CA1160920A - Engine braking system - Google Patents

Abstract

ABSTRACT
An engine braking system of a gas compression relief type is provided to control the timing of the sys-tem which includes a hydro-mechanical mechanism which opens the exhaust valve near the top of the compression stroke of the engine so that the energy absorbed by the engine during the compression stroke is not returned to the engine during the expansion stroke. In accordance with the invention, hydro-mechanical means are provided in the exhaust valve actuating mechanism to reduce the clearance in that mechanism to a value which maximizes the retarding power developed by the engine whenever the engine brake is activated.

Description

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ENGINE BRAKING SYSTEM
TECHNICAL FIELD
This invention generally relates to an engine braking system of a gas compression relief type The invention relates more particularly to engine brakes wherein the exhsust valves of the engine are opened near the top of the compression stroke of the engine so that the ene~gy absorbed by the engine during the compression stroke is not returned to the engine during the expansion stroke. The present invention relates specifically to a timing mechanism for an engine brake of the above type.
~ACKGRO~ND ART
For many years it has been recognized that the ordinary wheel brakin~ mechanisms, commonly of the disc or drum type fitted to commercial vehicles, while capable of absorbing a large amount of energy during a short period, are incapable of absorbing the lesser amounts of energy over an extended period of tLme which may be required, for example, during descent of a long decline. In such circumstances, the friction ma~erial used in the brake mechanism will become overheated (~causing "brake fading") and may be destroyed while the metallic parts may warp or buckle. In general~
the problem has been resolved either by using a lower gear ratio so that the engine can function more effectively as a brake due to its inherent friction ~or by employing some form of auxiliary braking system.
A number of such auxiliary braking systems, generally known as engine retarders, have been developed by the art, including hydrokinetic retarders, exhaust brakes, ~k ~3LG~9;~

electric brakes, and engine brakes. In each of these systems, a portion of the kinetic energy o~ the vehicle is transformed into heat as a result of gas compression, fluid friction, or electrical resistance and, there-after dissipated to the atmosphere directly or throughthe exhaust or cooling system. T~e common characteristic of such auxiliary braking systems is the abîlity to absorb and dissipate a certain amount of power con tinuously or at least for an indefinite period of time, Each of the types of engine retarder referred to above is described in detail in the publication "Retarders for Commercial Vehicles" published in 1975 by Mechanical Engineering Publications Limited, London, England The hydrokinetic and electric retarders are generally quite heavy and bulky since they require turbine or dynamo mechanism and thus may ~e undersirable from the viewpoint of initial cost as well as operating cost. The exhaust brake, while generally simple and compact, necessarily increases the exhaust manifold pressure and may occasion "floating" of the exnaust valves of the engine, a generally undesirable condition It has long been recognized that in the ordinary operation of an internal combustion engine employing the Otto or the Diesel cycle, for example, a considerable amount o~ work is done during the compression stroke upon the air or air/fuel mixture introduced into the cylinders. During the expansîon or power stroke of the engine the work of compression is recovered so that, neglecting friction losses, the net work due to compression and expansion is zero and the net power output is that resulting from the combustion of the fuel/air mixture. ~hen the throttle is closed, or the fuel supply interrupted, the engine will, of course, function as a brake to the extent of the friction inherent-in the engine mechanism.

