Operation of exhaust compression brake

Retarder operating (Fig. 12.31(a)) When the foot control (on/off) valve is depressed by the driver's left foot heel, compressed air from the brake system's reservoir is delivered to both the brake butterfly valve slave cylinder and the fuel cut-off slave cylinder via the pressure regulator valve, causing both slave pistons to move outwards simultaneously.

The outward movement of the butterfly slave piston causes the butterfly operating lever to rotate about its spindle to close the exhaust passage leading to the silencer. The engine then becomes a single stage low pressure compressor driven through the transmission by the road wheels. The air pressure established in the exhaust manifold and pipe due to blocking the exhaust exit reacts against the piston movement on most of the exhaust stroke, thus producing a retarding torque on the propeller shaft.

The outward movement of the fuel cut-off slave piston rotates the shift bell crank lever, causing the vertical link rod to pull down and fold the two governor link rods. As a result, the change speed lever of the injection pump is moved to the closed or full cut-off position and at the same time the accelerator pedal is drawn towards the floorboard.

The exhaust compression brake remains operative during the whole time the foot control valve is depressed.

Retarder inoperative (Fig. 12.31(b)) When the (on/off) foot valve is released during clutching or declutching, the compressed air in both slave cylinders is exhausted through the control valve. This permits the slave cylinders' respective return springs to open the butterfly valve, and to unfold the governor link rods so that their combined extended length moves the change speed lever to the full delivery position, and at the same time raises the accelerator pedal from the floorboards. The exhaust compression brake system is then inoperative and the engine can be driven normally once again.

12.4.3 Engine compressed air type retarder (Jacobs) (Fig. 12.33)

A cylinder compression retarder converts a power producing diesel engine into a power absorbing air compressor.

The compressed air engine retarder consists of a hydraulic circuit supplied by the engine oil pump which uses the existing injector rocker motion to open the exhaust valve at the end of the compression stroke via a pair of master and slave pistons actuated by a solenoid valve and a piston control valve.

These compressed air brake units are made to fit on top of the cylinder head and are designed for engines which incorporate combined pump and injector units such as the Cummins and Detroit Diesel.

Theory of operation With a conventional engine valve timing, work is done in compressing air on the inward compression stroke. The much reduced volume of air then gives out its energy by driving outwards the piston on its expansion stroke.

(a) Retarder operating

(a) Retarder operating

How Work Retarder
Fig. 12.31 (a and b) Exhaust compression (brake) type retarder

Therefore, except for frictional losses, there is very little energy lost in rotating the engine on overrun.

The adoption of a mechanism which modifies the exhaust valve timing to open the exhaust valves at the beginning of the expansion stroke causes the release of air into the atmosphere via the exhaust ports (exhaust blowdown) (Fig. 12.32), instead of making it do useful work in expanding the piston outwards. The effect of this is a net energy loss so that considerably more effort is required to crank the engine under these conditions.

Engine retarder engaged (exhaust blowdown) (Fig. 12.33(a)) When the accelerator pedal is released, the accelerator switch closes to complete the electrical circuit and thereby energizes the solenoid. The solenoid valve closes the oil return passage immediately and opens the passage leading to the control valve. Pressurized oil now pushes the control valve piston up to a point where the annular waste of the control valve piston uncovers the slave piston passage and unseats the ball. Oil at pump pressure therefore passes to the upper crown of the slave and master pistons. This forces the master piston downwards to take up the free movement between itself and the injector rocker adjustment screw. As the camshaft rotates, in the normal cycle of events, the cam lift forces the master piston upwards, causing the ball in the control valve to seat. The trapped contracting volume of oil

60 40 20 0 20 40 60 Before TDC After

Crartk angle movement fdeg)

Fig. 12.32 Typical pressure/crank diagram for a four stroke engine installed with an engine compressor retarder

60 40 20 0 20 40 60 Before TDC After

Crartk angle movement fdeg)

Fig. 12.32 Typical pressure/crank diagram for a four stroke engine installed with an engine compressor retarder between the control valve and master piston now increases its pressure sufficiently to force the slave piston and valve cross head downwards. Consequently both exhaust valves are unseated approximately 1.5 mm and so as a result the exhaust valves open just before the piston reaches TDC position at the end of the compression stroke.

Whilst the engine is on exhaust blowdown, the vehicle speed is reducing and the engine is being overrun by the transmission, so that with the accelerator pedal released and the centrifugal governor weights thrown outwards, fuel is prevented from injecting into the engine cylinders.

Engine retarder disengaged (Fig. 12.33(b)) When the accelerator pedal is depressed, the solenoid circuit is interrupted, causing the de-energized solenoid valve to open the oil pump return passage. The oil pressure under the control valve piston collapses, causing the valve to move to its lowest position. The trapped oil between the master piston and slave piston therefore escapes through the passage opened by the control valve piston. The slave piston is then permitted to rise enabling, the cross head and exhaust valve to operate as normal.

Retarder control A master 'on/off' dash switch in combination with automatic accelerator and clutch switches allows the driver to operate the engine pump retarder under most conditions. Complete release of the accelerator pedal operates the retarder. Depression of the accelerator or clutch pedals opens the electrical circuit, permitting gear changes to be made during descent and prevents the engine from stalling when the vehicle comes to a halt.

