Wrestling with modern diesels

Motor,  Jan 2002  by Marcy, Thomas

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Winning a bout with a diesel engine means using some heavy-duty skills. And as with any tough match, it pays to know the strengths and weaknesses of your opponent.

We don't know much about alligator wrestling (which looks dangerous) or mud wrestling (which looks like tons o' fun), but we do know something about diesel wrestling.

We say "wrestling" because working on diesel engines is different from working on gasoline engines-you don't get as many in your bays, so you're less familiar with them. This article is meant to change that. Next time you 11 go to the mat" with a diesel engine, you may be able to demonstrate a few new "holds."

As you know, a diesel engine requires no spark plugs to fire the fuel, although glow plugs are typically used to assist engine starts in cold weather. Often, the fuel accumulates slightly before light-off occurs. This is called the delay stage. Once the delay stage concludes, the fuel mixes with the hot compressed air in the cylinders and ignites, causing crunching blows to occur inside the engine. It's because of these crunching blows, plus the fact that the engine produces far greater cylinder pressures than its gasoline-fed cousin, that the diesel has long been the burly power champ that produces high torque.

Two Cummins Entries

Several contenders have climbed into the ring on the diesel light truck card. One is the Dodge Ram truck's 24valve Cummins turbo diesel, an inline Six. Featuring big breathing, this engine produces 245 hp with the sixspeed manual transmission, or 235 hp for automatics. Peak torque of 505 ft-- lb comes in at 1600 to 2300 rpm.

Cummins says the 24-valve cylinder head design increases airflow and improves low-speed performance. The head also features vertical, centered injection nozzles, which are claimed to boost combustion efficiency, cleanliness and fuel economy. Weigh in some electronic fuel control and you've got a tough opponent.

A diesel engine's fuel injection system must possess several characteristics to deliver maximum performance. These include accurate injection control, high-pressure atomization, fast fuel ignition and fuel temperature adjustment capability.

Fuel's fiery entry into the cylinders in the Cummins comes via a Bosch VP44 electronic injection pump and electronically controlled timing. First, the engine is cranked up for the match with an electric-powered lift (or supply) pump. Fuel flow begins as the lift pump pulls fuel from the tank and delivers it to the injection pump. As the engine spins over, the rotary, high-pressure injection pump is driven at half the engine speed by a simple front gear train. This simplicity helps cut operating noise.

The Cummins-developed ECM receives information from various sensors and controls on the engine, then translates that info into specific fuel quantity and timing commands that are sent to the injection pump metering control.

Note that the injection pump used on earlier (1994-98) Cummins-- equipped Dodge turbo diesel pickups is a Bosch P7100 inline design--what we sometimes call the jerk pump. The cam-operated six-plunger pump in the jerk pump sends fuel to the injector nozzles. The newer VP44, however, is a rotary job with the ability to vary fuel and injection timing based on input from the ECM. This pump incorporates a fuel pump control module (FPCM), which contains fueling, timing and diagnostic data. The FPCM communicates with the ECM to obtain information on desired fueling and timing. The FPCM responds by consulting the fueling data and timing in its memory, then commands fueling and timing solenoids.

The VP44 has a fuel temperature sensor inside it. The pump also contains a speed sensor that gives the FPCM information on the position and speed of the pump shaft. A crankshaft reference pulse that marks TDC of cylinder No. 1 is also sent once every pump revolution. By comparing this reference pulse from the ECM to the position signal from the speed sensor, the FPCM can reference the pump's position to that of the engine. This allows the FPCM to adjust pump timing to compensate for small position differences between the fuel pump shaft and the engine cam. If the difference becomes too great, a fault is logged in the FPCM.

The FPCM controls the timing and fueling through two solenoids located in the pump. The timing solenoid controls the position of a cam ring inside the pump by varying internal transfer pump pressure to a cam ring piston. The cam ring has evenly spaced lobes around its inner diameter. The pumping plungers ride on rollers that rotate around inside this ring. The rollers follow the inner diameter of the ring and push the pumping plungers inward whenever a cam lobe is encountered. This builds injection pressure on the fuel trapped between the plungers. By rotating the cam ring with the timing solenoid, the FPCM is able to advance and retard the injection timing by making the rollers contact the cam lobes either earlier or later in their rotation.

As its name implies, the fuel solenoid in the pump meters fuel to the injectors. It does this by opening a metering valve to allow fuel from the supply pump to flow into the pumping chamber. Once this chamber is charged with fuel, the solenoid valve closes, trapping the fuel in the chamber and allowing injection pressure to build.