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Many attempts have been made to increase the braking power of an engine by converting the engine to an air compressor and dumping the compressed air through the exhaust system. A simple and practical method of accomplishing this end is disclosed in U.S.
Patent 3,220,392. In that patent an auxiliary exhaust valve actuating means synchronized with the engine crankshaft is provided which opens the exhaust valve near the end of the compression stroke, without interfering with the normal actuating cam means for the exhaust valve, together with appropriate control means for the auxiliary exhaust valve actuating means.
While the engine brake means set forth in detail in U.S. Patent 3,220,3~2 is capable of producing a retarding power approaching the driving power of the engine under normal operating conditions, experience with this mechanism has reYealed that the retarding power may be affected significantly by-the timing of the opening of the engine exhaust valve.
If the exhaust val~e is opened too late a significant portion of-the retarding power may be lost due to the expansion of the compressed air during the initial part of the expansion stroke. On the other hand, if the exhaust valve is opened too early there may be insufficient compression during the compression stroke which, similarly, will reduce the amount of retarding power that can be developed.
The timing of the exhaust valve opening is affected to a significant degree by the temperature conditions in the engine which vary as a result of changes in ambient conditions as well as changes in operating conditions. It will be appreciated, for example, that the length of the engine exhaust YalYe will increase with increases in temperature, thereby reducing the clearances in the valve actuating mechanism. ~hile it is known to provide ad~usta~le ~6 3g~

elements in the valve actuating mechanism by means of which the clearance may be set (see, for example, U.S.
Patent 3,220,392, Fig. 2, element 301), the clearance as determined by the rocker arm adjusting screw (or equivalent element) must be at least large enough when the engine is cold ~o that some clearance will remain when the engine is hot. If there is inadequate clear-ance when the engine is hot, the exhaust valve may be held in a partially open condition. In this circumstance, the operation of the engine may be affected adversely and the exhaust valves are apt to be burned. To avoid such effects, it is c~mmon to provide a clearance in the actuating mechanisms for the exhaust (and intake) valves of an internal combustion engine on the order of Q.018 inch so as to compensate for dimensional changes in the mechanism resulting from temperature variations.
When using the exhaust valve actuating mechanism as part of an engine brake mechanism, it is highly advantageous to minimize the clearance or backlash in the valve actuating mechanism to provide a precise control of the valve timing where~y the retarding power of the engine is maximized.
DrSCLOSURE OF INVENTION
With the foregoing in mind, we provide in accordance with the invention an engine braking system of a gas compression relief type and comprising a combustion engine having exhaust valve means and an adjustable body engageable with a first piston for positioning said first piston to open said exhaust valve means at a selected predetermined time after hydraulic pressure fluid is applied to said first piston, characterized by a second piston mounted for reciprocating m~nt within the body,which is hollow,between a retracted and an extended position, said second piston being urged into its retracted posîtion against the counter force of a spring by the first piston in the absence of said -5~ Z~

hydraulic fluid pressure thereto,`and check valve means operatively related to the second piston to maintain said second piston in its extended position to prevent return of said first piston for engagement with said hollow body while said hydraulic fluid pressure is applied to said first piston.
By virtue o our braking system the clearance between the first piston and the valve stem is reduced to a value maximi~ing the performance whenever the en~ine brake is activated. B~ so reducing the clearance the exhaust valve is opened sooner and the timing o~
the valve opening coincides more nearly with the activation of the engine brake master piston so as to maximize the retarding pawer developed by the engine.
An additional advantage resulting fxom the present invention is that the maximum pushrod load may be decreased. The pushrod load is caused by the force required to open the exhaust valve against the pressure of the air compressed during the compression cycle and the force required to actuate t~e fuel injector. By effectively decreasing the clearance as aforenoted, the timing of the exhaust valve opening is advanced so as to increase the time interval between brake actuation load and the inJector actuation load, and thereby minimize the combined effect of the two events. Moreover, by advancing the timing of the exhaust valve opening, the peak engine cylinder pressure may be reduced which also serves to reduce pushrod load.
In certain engines, the design of the intaXe or exhaust valve pushrods or injector pushrods may be such that the maximum pushrod load induced when the engine brake is in operation may not sa~ely be tolerated. To accommodate such a contingency, it may be desired to provide a negative clearance in the exhaust valve operating mechanism during the engine braking mode of operation so that the exhaust -5a-valves will not close fully, thereby limiting the load imposed on the pushrods. Of course, during the fueling mode of engine operation, a positive clearance is required to avoid damage, such as burning, to the valves.

