Engine brake
The mechanical resistance that a motor opposes to an externally imposed torque is called the motor brake .
purpose
Purpose of the engine brake is converting the kinetic energy into heat or other forms of energy by counteracting friction and compression forces, called "delay" as well. The internal resistance of the driving machine that is running along is used for braking (see towing power ).
Executions
Simple applications
The braking effect is based solely on the removal of the driving energy.
Internal combustion engines
The braking effect is created when gasoline engine by suction charge throttling (by closing the throttle valve or throttle valve) and fuel-zero flow (ie lack of fuel supply), the simple diesel engine absence throttle member only by fuel zero delivery (exception: see section "effect Improved versions" below).
In today's motor vehicles with injection gasoline engines, the effect of the engine brake is improved by reducing or completely suppressing fuel injection ( overrun fuel cut-off ) compared to the previously common carburetor engines, the fuel supply of which even when the actuator (throttle valve, throttle valve) is closed e.g. B. could never be completely prevented by idle bores and nozzles. This also reduces the average fuel consumption. If the speed falls below a certain value, it is injected again. The braking torque of the engine brake increases with higher speed, which is why shifting down to a lower gear further reduces the speed and, if necessary, relieves the service brake more on long descents. Sometimes engaging the next lower gear (letting the clutch slip) is sufficient.
Steam engines
The braking effect is basically created by shutting off the steam supply. Furthermore, in steam locomotives, by reversing and reopening the steam supply, the effect of the counter- vapor brake can be used, the normal force-generating functional principle of the piston steam engine acting against the direction of travel. The counter-pressure brake , which is installed in steam locomotives for mountain routes, does not require a fresh steam supply, is adjustable and enables driving on long downhill stretches without additional brake block wear.
Electric motor
Simply switching off the energy supply alone does not lead to a significant braking effect in electric motors. While three-phase asynchronous motors rotate completely freely, there is a slight braking effect with commutator motors due to the friction of the brushes. In a possible generator operation , the kinetic energy is converted into electrical energy and either dissipated as heat via an electrical resistor ( resistance brake ) or fed back into a power grid or a storage device ( regenerative brake ).
An asynchronous motor can be braked (eddy current braking) by exciting it with direct current.
Effective versions
The braking effect due to internal friction is comparatively small in comparison to the types of conversion of kinetic energy described below, in which the effect of the engine brake is increased by further measures.
Engine dust brake
The engine dust brake system (also known as the exhaust flap brake ), in which the braking power is converted into compression work without subsequent fuel injection and combustion, exists especially for diesel engines in trucks and buses . While taking your foot off the accelerator pedal is sufficient for the engine brake, the engine dust brake must be actuated separately. Once switched on, it is very practical for the driver, especially in heavy traffic, as he does not always have to switch his foot back and forth between the accelerator pedal and the brake, but instead has a strong braking effect by simply removing his foot from the accelerator. When activated, the injection pump is reduced to zero delivery and a flap in the exhaust is closed. The braking power can be regulated via a valve in the exhaust system. This type of brake is mainly used when driving downhill. It reduces the stress on conventional brakes, reduces their heating and reduces the risk of brake fading .
In the case of newly registered commercial vehicles, it is no longer permissible to design the engine dust brake in such a way that the exhaust flaps are completely closed regardless of the driving condition and the fuel supply is reduced to zero, otherwise the engine will come to a standstill if this brake is operated too long, with all additional units how power steering fail and the vehicle becomes very difficult to maneuver. In the past, the engine stopping was a desired side effect because it was used to switch off the engines. In modern truck engines, the dust brake is activated electro-pneumatically, which also prevents the engine from coming to a standstill by applying the brake. So z. B. at a manufacturer below an engine speed of about 800 revolutions per minute, the exhaust flap opens automatically and brought the injection pump to idle delivery rate.
There are now systems in which the flaps only open on one of the four engine cycles. This has a similar braking effect, but the noise level is noticeably reduced.
In the case of diesel-mechanically driven railway vehicles, the engine dust brake is used as an additional brake for operation on steep sections . The best known example of this are the railcars of class 798 .
Decompression brake
Another way to increase the braking torque of the engine is to leave the work done by the engine in the compression cycle unused for the next cycle. For this purpose, at the end of the compression stroke, the exhaust valves or an additionally installed valve are opened and thus the pressure in the cylinder is reduced (decompressed). As a result, no more work can be transferred to the crankshaft during the expansion stroke, since the energy used for compression has already been dissipated by the relaxation.
The process of releasing the compressed air via the exhaust valves is widespread in North America and is known there under the name "Jake Brake" or "Jacobs Brake" . This braking system is called “freins jacob” in French-Canadian . The sudden relaxation of the compressed air leads to a very high level of noise pollution and easily exceeds the noise level of pneumatic hammers without additional silencers. At many town entrances in Québec , it is pointed out that the use of such brakes is not permitted because of the high level of noise, as is the case in many places in the USA (“No jake braking”, “No engine brakes”, “Engine brake mufflers required”).
When using the constant throttle , an additional valve with a small cross section is opened parallel to the exhaust valves not only at the end of the compression stroke, but during the entire engine braking operation. The decompression takes place continuously, but due to the small cross-section with still high back pressure. The engine braking effect is also high in the following expansion phase, as the air flows back into the cylinder against the resistance of the constant throttle.
Combination of engine dust brake and decompression brake
The braking effect of the engine can be further improved by combining the engine dust brake and decompression brake (e.g. EVB® / Exhaust Valve Brake).
See also
Web links
Individual evidence
- ↑ https://www.klibo.de/bremsen_16a.html Electronic brake from Klinger & Born GmbH, accessed on March 21, 2018.
- ↑ https://www.tripus.de/de/Elektronische-Motorbremse-news-5424-7737-11612-143.html Electronic brake at Tripus systems GmbH, accessed on March 21, 2018.
- ↑ Explanation of the Compression Release Brake ( Memento of the original from June 11, 2016 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. on the homepage of the manufacturer Jacobs Vehicle Systems
