How a VW TDI and Audi TDI diesel engine works


A basic understanding of your car and its diesel engine will increase your knowledge of how to drive efficiently, why it's special, and empower you when it comes time to work on it, either by you or by the mechanic.  This article is a basic "how things work" on the TDI engine and how it's different from a gasoline engine.  For details on differences between the TDI 1996-2003 Bosch injection pump, 2003-2007 pumpe duse, and 2009 and newer common rail engines, see 1000q: TDI engine fuel injection differences.

The Basic Parts of a TDI engine

The parts of the car that are discussed here are the engine and transmission.  All car engines are basically air pumps.  Pumping air and through the engine while burning fuel releases energy which is converted by the engine and drivetrain to turn the wheels.  The more air/fuel you burn, the more energy an engine can put out. 

There are a few ways to move more air to burn more fuel and make more power: spin the engine faster by revving it up, get a larger engine to move more air, or compress the air so that more air fits into a smaller engine.  Performance Honda engines are known for being smaller engines that rev high to make more power.  The advantage is that you use less fuel, less weight, less space, and normally better fuel economy.  Disadvantages are that these high revving small engines have less torque (also known as "pick up", that immediate push when you step on the gas).  Why not rev every engine higher to make more power?  Due to design limitations, some engine component will scatter or break at a high enough rpm.  This is why engines have redlines and is among the reasons why diesels cannot rev too fast.

A larger engine would make more power, but it takes up more room, usually costs more to build, weighs heavier, and gives worse fuel economy than a smaller engine, everything else being equal.  Larger, more powerful gasoline engines also work at a smaller percentage of potential power, forcing them to work under a greater intake manifold vacuum (because the throttle is mostly closed) and increasing pumping energy losses.

Turbodiesels all use turbocharging, the third method of making more power by compressing the air.  The goal of compressing the air is to make a denser air charge which increases volumetric efficiency (getting more power out of a given volume of engine).  Unfortunately, turbocharging also makes the air hotter through compression and radiant heat, which expands the air charge (air increases in volume as it gets hotter), contradicting the goal of maximum density.  A well designed turbocharged car will use an intercooler (a heat sink cooled with ambient air or water/coolant) to cool the air as much as possible, maximizing air density.  They are placed downstream of the turbo because that's where the air absorbs most of the heat.  An air intercooler cannot cool the air charge further than ambient air.  You also don't see intercoolers on non-turbo cars because the intake air is already at ambient temperature.  An air intake directly connected to an intercooler or anywhere not after the turbo would actually decrease performance by restricting airflow.  Below is a funny picture of an "interfooler", a non functional intercooler on a non turbo car, in the middle of installation.  They installed it because they think it looks cool but it shows ignorance of how an intercooler is supposed to work.

Because an engine is basically an air pump, it's also helpful to know the path air takes as it goes through a turbocharged diesel engine.  Here is a basic explanation: first, air enters through the air filter box.  It then goes into the intake side of the turbocharger where it gets compressed and heated, and then piped through the intercooler.  It cools and becomes denser in the intercooler and then goes into the intake manifold.  Air then passes through the cylinder head, which contains the intake and exhaust valves, and into the combustion cylinders.  The intake valves shut, sealing the combustion chamber. The engine piston moves up and compresses the charge.  Very high pressure fuel is sprayed into the combustion chamber which cools the air charge further.  In a gasoline engine, a spark plug ignites the charge.  In a diesel engine, the air/fuel charge combusts as it's injected due to the high compression.  The combustion pushes the piston down which turns the crankshaft.  The exhaust valve opens and lets the exhaust gas out the exhaust side of the cylinder head, through the exhaust side of the turbo, and out the exhaust pipe.  

When the piston is pushed down during combustion, the crankshaft turns.  This energy is transmitted to the transmission which turns the driveaxles, which turns the wheels.

The turbo is powered by exhaust gas, scavenging exhaust energy that would otherwise be wasted by going out the exhaust pipe.  A basic turbo has a compressor side, a turbine side, and a center housing.  After going out the engine, the exhaust gas goes into the turbine side of the turbo, spinning the turbine wheel, then going out the exhaust.  A center housing contains bearings and a shaft connecting the compressor and turbine wheel.  That energy from the turbine turns the shaft, which turns the compressor wheel, which compresses the intake air...then the cycle repeats.  If there is too much pressure build up, a gas wastegate on the turbine side lets excess exhaust gas out.  Some modern turbos relieve the excess gas pressure by changing the angle that the gasses are directed at the turbo wheels.  For more technical information regarding turbocharging, read 1000 answered questions: turbocharging.

Gasoline - Diesel engine differences

Among the biggest differences between gasoline and diesel engines are fuel and construction.  Diesel engines compress the air in a ratio, on average, of 19:1.  Compression ratios on gasoline average 9:1-11:1.  The static compression of a diesel engine is about 400-550 psi where a gasoline engine produces about 160-180 psi.  Never use a gasoline engine compression tester on a diesel engine because it will probably break the gauge!  The peak combustion pressures when the engine is burning fuel can reach 1000-1500 psi in a gasoline engine and 2000-3000 in a diesel engine.

