Engine brakes!!!!!!!!!!!!!

Engine brakes

Engine braking is the act of using the energy-requiring compression stroke of the internal combustion engine to dissipate energy and slow down a vehicle. Compression brakes is a common legal term for the same mechanism. Large trucks use a device called a jake brake to increase the effectiveness of engine braking.

Design

Most four stroke internal combustion engines require compression of the fuel-air mixture before ignition, in order to extract useful mechanical energy from the expansion. Diesel engines are adiabatic and have no spark plugs and use energy transferred to the fuel-air mixture during compression to directly ignite the mixture.

Regardless of engine type, compression of gas and vapor requires energy as described by theories in physical chemistry and thermodynamics. Compression in an engine is driven by the drive shaft which is driven by the moving wheels of a vehicle (as well as the flywheel). So, the engine ends up converting energy that was formerly kinetic energy of the vehicle into heat in the fuel-air mixture. These hot gasses as well as other heated components of exhaust are eventually ejected from the vehicle.

Advantages

The advantage of using the engine to dissipate energy is this immediate ejection of energy. Hot gasses are ejected from the vehicle very quickly and the gasses also transfer much of their heat directly to engine parts. In addition, friction produced within the engine system also adds heat to the engine parts.

This engine heat is taken away by the engine's integrated cooling system: usually a liquid circulation system and a radiator. Disk or drum brakes have no such energy dissipation mechanisms. They must rely on air flow to remove heat and they use their mass to retain heat without producing temperatures that would deform and damage the brakes.

Placing a vehicle in a low gear causes the engine to have more leverage (mechanical advantage) on the road and the road to have has less leverage on the engine. This is what allows multi-ton trucks to slow down using their relatively flimsy engine parts. The engine maintains a high RPM to dissipate a lot of power without forcing too much strain on the engine.

Applications

Active use of engine braking (shifting into a lower gear) is only advantageous when it is necessary to control speed while driving down very steep and long slopes. It should be applied after regular disk or drum brakes have been used to reduce speed to the desired speed. The desired speed is maintained by using engine braking to counteract the acceleration due to gravity.

Engine braking is otherwise always active in all non-hybrid cars with an internal combustion engine, regardless of transmission type. Engine braking passively reduces wear on brakes and helps a driver maintain control of the car. It is always active when the foot is lifted off the accelerator, the transmission is not in neutral, and the clutch is engaged.

Legal implications

Compression braking (jake brake), a form of engine braking, produces extreme amounts of noise pollution if there is no muffler on the exhaust system of the engine. Anecdotally, it sounds similar to a jackhammer, however the loudness is between 10-20 times the perceived loudness of a jackhammer.

http://www.youtube.com/watch?v=Kd9YyzOQ-qM

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...

• Lower operating costs- Use of an engine brake means less wear on service brakes, reducing maintenance and downtime costs.
• Reduced tire wear- Smooth retardation eliminates tire hop and lockup.
• Reduced trip time- Higher descend speeds on hills can be maintained without supplemental use of service brakes.
• Higher resale value- The vehicle retains a higher value when an engine brake is installed.
• Safety- Possibility of brake fade is reduced.

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.

it can save your life and your family
after your car brake failure

samirayad

G IFT4U

[syd]