Volkswagen TDI Diesel technology

Diesel technology is becoming more common as the vehicles spread across the country.

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The diesel-powered passenger car has had a rough go here in the United States. While the idea gains momentum from time to time, many OEMs that sell diesel cars (and lots of them) in Europe would not even consider introducing one to the U.S. market. Numerous factors have contributed to this, including the high cost of diesel relative to gasoline, tough emission control standards and the fickle nature of the American consumer in regards to diesel technology.

Despite all of these issues, one manufacturer that has not suffered a diesel identity crisis is Volkswagen. Its Turbocharged Direct Injection (TDI) engine is an industry icon and has a dedicated following in a diesel market that could be termed ambivalent at best. Starting with the Rabbit diesel in 1977, Volkswagen has made a name as the torchbearer for diesel passenger cars in North America. Case in point: There were years where VW was the only manufacturer selling diesel automobiles in the U.S. This tradition continues today with its series of 50-state diesels such as the Golf, Jetta, Passat, Touareg and more on the way. If a diesel passenger car makes its way into your service bays, chances are good that it will be a Volkswagen.

So what makes these little motors tick? For those automotive service professionals who have not ventured into diesel service, the good news is that diesel engines and gasoline engines are becoming more alike with each passing day. This is no different with the Volkswagen TDI, whose common-rail diesel technology looks very much like a gasoline direct injection system. Any competent drivability technician that is familiar with OBDII can learn TDI service with some upgrade training. You're off to a great start by just reading this article; read on if you're thinking that diesel service might be the next frontier for your shop.

TDI Evolution

The original Volkswagen diesels were based on the indirect injection (IDI) concept, which was quieter and produced fewer emissions than the direct-injected (DI) diesels of the time. However, the higher overall efficiency of the DI design and the benefits of turbocharging led to the introduction of the TDI. Volkswagen showcased some cutting-edge technology with the 4-cylinder TDIs, such as an electronically-controlled distributor injection pump (found on the ALH version1999 to 2003), which sported a number of advanced features that set it apart from its peers.

One example was a needle lift sensor that was built into the No. 3 cylinder injector nozzle. This sensor signaled the ECM of the exact moment the injector needle started to move, which allowed the ECM to adjust injection timing more accurately. The ALH was an efficient and reliable powerplant, but eventually had to undergo a redesign because of tightening emission control regulations.

The Pumpe-Düse (pronounced "pumpadoos") TDI replaced the ALH in 2004. This engine had the same displacement as the ALH and used a similar block, but the cylinder head was redesigned. The Pumpe-Duse head was still a two-valve design, but used camshaft-actuated unit injectors that were controlled by an integrated solenoid module. Bosch originally designed the injection system, which was able to produce higher injection pressures than the common rail systems of the time. However, Pumpe-Düse's emissions were rated at Tier 2 Bin 10, much dirtier than the Tier 2 Bin 5 rating that it needed in order for it to be sold in California and the "Green States." This meant that the TDI (including the 5.0 liter V10 TDI in the Touareg SUV) was shut out of several of the biggest automotive markets in the U.S., which by themselves represent 40 percent of new car registrations.

Ultimately, its limitations in regards to injection timing killed the Pumpe-Düse. Diesel particulate filters (DPFs) were going to be installed in all on-road diesel engines starting with the 2007 model year in order to meet the new particulate matter (PM) emission regulations. DPFs rely on a diesel engine's injection system for post-injection (injecting fuel during the exhaust stroke) in order to initiate active regeneration. Unfortunately, Pumpe-Düsewas not nearly as flexible as common rail systems in regards to post-injection timing. Another redesign of the 4-cylinder TDI was undertaken, and the 2007-2008 model years were skipped entirely in order to develop a diesel engine that could be sold in all 50 states.

50-State TDI

The current 4-cylinder TDI is a 4-valve 2.0 liter with a high-pressure common-rail injection system. True to form, Volkswagen has pushed the technological envelope with some features that take the TDI to a new level. A huge step forward was achieved with the use of pressure-sensing glow plugs (PSGs) for closed-loop combustion control.

Using a piezo-resistive element and an application-specific integrated circuit (ASIC) built into the glow plug body, combustion pressures in the cylinder can be measured in real time. This information then can be used to adjust injection timing, thus enabling compensation for varying cetaneratings in diesel fuel. Because combustion pressure is directly related to temperatures in the cylinder, the PSG feedback loop can be used to adjust fuel delivery to limit peak combustion pressures and thus, NOx formation. Misfire detection also is much more accurate using a PSG.

While diesel engine misfire monitors (using a crankshaft position sensor) typically run only at idle, the PSG can be used to detect misfire across the engine's operating range.

