Volkswagen's Diesel Showpiece

However, the diesel Toureg was pulled from the market the next year, reportedly due to a problem related to the high-sulfur diesel fuel sold in this country, so the chances of seeing one in your bay might be pretty slim. If you do, they need the same basic fluid and filter service as any other diesel, and because the engine is so complex, that's about all you'll want to tackle anyway.

But for anyone interested in engines, this new oil burner is an engineering tour de force that's worth a closer look.

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Engine code BKW is a 4.9-liter 90-degree V10 with two turbochargers, aluminum block and heads, single overhead camshafts and electronically adjusted direct fuel injection. While these features are common to many engines, even diesels, nothing else about this engine is common.

For instance, instead of cylinder liners, the bores in the aluminum block are plasma-sprayed. In this process, an inert gas is blown through a torch nozzle past an electric arc, creating an extremely high-temperature, high-speed plasma stream. As the hot stream exits the torch, a mixture of powdered metals is fed into the stream. The melted alloy sprays onto the aluminum cylinders, immediately solidifying to form a hard, wear-resistant coating on the wall. It's an expensive process, but in this application, it saves weight and space, allowing the cylinder bores to be closer together for a smaller and even lighter engine block.

The block also has no main bearing bores. The crankshaft is carried in a separate two-piece, cast-iron main bearing web, which VW calls an "end bracket." The upper web piece is shaped like a 90-degree triangle, and the right angle fits up into the block so the flat sides of the triangle mate with the bottom of the cylinders.

When the cylinder heads are mounted, the long head bolts go all the way through the block and thread into the cast iron main bearing web. This allows the high clamping force necessary for a diesel cylinder head without the risk of pulling threads out of the aluminum block. The lower half of the web contains the four-bolt main bearing caps. On the bottom of one rail of the main bearing web are two more bearing caps for the balance shaft.

The connecting rod journals on the forged steel crankshaft are offset, so the firing pulses on this 90-degree V10 are evenly spaced. Looking at the rod from the side, the smaller end is shaped like a pyramid to fit the shape of the wrist pin boss in the piston. The upper half of the wrist pin boss is extended towards the center of the piston. This spreads the pressure of combustion over a greater length of the wrist pin, improving durability and reducing vibration. Vibration is reduced further by a gear-driven balance shaft and by a silicone-filled vibration damper bolted to the front of the crankshaft. When mounted in the vehicle, this is reported to be a very smooth-running diesel engine.

There are no belts or chains on this engine. All of the pumps, accessories and camshafts are driven through a gear train that's bolted as a complete assembly to the flywheel end of the block. Except for the crank and cam gears and the power steering pump gear, all the gears are sandwiched between two cast-iron carrier plates, and the plates have external pipes and internal oil passages to pressure-lube the gear shafts' bearings.

One of these gears drives one half of a coupling called a Gates Drive, which couples to the water-cooled alternator nestled in the V between the cylinder heads. The gear-driven power steering pump is mounted directly to the block, and the pump's own driveshaft extends out the back of the pump to another coupling that drives the air conditioning (A/C) compressor. Another gear just below the V drives a long shaft that extends all the way through the block and turns the water pump. Below the crankshaft gear is the drive gear for the balance shaft and oil pumps.

When using gears to drive camshafts, gear tooth contact depth must remain constant to avoid noise and wear and changes in valve timing. When all the gears are mounted on the same casting, this is a minor issue. But in an overhead cam engine, the camshafts move away from the crankshaft as the engine grows with temperature. It's only a few thousandths, but it's enough to influence cam timing.

Volkswagen has devised a cam drive system with four gears: the crankshaft gear, a reduction gear, an idler gear and finally, the camshaft gear. What's unique about this train is that the idler rides in a carrier called a shackle that is hinged in the center like a pocketknife. The shackle attaches to the head at one end and to the block at the other, and the idler gear rides with the hinge.

When cold, the shackle is compressed so the idler is offset to one side. When the engine warms up, the shackle is pulled apart and the idler moves closer to the centerline of the three gears, keeping gear contact depth constant. The movement is only a few thousandths of an inch, but it's enough to maintain exact cam timing. Because the camshafts also operate the injectors, even a half-degree of cam timing change would have a big effect.

LUBRICATION

On top of the block in the V between the heads is a housing for the oil filter and oil-to-coolant heat exchanger. Below the block is a two-piece cast aluminum oil pan with three separate compartments; one for oil returned from the gear train and turbochargers; one for the oil/air separator; and one for the main sump and oil pumps.

