New A 200 CDI with 180-bar ignition pressure and a VNT turbocharger

The impressive increase in output and torque which clearly distinguishes the CDI engines of the new A-Class from those in the preceding model is not just the result of increasing the engine displacement by almost 18 percent. At the same time the Mercedes engineers improved the gas cycle in the cylinders and increased the ignition pressure from the previous 145 to 180 bar. As a result the new engines develop even more tractive power in the lower engine speed range. Specifically this means that in the A 180 CDI, 198 Newton metres no less than 79 percent of the maximum torque – is already available from 1300 rpm.

Even more impressive torque characteristics are shown by the new A 200 CDI, whose engine operates with an ignition pressure increased to 180 bar and distinguishes itself from the other diesel units for the new A-Class by featuring a VNT (Variable Nozzle Turbine) turbocharger. In a VNT charger the angle of the turbine vanes can be varied according to the engine operating parameters, enabling the greatest possible volume of exhaust gas to be used to compress the intake air and build up the charge pressure. At low engine speeds the vanes reduce the flow cross-section and the charge pressure increases, while the cross-section is increased at higher engine speeds to reduce the rotation speed of the turbocharger. The major advantages of this variable turbocharger control include better cylinder charging and therefore more torque.

In all the diesel engines for the A-Class, the air compressed by the turbocharger flows through an intercooler which has now been tripled in size, from where it enters a mixing chamber and meets the recirculated exhaust gases. The exhaust gases are fed directly from the exhaust manifold and flow through a water-cooled heat exchanger. They mix with the compressed fresh air, with an automatic intake air throttling system ensuring the most favourable ratio of charge air to recirculated exhaust gases. In the A 200 CDI the VNT turbocharger has the task of setting the optimal pressure conditions for exhaust gas recirculation.

The mixing chamber is part of a complex air distribution module. From here each cylinder is supplied with the air/exhaust gas mixture via two ducts. One of the two intake ducts can be shut off in keeping with the engine speed and engine load, thereby increasing the swirl rate of the mixture. At low engine speeds a system of variable swirl flaps is also in action. These further increase the air speed in the four spiral ducts of the cylinder head, thereby improving the output and torque under partial load.