is a distance between the side 
wall sections 26a, 26b of the lower wall member 26 in the direction of the 
width of the radiator 18. The reference character a' in FIG. 6 denotes the 
height of the side wall sections 26a, 26b. 
The reason why the sound insulating cover forward extending part 30a is 
preferably the value of the above-mentioned distance b'/.pi. will be 
theoretically discussed hereinafter with reference to FIG. 7. 
On the assumption that there is a face sound source having an area of 
F=a.times.b (a&lt;b) as shown in FIG. 7, a consideration will be made on a 
distance-damping (or damping due to distance-separation) in the direction 
of right angles with respect to the center of this face sound source. It 
will be understood that the distances a and b generally correspond 
respectively to the above-mentioned height a' of each side wall section 
26a, 26b of the lower wall member 26 and the distance b' between the 
opposite side wall sections 26a, 26b. As shown in FIG. 7, (similarly in 
the case of a line sound source) the damping appears to be that of the 
face sound source in a vicinity of the face sound source, i.e., extending 
as far as a location a/.pi.. However, thereafter the damping becomes that 
of a line sound source extending as far as the location b/.pi., and then 
it becomes that of a point sound source at a location far from the 
location b/.pi.. 
Accordingly, as shown in FIG. 7, a bended line is drawn such that three 
line segments 0 dB/DD (dB at a location of two times the distance from the 
face sound source), -3 dB/DD, and -6 dB/DD of the line lie respectively 
within three ranges which are separated from each other by vertical lines 
a/.pi. and b/.pi.. This bended line indicates distance-damping of the face 
sound source such that the value of the bended line is L.sub.0 (dB) at a 
distance r.sub.0. Also in this case, the distance-damping characteristics 
strictly appears to become that indicated by the dark curve. 
Additionally, the sound pressure levels for the distances r.sub.0, r.sub.1 
. . . are L.sub.0, L.sub.1 . . . (dB), and the level at points A and B in 
FIG. 7 are L.sub.a, L.sub.b (dB) in which L.sub.a =L.sub.1 =L.sub.0. 
Accordingly, 
EQU L.sub.1 =L.sub.0 for r.ltoreq.a/.pi. (1-1) 
##EQU1## 
wherein, 10 log .pi..apprxeq.5 
##EQU2## 
by combining equations (1-3) and (1-4), 
##EQU3## 
where F=ab. 
As a result, the sound pressure level or noise level at any distance from 
the face sound source can be obtained. It will be understood from the 
above discussion, that the reduction of power unit noise emitted from the 
vehicle front is very effective by extending the noise indulating duct at 
least by a distance of b'/.pi.. 
Besides, it is more preferable that the length L of the noise insulating 
cover forward extending part 30a is about the same as the above-mentioned 
distance b' between the opposite side wall sections 26a, 26b from the 
practical standpoint. Additionally, the inventors' experiments revealed 
that the sound insulating cover 30 having the forward extending part 30a 
of the length L same as the distance b' was very effective for reduction 
of the power unit noise emitted from the vehicle forward portion F through 
the noise insulating duct D, as shown in FIG. 8. In FIG. 8, a curve M 
indicates the sound pressure level variation on various frequencies when 
no sound insulating cover 30 was used, whereas a curve N indicates the 
sound pressure level variation on various frequencies when the sound 
insulating cover 30 was used. The cover has the forward extending part 30a 
of the same length L as the above-mentioned distance b'. As is apparent 
from the data of FIG. 8, the measured sound pressure levels in case of the 
curve N are lower by about 2 dB(A) than in the case of curve M. This shows 
that, by utilizing the sound insulating cover 30 having the forward 
extending part 30a of the above-mentioned length, the power unit noise 
emitted from the vehicle forward portion F is reduced to the value of 
about 63% relative to the noise generated from the power unit 16, thereby 
greatly contributing to the total vehicle noise reduction. 
