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. In most cases, an exhaust brake is so effective that it can slow a heavily loaded vehicle on a downgrade without ever applying the vehicle's service brakes. Exhaust brakes are manufactured by many companies. The brakes vary in design, but essentially operate as described above. More advanced exhaust brakes have exhaust pressure modulation (EPM) that controls the back pressure which in turn improves the braking performance across a range of engine speeds.

Description

An exhaust brake is a valve which essentially creates a back-pressure in the exhaust system, which applies enough force onto the engine's pistons to slow the engine. In most cases, an exhaust brake is so effective that it can slow a heavily-loaded vehicle on a downgrade without ever applying the vehicle’s service brakes. Under these conditions, the exhaust flow from the cylinders is bottlenecked and rapidly builds pressure in the exhaust system upstream from the exhaust brake. Depending on engine speed, this pressure can easily reach up to 60 PSI maximum working pressure. Maximum working pressure is limited as part of the design of an exhaust brake. In this example, that same 60 PSI also remains in the cylinder for the entire exhaust stroke (exhaust valve open) and exerts 60 PSI on the piston top to resist its upward movement. We can think of this as negative torque, slowing the engine for a braking effect. This might be even thought as just the opposite of the power stroke, and in effect, it is. Thus, simply restricting the exhaust flow can generate substantial braking. That’s what makes an exhaust brake so effective.

Some innovations increase the exhaust back-pressure by various means, leading to more torque at the flywheel, and therefore more braking power. Braking effectiveness is measured in units of power and is about 60 to 80% of the engine's maximum power output. More performance is usually easily had by down shifting the vehicle (increasing the leverage, or gear ratio of the engine over the wheels). See also Jake brake.

Legal implications

Compression brakes, a form of engine brakes, produce greater noise pollution than exhaust brakes. For this reason, some vehicle original equipment manufacturers prefer to use exhaust brakes despite their lower braking power. However, combining exhaust and compression brakes increases their effectiveness while significantly reducing noise.

Numerous cities, municipalities, states, and provinces ban the use of unmuffled compression brakes.

Pedal-operated Butterfly Valve

The butterfly valve made up of acrylic material made in the circular shape, having three peripheral circular cut on it. These circular cuts are given because it is not expected to block whole cross section of the exhaust pipe. If there were no holes, the engine would stop instead of slowing down. In the project, actual engine is not used but a blower; still this type of the valve is used to correlate it with actual arrangement.

The butterfly valve is actuated through hydraulic linkages in actual working model. This consists of a hydraulic pressure pump which is connected to the butterfly valve. This pump controls the operation of butterfly valve in following manner: When brakes are applied by the driver, the cylinder reduces its pressure so that valve gets closed and restricts the path of exhaust gases. In this position butterfly valve remains perpendicular to the flow of exhaust gases and thus creates back pressure on the engine. This butterfly valve has some holes (may be one to three) into it so that there would not be complete blockage in the exhaust pipe. This assures the avoidance of abnormal damage due to high pressure.

When brakes are removed by the driver, cylinder generates pressure into it so that butterfly valve gets opened and allows the exhaust gases to flow into exhaust pipe. In this position butterfly valve remains parallel to the path of exhaust gases and thus releases the pressure on the engine and again speed increases.

In the project, there is not such cylinder pressure mechanism but a simple linkage. To create a tension to apply brakes, a pedal brake is connected to the elastic rubber so that there will be pressure force to apply and release butterfly valve like in actual arrangement.

Turbocharged Exhaust Brakes

A turbocharger is commonly used to improve power and reduce fuel consumption ratio. Study on effect of turbocharger on exhaust brake for diesel engines had practical significance. When butterfly valve is closed, in the exhaust manifold there is no exhaust emissions so the turbine blades of the turbocharger cannot work, but because turbine blades has the blocking effect, and this effect can make lower inlet pressure of the diesel engine than it with no turbocharger, so air intake into the engine reduces which not only reduces the fuel consumption but also speed of the engine.

Two stage pressure control

Increase in the back pressure with the help butterfly valve can also be achieved by operating in two stages i.e. two valves can be used. This also ensures the efficient working of valve and long its long life by avoiding valve failure. Such type of arrangement is found to be very useful in case of heavy duty trucks. Because to create sufficient back pressure to slow down the engine speed, it requires large force from the valve which is possible by two valves arrangement.

ARIS Actuator Valve

Usually pneumatic or hydraulic actuators are used to operate the butterfly valve. But with the large force of exhaust such type of actuators turns to be inefficient. Hence ARIS type of valve actuator which is widely used in the industries provides effective valve-operating force.

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    This article is issued from Wikipedia - version of the 10/3/2016. The text is available under the Creative Commons Attribution/Share Alike but additional terms may apply for the media files.