Description of the Drawin~s Further objects and advantages o the invention will become apparent from the following detailèd description of the invention and the accompanying drawings in which: :

Figure is a schematic view of an engine brake incorporating the timing advance mechanism according to , ~ 609ZO

the present invention, Figure 2 is an enlarged fragmentary cross-section of a portion of the engine brake mechanism shown in Figure 1 showing the timing mechanism of the present invention in more detail;
Figure 3 is a bottom plan view taken along line 3-3 of Figure 2; and Figure 4 is a graph showing a compari~on of the retarding power developed by two engines incorporating the timing advance mechanism of the present invention and the same tw~ engines without the inventive device.
DETAILED DESCRIPTION OF THE INVENTION
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Referring to Figure 1, the numeral 10 descri~es various fragmentar~ portions of the engine brake housing while 12 is a schematic view of the engine sump contain-ing oil 14. Oil 14 may ~e withdrawn from the sump 12 through a duct 16 ~y an oil p~mp 18 and then directed into a solenoid valYe 20 via duct 22. The solenoid valYe 20 comprises a valve body 24 secured to the englne brake housing 10, having an inlet port 26, an outlet port 28 and a dump port 3Q, The lnlet port 26 and d~mp port 30 communicate wlth the Yalve cavity 32 respectively at the upper and lower ends thereof while the outlet port 28 communicates with an enlarged central portion of the valve cavity 32. A valve stem 34 is journalled for reciprocating movement within the valve body 24 and carries a cylindrical valve seat 36 adapted to seat against the shoulders formed ~y the enlarged central portion of the valve cavlty 32. A spring 38 normally ~iases the valve stem 34 so as to prevent the flow of oil from the inlet port 26 to the outlet port 28 of the solenoid valve.
A solenold coil 40 surrounding the upper end of the Yalve stem 34 is deslgned to open the solenoid valve 20 agalnst the ~ias of the spring 38 wheneyer electrical current flows through the coil, The solenoid ~11 6(~9;20 coil circuit includes, in series, a fuel pump switch 42, a clutch switch 44, a dash switch 46, a fuse 48 and the vehicle battery 50 The purpose of each of the logic switches 42, 44 and 46 will be set forth in connection with an explanation of the operation of the engine brake.
The dump port 30 communicates through a duct 52 with the engine sump 12 while the outlet port 28 communicates with a control valve 54 through a duct 56. The control valve 54 is generally in the form of a piston mounted for reciprocating motion wîthin a control valve cylinder 58 formed in the engine brake housing 10. The control valYe contains an inlet port 60 which communicates with an outlet port 62. The control valve inlet port 6Q is normally closed by a ball check valve 64 biased by a valYe spring 66.
~ hen the solenoid valve 20 is in the open position as shown in Figure 1, oil 14 from the sump 12 flows through the solenoid YalYe 20 and outlet duct 56 to the inlet port 60 of the control valve 54, The oil then lifts the control Yalve 54 against the bias of a control valve spring 68 until the annular outlet port 62 registers with the control Yalve cylinder outlet port 70. Thereafter the pressure of the oil opens the check valve 64 permitting oil to pass through the control valve 54 and înto the duct 72 which c~mmunicates between the outlet port 7Q of the control valve and the inlet port 74 to the slave cylinder 76.
The slaYe c~linder 76 is formed in the engine brake housing lQ so as to be aligned with an exhaust valve 78 which is biased to a closed position by :~ an exhaust valve spring 80. A slave piston 82 is positioned for reciprocating movement within the cylinder 76. One end of the slaYe piston 82 is adapted to contact the eXhaust Yalve stem cap 84 while the opposite end of the slaYe piston contacts an adjustable timing mechanism -8- ~ ~ 6~ g 2 O

