Dynamic pressure limitation with safety valve

A dynamic pressure-limiting device in an engine exhaust brake, wherein the edge surface of a butterfly disc valve in its closed position is bypassed at one point by a poppet valve bore in which is slidably arranged a poppet valve head which is resiliently urged into the closed position by a spring to provide a safety valve which responds with greater reliability when the upper dynamic pressure limit is exceeded, the response being matched to the amount the limit is exceeded, includes a narrow gap between the butterfly valve disc in its closed position and the poppet valve head in its closed position, and the characteristic of the spring rises at a ratio of spring force (N) to spring deflection (mm) of 1.5-2.5:1. Allowance is made in determining the spring characteristic for the suction effect of the flowing gas on the poppet valve head.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a device to limit the dynamic pressure in an 
engine exhaust brake wherein a butterfly valve is movably supported in an 
exhaust pipe to be deflected from an open position into a closed position. 
2. Description of the Prior Art 
In a prior-art device of this type German Utility Pat. No. 84 24 212, which 
corresponds to co-pending U.S. patent application Ser. No. 763,216, filed 
Aug. 7, 1985, now U.S. Pat. No. 4,682,674, the butterfly valve in its 
closed position also contacts the end face of the poppet valve head and 
the spring has a characteristic about whose rate of rise nothing is 
stated. During tests using this device, the safety valve failed to respond 
either when the upper dynamic pressure limit was exceeded by a small 
amount or when the upper dynamic pressure limit was considerably exceeded. 
BRIEF SUMMARY OF THE INVENTION 
It is an object of the present invention, therefore, to provide a device of 
the type initially referred to whose safety valve will respond with 
greater reliability on the upper dynamic pressure limit being exceeded, 
the response being matched to the amount by which said limit is exceeded. 
In solving this problem, the invention provides a device wherein a poppet 
valve bore bypassing the disc of the butterfly valve when situated in the 
closed position is provided in the exhaust pipe, a poppet valve head which 
is slidable in the poppet valve bore is forced by means of a spring into 
its closing position, the cross-section of the exhaust pipe is round, the 
butterfly valve disc is elliptical and is in a slightly oblique position 
when closed, the poppet valve bore axis being offset from the pivot axis 
of the butterfly valve disc, the edge face of the butterfly disc being 
situated only in its closed position at the poppet valve bore or poppet 
valve head respectively and the end face of the poppet valve head being 
matched to the curvature of the inside surface of the exhaust pipe. The 
invention is also characterized in that a narrow gap exists in between the 
butterfly valve disc situated in its closed position and the poppet valve 
head situated in its closed position and in that the characteristic of the 
spring rises at a ratio of spring force (N): spring deflection (mm) from 
1.5-2.5:1. 
The narrow gap extending across the poppet valve head prevents the poppet 
valve head from being sucked down by the flowing gas and hindered from 
opening any further under conditions where the poppet valve head is 
slightly open. The rise of the spring characteristic is selected so that 
the poppet valve head will, with due consideration of the suction effect, 
recede in proportion to the amount by which the dynamic pressure exceeds 
the desired limit. 
As a rule, engine exhaust brakes operate with a dynamic pressure of 4-7 
bar. Matched to this pressure, the narrow gap is approximately 0.25-0.5 mm 
wide. The ratio of spring force to spring deflection is as a rule of the 
order of approximately 2:1.

DETAILED DESCRIPTION 
In the drawing, the numeral 1 denotes a piece of an exhaust pipe which is 
circular in cross-section and in which the disc 2 of a butterfly valve is 
arranged pivotably an a shaft 3 having its axis located to coincide with 
the inside diameter of the exhaust pipe. The butterfly valve disc 2 is 
formed with an elliptical shape and, as can be seen in FIGS. 1, 2 and 4, 
is situated in a slightly oblique position when it is in its closed 
position. In its closed position, the butterfly valve disc 2 faces the 
inside of the exhaust pipe 1 with a peripheral edge surface 4. Turning the 
butterfly valve disc 2 from its closed position indicated in FIG. 4 by 
dash-dot lines to its horizontal open position and back again is effected 
via a crank mechanism 5 by a piston/cylinder device 6. 
On top of the exhaust pipe 1, there is provided a poppet valve body 7 which 
extends onto the wall of the exhaust pipe and covers the latter on the 
outside. The poppet valve body 7 is formed with a valve bore 8 which is of 
circular cross section and whose diameter is reduced at the end facing the 
inside of the exhaust pipe 1 in the manner of a step which forms an 
inner-end stop 9. The valve bore 8 houses a poppet valve head 10 whose 
diameter increases in the manner of a step in the direction from the 
inside of the exhaust pipe towards the outside and which, in its closed 
position, contacts the stop 9 of the poppet valve body. The poppet valve 
head 10 is formed at its end facing the inside of the exhaust pipe with a 
spherically-shaped concave end face 11 and, as seen in the direction of 
the axis of the exhaust pipe 1 when closed extends beyond the edge surface 
4 of the butterfly valve disc 2 over a considerable length in either 
direction. 
The poppet valve head 10 is provided at its back with an elongate stem 12 
which extends into a bonnet 13. This bonnet is attached to the valve body 
7 and secured to it by means of a circlip. A front-end flange strip of the 
bonnet forms a rear stop 14 for the poppet valve head 10 which delimits 
the travel of the poppet valve head at the back. At the rear end of the 
stem 12 is provided a thrust disc 15 which is slidable guided in the 
cylindrical bonnet 13. Opening into the exhaust pipe 1 on the discharge 
side of the closed butterfly valve disc 2 is a balance passage 16 of 
relatively large cross-sectional area which joins the poppet valve bore 8 
in a manner that with the poppet valve head 10 in the extended closed 
position (FIG. 1) it is only partly closed by the latter, i.e. the passage 
16 is partly open, and that, according to FIG. 2, it is also at least 
partly open with the poppet valve head fully retracted when open, i.e. it 
will be only partly closed by the poppet valve head. 
Bearing on the back of the thrust disc 15 is a coiled wire spring 17 which 
is supported at its outer end at the end of the bonnet. The support is 
effected via an adjusting device 18 which, however, is not mandatory. The 
characteristic of the spring 17 is shown as linear in FIG. 5. According to 
FIG. 6, the poppet valve head in its extended position is set back with 
its end face 11 relative to the inner cylindrical surface of the exhaust 
pipe 1 by a narrow gap 19 and this gap 19 also exists between the poppet 
valve head and the edge surface 4 of the butterfly valve disc 2 both are 
in the closed position as shown in FIG. 1. The gap is preferably about 
0.25 to 0.5 mm in width. 
With the poppet valve head 10 in its extended closed position according to 
FIG. 1, the spring 17 is already loaded; in the retracted position of the 
poppet valve head 10 shown in FIG. 2, the loading of the spring 17 is 
increased. 
As can be seen from FIGS. 3 and 4, the butterfly valve disc 2 can be turned 
from its horizontal open position which is parallel to the centerline of 
the exhaust pipe 1, to the closed position in which it is positioned 
transverse to the centerline.