A-Class Mercedes Benz offers two turbo diesel options

By Barry Lake

MERCEDES-BENZ has a long history of producing diesel engines, for both cars and trucks, so it is no great surprise to see two diesel options offered with the new, small A-Class Mercedes-Benz.

One engine is called the 160 turbo diesel, and the other 170 turbo diesel (td).

Despite the different names, both use identical capacity four- cylinder engines.

The 160td is rated at 44kW and the 170td at 66kW. The higher output 170td engine produces a very impressive 180Nm of torque between 1600rpm and 3200rpm, while the lower powered 160td makes a very respectable 160Nm of torque between 1500rpm and 2400rpm.

More of an eye-opener is the fact these are the world's smallest -- at just 1.7-liter capacity -- direct-injection, 16 valve turbo-diesel power units.

Direct injection offers greater power and economy over indirect (prechamber) injection but, until recently, there have been problems reducing the emission of particulates -- what you see as black smoke, or soot, coming from the exhaust pipe, especially under hard acceleration.

Studies show these particulates can be greatly reduced simply by increasing the pressure at which the fuel is injected into the engine.

The normal method is for the fuel pump to provide fuel to each cylinder via separate pipes.

Pressure for each pulse of fuel fed to these individual pipes has to be provided by this pump almost instantaneously.

By increasing fuel-injection pressure from 300 bar to 600 bar, particulate emissions were reduced up to 90 percent in some operating conditions.

Greater pressure offered even larger gains and Mercedes-Benz engineers were aiming for 1350 bar.

They achieved this by creating a "common rail" system, in which fuel is carried to all injectors via one fuel rail.

The new high-pressure pump runs at a constant speed and maintains the required pressure within this rail for all conditions (dependent on engine speed and load) up to a maximum of 1350 bar.

Quick-acting solenoid valves had to be developed so the high- pressure fuel could be accurately metered to each cylinder at the precise time for optimum performance.

The turbo diesel engines run at a very high 19.5:1 compression ratio. By comparison the A-Class petrol cars run at 11.0:1.

A further refinement is that a small precharge is injected to preheat the combustion chamber just before each main injection of fuel, thereby reducing the rapid heat build-up that creates harmful emissions of nitrogen oxides (NOx).

Another benefit is a claimed 15 percent fuel saving. The 160td engine is said to return 4.7l/100km in mixed urban and suburban driving, while the 170td is almost as frugal 4.9l/100km.

The 160td's top speed is said to be 153km/h while the higher output 170td reaches 175km/h, just 7km/h shy of the biggest petrol engined A-Class at 182km/h.

The four valves per cylinder layout was developed by Benz for its A-Class diesel engine. The valves are controlled by twin camshafts.

The turbo diesel engines are substantially similar to the petrol versions, especially in the radical forward tilting layout.

Cylinder liners are made of cast iron not aluminum silicon alloy as for the petrol engines. The pistons are cooled by pressure-controlled oil injectors.

The turbo diesel engines are so efficient in fact that Mercedes has had to fit a 5kW secondary diesel heater to provide warm air to the cabin in extremely cold weather.

The engines are computer controlled, from the starting sequence, to the high pressure fuel pump, the common rail valve system, solenoid valves on the injectors exhaust gas circulation and turbocharger boost pressure.

The diesel engines can also go up to 40,000km between services, lowering the cost of ownership.