Over the years, off-road equipment has maintained a reputation as being somewhat primitive. They were bigger, heavier and noisier, and had much dirtier emissions than the engines found in light-duty applications. While light-duty diesels have held a technological edge for many years, the gap is closing as emission regulations get tighter for the high-horsepower machines.
The mining industry is case in point, where emission control regulations are a relatively recent phenomenon. Off-road machines didn’t get a lot of scrutiny from environmental regulators in the past, mostly because they were few in number and represented a much smaller share of the overall emissions inventory. It was only a matter of time, however, before big diesels would have a much higher set of expectations to live up to.
Caterpillar’s line of mine haul trucks is a good example of how far the off-road equipment industry has come in recent years. The newest models have higher power, lower fuel consumption and reduced emissions, and these goals were primarily achieved using technology that was developed in light and medium-duty diesels. In this article, we will take a look at the Caterpillar 793F off-highway truck, and compare its technology and service procedures with those of light-duty diesels.
793F overview
The Caterpillar 793F is a heavy-duty mine haul truck with a 250-ton payload. The 793F is 25' width by 44.9' depth, by 21' height when the dump bed is down. When the bed is raised, its forward edge rises to 45.5'. When service is performed with the dump bed up, the bed must be “pinned,” meaning that a large cable is used to prevent the body from dropping if the hydraulics were to fail.The powertrain is unusual by automotive standards. The torque converter is attached directly to the engine, but the torque is then transmitted by a driveshaft to the transmission, which is incorporated into the rear drive assembly. The transmission has six forward speeds, which propels the 793F to a top speed of 37.3 mph. For machine management purposes, the higher speeds can be locked out electronically.
Draining and filling the 291-gallon cooling system also is easier than you’d think. Coolant is drained into a portable holding tank below the truck and then is pumped back in to the system through the same drain port. Pushing the coolant in from underneath eliminates trapped air and puts the machine back into service that much faster.
The standard fuel tank holds 750 gallons, but an optional fuel tank can be installed that brings the capacity up to 1,300 gallons. There are a total of five fuel filters in the system; fuel is first sent through two 10 micron primary filter/water separators near the fuel tank. After passing through the engine-driven transfer pump, the fuel is then sent through two 4 micron secondary filters and finally a single 4 micron tertiary filter. The fuel can’t be too clean!
After replacing the filters, priming of the fuel system is accomplished using an electric pump that is connected in parallel with the transfer pump. The engine ECM directly controls the electric primer, but also has a manual switch located near the secondary fuel filters. The filters are installed dry and then fuel is pumped through from the tank using the electric primer. Aside from priming the system, the electric primer is also used to increase fuel system pressure during engine starting.
Service philosophy
In the mining industry, the big machines pay the bills. These machines (such as haul trucks and loading units) cost millions of dollars and generally run 24 hours a day, seven days a week. Failures are extremely costly, primarily due to lost production. Thus, every effort is made to catch failures long before they come to pass and minimize unscheduled downtime.
There are two schools of thought regarding scheduling of major service:
- Time-based: A time-based approach would allow only a certain number of hours to be run on a component before it would undergo a Planned Component Replacement, or PCR. The number of hours a component would accumulate before a PCR is often based on experience. Allowing a component to run to destruction may appear to wring more value from it, but introduces a whole dimension of uncertainty to the equation. There is no telling when it will fail, or where the machine will be when it fails, and could also damage a valuable component core.
- Condition-based: While time-based PCRs work, a more sophisticated approach is to monitor the condition of the component using oil sampling, vibration analysis, etc. and then replace it when it starts to show signs of failing. This can get more hours out of a component and has the added advantage of catching a failure that would have occurred before a component reaches its normal time-based limits.
An engine is typically ready for a PCR around 18,000 hours, and will undergo a mid-life service around 9,000 hours. A mid-life involves changing out of certain critical assemblies while the engine is in-frame, including fuel system components such as the high-pressure fuel pump and a complete set of injectors. This is all scheduled service; predictable and relatively easy to manage as opposed to sudden breakdowns.
C175 fuel system
The C175 fuel system is a high-pressure common rail (HPCR) design, which is capable of producing injection pressures of up to 26,100 psi. HPCR is a technological quantum leap and allows for tremendous flexibility in injection timing, rate, and quantity. This gives it a major performance advantage over the mechanically-actuated EUI systems that were used with previous generations of high-horsepower Caterpillar diesels.One of the distinct advantages of an HPCR system is its ability to break a diesel injection event into five or more separate pulses. Back before electronics were incorporated into diesel injection systems, fuel was dumped into an engine cylinder in one shot. This lack of control resulted in tremendous pressure rises in the cylinders, and the loud cylinder knock that diesel engines were once known for. The high cylinder pressures also produced high combustion temperatures and excessive NOx emissions.