86 which îs threaded into the engine brake housing 1 in alignment with the slave piston 82 and locked in position by locknut 88. A slave piston return spring 77 is located within the slave piston 82 so that one end of the spring biases the slaYe piston upwardly against the timing mechanism ~6, The opposite end of the spring 77 is carried ~y a retainer 79 seated in the engine brake housing. The slave cylinder 76 contains an outlet port 90 which communicates with the inlet port 92 of a master cylinder 94 formed in the engine ~rake housing 10 through a duct 46, A master piston 9.8 is mounted for reciprocating motion in the master cylinder 94 and its outer end is adapted to contact the rocker arm adjusting screw 100 of the appropriate fuel injector or intake valve rocker arm 102 which, in turn, is actuated by the pushrod 104. The master piston 98 is held in the housing hore ~y a light leaf spring 106.
It will be understood that, ordinarily7 there will be a slave piston ~2 associated with each exhaust valve so that a six cylinder engine will have six slave pistons while a four cylinder engine will ha~e four slave pistons. In addition, each slave piston is inter-connected wi~h a master piston as.sociatedwith an intake or fuel lnjector rocker arm and pushrod. ~f course, the master and its related slaYe piston may be associated with different engine cylinders. An exemplar~ relation-ship for this alternative is shown in Table l below for a six cylinder engine:

30 Location of Master Pi'ston L'ocation of'Slave Piston ... . .. .
No. 1 Pushrod No. 3 Exhaust Valve No. 5 Pushrod No. 6 Exhaust Valve No. 3 Pushrod No. 2 Exhaust Valve No. 6 Pushrod No. 4 Exhaust Valve No. 2 Pushrod No, 1 Exhaust Yalve No. 4 Pushrod No, 5 Exhaust Valve \ 9 ~9~o It will be understood that when the solenoidvalve 20 is opened, oil 14 flows through the solenoid valve and control valve 54 and fills the ducts 72 and 96 as well as the slave cylinder 76 and master cylinder 94.
Check valve 64 prevents a reverse flow of oil 14 from the slave and master cylinders. Thereafter, activation of the pushrod 104 will move the master piston ~8 upwardly in the master cylinder 94 causing a rapid rise in the pressure of the oil. The hydraulic pressure in the slave cylinder 76 w;ll force the slave piston 82 downwardly so as to open the exhaust valYe 78 It will be appreciated that if the slave piston 82 is not seated against the exhaust valve stem cap 84 when the master piston ~8 begins to move, the opening of the exhaust valve 78 will be delayed by the time required to take up the clearance in the system However, it is necessary to acc~mmodate dimensional changes in the mechanism, such as the exhaust valve stem, due to temperature changes In the prior art deYice J the clearance was controlled ~y an adjusting screw located in the position of the timing mechanism 86 and set to a clearance of, for example, 0.018 inch when the engine was cold. In accordance with the present invention, a timing mechanism 86 is provided which effectively maintains a zero clearance in the exhaust valYe actuating mechanism so that movement of the exhaust valYe will begin as soon as master piston begins to move whenever the brake mechanism is operated.
Referrîng now to Figures 2 and 3, the timing mechanism 86 comprises an adjustable body 108 having threads formed on its external cylindrical surface which is threadably engaged with the engîne brake housing 10 in alignment with the slave cylinder 76. The body lQ8 may be provided with a slot llO or other appropriate recess for convenience of ad~ustment and ~ay be locked in the desired position by a locknut 88. While the .`10~ 0 timing mechanism 86 may be located elsewhere, for example t between the slave piston 82 and exhaust valve stem cap 84, or within the slave pîston, it is prefera~le for purposes of convenient ad~ustability to position one end of the mechanism exteriorly o the exhaust valve mechanism.
A series of three coaxial bores 112, 114 and 116 are formed in the body 108 extending partially through the body 108 from the end opposite that contain-ing the adjusting slot 110, The first, and largest, bore 112 extends approximately halfway through the body 108 and is adapted to receive a timing piston 118.
The intermediate bore 114 extends approximately halfway through the remaining length of the body 108 and contains therein a compression spring 120, The third and smallest bore 116 extends slightly deeper than the intermediate bore 114 to provide a seat for check valve spring 122.
It will be understood, of course, that a single bore having the diameter of bore 112 and the depth of bore 116 may be used in place of the three bores shown and described above.
The timing piston 118 is formed with an axial bore 124 extending entirely through the piston 118 At the inner or upper end of the piston an enlarged bore 126 is formed to provide a seat 128 for a ball check 25 valve 130. The ball check Yalve 130 is normally urged against the seat 128 by the compression spring 122 A
transverse bore or slot 132 is formed across a diameter of the piston 118. The body lQ8 contains a transverse bore 134 within which is pressed a pin 136, The bore or 30 slot 132 is substantially wider than the pin 136 so as to permit the piston 118 to move axially relative to ; the body 108 within a limited range, In the present embodiment, the range of movement of the piston 118 i8 : from a first position slightly within the body 108 to a second position wherein the piston 118 extends sIightly beyond the end of the body 108, for example, 0.018-0.028 inches.

