How a diesel works

Introduction - How a diesel works

A very basic understanding of your car and it's special diesel engine will increase your enjoyment from your car and will also empower you when it comes time to work on it, either by you or by the mechanic, so take some time to read this.   It's basically a "how things work" article on how your diesel car works and how it may be different from a gasoline car.

The Basic Parts

The basic parts of the car that will be discussed here are the engine, transmission, hydraulic clutch.  All car engines are basically air pumps. They require air and fuel to burn and use some of the energy released to move your car.  The more air/fuel you burn, the more energy an engine can put out. There are a few ways to move more air 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 and weight, usually costs more to build, and usually has worse fuel economy than a smaller engine.

Turbodiesels all use turbocharging, the third method of making more power by compressing the air.  The goal of compression is to make a denser air charge and increasing volumetric efficiency, which means 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), defeating 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 it's density.  They must be placed downstream of the turbo because that's where the air is heated.  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 silly picture of an "interfooler", someone who put an intercooler on a non turbo car.  It's there because they want to look cool and are ignorant of what it's function is.

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, gets compressed and heated, then piped into the intercooler.  It cools and becomes denser in the intercooler, and then goes into the intake manifold.  Then, it 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. The fuel, under very high pressure, is sprayed into the combustion chamber which cools the 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 and pushes the piston down.  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 from combustion, that turns the crankshaft which turns the transmission, which makes the wheels on the car go 'round and 'round.

The turbo is powered by exhaust gas, scavenging exhaust energy that would otherwise be wasted.  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 pointing them at the turbo wheel at a different angle.  For more technical information regarding turbocharging, click here for 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.  These compression ratios produce on average, 160-180 psi in gasoline engines, and 400-550 psi in diesel engines in a static compression test.  Yes, you read that right, and if you use a gasoline engine compression tester on a diesel engine, it will break and the gauge's needle will pop off so fast that you are hopefully wearing safety goggles.  The peak combustion pressures as a result of combustion 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 diesel fuel/air mixture to autoignite in micro explosions.  Gasoline burns with a flame front starting when the spark plug ignites the fuel/air mixture.  While the diesel cycle results in less energy into waste heat, a gasoline can rev higher partly due to how fast the diesel fuel can cleanly burn.

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 formed under pressure to make the metal denser, stronger, and "better formed".  Diesel cars have many cast parts and some gasoline cars have many forged parts, so construction really varies from car model to car model.  

Casting metal by pouring molten metal into a mold, is a cheaper and easier way to form metal.  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 are part of the reason why a diesel engine cannot rev as high as most gasoline engines before the engine parts decide to 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.  The short answer why is because aftermarket piston makers don't have an army of engineers to predict, test, and make a piston that fits perfectly in the engine and test how it heats, expands, and changes shape.  It never compares to the level of engineering that an OEM engine sees.  Another reason is because if you are using aftermarket forged pistons, you are using them in a performance engine that will see much higher stress than a stock engine and needs more room for expansion due to heat and pressure.  The advantage of using aftermarket forged pistons is that once they heat up and expand, closing the clearances, the forged piston is stronger and can tolerate greater stresses than a comparable cast piston.  The 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 these reasons and due to economy.

All modern diesel engines also use direct fuel injection.  Various manufacturers may call it "pumpe duse", CDI, or common rail, but they all mean direct injection.  Direct injection means that unlike most gasoline engines which spray fuel into the intake manifold or ports, direct injection sprays fuel directly into the hot combustion cylinder.  Not only does this have reduced emissions from a more complete burn due to finer atomization, it also cools down the engine by absorbing heat and letting you compress the air even more than with a non-direct injection engine.  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.  Direct injection lets these cars use some valve overlap and still meet emissions standards because of control over fuel injection.  See 1000q: direct injection vs pumpe duse vs common rail for more details.  

The biggest difference between older direction injection, newer pumpe duse, and common rail direct injection is the fuel pressure.  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.  Mercedes' direct injection uses small, then large multiple shots of fuel into the cylinders under very high pressure.  The new common rail technology uses multiple shots of fuel sprayed out of piezoelectric fuel injectors and is quickly becoming the new standard in diesel engines due to more precise control over fuel injection.  Common rail is the technology used on the new 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 because they do notignite the fuel/air charge.

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 spec 507.00 oil and there is not enough information for aftermarket oils yet so I suggest only using the 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.

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.  The biggest difference between a diesel and gasoline transmission is the rpm range for which it is geared, due to the operating rpm of the engine.  I will first mention automatic transmission maintenance, then manual transmission maintenance.

An automatic transmission is almost entirely 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".  VW/Audi auto transmissions require special automatic transmission fluid, refer to your model for more details.  Note that 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 damages the transmission, it ends the economically useful lifetime of your car, so come back and just 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, heat damaged, 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.  

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 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.  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 what you like.

The clutch

To work on and maintain the clutch, I'll discuss briefly a hydraulic clutch like the one used in VW's and most other modern cars.  The clutch is a disc made of brake-like material that transmits the energy of the engine to the transmission.  The engine has a weighted disk called the flywheel which is turning at the engine's rpm.  The flywheel is connected to the crankshaft, which is connected to the pistons, which if you recall, are where the energy is being created through internal combustion.  See the detailed clutch, flywheel, and pressure plate FAQ at 1000q: clutch FAQ.

When you step on the clutch pedal, the pedal moves a piston inside the clutch master cylinder.  The clutch master cylinder is filled with hydraulic fluid which then moves through tubes and pushes a piston inside the clutch slave cylinder.  This piston pushes a pushrod, when then levers the clutch fork lever against a clutch pivot point, pushing a clutch throwout bearing against the pressure plate.  A bearing is necessary here because the clutch fork lever is stationary, the pressure plate is bolted to the flywheel, which is spinning.  The pressure plate has springs which release the clutch from the flywheel.  The engine's force is now disconnected from the transmission.  Releasing your foot from the clutch pedal lets the clutch grab because the fluid pressure is no longer pressing the various pistons and springs.  Here is a diagram of a clutch and flywheel exploded:  clutch and flywheel diagram

Here is another picture of a clutch and flywheel separated.  You can see the pressure plate springs on one side bolted to the clutch and flywheel.  The other side has the clutch fork and bearing that slides on the pressure plate springs.

The maintenance required of this system is that the hydraulic fluid must be regularly changed and that the various pistons and seals must be replaced when their seals are worn out.  If the fluid is old, it can absorb water, which rusts the pistons and tubes, preventing hydraulic fluid from building pressure because of air bubbles..  If an air bubble is introduced into the system, the clutch pedal will no longer release the clutch.  This is because when you press the clutch pedal, the air bubble compresses and absorbs the force of the pedal going down instead of moving the fluid down the tubes.

The other components such as the pressure plate, clutch fork, clutch fork pivot ball, throw out bearing, and clutch all also have limited life, but replacement interval varies greatly.  Replacement of any of these items require removing the transmission.  If your shifting technique slips the clutch often, the engine's power upgrades overwhelm the clutch's (torque rating) grabbing capacity, or there is a mechanical problem, then the clutch's brake-like material will wear out faster than normal.  If you rev-match every shift perfectly and never overstress the clutch, then the clutch could last a long, long time, in theory, maybe even 1,000,000 miles, by which everything else will be worn out.  Each time you press the clutch pedal, the clutch fork rubs against the clutch fork pivot ball.  The throw out bearing rotates and spins against the pressure plate.  The pressure plate springs get worn out a little more.  If any one of these components were to break, your car would not be able to normally shift, and you would be stranded. 

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