Kenworth Offers Advice on Choosing Specs for Dump Trucks

Wed December 08, 2010
Aggregate Equipment Guide

If you’re in the market for a dump truck, it’s important to do your homework. Unlike some other vocations, dump truck specifications are very regionalized. What works in one area of the country will not work in another.

Your first piece of homework: Find out what the length and weight regulations are in your state. Try to take maximum advantage of the weight laws to maximize payload. Some states, mostly in the West, require compliance with the Federal Bridge Formula; others don’t. This will have a big influence on how the axles are set up and spaced.

“A Bridge Formula truck will tend to be longer to spread the weight,” said Samantha Parlier, vocational marketing manager of Kenworth Truck Company in Kirkland, Wash. “You may need to have lift axles, but there are different rules on how much load you can add with lift axles. And some states don’t allow lift axles. Your local Kenworth dealer will know the rules and regulations.”

In states where you don’t need to comply with the Bridge Formula, you can spec trucks shorter and with higher axle ratings, making them more maneuverable on job sites.

2010 Engines - SCR or EGR

It’s also important to understand how the 2010 federal engine emissions standards may require some changes to be made when spec’ing for new truck purchases compared to your current dump truck spec.

“The extent of these changes depends upon each dump truck operator’s choice between two available engine technologies, which may also affect truck performance and operating costs over its lifetime,” Parlier said.

Operators can choose an engine aftertreatment approach that utilizes selective catalytic reduction (SCR) technology in combination with exhaust gas recirculation (EGR), or an in-cylinder approach through increased EGR.

Both technologies use EGR to circulate a portion of an engine’s exhaust gas back to the engine cylinders and a diesel particulate filter (DPF) to remove particulate matter from the exhaust. A critical difference is the amount of exhaust gas that is recirculated back to the engine; the enhanced EGR approach uses a significantly higher level of recirculated exhaust gases.

SCR also mixes a reactant — most commonly a solution of urea and de-ionized water known as diesel exhaust fluid (DEF) — with the nitrogen oxides (NOx) in exhaust gases. The exhaust then passes through a catalyst, where the DEF reacts with the NOx to convert it into nitrogen and water.

Increased EGR reduces NOx by boosting the amount of exhaust gases in the engine cylinder, then slowing and cooling the combustion process and burning off pollutants. The increased heat created with the enhanced EGR approach requires greater engine cooling capacity. Increased EGR also requires more fuel to be injected into the DPF for active regenerations.

“SCR doesn’t rely on engine heat to treat emissions, so SCR-based engines offer the advantage of higher fuel economy,” Parlier said. “Since SCR doesn’t narrow the engine’s maximum speed range for optimum efficiency, or its ’sweet spot,’ to attain emission reductions, fleets also can still maintain fuel economy at lower or higher engine speeds.”

According to Parlier, it is important for operators choosing SCR to consider DEF tank capacity and placement.

“To support SCR system integration, Kenworth has a range of exhaust and DEF tank sizes and locations designed for dump trucks. This helps dump operators to maintain their wheelbase and body configuration when spec’ing Kenworth trucks with 2010 engines,” Parlier said.

“For dump trucks where truck frame space is often critical, Kenworth provides a 5.6-gallon tank with a clear back of cab option. The tank has a range of more than 1,500 miles between refillings. When combined with a Kenworth SCR and DPF package under the cab access step, there is essentially no impact to the customer.”

Not all SCR technology engines are the same, however.

“An aftertreatment catalyst using copper zeolite is much more efficient than one with iron zeolite at reducing NOx at normal engine operating temperatures,” Parlier said. “Engines using copper zeolite may enjoy up to an additional 2 percent fuel economy improvement over engines using iron zeolite.”

PACCAR engines and Cummins engines both use copper zeolite.

A Third Option: Natural Gas

For those operators looking for an alternative to a standard diesel engine, natural gas-powered engines such as the Westport Innovations GX 15-L engine and LNG system for heavy duty Class 8 trucks can offer a third option.

For those operators hauling particularly heavy bulk loads, the Westport GX, based on the Cummins ISX diesel engine, is available in power ratings of 400 to 475 hp (298 to 354 kW) and torque ratings of 1,450 to 1,750 lb.-ft. Plus, the LNG fuel tanks can be configured to suit the operators’ range requirements.

Operators who don’t need that high of horsepower or torque can spec the Cummins ISL G engine, which is rated at 320-hp (238 kW) and 1,000 lb.-ft of torque. The ISL G engine operates on either LNG or compressed natural gas (CNG). It uses a maintenance-free, three-way catalyst and is 2010 EPA- and CARB-compliant without the use of SCR technology or a DPF.

“Deciding on whether to go with CNG- or LNG-powered trucks may be determined by the availability of the fuel in your area,” Parlier said. “With many local transit and government agencies using compressed natural gas to power buses and trucks, sources of CNG fuel may be easier to find in some areas than LNG.”

Operators also should consider that fueling CNG-powered trucks doesn’t require special training as it does with LNG trucks, she added. However, LNG fuel has a higher energy density than CNG since it is a liquid, so an LNG-powered truck can go further on the same amount of fuel.

