"There are some fleets in the Midwest that have never seen value in an engine brake, but that's a diminishing number now. Flat lands, low speeds, low weights… A 55 mph fleet rolling down the interstate hauling triples on flat ground, they really don't see the ROI that someone running in the mountains does. As those trucks come into the marketplace now as trade-ins, and they do not have retarders on them, they get equipped with retarders before the second or third owner buys them. As they move around the country—a fleet of trucks from Ohio might wind up in Los Angeles—suddenly they're in the mountain west and they absolutely need engine brakes. Sometimes the second or third owner has the engine brake installed before putting the truck back in service."
24 VOLT DILEMMA
It will be a while before any technician performs an engine brake tune-up on an '07 engine, but when that time comes the technician may be in for a small surprise, according to Detroit Diesel.
"What is different about the 2007 engine brake from the '02 and '04 brakes is the fact that now the solenoid valve has two lead wires instead of one," says Al Warner, staff project engineer for Detroit Diesel. "In the past, for the older 'Jake brakes' that had two cylinders per housing, that had just a single lead that we call a high-side driver. It was turned on by supplying electricity to the solenoid, and then it was a block return, or block ground.
"Well, with '07, with the new electronics, it gave us the opportunity—early in the program, when we first started this about three years ago—to eliminate the 24V solenoid models from the brake," Warner explains. "We grabbed for that opportunity. So now there is a high-side power source which comes from the switched ignition. Then the solenoid is driven on the low side by PWM-ing—aiding the current in a duty cycle fashion. That way with one solenoid it could be used on either 12V or 24V models of the brake.
"PWM-ing," Warner explains, "is high speed (350 hz) turning on and off the electrical ground. For example, if you run 50 percent duty cycle, then the ground is on 50 percent of the time. Subsequently, you get 50 percent of the current that you would if the ground was on 100 percent. That is about where we currently run the solenoids even though they are perfectly capable of running 100 percent all of the time.
"Also, the circuit is a little smarter than this," he says. "When the brake first comes on, there is a brief surge of 'pull in current' to get the armature moving from it's off position. Once again, the benefits of the reduction in heating due to the current flow are very minor since the oil temp is the real driver of solenoid temperature and it's wear and tear. The whole approach was to reduce cost by elimination of the 24V brake part."
However, after the two wire solenoid design was finalized, the company decided to abandon the 24V option for the '07 EGR engine.
"It's a straight 12V engine, and therefore, the opportunity that we tried to go for with the two-lead solenoid disappeared," Warner says. "Everything was cast in stone at that time, so we still have a two-lead solenoid. And there are minor advantages, because with the pulsing of the current there is somewhat of a reduced thermal load on the solenoid. But that is not the reason why—that is an 'Oh, by the way' advantage."
TWO WIRES TO REMEMBER
In addition to having to hook up two leads to the solenoid instead of one, the technician working on a Detroit Diesel will have to become familiar with a newly-designed injector harness that carries the electrical leads for the engine brake.
"Its arrangement is very different than what we've had in the past, where we feed the lead wires to a hole in the head in the back of the engine," Warner explains. "You know how tight it can get with the firewall there. So we've developed a new injector harness that comes in through the side of the rocker cover, in what's called the intermediate housing—the lower spacer in the rocker cover. It goes up and over the brake to get to the injectors, and we have very elaborate parts that support the injector harness, because we've had some issues in the past."
Detroit Diesel's goals for the new injector harness design were to increase reliability and reduce the already small amout of warranty returns on the engine brake wiring. Warner reports that early feedback from technicians on the new layout has been all positive.
"It has no effect at all on the maintenance of the engine, outside of the fact that you've got to lash it, you've got to connect the solenoids, etc.," he says. "As a cautionary, the one hot lead—and it's only going to be hot with ignition on—if that lead ever shorts the block, or shorts the solenoid, it does result in an engine shutdown situation. That high-side sourcing is shared by other critical actuators on the engine, and it's actually split—with the front solenoid, it shares power with, I believe, the turbo actuator, and the rear solenoid shares its power with the EGR valve.