Overtaxed

| July 30, 2006

A technician tightens a fitting at the discharge port of a Bendix AD-9 Air Dryer.

The obvious impacts from driving in congestion are bad enough – wasted hours and nerve-frying stress. The wear and tear on your equipment is less apparent, but just as real, and it’s not limited to urban applications. “You about wear out your brakes on I-85 in Georgia,” says Phyllis Krueger of Krueger Trucking in Buford, Ga. “That’s crazy to have to ride your brakes on an expressway.”

Assessing the actual cost of wear is difficult, given that some trucks are spec’d more for heavy stop-and-go driving, and that some drivers multiply the effects of congestion through improper shifting, accelerating and braking.

“City stop and go is essentially the worst environment for a diesel engine,” says David McKenna, powertrain sales and marketing manager for Mack Trucks. For starters, fuel efficiency plummets when an engine designed for on-highway use has to slowly grunt through too much traffic.

“Highway engines are optimized to run in a general rpm range,” McKenna says. “When you’re appreciably above or below that, your fuel economy will suffer. As you accelerate, you use more fuel than you would by maintaining a governed cruise limit.”

A very light load that inches along in the lowest gears also wastes fuel. Such driving is akin to idling.

Poor driving wastes even more fuel and causes unneeded damage to the truck.

“I watch some of these truck drivers get right on the tail of somebody else and grab as much brake as they can and come to a grinding stop,” McKenna says. “When the traffic’s slow, the driver gets on the throttle and you can see the chassis twisting, and that is extremely hard on equipment. Basically, you’ve got to smooth down the operation of the equipment.”

One way to minimize the equipment stress is to use the engine brake more, even at low speeds, though an increasing number of municipalities prohibit engine brake use. “Every little bit you can take away from the service brakes helps immensely,” McKenna says.
Service brakes wear much faster when overused because they build too much heat.

“Many brake applications close together with little time for cooling, and no air flow in between the lining and drum builds temperature and causes wear to the brakes,” says Tom Runels, foundation drum brake engineering manager for Bendix Spicer Foundation Brake. Adding 100 degrees to the normal operating temperature of a drum – say, from 250 degrees to 350 – will cut brake lining life in half, he says.

Other brake system wear is due to pushing the air supply beyond its spec, says Chuck Eberling, principal engineer for Bendix Commercial Vehicle Systems. “Historically, the customer will usually spec’ the basic vehicle, until he learns that it’s unable to deal with the conditions he’s putting the vehicle under,” Eberling says. In some cases, the unanticipated conditions include increased amounts of driving in highly congested areas with repeated brake applications. Other cases involve an owner adding an axle without upgrading the brake air supply system.

“Bendix doesn’t like to see a duty cycle higher than 25 percent on most of our compressors,” Eberling says. One exception is the company’s BA922 compressor, which can go up to 35 percent with the proper application review and approval. Duty cycle is the amount of time the compressor spends building air pressure.

“We’ve seen people who have not selected the right size components push 50, 60 percent duty cycles,” Eberling says. “Usually we will try and assist people with proper product sizing and selection such that duty cycles wind up being within allowable limits.” If you typically run in congested conditions, be sure to take this into account when spec’ing so you won’t experience premature compressor failure.

Like brake temperature, brake air compressor duty cycle percentage is not among the ECU data that shows up on the dash, though manufacturers could add it, Eberling says.

When the brake air supply system is overworked, it accumulates more moisture than the dryer can process. This moist air “will reduce the life of all components in the air brake system. Moisture tends to flush lubricant out of the valves,” Eberling says. The increased duty cycle means the desiccant inside the dryer may become saturated, reducing efficiency.

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