Not only does higher compression result in better fuel economy, it causes the diesel fuel/air mixture to autoignite in micro explosions when injected into the engine.  Gasoline burns with a flame front starting when the spark plug ignites the fuel/air mixture.  A gasoline can rev higher partially due to the limits of how fast diesel fuel can cleanly burn.  Gasoline vapors can also easily ignite from spilled fuel in a car crash or while pouring gas into a spare can (from a static electricity spark).  Diesel vapors don't easily ignite at room temperature/pressure from an open flame since its flash point is at about 64oC.  Biodiesel's flash point is about 130oC.  An example of this is shown at the 1:00 minute mark in the video below.  Don't try to replicate the experiment shown!  This higher resistance is one reason diesel was used in tanks and military vehicles.  Some still use diesel in modern turbine powered tanks.

Here are some pictures of a gasoline engine piston (left) and a diesel engine piston (right).  In most gasoline engines, the piston face is often flat because the space above it acts as the combustion chamber.  There are some pistons with dome or roof shaped tops, but the picture below is typical of most gasoline pistons.  The air/fuel mixture is also mixed outside of the cylinders in most gasoline engines.

VW TDI diesel pistons (pictured below) has a bowl into which the fuel is sprayed and swirls into.  That bowl acts as the combustion chamber due to the need for higher compression.  The face of the piston nearly touches the cylinder head while the engine is running.  Some older diesel engines have the bowl in the cylinder head and a flat piston but this type is standard for modern TDI engines.

Because a diesel engine has much higher compression and pressure than a gasoline engine, they are also typically built with more forged metal parts and sturdier construction than gasoline engines which are built mostly with cast metal parts.  Forging means the metal is "stamped" and formed under pressure to make the metal denser, stronger, and "better formed" on a microscopic level.  Diesel cars have many cast parts and some gasoline cars have many forged parts, so the exact construction really varies from car model to model.  Casting metal by pouring molten metal into a mold is a cheaper, faster, and easier way to form metal.  As an example, the mk6 VW Jetta TDI weighs about 125 lbs heavier than the 2.5L engine gasoline Jetta.  Part of the extra weight is from the turbo, intercooler, piping, and emissions treatments, so I'm guessing the 2.0L 4 cylinder TDI engine weighs about 80 lbs more than the 2.5L 5 cylinder gas engine.

A downside of a diesel's sturdier construction is that the parts have more mass and cannot move as quickly as lighter gasoline engine parts.  This is mostly due to beefier construction and not necessarily due to forged parts.  Heavier components in a diesel engine limit how high the engine can rev before the engine parts decide to fail and scatter.  The other main reason is the burning properties of diesel fuel.  While forged metal parts are stronger, a limitation is the slightly different rates at which forged and cast metal parts expand from heat due to the forging process and the metals used.  Generally speaking, aftermarket forged pistons will expand more than OEM forged pistons because they're used in tuner engines and engineering limitations.  While quality control can be higher, engineering can't compare to the amount of work that an OEM engine sees.  Also, if someone switches to aftermarket forged pistons, they did so because they need a looser fit to expand from higher heat and pressure that the performance engine will see in racing conditions.  Once they heat up and expand, it closes the clearances and the stronger forged piston can tolerate greater stresses than a comparable cast piston.  The big advantage of using a cast piston is that it is cheaper and easier to make, less vulnerable to cold engine start abuse, and works fine for OEM applications.  As a result, even though most gasoline and diesel turbocharged passenger cars have forged crankshafts and connecting rods, most of them use cast pistons, for the aforementioned reasons and economy.

All modern diesel engines also use direct fuel injection (DFI or DI).  DFI engines spray fuel directly into the hot combustion cylinder.  Various manufacturers may call it pumpe duse, CDI, or common rail - these are just different methods and brand names of DI.  Most gasoline engines spray fuel into the intake manifold or ports.  

Not only does DFI reduce emissions from a more complete burn (finer fuel atomization), it also lets you compress the air more (greater compression ratio = more power and efficiency) due to greater resistance to detonation.  DFI cools down the engine by the latent heat of fuel vaporization further helps prevent detonation because the fuel isn't injected until right before the spark plug ignites it.  Some gasoline turbo direct injection cars also time the opening and closing of the exhaust valves to create a small extra draft to help spin the turbo without burning up the exhaust valves or turbo.  Direct injection lets these cars use some valve overlap and still meet emissions standards because of great control over fuel injection.  By not injecting fuel during valve overlap no fuel is wasted or is released as excess emissions.

In addition to higher compression, diesel engines are not throttled by air, they are throttled by fuel.  Because they're always running air through them, they can run in a very lean (low fuel) state which conserves fuel.