The oil-return compartment is at the rear, and two scavenger pumps pull oil from there and send it directly to the oil/air separator. Flooding of that compartment would create backpressure in the lube systems for the turbos and gear train, and it would let the oil level fall below the pick-up tube in the main sump. So during acceleration or uphill driving, a flapper gate closes off that compartment to prevent oil from sloshing back into it. During level driving, braking and downhill driving, the gate is open and the return compartment drains into the main sump by gravity, so the pick-up tube is always submerged.

It sounds complicated, but having a separate return sump for the turbos allows a shallower oil pan so the engine can sit lower in the chassis. The shallow pan is also why VW used four small-diameter oil pumps instead of two large ones.

FUEL SYSTEM

This engine has 10 mechanically actuated unit injectors equipped with solenoid valves to vary the injector opening pressure. In a unit injector, the injection pump and nozzle for one cylinder are together in one housing. Each unit injector is mounted in the head between the two valves, much like a spark plug would be in a gasoline engine, except everything is inside the valve cover.

Each pump is operated by a third lobe on the engine's camshaft, making it possible to develop the extremely high pressures needed for injecting fuel directly into a diesel combustion chamber. The downside is that injection timing and rate-of-injection are fixed – literally carved into the cam profile – as opposed to electronic injection where the Powertrain Control Module (PCM) controls the injectors to suit current operating conditions.

However, the injectors on this engine have a solenoid valve to open an internal passage that changes nozzle-opening pressure. Even though this engine has fixed injection timing, the PCM can at least choose between two different injection pressures. During part-load operation, injection pressure can be increased to reduce emissions and improve fuel economy.

Fuel is moved from the tank by twin electric pumps, supplying fuel to twin high-pressure mechanical pumps mounted on the heads and driven by lugs on the camshaft drive gears. These send fuel at 125 psi through two fuel supply rails to the injectors. Overflow fuel is not sent directly back to the tank, but to the filter housing/heat exchanger assembly mounted in the V between the cylinders (getting crowded up there, isn't it?).

A thermostatically operated valve in the filter housing and three check valves control fuel flow through this assembly. Fuel returning from the engine is circulated through the fuel/coolant heat exchanger before being sent back to the tank. When cold, return fuel is recirculated through the filter housing before being returned to the tank, thus limiting the amount of cold fuel flowing into the filter housing. Along with the heat exchanger, this keeps the fuel filters warm to prevent the clogging and fuel gelling that is always a danger in cold weather.

ENGINE MANAGEMENT

Along with the glow plugs and the fuel injector solenoid valves, the PCM also operates an electric thermostat in the cooling system, auxiliary coolant pumps, electric motors for the turbocharger boost controls and an exhaust gas recirculation (EGR) system. This system includes EGR valves and intake air throttle valves that assure a positive difference between the exhaust and intake systems.

There is a full suite of engine sensors similar to those used on gasoline engines, plus two fuel temperature sensors and two oxygen sensors that the PCM uses for operating the EGR system. In 2004 models, there are four oxidation catalysts between the engine and first muffler. Later models also have two particulate traps.

At this point, existing PCM platforms are not built to operate more than eight cylinders. Therefore, like other V10 and V12 engines, this one has two PCMs. One operates one bank of injectors and glow plugs and that one bank's EGR system. The other operates the other five cylinders, all the pumps and control relays and everything else in the powertrain. Both PCMs share all the input sensors, including the built-in altitude sensor of PCM number one. They also share one diagnostic connector, and either PCM can operate both turbocharger waste gates to control turbo boost for the whole engine.

From 300 cubic inches (4921cc), this engine generates 308 horsepower at only 3,750 rpm and a whopping 553 lb.-ft. of torque at 2,250 rpm. It will propel the 5,825-pound Toureg to 60 mph in 6.2 seconds, and it returns 17/23 mpg (city/highway).

Like the gasoline-powered Toureg, this vehicle performs and feels more like a big car than like a truck, even though it can tow 7,700 pounds. VW will not sell any diesel cars here in 2007, but the Toureg is a truck and therefore subject to different regulations.

With the availability of low-sulfur fuel and an exhaust particulate trap, the automaker is ready to launch the diesel Toureg again this fall, and it will meet emissions regulations in all 50 states. Don't be surprised to start seeing more of them in 2007.