As shown, the noise insulating duct D includes forward and rearward duct 
sections D.sub.1, D.sub.2 which are located respectively forward and 
rearward of the engine 10 and radiator 18. It is to be noted that the 
minimum effective cross-sectional area of the duct opening of the rearward 
duct section D.sub.2 is preferably not less than 50%, more preferably 70%, 
of that of the forward duct section D.sub.1. What is meant by the minimum 
effective cross-sectional area is a minimum cross-sectional area of the 
duct opening except the cross-sectional area of members such as the 
transmission rear end section 24a. This relationship between the duct 
openings of the forward and rearward duct sections D.sub.1, D.sub.2 was 
determined on the basis of the date obtained by the inventors' 
experiments, as shown in FIG. 9. The data of FIG. 9 depicts the 
temperature variation of fuel within a float chamber of a carburetor (not 
shown) during so-called hot soak condition, i.e., when engine operation 
was completely stopped and a predetermined time elapsed, immediately after 
an automotive vehicle was cruising at 120 Km/hr under road-load 
conditions. In the graph of FIG. 9, curves P, Q and R indicate the 
temperature variations in cases where the minimum effective 
cross-sectional areas of the rearward duct section D.sub.2 are 25%, 50%, 
and 70% of that of the forward duct section D.sub.1, respectively. A fuel 
temperature limit means a fuel temperature above which so-called vapor 
lock and percolation will occur. As is apparent from the graph of FIG. 9, 
in case where the minimum cross-sectional area of the rearward duct 
section D.sub.2 is less than 50% of that of the forward duct section 
D.sub.1, the fuel temperature within the carburetor float chamber exceeds 
the above-mentioned fuel temperature limit during the hot soak. On the 
contrary, in case where the minimum effective cross-sectional area of the 
rearward duct section (D.sub.2) opening is not less than 50% of that of 
the foreward duct section (D.sub.1) opening, the fuel temperature within 
the carburetor float chamber does not reach the above-mentioned 
temperature limit even during the hot soak condition, which results from 
the fact that a sufficient air flow for engine cooling can be obtained 
throughout the noise reducing duct D. It is to be noted that an air stream 
indicated by arrows in FIG. 1 is generated in the duct D during engine 
operation or vehicle cruising since air is induced from the forward 
opening A and discharged through the rearward opening B. The reference 
character M (in FIG. 3) represents a cross-member for supporting the 
transmission 24. 
With such an arrangement, the noise insulating effect is greater in the 
lateral direction of the vehicle and the noise can be effectively 
prevented from being emitted out of the vehicle. Additionally, due to the 
long formation of the noise insulating duct D over the range from the 
vicinity of the front end of the vehicle to the vicinity of the rear end 
portion of the transmission, the noise going round outside the vehicle is 
rendered minimal and the leakage of the noise which is directional forward 
and backward is also rendered minimal. Further, since the support plate 
members 28a, 28b and the sound insulating cover 30 are securely connected 
to the vehicle body 14, no vibration transmission from the engine 10 
occurs and accordingly the sound insulating cover 30 does not serve as a 
secondary noise source. 
Furthermore, since the noise reducing duct D extends away from the radiator 
18 in the vehicle forward direction, the power unit noise can be 
effectively prevented from being emitted from the vehicle front portion F, 
which greatly contributes to the total vehicle engine noise reduction. 
Moreover, since the rearward duct section D.sub.2 has a relatively larger 
area opening, smooth air flow and good engine temperature radiation can be 
gained, thereby preventing engine overheating and problems in the fuel 
system. 
The cooling air for the radiator 18 and engine 10 is taken in from the 
forward opening A with little resistance and the air flow is effectively 
guided therethrough, thereby increasing the cooling ability thereof, and 
the hot air is exhausted out of the rearward opening B. The opening areas 
of the respective forward and rearward openings A, B of the noise 
insulating duct D and the ratio of these two areas are determined 
appropriately by consideration of the air amount passing through the duct 
D. 
Checking and adjustment of the engine 10, transmission 24 etc. can be 
easily carried out by removing the sound insulating cover 30. 
Through-holes 38 arranged on the noise insulating duct D shown in FIGS. 2 
and 5 are provided for such parts (not shown) as installed by piercing 
through these holes, and if these holes 38 are provided on the jointing 
part of the supporting plate 28a, 28b and sound insulating cover 30, the 
sound insulating cover 30 can be engaged and disengaged irrespective of 
such piercing parts. Additionally, the piercing parts also can be easily 
engaged and disengaged only by removing the sound insulating cover 30, 
thereby saving time for adjustment and the like. Recesses 40 arranged on 
the support plate member 28a or 28b shown in FIG. 5, are provided to 
dispose therein a bracket on the lower surface of the frame 12, to secure 
piping and wiring, though not shown. As will be appreciated, by providing 
the support plate member 28a, 28b separately from the sound insulating 
cover 30, the engagement and disengagement of above-mentioned various 
accessories can be rendered much easier in maintenance of the vehicle. 