12.4.4 Multiplate friction type retarder (Ferodo) (Fig. 12.34)

The retarder is an oil-cooled multiplate brake mounted against the rear end of the gearbox casing. It consists of four steel annular shaped plates with internal locating slots. Both sides of each plate are faced with sintered bronze (Fig. 12.34). These facings have two sets of parallel grooves machined in them at right angles to each other for distribution of the oil. The drive plates are aligned and supported on the slotted output hub, which is itself internally splined at one end to the input shaft and bolted to the output shaft at the other end. This provides a straight-through drive between the gearbox main shaft and the propeller shaft. Support is provided for the output hub and shaft by an inner

Fig. 12.33 (a and b) Engine compressed air type retarder

Exhaust valves closed

Fig. 12.33 (a and b) Engine compressed air type retarder

Fig. 12.34 Multiplate friction type retarder

and outer roller and ball bearing respectively. Interleaved with the driven plates are five cast iron stationary counter plates, also of the annular form, with four outer radial lugs. Four stator pins supported at their ends by the casing are pressed through holes in these lugs to prevent the counter plates rotating, and therefore absorb the frictional reaction torque.

Between the pump housing flange and the friction plate assembly is an annular stainless steel bellows. When oil under pressure is directed into the bellows, it expands to compress and clamp the friction plate assembly to apply the retarder.

The friction level achieved at the rubbing surfaces is a function of the special oil used and the film thickness, as well as of the friction materials.

The oil flow is generated by a lobe type positive displacement pump, housed in the same inner housing that supports the stator pins. The inner member of the pump is concentric with the shaft, to which it is keyed, and drives the outer member. The pump draws oil from the pump pick-up and circulates it through a control valve. It then passes the oil through a relief valve and a filter (both not shown) and a heat exchanger before returning it to the inlet port. The heat exchanger dissipates its heat energy into the engine cooling system at the time when the waste heat from the engine is at a minimum.

Output torque control (Fig. 12.34) When the spool control valve is in the 'off' position, part of the oil flow still circulates through the heat exchanger, so that cooling continues, but the main flow returns direct to the casing sump. The bellows are vented into the casing, releasing all pressure on the friction surfaces.

When the control valve is moved to the open position, it directs some oil into the bellows at a pressure which is governed by the amount the spool valve shifts to one side. This pressure determines the clamping force on the friction assembly. The main oil flow is now passed through the heat exchanger and into the friction assembly to lubricate and cool the friction plates.

12.4.5 Electro-magnetic eddy current type retarder (Telma) (Fig. 12.35(a, b and c)) The essential components are a stator, a support plate, which carries suitably arranged solenoids and is attached either to the chassis for mid-propeller shaft location or on the rear end of the gearbox (Fig. 12.35(a)), and a rotor assembly mounted on a flange hub. The stator consists of a steel dished plate mounted on a support bracket which is itself bolted to a rear gearbox flange. On the outward facing dished stator plate side are fixed eight solenoids with their axis parallel to that of the transmission. The rotor, made up of two soft steel discs facing the stator pole pieces, is bolted to a hub which is supported at the propeller shaft end by a ball bearing and at its other end by the gearbox output shaft. The drive from the gearbox output shaft is transferred to the propeller universal joint via the internally splined rotor hub sleeve. The rotor discs incorporate spiral shaped (turbine type blades) vanes to provide a large exposed area and to induce airflow sufficient to dissipate the heat generated by the current induced in the rotor and that produced in the stationary solenoid windings.

Four independent circuits are energized by the vehicle's battery through a relay box, itself controlled by a fingertip lever switch usually positioned under the steering wheel (Fig. 12.35(b and c)). These solenoid circuits are arranged in parallel as an added safety precaution because, in the event of failure of one circuit, the unit can still develop three-quarters of its normal power. The control lever has four positions beside 'off' which respectively energize two, four, six and eight poles. The solenoid circuit consumption is fairly heavy, ranging for a typical retarder from 40 to 180 amperes for a 12 volt system.

Operating principles (Fig. 12.36) If current is introduced to each pole piece winding, a magnetic flux is produced which interlinks each of the winding loops and extends across the air gap into the steel rotor disc, joining up with the flux created from adjacent windings (Fig. 12.36).

When the rotors revolve, a different section of the disc passes through the established flux so that in effect the flux in any part of the disc is continuously varying. As a result, the flux in any one segmental portion of the disc, as it sweeps across the faces of the pole pieces, increases and then decreases in strength as it moves towards and then away from the established flux field. The change in flux linkage with each segmental portion of the disc which passes an adjacent pole face induces an electromotive force (voltage) into the disc. Because the disc is an electrical conductor, these induced voltages will cause corresponding induced currents to flow in the rotor disc. These currents are termed eddy currents because of the way in which they whirl around within the metal.

Collectively the eddy currents produce an additional interlinking flux which opposes the motion of the rotor disc. This is really Lenz's Law which states that the direction of an induced voltage is such as to tend to set up a current flow, which in turn causes a force opposing the change which is producing the voltage. In other words, the eddy currents oppose the motion which produced them. Thus the magnetic field set up by these solenoids create eddy currents in the rotor discs as they revolve, and then eddy currents produce a magnetic drag force tending to slow down the rotors and consequently the propeller shaft (Fig. 12.36).

The induced eddy currents are created inside the steel discs in a perpendicular direction to the flux, and therefore heat (I2Rt) is produced in the metal.

The retarding drag force or resisting torque varies with both the rotational speed of the rotor and propeller shaft and the strength of the electromagnetic field, which is itself controlled by the amount of current supplied.

Do It Yourself Car Diagnosis

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Responses

  • Pamphila
    How does a retarder work?
    2 years ago

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