Jacobs Brake Used on Diesels To Retard Vehicle Motion
The general acceptance of the diesel engine by fire departments to power fire apparatus came after much refinement of the diesel engine resulting from operating experience under all types of road conditions. During these years of operating experience it was found that when descending long or steep grades the diesel engine was inherently lacking in good enginebraking ability.
To provide this essential braking ability to assist or conserve the vehicle wheel brakes, a new braking system was developed to permit conversion of some of the kinetic energy of the moving vehicle into engine-braking ability. An ingenious design, combining hydraulic actuation with electric solenoid control, permitted using the diesel engine as an air compressor only to effect braking by the absorption of energy.
The development and refinement of this braking system were done by Clessie Cummins and the Jacobs Manufacturing Company. The system is commonly referred to as the Jake brake. There are over 30,000 diesel engines in commercial, industrial and fire service equipped with the Jacobs brake, indicating the wide acceptance of the effectiveness and reliability of this engine-braking system.
Fire service experience: In the interest of greater vehicle braking safety with a marked reduction in maintenance of wheel brakes, it is essential to include the Jacobs brake in our discussions of braking systems in fire service use. Many fire departments will support this statement based on their own experience with the Jacobs brake.
In developing a diesel engine-braking system for retarding vehicle motion, the following objectives had to be achieved:
- Engine maintenance procedures must not be affected.
- The brake design should utilize the full retarding potential of the engine.
- The braking system must not affect the service life of the engine.
- The braking system must permit quick activation and deactivation without interfering with normal driving operation.
- All fuel must be shut off at the injector when the braking system is activated or in use.
- Control of the braking system must be simple and foolproof and not contribute to any hazard of driving.
- The braking system must be designed to adapt to diesel engines now in service.
All of these objectives have been achieved in the Jacobs brake.
How Jacobs brake works: Very briefly, the principle of operation is to open the engine exhaust valves on the compression stroke, at approximately 10° to 30° before top dead center, releasing the compressed air to the atmosphere through the exhaust system.
A vehicle moving downgrade provides power through the drive line to rotate the engine and compress air. This compression may represent a power absorption of 70 to 75 percent of the rated engine power output for a naturally aspirated engine. For example, such an engine rated at 220 blip will absorb 165 bhp at 2100 rpm. A turbocharged diesel engine will absorb approximately 85 percent of its rated power output. An example is an engine rated at 250 blip which will absorb 225 bhp with the brake operating.
All illustrations courtesy Jacobs Manufacturing Co.
Although heat is produced by compression of the air, cooling is always adequate as the absorbed power is less than full rated power. Approximately 45 percent of the braking horsepower is dissipated by the engine cooling system and 55 percent is discharged through the exhaust system. This heat of compression transferred to the cooling system maintains a reasonably stable engine temperature, which is essential for instant power response when the engine braking is no longer required.
Fuel injection halted: Activation of the Jacobs engine brake is by a switch controlling an electric solenoid valve (Figure 1). This valve admits engine lubricating oil under pressure to the master piston, and the hold down piston. Engine oil pressure causes the hold down piston to follow the injector plunger to prevent fuel injection. As cam rotation moves the injector rocker arm, a ball check valve closes the hydraulic fluid passage due to an increase in hydraulic pressure produced by the rocker arm movement at the master piston. This increased hydraulic pressure activates a slave piston to open the exhaust valve and release the compressed air to atmosphere.
There is no washing of the cylinder walls with fuel or dilution of the engine lubricating oil.
The engine cylinder pressure is lower when the brake is operating than when under its own power due to the exhaust valves opening before top dead center on the compression stroke.
The high compression ratio of the diesel engine permits only a limited clearance between the top of the piston and the valves. This limits the exhaust valve opening to release the compressed air. The intermittent release of clean air through the exhaust system acts as a muffler scrubber and quite effectively cleans loose carbon from the system.
When the control switch is moved to the “off” position, the braking system is deactivated and the engine responds to normal operation within one-half second.
The general appearance of the Jacobs brake assembly is shown in Figures 2 and 3.Each assembly mounts on top of the engine over the camshaft, as shown in Figure 3. Sections are cut away to illustrate the relative locations of the master piston and the slave pistons.
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Fire service acceptance: To the fire service, this type of brake is important, not only for safer braking, but for reduced brake system maintenance. Chief Gordon Vickery of Seattle, Wash., stated he would not want to operate his equipment without this braking system. We also note that the San Diego, Calif., Fire Department recently took delivery on four new pieces of apparatus, all engines being equipped with the Jacobs brake.
Actual operation over a period of time has shown the following benefits to the ; fire department using this brake:
- Two to four times the mileage or life of wheel brake linings.
- No brake fade on long or steep grades.
- Longer brake drum life.
- Better control of vehicle, less tendency to skid on wet and slippery pavement.
- Reduced number of flat spots on tires.
- Longer tire life due to reduced brake heating
- Cleaner exhaust system.
To these we can add complete vehicle control when going down grades. Also, the engine-braking system is completely independent of the wheel-braking system whether air-over-hydraulic or straight air brakes.
The Jacobs brake is available equipment on several makes of diesel engines, including Cummins, Mack and Detroit Diesel. It can also be installed on these engines now in service, if desired.