Furthermore, such an arrangement is advantageous from a point of view that 
the frame 12 is difficult to be combined directly with the sound 
insulating cover 30 due to its shape and that the elastic support of the 
sound insulating cover 30 as referred to later can also be rendered much 
easier. 
FIGS. 10 and 11 show another example of the connection between the support 
plate member 28a, 28b and the sound insulating cover 30, in which a 
cylindrical member or spacer 42 is disposed between the flange portion of 
the support plate member 28a and an annular flat washer 44 through which 
the bolt 32 passes, in order to maintain a certain distance between the 
flange portion of the support plate member 28a and the washer 44. As 
viewed, the bolt 32 passes through the cylindrical opening (no numeral) of 
the cylindrical member 42 and threaded into the nut 34 located at the 
opposite side of the head portion (no numeral) of the bolt 32 relative to 
the flange portion of the support plate member 28a. The flange portion of 
the sound insulating cover 30 is securely connected through an annular 
insulating rubber 46 on the outer surface of the cylindrical member 42. 
The member 42 and insulating rubber 46 forms a vibration insulating device 
(no numeral). It is to be noted that the flange portion of the sound 
insulating cover 30 does not directly contact with, the cylindrical member 
42, but rather elastically contacts the cylindrical member 42 to prevent 
the vibration of the vehicle body from being transmitted to the sound 
insulating cover 30. Additionally, an elastic sound insulating seal member 
48 such as a sealing sponge rubber is disposed in a space between the 
flange portion of the support plate member 28a and the insulating rubber 
46 to prevent the noise within the noise insulating duct D from being 
radiated outside of the duct D. 
With this arrangement, vibration of the vehicle body 14 is prevented from 
being transmitted to the sound insulating cover 30 and accordingly the 
cover 30 will never become a secondary noise source although the surface 
area of the sound insulating cover 30 is considerably large. 
FIG. 12 illustrates another preferred embodiment of the vehicle in 
accordance with the present invention, which is substantially the same as 
the embodiment shown in FIG. 1 with the exception that a sound absorbent 
material 50 is attached on the inner surface of the structural members 
constituting the noise insulating duct D to absorb the noises generated 
within the duct D in which the engine 10 lies. 
In this regard, by virtue of the sound absorbent material, the noise 
reduction can be effectively achieved as will be apparent from the 
following Sobine expression which was established experimentally and by 
which the reduction amount R is calculated: 
##EQU4## 
where l: peripheral length (m) 
S: sectional area (m.sup.2) 
L: duct length (m) 
K: 3.05.times..alpha..sup.1.4 (.alpha.: sound absorption coefficient) 
For example, when the reduction amount R is calculated on the assumption of 
rough estimates l=2 m, S=0.2 m.sup.2, L=0.8 m in the forward duct section 
(D.sub.1) and 0.7 m in the reaward duct section (D.sub.2), and .alpha.=0.8 
with respect to f (noise frequency)=1000 HZ, the following expression can 
be obtained, 
##EQU5## 
the forward duct section (D.sub.1) 
##EQU6## 
the rearward duct section (D.sub.2). 
Such a significant effect has been confirmed with experiments carried out 
by the inventors of the present invention. 
With the arrangement of FIG. 12, the noises generated by the engine 10 and 
the transmission 24 can be absorbed by the sound absorbent material 50 
lined on the inner surface of duct D and therefore the such noises are 
prevented from being radiated outside the noise insulating duct D, 
reducing the level of the noise emitted outside the vehicle. 
As viewed in FIG. 12, the surface of the sound absorbent material 50 is 
supported or covered with a perforated member or material 52 such as 
cloth, net or punched board. Otherwise, the sound absorbent material may 
be covered with an extremely thin heat-resistant material such as 
aluminium foil though not shown, by which fires caused by the oil 
permeation into the sound absorbent material can be effectively prevented. 
While only the cab-over engine type truck has been shown and described, it 
will be understood that the principle of the present invention is 
applicable to other types of motor vehicles. 
As appreciated from the foregoing, according to the present invention, 
sufficient noise insulating effect can be attained since the vibration of 
the sound insulating cover 30 is prevented, and, in addition, engine noise 
itself is prevented from being radiated outside the vehicle. Further, the 
thermal load problem can be solved, improving the cooling effect of the 
engine. Furthermore, assemblying and maintenance of the vehicle become 
much easier.