~ ~ g 20 It will be appreciated that the compression spring 120 normally urges the piston 118 to its extended position while the lighter, i.e lower rate, compression spring 122 urges the ball check valve to a closed position.
While it is convenient to use a ball check valve 13Q and compression spring 122, it will be understood that other check valve means may be employed.
For example, a leaf valve or other form of check valve may be located either on the timing piston 118 or in a separate duct communicating between the slave cylinder 76 and the region of the bores 112, 114, 116 a~ove the timing mechanism pîston 118. Similarly, means other than the pin 136 and slot 132 may be employed to provide limited axial movement of the piston 118 within the adjustable body 108, Such alternate means may include a reduced diameter at the lower end of the piston 118 and a mating inwardly directed flange or lip on the lower end of the ad~ustable body 108.
The operation of the mechanism will now be described. First, the ad~ustable body 108 may be set, as with the aforementioned prior art device, to provide any desired clearance, for example, 0.018 inch between the slave piston 82 and the exhaust valve stem cap 84 to insure that, under all operating conditions, there will be sufficient clearance to prevent unintentional partial opening or lifting of the exhaust valve 78.
Under these conditions, the timing mechanism piston 118 of the invention will not extend from the body 108 and the body 108 will be in direct contact with the top of the slave piston 82.
When it is desired to operate the engine brake, the solenoid valve 20 and the control valve 54 are actuated. This results in a flow of oil 14 through 35 the ducts 72 and 96 and into the slave cylinder 76 and master cylinder 94. When the master piston 98 begins to -12~ 2 0 move, pressure is immediately built up in the hydraulic circuit as that circuit has ~een fully filled with oil 14. Thus, the movement of the master piston 98 will immediately result in movement of the slave piston 82 and, as with the prior art device, after clearance in the mechanism has béen taken up, opening of the exhaust valve with which the slave piston is associated.
In accordance with the invention, as the slave piston 82 moves away from the timing mechanism body 108, the timing mechanism piston 118 extends a predetermined distance, governed by pin 136, from the timing mechanism body due to force from compression spring 120. This creates a pressure differential sufficient to cause the ball check valve 130 to unseat and admit oil to the region lS of the bores 112, 114 and 116. Upon return o~ the slave piston 82, undér action of spring 77, toward its initial position it encounters the timing mechanism piston 118.
The oil which entered the timing mechanism through the chéck valve 130 being trapped, and being relatively incompressible, opposes the force applied to the slave piston 82 by the slave return spring 77 Thus the slave piston 82 assumes a new initial position for all subsequent operating cycles, controlled by the predeter-mined stroke of the timing mechanism piston 118 and vary-ing only by the timing mechanism piston 118 movementdue to leakage through such piston and bore clearance ~etween cycles. This leakage loss is replaced through the ball check valve 130 during each cycle.
When the engine ~rake solenoid valve 20 and control valve 54 are deactivated, the hydraulic circuît is vented to drain. As the cyclic motion of the slave piston 82 ceases and it comes to rest on the timing mechanism piston 118, leakage past the timing mechanism piston, due to ~ore clearance, will permit full retraction of the timing mechanism piston and reloca-tion of the slave piston to its original position against ~g2~
