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What Makes a Diesel Engine Go?

Diesel engine technicians.

The basic difference between a diesel engine and a gasoline engine is that in a diesel engine, the fuel isn't ignited by an outside power source like a spark plug. Instead, the fuel is sprayed into the combustion chambers through fuel injector nozzles just when the air in each chamber has been placedunder such great pressure that it's hot enough to ignite the fuel spontaneously.

Most conventional gasoline engines have compression ratios of around 8:1, which means that the volume of each cylinder is eight times larger when the piston is at the bottom of the cylinder than when the piston is at the top of the cylinder. Diesel engines may employ compression ratios of above 20:1. Because of this volume, and because the compressed air can reach very high temperatures, diesel engines must be built for greater strength and endurance.

The following details may vary from one vehicle to another, but the action remains pretty much the same.

1. When you first turn the key in the ignition, you're asked to wait until the engine builds up enough heat in the cylinders for satisfactory starting. (Most vehicles have a little light that says "Wait," but a sultry computer voice may do the same job on some vehicles.) Turning the key begins a process in which fuel is injected into the cylinders under such high pressure that it heats the air in the cylinders all by itself. The time it takes to warm things up has been dramatically reduced — probably no morethan 1.5 seconds in moderate weather.

Figure 9-2: Glow plugs provide extra heart to burn fuel more efficiently.

Diesel fuel is less volatile than gasoline and is easier to start if the combustion chamber is preheated, so manufacturers originally installed little glow plugs that worked off the battery to pre-warm the air in the cylinders when you first started the engine. Better fuel management techniques and higher injection pressures now create enough heat to touch off the fuel without glow plugs, but the plugs are still in there for emissions control: The extra heat they provide helps burn the fuel more efficiently. Figure 9-2 shows a glow plug in a precombustion chamber, which allows the glow plug to heat a smaller amount of air more quickly and efficiently. Some vehicles still have these chambers, others don't, but the results are still the same.

2. When everything is warm enough, a "Start" light goes on. When you see it, you step on the accelerator and turn the ignition key to "Start."

Figure 9-3: A diesel fuel filter.

3. Fuel pumps deliver the fuel from the fuel tank to the engine. On its way, the fuel passes through a couple of fuel filters (see Figure 9-3) that clean it before it can get to the fuel injector nozzles (see Figure 9-4). Proper filter maintenance is especially important in diesels because fuel contamination can clog up the tiny holes in the injector nozzles.

Figure 9-4: Anatomy of a fuel injector.

4. In the most common type of modern diesel fuel system, called common rail direct injection (CDI) and shown in Figure 9-5, the fuel injection pump pressurizes fuel into a delivery tube called a rail and keeps it there under constant high pressure of 23,500 pounds per square inch (psi) or even higher while it delivers the fuel to each cylinder at the proper time. (Gasoline fuel injection pressure may be just 10 to 50 psi!) The fuel injectors feed the fuel as a fine spray into the combustion chambers of the cylinders through nozzles controlled by the engine's engine control unit (ECU), which determines the pressure, when the fuel spray occurs, how long it lasts, and other functions.

Other diesel fuel systems use hydraulics, crystalline wafers, and other methods to control fuel injection, and more are being developed to produce diesel engines that are even more powerful and responsive.

5. At this point, the action moves to the cylinders, where the fuel, air, and "fire" meet. While the preceding steps get the fuel where it needs to go, another process runs simultaneously to get the air where it needs to be for the final, fiery power play.

On conventional diesels, the air comes in through an air cleaner that's quite similar to those in gas-powered vehicles. However, modern turbochargers can ram greater volumes of air into the cylinders and may provide greater power and fuel economy under optimum conditions. A turbocharger can increase the power on a diesel vehicle by 50 percentwhile lowering its fuel consumption by 20 to 25 percent!

6. Combustion spreads from the smaller amount of fuel that's placed under pressure in the precombustion chamber to the fuel and air in the combustion chamber itself.

Figure 9-5: A common rail fuel injection system.

From Auto Repair for Dummies, copyright © 2009 by Wiley Publishing, Inc., Indianapolis, Indiana. Used by arrangement with John Wiley & Sons, Inc.

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