Transcript
Making heavy vehicle fleet management easy for you
REAL
Contact maintenance for any questions on 0800 80 80 69
Torque May 2017
Exhaust brakes, engine brakes and retarders When the driver releases the accelerator on a moving vehicle powered by a diesel engine, the vehicle’s forward momentum continues to turn the engine’s crankshaft. Diesel engines, by design, have no throttle butterfly in the intake so regardless of throttle setting a full charge of air is always drawn into the cylinder. Each time air is compressed in a cylinder virtually 100% of that energy is returned to the crankshaft providing very little in the way of engine braking to the vehicle. There are three types of equipment fitted to a vehicle to help with slowing or holding a vehicle at a certain speed when the accelerator is in the off position.
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Exhaust brake An exhaust brake is a means of slowing a diesel engine by closing off the exhaust path from the engine, causing the exhaust gases to be compressed in the exhaust manifold, and in the cylinder. Since the exhaust is being compressed, and there is no fuel being applied, the engine works backwards, slowing down the vehicle. The amount of negative torque generated is usually directly proportional to the back pressure of the engine. An exhaust brake is a device that essentially creates a major restriction in the exhaust system, and creates substantial exhaust back pressure to retard engine speed and offer some supplemental braking.
This picture shows the exhaust brake in the on position blocking/restricting the exhaust gases from leaving the system.
This is the exhaust brake in the off position and exhaust gases can flow freely through the system.
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Engine brakes
The engine brakes, frequently called Jake brakes or Jacobs brakes, properly referred to Jacobs brand of engine brake. This is actually a compression release system. The typical compression release engine brake, uses an add-on hydraulic system using engine oil. When activated, the motion of the fuel injector rocker arm is transferred to the engine exhaust valve(s). This occurs very near TDC and releases the compressed air in the cylinder so that the energy is not returned to the crankshaft. Energy is now absorbed and the engine becomes an excellent “brake.” If used properly, this energy can be used by the truck driver to maintain speed or even slow the vehicle with little or no use of the service brakes. The power of this type can be around the same as the engine power. Diesel compression release brake controls consist of an on/off switch and, often, a multi-position switch that controls the number of cylinders on which the brake is active. Throttle and clutch switches are integral with the system. Activation occurs when both the clutch is released with transmission in gear and the throttle is released. It is the driver’s job to ascertain the correct transmission gear to use depending on, for example, the steepness of the grade and whether the truck is loaded or empty, to obtain maximum engine braking.
Left switch turns the engine brake on. R/H switch will turn on two cylinders, four cylinders or all six on a six cylinder engine.
This picture shows the cylinder that comes in contact with the exhaust valve and opens it at near the top of the compression stroke.
Making heavy vehicle fleet management easy for you
Engine brakes mounted to the engine.
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Hydraulic retarders A simple retarder uses vanes attached to a transmission driveshaft between the clutch and roadwheels. They can also be driven separately via gears off a driveshaft. The vanes are enclosed in a static chamber with small clearances to the chamber’s walls (which will also be vaned), as in an automatic transmission. When retardation is required, fluid (oil) is pumped into the chamber, and the viscous drag induced will slow the vehicle. The working fluid will heat, and is usually circulated through the vehicle cooling system. The degree of retardation can be varied by adjusting the fill level of the chamber (the more oil in the chamber the faster the vehicle will slow).
Retarder mounted to the transmission in a vehicle.
Hydraulic retarders are extremely quiet, often inaudible over the sound of a running engine, and are especially quiet in operation compared to engine brakes.
Tips and tricks
TR Tips
ff Allow engine and transmission to be at operating temperature before using engine brakes and retarders. ff Engine braking and retarder is not a replacement for the use of a service brake and will not completely stop the vehicle. ff It pays to use engine brakes and retarders when you have a load on the vehicle. ff It is not always legal to use engine brakes in certain places due to noise levels so it pays to know when and where you can use them.
This shows oil entering the retarder and starting to slow the drive shaft.
ff Exhaust brake relies on exhaust back pressure so get any exhaust leaks repaired.
Contact maintenance for any questions on 0800 80 80 69
REAL Torque May 2017
ff Using transmission retarders generates a lot of heat in the oil and causes it to break down faster. ff While these will all help save on the wear of the foundation brakes they do come at a cost. Exhaust brakes keep heat around the exhaust valves for a longer period on time which increases wear on the valves. ff Engine brakes cause an extra load on the engine’s camshaft and if operated outside of normal condition can cause damage to the valve and gear train. ff Transmission retarders are one of the best ways of slowing or holding a vehicle while travelling down a hill but the retarder creates heat and places it on the vehicle cooling system so the cooling system needs to be in good condition.
