Engine braking
When engine braking, the mechanical resistance is known, the one motor one opposes externally imposed torque.
- 2.2.1 exhaust brake
- 2.2.2 decompression brake
- 2.2.3 Combination of exhaust brake and decompression brake
Purpose
The purpose is to convert the kinetic energy by friction and counteracting forces in the compression heat, or other forms of energy.
In this case, the internal resistance of the follower drive unit is used for the deceleration (see trailing performance).
Simple applications
The braking action is based solely on the removal of the traction energy.
Combustion engines
Braking is achieved by zero promotion, so shutting off the fuel supply.
In conventional cars with fuel-injected engines, the engine brake is increased by decreasing or exposing the fuel injection in their effect ( fuel cut ), at the same time, the average fuel consumption is also reduced. If the speed drops below a certain value, it is re-injected. The effect of the engine brake is greater at high engine speeds, and therefore a downshift to a lower gear further restricts the speed.
Steam engines
Braking is achieved by shutting off the steam supply.
Electric motor
In a possible generator mode, the kinetic energy is converted into electrical energy and dissipated either via an electrical resistance as heat ( dynamic brake ) or into a power grid or a memory fed back ( regenerative brake).
Effect engine versions
The braking effect by internal friction is relatively low as compared to the types described in the following the conversion of kinetic energy in which the effect of engine brake is increased by other measures.
Exhaust brake
Especially for trucks and buses, there is the exhaust brake system (also known as exhaust brake ), in which the braking power is converted into compression work without subsequent fuel injection and combustion. While the engine brake is removing the foot from the accelerator pedal is enough, the exhaust brake must be operated separately. Once turned on, it is particularly in heavy traffic very convenient for the driver because it does not always have the foot between the accelerator and brake switch back and forth, but is already using a strong braking effect by mere taking the foot off the gas. Pressing the injection pump is reduced to zero promotion and closed a flap in the exhaust. The braking power can be regulated via a valve in the exhaust system. This type of brake is used especially when driving downhill. It reduces the load on the conventional brakes, reduce their heating and reduces the risk of brake fade.
In new vehicles, it is no longer allowed to design the exhaust brake so that the exhaust valves are completely closed and the fuel supply is reduced to zero, since there is otherwise too long pressing this brake to a stop the engine comes with all ancillaries fail as power steering and the vehicle is very difficult to maneuver. Previously, until the motor was a positive side effect, because so the engines were shut down. In modern truck engines, the exhaust brake is electro- pneumatically operated, which also prevents the engine to stop. Thus, the exhaust valve is opened automatically, for example at a manufacturer below an engine speed of approximately 800 revolutions per minute and the injection pump is brought to idle speed.
Meanwhile, there are systems in which the damper opens only one of the four engine cycles. This causes a similar braking effect, but it reduces noise level considerably.
For diesel mechanically driven rail vehicles, the exhaust brake is used as an auxiliary brake for the steep haul operations. The best known example of this is the railcars of series 798
Decompression brake
Another way to increase the braking torque of the engine, is to let the work done by the engine during the compression stroke of work for the following clock unused. For this, the exhaust valves, or an additionally built-in valve can be opened and thus the pressure in the reduced cylinder ( decompressed ) at the end of the compression stroke. As a result, in the expansion stroke is no more work to be output to the crankshaft, since the time taken for the compression energy has been dissipated by the unwind.
The process to relax the compressed air via the exhaust valves, is widely used in North America, there is known under the name " Jake Brake " or " Brake Jacob ". In the French- Canadian, this braking system " freins jacob". The sudden relaxation of the compressed air leads to a very high noise and exceeds the noise level without additional silencer jackhammers problems. At many local inputs in Quebec should therefore be noted that the use of such brakes is not allowed because of the strong noise, as in many places in the United States ("No jake braking ", "No engine brakes", " engine brake mufflers required" ).
When using the constant throttle valve, an additional valve of small cross section is opened in parallel with the exhaust valves, not only at the end of the compression stroke, but during the entire engine braking operation. Decompression is performed as continuous, but due to the small cross-section with further high backpressure. In the following phase of expansion, a high engine braking available, since the air flows back against the resistance of the constant throttle valve into the cylinder.
Combination of exhaust brake and decompression brake
The braking energy of the motor can be further improved by combining exhaust brake and decompression brake (eg EVB ® / Exhaust Valve Brake ).