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What the heck is a TDI? by ckatkinson
July 07, 2004

Diesel - an internal combustion engine developed by Rudolf Diesel in the late 1800’s in which air is compressed to a temperature sufficiently high to ignite fuel injected into the cylinder where the combustion and expansion actuate a piston.

Now that you know Webster’s take on it, what does it mean to the newly released US spec 2004 Passat TDI? How about 500-600 miles per tank of fuel? How about 247 ft-lbs of torque at 1900 rpm? You won’t win many races at the track, but you will probably win a few at the stop light. TDI stands for Turbo Direct Injection in a diesel engine. The 2.0-liter, 134 hp, 16 valve, 4 cylinder, turbocharged diesel engine used in the US spec 2004 Passat TDI uses the latest technology to provide a clean, fuel efficient, and reliable power plant for transporting you and your Passat down the highway.

Diesel engines are different than gasoline engines for two basic reasons. First and most obvious, they use different fuels. Less obvious is the difference between spark ignition in the gasoline engine and compression ignition in the diesel. Both engines use a four stroke cycle of the pistons in the cylinders. The strokes can be better understood as the four steps required to produce power. The first step is the intake stroke. In a gas engine, the intake valve opens and the downward movement of the piston draws in a mixture of gas and air. In a diesel, the intake stroke draws only air into the cylinder. The second step is the compression stroke where the piston travels upward and compresses the air or air/fuel mixture in the cylinder. A term commonly associated with this stroke is the compression ratio, which is the ratio of the volume of the cylinder chamber at the end of intake stroke (the engine has just sucked in the air or air/fuel mix) versus the volume of the cylinder chamber at the end of the compression stroke (all that air or air/fuel mix has been compressed into the top end of the cylinder). VW’s 1.8T gas engine has a compression ratio of 9.3:1. The 2.0-liter TDI engine has a compression ratio of 19.5:1. Compression raises both the pressure and temperature of the air or air/fuel mix in the cylinder chamber. In the gas engine, the designers must take care not to raise the temperature too high because the air fuel mixture will explode without a spark if it reaches around 250 deg C. However, this is exactly what the additional compression in the diesel engine does to the air in the diesel cylinder chamber. The air is compressed to a pressure and temperature high enough that the diesel fuel will ignite when it is injected into the hot air. The third step is the power stroke. In a gas engine, the air/fuel mixture is ignited by a spark from the spark plug when the piston reaches the end of the compression stroke. The ignited air and fuel explode and drive the piston downward which transmits power into the crankshaft which in turn drives the wheels of the car and the other pistons through their other strokes. In a diesel, the diesel fuel is sprayed into the very hot compressed air in the chamber. The hot air auto ignites the diesel fuel which then explodes and drives the piston down just like in the gas engine. The fourth and final step is the exhaust stroke. In both engines, the exhaust valve opens and the piston pushes the spent combustion gases out of the cylinder chamber and into the exhaust system. Please click the links below to see animations of each cycle.

http://static.howstuffworks.com/flash/engine.swf -
Animation of Gasoline Engine Cycle

http://static.howstuffworks.com/flash/diesel.swf -
Animation of Diesel Engine Cycle

In the case of a turbocharged engine, whether gas or diesel, the air that is drawn in has already been slightly compressed by the turbocharger, which is a small air compressor that is driven by the exhaust gas from the engine. This has the effect of packing a little more air or air/fuel mix into the cylinder which will allow for a more powerful combustion.

So now you know the difference between a gas engine and a diesel, but what does that really mean for an owner? The first thing a gas engine driver notices about the TDI is the small number in front of the horsepower… only 134 for the new Passat. The big difference is the engine speed at which the maximum horsepower is developed. In the 1.8T, the 170 max hp occurs at 5900 rpm. At cruising speed of 60 mph, at about 2200 rpm, it develops 70-80 hp. The TDI, on the other hand, has 134 max hp at 4000 rpm which puts its cruising speed horsepower at about the same 70-80 hp. The TDI will not win any drag races with 0-60 mph times in the 10-11 second range, but its 247 ft-lbs of torque at 1900 rpm will rocket the TDI past traffic at highway speeds without downshifting. For comparison, the 1.8T develops only 166 ft-lbs of torque for engine speeds between 1950 and 5000 rpm. For all of you “mod maniacs” out there, you will be relieved to know that TDI’s are very mod friendly, and tuners all over the world will be happy to support your habit even if you own a TDI. Those 10-11 sec times can be reduced significantly with reasonable modifications.

What other differences stand out between the TDI diesel and gasoline engines? Diesels are known for being reliable which is in part related to having fewer parts than a comparable gasoline engine. Diesels do not have spark plugs, ignition coils, distributors or plug wires. The TDI also has no oxygen sensors in the exhaust system. That might spell reliability right there for some of us with multiple oxygen sensor failures. Another difference is the fueling ritual. You may have to scout out a few unusual gas stations to find diesel. It is becoming more popular, but there will always be a few that do not have it. Also, diesel pumps at the fueling stations tend to be a little more messy and smelly since they do not require the emissions style nozzles and they pump faster to accommodate the larger tanks on trucks. It’s always a good idea to keep some latex gloves in the car to keep the mess off your hands. One difference that some consider to be an inconvenience is the perceived ineffectiveness of the heater in a diesel. Since diesels are more efficient engines by design, they do not generate as much waste heat to be blown into the cabin on a cold morning. Seat heaters are a valuable option for diesel owners in cold climates. The last notable difference is the maintenance of diesels versus gas. Overall, you should expect similar maintenance costs. You will not need spark plugs or other ignition items but you still have to change the oil and the air filter and expendable items like wipers and tires and brakes. The timing belt change on diesels used to be an expensive maintenance item, because it was required more often, like every 50,000-60,000 miles. The higher temperatures and the catastrophic failure of the interference engine associated with a timing belt failure justified the shorter intervals. However, the new Passat TDI has changed this by extending the normally low maintenance interval to a comparable 100,000 miles.

As with any car, there are plusses and minuses. For those of you that already drive a B5 or B5.5 Passat, the introduction of the TDI engine provides just one more option for powering you down the road in an excellent automobile. This option just happens to be diesel powered and more fuel efficient than its gasoline counterparts. In today’s topsy-turvy fuel market, a Passat that gets 600 miles to a tank of gas is certainly welcome in my garage.

For more information on diesel and gasoline engines, please visit www.howstuffworks.com and click on the Auto Stuff tab.

For more information on TDI engines including the new Passat, please visit www.tdiclub.com.

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