adjusting screw body 108, The result of effectively eliminating clearance or backlash in the valve actuating mechanism is demon-strated in Figure 4. Figure 4 is a graph showing the relationship between engine speed and absorbed or braking power for a sîx cylinder and a four cylinder engine with and without the timing mechanism of the present invention. Curve 138 is a plot of the braking horsepower obtained from a six cylinder diesel engine fitted with a Jacobs engine brake and having the clearance adjustment set at 0,018 inch in accordance with the prior art. Curve 140 is a plot of the braking horsepower obtained fr~m the same engine wherein the clearance adjusting screw was replaced with a timing advance mechanism in accordance with ~he present invention. In a similar manner curve 142 is a plot of the braking horsepower developed by a four cylinder diesel engine equipped with a standard Jacobs engine brake*while curve 144 shows the effect of su~stituting the timing advance mechanîsm of the present invention for a standard adjusting screw set for a cleaxance of 0.018 inch. It will be o~served that at normal engine operating speeds in the range of 20Q0 r.p,~,, a very substantial increase in retarding horsepower is attained by the practice of the present invention, As indicated schematically in Figure 1~ the engine brake of the present invention is operated by a solenoid valve which is wired in series with three switches, a fuel pump switch 42, a clutch switch 44 and a dash switch 46. It will be appreciated that if any one or more of these switches is in the open posi-tion, the brake cannot be operated. The fuel pump switch 42 disables the brake system whenever the engine is being fueled, i.e. whenever the throttle is opened.
The clutch switch 44 opens whenever the clutch is dis-engaged in order to prevent stalling o~ ~e engine. Theda&h switch 46 is a manual control to enable th~ operator *see for example U.S. Patent 3,220,392 t . ~ ~
`~
-14~ 920 to disengage the brake system if he should wish to do so. The dash swit~h 46 may also be of the multi-position type which deactivates a portion of the system so that the operator can select partial or full braking power S as may be desired under particular operating conditions.
In addition to th~ primary advantage of substantially increasing the braking horsepower of the engine as shown by Figure 4, the timing advance mechanis~
o the present invention can be retrofitted on engines having engine brakes of the type herein disclosed without requiring any modification of the engine. A
secondary advantage of the timing advance mechanism here disclosed is a decrease in the pushrod loading when the device is employed in an engine having mechanical fuel injectors operated from pushrods. This advantage results from the increase in the time interval between the opening of the exhaust valve and the actuation o the fuel injector~ Applicant has found that for each 0.001 inch decrease in clearance the pushrod load is decreased by about 50 pounds in engines of the character tested for Figure 4.
Up to this point, the present invention has been described with respect to an embodiment wherein the normal exhaust ~alve operating mechanism clearance is effectively reduced to zero during the braking mode of operation. In such an embodiment, if the normal cold clearance is 0.018 inch, the timing mechanism is designed 50 that the piston 118 extends outwardly, during operation, a distance of 0.018 inch from the end of the body 108.
This may be accomplished by controlling the width of the slot 132 to permit the desired extension of 0.018 inch.

