Daimler has highlighted a number of improvements over the outgoing versions, notably a 400bar increase in fuel rail pressure, with the system now running at 2000bar. This is well short if a unit injector, but it’ impressive for common-rail. Like the current Volkswagen group common-rail engines, the new CDIs use piezo crystals to control the injector needles. Significant gains in low-end torque are claimed. In their current form, the engines comply with Euro 5 emissions.
The four-cylinder 2143cc OM651 engine is compact, and can be installed longitudinally or transversely. When demand permits, production can hit 700,000 engines a year. Production processes are divided between the Untertürkheim and Kölleda plants.
The C250CDI is Daimler’s first crack at two-stage turbocharging. The intention with two-stage systems is to improve torque at very low engine speeds. There are two blowers: a high-pressure (small) unit, and a larger low-pressure turbo. They are connected in series.
The small, high-pressure turbine has a diameter of 38.5mm, and is positioned directly in the exhaust manifold. Exhaust
gases flow through this unit first. Its low mass makes it easy for relatively low volumes of exhaust gases to get the unit spinning. This small blower’s maximum design speed is 248,000rpm.
Integrated into the housing of the small turbo is a bypass duct. This can be opened or closed by means of a charge-pressure control flap triggered by an actuator. If the duct is closed, all of the exhaust gases flow through the high-pressure turbine: all of the energy contained in the exhaust gases is directed towards rotating the small high-pressure blower. The larger, low-pressure turbo is not receiving any exhaust gas, and is doing nothing. In this way, charge air pressure can be built up at low crankshaft speeds and modest loads.
As the engine speed and load increase, the charge-pressure control flap opens to prevent the small turbo from becoming overloaded. This is important, because the maximum gas-flow through the housing of the high-pressure unit is quite restricted. With the control flap open, a portion of the exhaust gas flows through the bypass duct to relieve the load on the high-pressure stage.
Downstream from the high-pressure turbine, the two exhaust gas streams join up again,
and any remaining exhaust energy drives the larger, 50mm low-pressure turbine. This unit has a maximum design speed of 185,000rpm. To protect this unit against overload, the low-pressure blower also features a bypass duct, this time directly to the exhaust.
Once the engine reaches middling revs and loads, the high-pressure turbine’s charge pressure control flap has opened so wide that the high-pressure turbine ceases to perform any appreciable work.
All exhaust gases are directed with low losses into the low-pressure turbine, which then does all of the supercharging.
Just as the two turbochargers’ primary turbines are connected in series to make the best use of exhuast gas energy to drive them, so the secondary turbines — which compress the engine’s intake air and supercharge the cylinders — are also connected in series. The only difference is that the charge air is compressed by the larger, low-pressure turbine first. This has a diameter of 56.1mm. Once the intake air has been ‘pre-compressed’ (compressed a bit) by the low-pressure blower, it then passes to the high-pressure unit. This smaller turbine has a diameter of 41.0mm. At low to middling engine speeds and loads, the small, high-pressure blower does a lot of the work, as it is spinning a lot faster than the larger device. But as engine loads and speeds rise, and the flow capacity of the small turbine is reached, a bypass duct opens to divert charge air past the high-pressure stage and directly to the intercooler. So at higher engine speeds and loads, it is the larger, low-pressure blower that is doing the work.