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The Theory Behind the Engine Brake
What An Engine Brake Does...
A compression engine brake is a device that can convert a diesel engine into an air compressor, a power absorbing device. These devices are used in trucking applications to provide a means for retarding the vehicle, supplementing the normal vehicle braking system. The retarding operation is under full control of the vehicle operator and can be used simultaneously with the normal service brakes. Engine brakes can be installed at any time to a new or used vehicle for most applications.
Benefits Of An Engine Brake...
Theory Of Operation...
In normal operation, a diesel engine provides power through compression of air in a cylinder into which diesel fuel is injected and ignited. When fuel is cut off by releasing the throttle, the engine produces no power. It does, however, continue to compress air during the compression stroke of the piston. The "work" required to compress this air is obtained from the inertia of the vehicle. Upon reaching the top of the compression stroke, compressed air forces the piston down, canceling out all of the "work" performed on the upward stroke. The result is that power is returned to the vehicle via the crankshaft during the expansion stroke. The only retardation offered by an engine without an engine brake is friction from the moving parts.
Operation of the compression brake alters what happens at the top of the compression stroke. When the piston reaches the top of its stroke the exhaust valves are opened, allowing compressed air to exhaust to the atmosphere. Because energy stored in the compressed air is no longer retained in the cylinder, no power producing "work" is returned to the engine during the downward stroke.
How An Engine Brake Works...
When an engine brake is operating, at top dead center of the compression stroke a "Slave" piston pushes on the exhaust valve crosshead which opens the exhaust valves, allowing the compressed air to exhaust to the atmosphere. Typically, this slave piston is operated through a closed hydraulic circuit by a "Master Piston". the master piston is driven by the motion of the fuel injector rocker lever.
Referring to the drawing, you can see that the master piston is positioned above the injector rocker lever screw. Upward motion of the injector push tube moves the master piston, creating a pressure in the closed hydraulic system. This causes the slave piston to move downward, contacting the exhaust valve crosshead, opening the exhaust valves. The "timing" of the fuel injection requires that the injector push tube begin it's movement near top dead center on the compression stroke, precisely when we wanted the exhaust valves to open for brake operation.
The hydraulic circuit, comprised of a master piston and a slave piston is closed only during braking operation. Under normal engine operating conditions, when the engine is producing power, the hydraulic circuit is open. The master and slave pistons have no movement and are held away from the rocker lever and crosshead by springs.
The opening and closing of the hydraulic circuit is accomplished by a control valve which allows engine lubricating oil to enter the circuit, retaining it with a check valve. The control valve moves vertically in a bore which connects an oil passage, allowing lube oil to enter the master piston - slave piston hydraulic circuit. A check valve inside the control valve allow replenishing oil to enter the circuit but prevents it from leaving.
The position of the control valve which turns the brake on and off is controlled by an electrical solenoid. The solenoid allows lube oil to feed the control valve when it is electrically energized. The brake, therefore, is electrically controlled.
Other typical components of the engine brake include: control switches in the dash, a throttle position switch, and a clutch switch.
A dash switch is used to deactivate the entire compression brake system or, depending on wiring options, control the level of braking power by deactivating one or more brake units. The throttle position switch is used to prevent brake operation while the throttle is depressed. A brake can only be operated when the throttle is in the idle position. A clutch switch is used to prevent the brake from operation when the clutch pedal is depressed. Without a clutch switch installed, the engine brake could stall an engine as the operator shifted gears or as the vehicle came to a stop. Optionally, a foot switch can be installed to operate the brake, eliminating the need for a clutch switch.
Opening the exhaust valves at precisely the correct time is important. A slight change in timing has a significant affect on braking capability. The valve opening time in typical applications can be affected by changing the clearance between the slave piston and crosshead. In practice sophisticated hydraulic lash adjusting mechanisms are used.
In some instances, the pressure required to open exhaust valves against compressed air in the cylinders is extremely high. In these instances the resulting load through the push tube back to the camshaft exceeds valve train loading capabilities. These excessively high pressures can quickly damage a valve train system. Increasing slave piston lash will cause the exhaust valve opening to be delayed and open closer to top dead center when pressure is higher.
Manufacturers use various devices to change the valve opening time or to hold the valve open slightly to reduce pressures. These devices optimize braking capability while keeping push tube loads at an acceptable level. these systems come under various names such as: Auto-Lash® (Jacobs Engine Brake®), Paclash® (PacBrake®), Vari-lash® (Cummins®).
Control of the valve train loading is critical and is determined by the configuration of the engine and compression brake. Installation of a twin entry turbocharger in place of a single entry turbo is a typical upgrade performed on Cummins engines. This change increases the compression pressures and the push tube loading. Making this change without installing the appropriate lash adjusting mechanism can quickly damage the valve train.
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