Neither LNG nor CNG has the high energy density of diesel fuel, but both are cleaner fossil fuels, so they produce less carbon pollution than diesel fuel. Also, both are domestically produced, so they have the potential to reduce reliance on foreign oil. The drawback can be a significantly higher cost engine, Parlier said. But some state and local air quality control agencies may have limited programs with grants to help offset the additional cost of the engine technology. Plus, by deleting the additional weight associated with the SCR or EGR emission control systems, a natural gas-powered truck may be able to carry more payload.

Load and Hauling Questions

Some key questions need to be answered concerning the loads that you expect to haul. For example, you will need a different chassis spec when hauling bulk loads such as asphalt, sand or gravel than you would if you hauled mostly demolition debris.

If you’re planning on visiting a lot of demolition sites or are hauling heavy rock, you will need to have the body and suspension beefed up to handle the pounding it will take from the large masses going in the dump body. Your body supplier will have input on this.

The hauling question relates to the environment or roads you are operating in. Are you going to spend a lot of time on very rough job sites or will most of the hauling be long distances on smooth gravel and sealed roads?

“If you will be going off-road a lot into rough terrain, you’ll need a suspension that is heavier duty and has more articulation,” Parlier advised. “But if you’re hauling longer distances, you’ll need to consider the trade-off between the ease of dumping and the ability to haul more load per trip. For example, a transfer dump will allow you to haul more with one driver, but it will take longer to unload. Double bottom-trailers carry a lot of payload, too, but with those you’re limited on where you can drop the load — it’s a lot harder to dump gravel into a hole for a swimming pool, for instance, with bottom dumps.”

The Drivetrain

One of the big mistakes many people make with dump truck engines is they spec too much power, said Parlier.

“You should get just enough horsepower to do the job. Generally, 350 to 400 hp is plenty for most applications. Extra horsepower just uses more fuel, puts more strain on the rest of the drivetrain, and adds cost up front.

“If you go with a smaller 13-liter block, you save around 700 pounds over a 15-liter block,” Parlier said.

The transmission installed with the engine needs a lot of ratio range. You need a low enough gear to get out of a hilly jobsite and high enough top gear to attain decent highway speeds. The Eaton Fuller 8LL transmission is a common truck spec, but she suggested an 18-speed transmission for larger and heavier trucks.

“The 8LL gives you two low gears for startability off-road and enough top-end range for the highway. But if you are hauling over 90,000 pounds, you should consider an 18-speed because you get much closer splits from bottom to top.”

The typical dump truck uses rear axles rated at 46,000 lbs. (20,865 kg). This covers most trucks, from 14/16-yard solo dumps through combinations up to 110,000 GCW.

Another thing to remember for operating off-road is air filtration. Lots of drivers love the look of dual polished external air cleaners, and these provide excellent filtration with low air restriction. But they are quite expensive compared with under-hood air cleaners. A little money spent up front on a better air cleaner is cheap compared to a dusted engine. And better filtration will usually mean longer life for the filter elements. For example, dual 15-in. air cleaners will last more than 7 times as long as a single 11-in. underhood air cleaner before needing replacement.

Weight Watching

If you are hauling a lot of loads per day, cutting vehicle weight can be profitable. You can slim down by spec’ing components — such as wheels, air tanks and clutch housings — in aluminum rather than steel.

“Use the smallest fuel tank you can get away with,” Parlier added. “Some operators can get away with a 56-gallon tank, but most will need at least 75 to 90 gallons to get through a day.”

You can also save valuable pounds by choosing the right suspension.

“The difference can be as high as 400 pounds,” Parlier said of the different suspension options.

To avoid hauling around extra steel in the vehicle frame, have the dealer work with a Kenworth application engineer so that you only get enough frame where you need it. You will typically need a strong crossmember at the back of the cab to strengthen the hoist mounting area. If you are planning to add lift axles later, make sure the dealer adds that information to the order so that the frame can be prepared for them.

But remember that many of these weight savers will cost more up front. You will need to balance that against the gains you expect to make hauling more payload.

Driver Performance Items

Finally, consider a few driver performance-related items. To get the best turn performance and road feel from steering, Parlier recommended dual small gears rather than a large single steering gear. The dual system also will last longer than a single system.

Try to spec as much glass area as possible.

“Kenworth DayLite doors give a larger glass area in the main door window,” Parlier said. “You also want plenty of mirrors.”

On a topic related to windows, Parlier suggested picking low-replacement cost windshields when they’re available.

“Most vocational fleets replace at least one windshield side per truck per year. Two-piece flat-glass windshields with roped-in seals can be replaced in half an hour for a total cost of under a hundred dollars. This can save thousands of dollars over the life of the truck.”

With lift axles, it’s smart to get a six-channel ABS system.

“Lift axles, especially steerable ones, are normally over-braked for the load,” Parlier said. “By including them in the ABS system, they’re less likely to lock up and you reduce tire flat spotting.”

To enhance truck productivity and the driving experience, consider adding a navigation system Parlier said.

This story also appears on Truck and Trailer Guide.