Exhaust gas recirculation (EGR) is being used by virtually all diesel engines as part of their NOx control strategy. The idea is to use inert exhaust gases to cool the flame in the combustion chamber and limit NOx formation. The majority of diesels use a high-pressure (HP) EGR system design, which routes exhaust gases from upstream of the turbocharger and sends them through a cooler on their way to the engine's intake manifold. The 2.0 liter common-rail TDI uses both high-pressure EGR (with no cooler) in combination with a low-pressure (LP) system.

The low-pressure loop takes exhaust gases from the outlet of the diesel particulate filter (DPF), sends them through a cooler, then into the intake of the turbocharger compressor. To increase LP EGR flow, a throttle valve is located at the exhaust system outlet. When the exhaust throttle starts to close, exhaust backpressure rises and causes the LP EGR system to increase its flow. The combination of the HP and LP systems in the TDI makes it possible for EGR gases to flow under all engine operating conditions, including high engine loads.

The 2.0 liter common-rail TDI also uses a NOx adsorber catalyst (NAC) as part of its exhaust aftertreatment system. The NAC is located downstream from the DPF, and is responsible for reducing NO2 particles to N2 and CO2. The NAC adsorbs (traps) NOx particles during lean engine operation (also known as the load phase), and then is regenerated by driving the exhaust rich. Throttling the intake air and retarding the fuel injection timing accomplishes a rich exhaust. The TDI's load phase usually lasts from five to 10 minutes, where regeneration takes from two to 10 seconds.

While heavier vehicles are using urea SCR as a NOx aftertreatment strategy (such as the 3.0 liter V6 TDI), the 2.0 liter common-rail TDI does not require any customer intervention to operate its emission control system. On the other hand, there is reduced fuel economy associated with the operation of the NOx adsorber due to its regeneration process.

All of these features of the TDI add up to a passenger car diesel that meets Tier 2 Bin 5 standards, which means it can be sold in all 50 states. Volkswagen started by offering this version of the TDI in the 2009 Jetta, but now has expanded that to almost every car in its lineup.

TDI Service

Service on the fuel system of a TDI must be conducted using the cleanest of work habits due to extremely tight tolerances in the common-rail injection system. Before opening any part of the fuel system, be certain to clean the surrounding area carefully. Any removed parts that will be reinstalled should be placed on a clean work surface and covered with a lint-free cloth. Use sealing caps to cover open fuel fittings, and don't remove protective packing from new parts until immediately before installation. Also, don't spill diesel fuel on coolant hoses! Be sure to wash hoses immediately if you do, because diesel will severely degrade the rubber.

A common service item on a TDI fuel system is replacement of the fuel filter. On the 2.0 liter common-rail, there is one large fuel filter located on the passenger side of the engine compartment. Before changing the filter, clean any dirt from the filter housing and surrounding area. Some fuel should be removed from the housing before taking the cover off. Remove the T30 screw from its fitting on top of the filter cover, and then use a vacuum extractor to remove a minimum of 100 cc of fuel. This also will remove any water that has accumulated at the bottom of the housing.

Take precautions to avoid spilling fuel on nearby coolant hoses, then remove the cover and replace the filter along with the seals in the kit. Once the cover has been reinstalled (use a torque wrench to tighten the housing bolts in a star pattern), use a scan tool to operate the in-tank fuel pump for 60 seconds in order to bleed the system.

Volkswagen has introduced numerous coolants over the years including G11, G12, G12+ and the most current G12++. This has created challenges for automotive service professionals, because color wouldn't necessarily tell you which coolant you had in the vehicle, and it wasn't clear which coolants could be mixed. A rule of thumb is that G12++ can be used to top off any of the aforementioned VW coolants, but the corrosion protection will be reduced when coolants are mixed. For maximum protection, flush the system using recommended service procedures and install G12++ on any VW TDI.

We'll leave you with one last service precaution: be very careful when using silicone sealants or lubricants on a TDI. Make sure to allow them to cure prior to starting and running the vehicle's engine. Silicone fumes that make their way into the engine's air intake can damage the mass airflow (MAF) sensor and oxygen sensors.

You Can Do It

Certainly, Volkswagen TDIs are a niche market in today's automotive marketplace. However, they represent 23 percent of total Volkswagen sales and would be more except for capacity-constraints. The number of shops that perform TDI service are relatively few and far between, and the good ones are rarer still. Take a look around your area: how many TDIs drive past your shop every day? With today's diesels looking more and more like gasoline engines, it might not be a big stretch to expand into diesel service.

Tony Martin is an associate professor of automotive technology at University of Alaska Southeast in Juneau, Alaska. He holds Canadian Interprovincial status as a Journeyman Heavy Duty Equipment Mechanic. He also has 19 ASE certifications, including CMAT, CMTT, L1 and L2.