What are trim files?
Trim files are high-pressure common rail injection systems are designed for precision in every sense of the word, and it is extremely important that the injectors all flow fuel at the same rate. Building an HPCR injector is expensive to begin with, but if the manufacturing tolerances are tightened to the point where every injector flows almost the same, it becomes costly enough to be impractical. In order to bring costs down, an alternative is to loosen the manufacturing tolerances and then use software to compensate for flow variances.
This is the strategy used in the manufacture of the HPCR injectors for the C175. Each finished injector is tested at the factory, and is rated according to its flow characteristics. A “trim file” is then created for each injector, which is software that is uploaded to the engine ECM when an injector is installed. The trim file tells the ECM how to adjust the pulses to the injector to balance it with the others. For example, an injector that flows less than the baseline number would have a longer pulse sent to it by the ECM. This strategy reduces the cost of the injector hardware but makes them perform as though they all had the same flow rate.
HPCR made it possible to add fuel little bits at a time, moderating the cylinder pressure rise and taking the sharp edge off the diesel’s personality. Now, an HPCR diesel engine is quieter, produces fewer emissions, and yet is more powerful.
HPCR was pioneered in light-duty diesels and has gradually made its way into high horsepower applications. The major challenge with HPCR is the tight clearances between moving parts. This precision requires ultra clean fuel and sterile service procedures, which are always desired but not necessarily achieved in the mining industry. As you can imagine, mining is a challenging environment for any technology that requires strict cleanliness during service.
The injectors used in the C175 are configured similar to those used in the HPCR-equipped Cummins turbodiesel. Like the Cummins, they utilize a solenoid actuator and a quill tube to connect the injector body to the common rail fuel manifold. The quill tube and other high-pressure connections in the C175 fuel system use spherical ball and conical sealing joints, which must be inspected carefully before they are reused.
HPCR service
To diagnose a C175 fuel system issue, the technician can retrieve diagnostic data in two different ways. The quickest method is to use the Advisor panel located on the 793F dashboard. Advisor is connected to the CAT Data Link, which is one of three networks used on the 793F and will provide access to the machine and engine ECMs. Navigating to the Service menu in Advisor gives the technician access to live data, diagnostic codes, as well as a data logger function.For more comprehensive diagnostics, the technician will need a laptop-based scan tool with Caterpillar ET (Electronic Technician) software. The computer’s USB port connects to the nine-pin data link using a communications adapter, similar to those used with laptop-based automotive scan tools. Caterpillar ET has similar functionality to other automotive factory scan tools, such as viewing live data, retrieving and clearing trouble codes, bidirectional controls and reflashing modules.
Caterpillar ET plays a critical role during an injector replacement on a C175. Before the high-pressure fuel system is opened up, ET is used to determine the pressure in the fuel system common rail. Pressures in an HPCR system can be dangerously high and could severely injure a technician if precautions are not observed. Once ET indicates that fuel pressures are reduced to safe levels, the job can continue.
Replacing a C175 injector requires strict attention to detail. Above all, everything must be clean! This includes the technician’s clothing; if the tech is dirty from previous jobs, they should put on a new set of coveralls and a fresh set of rubber gloves. The top end of the engine should be pressure washed and dried off with compressed air before the cylinder’s valve cover is removed.
Torque specs and sequences are critical when installing the new injector. The O-rings on the injector body should be lubricated with clean motor oil, as well as the ends of the injector quill tube. Experienced technicians keep a cheat sheet handy (a smart phone works good) with the torque instructions as they are too detailed for most to memorize. Before the injector is installed, make a note of its serial number (available on the injector hold-down bracket), as well as the cylinder it is being installed in.
Once the injector(s) have been installed, the next step is to perform a trim calibration with Caterpillar ET. A CD with the necessary trim files is packaged with each new injector, but these files can also be downloaded from the Caterpillar service information website. This procedure is used whenever injectors are replaced or swapped between cylinders. Once the trim calibration is complete, the fuel system can be primed using the onboard electric pump. Since the scan tool is already connected, Caterpillar ET can be used to command the pump on to finish the job. Wrap it all up by clearing the trouble codes, and the truck is ready to go back into service.
Certainly, there are significant differences in the maintenance procedures used with high-horsepower diesels. However, the big engines are becoming much more sophisticated as they adopt technology that was refined in automotive diesels. This trend will only continue as emission control regulations get tighter for off-highway equipment.