The biggest difference between DFI in older VW and Audi diesels, newer pumpe duse, and common rail DI is the fuel pressure and control over the fuel injection events.  See 1000q: direct injection vs pumpe duse vs common rail for more details.  Older direct injection sprayed a shot of fuel into the cylinders at about 3300 psi.  Pumpe duse compresses the fuel to a much higher pressure of about 27,000 psi.  Common rail DI uses multiple small, then large shots of fuel sprayed out of piezoelectric fuel injectors and is quickly becoming the new standard in diesel engines due to very precise control over fuel injection.  Unlike earlier TDI, the timing of the fuel injection in common rail is independent of the cam or timing belt.  Common rail TDI engines were introduced to North America in the 2009 VW Jetta TDI

A diesel engine also uses glow plugs to preheat the cylinders on cold engine starts.  On a gasoline engine, spark plugs are in about the same position in the cylinder head but are not analogous to glow plugs since they ignite the fuel/air charge, glow plugs only preheat the cylinder.

A diesel engine should also use diesel specific engine oils.  These oils are designed to control and contain the soot of diesel engine.  Gasoline specific engine oils have different specifications  A list of diesel specific engine oils can be found here:  1000q: 1996-2003 TDI engine oil or 1000q: 2004-2006 TDI engine oil.  The 2009+ Jetta TDI uses VW/Audi spec 507.00 oil.  There is not enough information for aftermarket oils in the 2009+ yet so I suggest sticking to the manufacturer's specification of VW spec 507.00 oil.

The transmission

A transmission is a series of gears and other parts to transmit the energy produced from the car's engine to the drive wheels of the car.  Because it is too complicated to discuss every detail of the transmission, this section will focus on the basic mechanics, operation and maintenance of the transmission. If you want to know the details of how a transmission works, there is a good article on the site "How stuff works" which has lots of good pictures.  I've written an article on how the DSG transmission works because it's specific to the TDI and VW/Audi, see 1000q: DSG FAQ.  To see specifics on how the clutch works, see 1000q: clutch FAQ.

Basically, the transmission transmits the energy of the engine, which operates from idle to 5000rpm in VW diesels, to the car's wheels, which operates between 0 and 2500rpm.  In your front wheel drive VW, it's called a transaxle because it combines the transmission and transaxle in one piece.  It also sits sideways so that the output shafts point at the front wheels.  The biggest difference between a diesel and gasoline transmission is the rpm range for which it is geared because of the operating rpm of the engine.  Here's a video showing some more explanation.  It's not a TDI transaxle but the basic explanation is the same.

An automatic transmission is very different from a manual transmission.  Inside the auto trans is a torque converter, a hydraulic fluid coupling that looks like a propeller and transmits the engine energy to the rest of the transmission, which then drives the wheels.  If you want to see pictures, refer to "How stuff works".   A recent trend in auto transmissions is "lifetime" fluid.  If you go to the dealership, they may tell you that your car has lifetime fluid and is not serviceable.   I think what they mean is that when the fluid breaks down and the transmission becomes rough or worn, it ends the economically useful lifetime of your car, so come back and buy a new car.  Lifetime might also mean the lifetime of your car's warranty.  There is no such thing as fluid that has an infinite life.  In theory, lifetime fluids could last anywhere between 50,000 mi and 500,000 mi without issue.  The problem is that it gets contaminated with water, new car break-in debris, age related debris, faulty auto transmission filters, or leaks/seeps out.  Here is a picture of a "lifetime" fluid's filter pan from an Audi A8 with lifetime fill.  Those are metal shavings stuck to a magnet and changing the fluid will at least greatly extend the life of the car.

Even being very conservative/frugal, a fluid change every 60-80,000 mi or 5 years is not asking a lot and doesn't cost very much per mile considering how long the service interval is.  For your car, refer to your owner's manual or find your model's how to article on changing the ATF in 1000 answered questions: index. 

A manual transmission uses gear oil, not ATF fluid, so do not mix auto transmission fluids and manual gear oil. A manual transmission has a pressure plate attached to the flywheel with a clutch sandwiched in between.  When you step on the clutch pedal, you release the pressure plate's clamp on the clutch and disengage the engine from the transmission. All modern passenger manual transmissions are synchronized, they use "rings or cones" which match the speed of the gears.  If your synchronizers are worn out due to poor shifting technique, defect, or lack of maintenance, the gears will not be "matched" and will grind.  For a better description of a manual transmission with pictures, see "How stuff works".  Fluid should be changed according to your car's service interval, refer to the your model's detailed specs in 1000q: buyers guide for more information on change intervals or 1000 answered questions: index. 

The DSG transmission is a dual clutch automatically shifted manual-like transmission.  It is not anything like a conventional torque converter automatic transmission.  The advantages are ease of use and better performance than a conventional automatic.  However, it may give lower mileage and as a group, give lower reliability than a manual transmission.  See a detailed description at 1000q: DSG FAQ

There is a lot of confusion about which manual transmission gear oil brand to use.  Refer to your owner's manual for the most precise answer.  Older VW transmissions should only use a GL-4 gear oil, not a GL-5 gear oil which includes Mobil 1 75-90.  Many people have success with Redline MTL or MT 90, or Royal Purple Max gear.  VW often changes and contradicts itself on the official spec oil, so try a few different GL-4 oils and see which results in the best shifting and transmission feel for you.

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