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-14a~ g 20 It will be appreciated that by an appropriate change in the width of the slot 132, the motion of the piston 118 may be increased or decreased.
Thus, if it were desired to provide a small positive clearance of, ~or example, 0.005 inch during braking in an engine having a clearance when cold of 0.018 inch in the fueling mode, the extension of the piston 118 would be designed to be 0.013 inch. Similarly, if it were desired to provide a small negative clearance of, for example, 0.005 inch during braking in an engine having a clearance when cold of 0.018 inch in the fueling mode, the extension of the piston 118 would be designed to be 0.023 inch.
The foregoing examples are, of course, merely illustrative of the principle involved because, in each case, consideration must be given to the actual~exhaust valve clearance during normal engine operation, which clearance depends upon the design of ~he particular en~ine involved and the conditions under which it is operated. Once these factors are established or specified, the timing mechanism according to the present invention can be designed to give the desired braking clearance which may be positive, negative, or essentially zero. It will be understood that a number of timing -mechanisms may be supplied to provide a number of standard extensions so that the engine owner may select the appropriate timing device to optimize the braking operation of his particular engine.

Claims (8)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED, ARE DEFINED
AS FOLLOWS:

1. In an engine braking system of a gas compre-sion release type including a combustion engine having exhaust valve means, hydraulically actuated reciprocating first piston means associated with said exhaust valve means to open said exhaust valve means at a predetermined time, and means for applying hydraulic pressure fluid to one end of said first piston means, the improvement comprising a timing means including body means adjustably positioned within said engine so as to locate one end of said body means to provide a first predetermined clearance in said exhaust valve means when said first piston means is in contact with said body means and said exhaust valve means are in a fully closed position, said body means having a cavity formed therein, second piston means having first and second ends and closely fitted for reciprocating move-ment with respect to said cavity of said body means between a first position in which said second piston means is located entirely within said cavity and a second position in which said second end of said second piston means extends outwardly a predetermined distance from said one end of said body means, said first end of said second piston means defining a closure for said cavity in said body means, said second end of said second piston means adapted to provide, in its second position, a second predetermined clearance in said exhaust valve means whenever said means for applying hydraulic pressure fluid is actuated, check valve means communicating between said cavity of said body means and said one end of said body means whereby said hydraulic fluid is directed into said cavity whenever the hydraulic pressure on said first piston exceeds the. hydraulic pres-sure in said cavity and spring means biasing said second piston means toward said one end of said body means.

2. An apparatus as. described in Claim 1 in which said check valve means comprises a bail check valve, said second piston means having an axial duct formed there-through and a valve seat formed at one end of said duct, and spring means. biasing said ball check valve against said valve seat.

3. An apparatus as described in Claim 1, wherein said second predetermined clearance in said exhaust valve means is smaller than said first predetermined clearance.

4. An apparatus as described in Claim 2, wherein said second predetermined clearance in said exhaust valve means is smaller than said first predetermined clearance.

5. In an engine braking system of a gas compre-sion relief type including a combustion engine having exhaust valve means, hydraulically actuated reciprocating first piston means associated with said exhaust valve means to open said exhaust valve means at a predetermined time, and means for applying hydraulic pressure fluid to one end of said first piston means, the improvement comprising a timing means, including body means adjustably positioned within said engine so as to locate one end of said body means to provide a first predetermined clearance in said exhaust valve means when said first piston means is in contact with said body means and said exhaust valve means are in a fully closed position, said body means having a cavity formed therein, pin means fixed to said body means transversely across the said cavity of said body means, second piston means having first and second ends and closely fitted for reciprocating movement with respect to said cavity of said body means between a first position in which said second piston means is located entirely within said cavity and a second position in which said second end of said second piston means extends outwardly a predetermined distance from said one end of said body means, said second piston means having formed transversely therethrough a slot having a width in the axial direction of said second piston means greater than the axial dimension of said pin means so as to define said first and second positions of said second piston means, said first end of said second piston means defining a closure for said cavity in said body means, said second end of said second piston means adapted to provide, in its second position, a second pre-determined clearance in said exhaust valve means whenever said means for applying hydraulic pressure fluid is actuated, check valve means communicating between said cavity of said body means and said one end of said body means whereby said hydraulic fluid is directed into said cavity whenever the hydraulic pressure on said first piston exceeds the hydraulic pressure in said cavity and spring means biasing said second piston means toward said one end of said body means.

6. An apparatus as described in Claim 5 in which said check valve means comprises a ball check valve, said second piston means having an axial duct formed there-through and a valve seat formed at one end of said duct, and spring means biasing said ball check valve against said valve seat.

7. An apparatus as described in Claim 5, wherein said second predetermined clearance in said exhaust valve means is smaller than said first predetermined clearance.

8. An apparatus as described in Claim 6, wherein said second predetermined clearance in said exhaust valve means is smaller than said first predetermined clearance.