Principle of exhaust compression retarder

object of the exhaust brake (retarded) is to convert the exhaust gas expelling stroke into an air compressing power, absorbing upstroke when the accelerator pedal is released and the vehicle possesses a surplus of kinetic energy. This happens on overrun and tends to propel the vehicle forward, thus causing the transmission to rotate the engine. To achieve this negative work, a butterfly valve or slide valve is incorporated into the exhaust pipe as near to the manifold as possible, so that during the exhaust stroke the volume of air in the engine's cylinder will be displaced and contained within the exhaust system, causing its pressure to rise (Fig. 12.30). Just after the piston has reversed its direction and commences to move outwards on its induction stroke, the exhaust valve closes, trapping

TDC BDC

Cylinder volume

(1i Induction Compression (31 E*fJ3risicjn f4) Compression a^rist exhaust vslvt

TDC BDC

Cylinder volume

(1i Induction Compression (31 E*fJ3risicjn f4) Compression a^rist exhaust vslvt

(SI Pressure eqUcliiEclIion

Fig. 12.30 Pressure/volume diagram for a four stroke engine installed with an exhaust compression the air remaining between the exhaust port poppet valve and the exhaust butterfly brake valve in the exhaust system. Therefore approximately 90% of the exhaust upstroke is devoted in compressing air and absorbing energy (Fig. 12.30) before the inlet valve opens. This energy is then released to the atmosphere and cannot do useful work in pushing down the piston on the following induction stroke. Hence there is a net gain in negative work done by the engine whilst resisting the rotation of the crankshaft, which considerably helps to reduce the vehicle's speed.

Air will therefore be drawn into the cylinder on each induction stroke as normal, but on subsequent exhaust strokes the pressure in the exhaust manifold and pipe increases to a maximum of between 2.5 and 4.5 bar, depending upon the valve overlap compression ratio and engine speed.

With increased engine speed, the pressure buildup in the exhaust system eventually overcomes the exhaust valve spring closing force and unseats the exhaust valve. Pressurized air will then flow back into the cylinder and escape out of the induction port the next time the inlet valve opens. Consequently the upper limit to the pressure build-up on the exhaust upstroke is controlled by the amount of trapped compressed air stored in the exhaust system behind the exhaust valve when the exhaust valve opens and the piston commences on its modified exhaust upstroke.

With several cylinders feeding into a common exhaust manifold, pressure fluctuations are considerably reduced and a relatively high average pressure can be maintained. Subsequently a great deal of negative work can be done by all the cylinders collectively.

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