It’s always been hot under the hood of a big truck, but it can be more so with aerodynamic designs and, more recently, new diesels that run hotter than before. To cope with higher temperatures generated by their emission-fighting hardware, most engines now require extra cooling that can put more strain on hoses and belts. Meanwhile, many truck owners are keeping vehicles longer, which can make spec’ing premium hoses a sensible idea.
Prior to these new engines, some truck makers were alarmed about the prospects of additional under-hood heat. Tests have shown it’s not as bad as they feared, and engineers concluded that no changes were needed in hoses or belts. Manufacturers agreed, largely because rubber products had already improved to cope with higher temperatures under the sloping, sometimes narrower hoods of aero models.
Helping with heat dissipation on new truck models is higher under-hood air flow from more powerful fans. Special baffles direct air to hot spots. Heat shielding protects sensitive parts, while others were redesigned to live in the higher temperatures. Examples are windshield-washer-fluid reservoirs, which should be vented to avoid boiling, and alternators, some made of tougher materials to resist heat’s debilitating effects.
How hot is hotter? Testing by Leece-Neville Heavy Duty Systems shows its products must now withstand under-hood temperatures of 110 degrees F, versus 93 F before. So its alternators are made with tougher varnish and wiring insulation and more efficient fans to carry heat away, according to Dan Bartman, the company’s new products development engineer.
Testing by Flexfab LLC shows that double turbochargers on Caterpillar’s ACERT diesels compress air to where its temperature can go as high as 450 F – 75 to 100 degrees hotter than before, says Rod Ward, Flexfab’s director of sales engineering.
Higher turbo boost pressures were a concern for a time, but in-service engines have yet to exhibit any problems with the hoses leading into and out of the charge air cooler.
But the hoses and double turbos themselves now radiate more heat, so adjoining parts must be protected. That’s also true of the jacketwater aftercoolers that remove some of the Btu’s from recirculated exhaust gas on Detroit’s Series 60s. These things introduce more heat than you used to find in that area of the engine compartment, so wiring and hoses must be shielded or wrapped with reflective materials. Engine cooling systems are getting better at carrying away more heat, so engine operating temperatures remain at 210 to 220 F as before.
Hoses carrying exhaust gas to the jacketwater aftercoolers can see temperatures as high as 500 F, says Flexfab’s Ward. To stand up to the heat, the company’s hoses have Viton fluorocarbon liners and Nomex woven-fabric reinforcement. Ordinary coolant conduits, such as the radiator upper and lower hoses, are probably okay as provided by the truck builder. These are usually so-called EPDM (ethylene propylene dienemonomer) hoses, which are popular because they cost less than silicone hoses, but still resist coolant loss from permeation of water molecules through the hose walls.
Silicone hoses are known for their longevity, but EPDM hoses can last a long time if secured by spring-type or constant-tension clamps. Kenny Bridges, marketing manager for heavy-duty fleet products at Gates Rubber Co., says field tests by a major leasing fleet saw some EPDM hoses last a million miles.
Silicone hoses are worth some extra money in certain applications. Bridges says they might be called for in certain high-heat locations, like near a turbocharger, or somewhere that’s hard to get to. Ward says silicone – a hard material made of granite and beach sand – gives hoses extra strength. They’re now being spec’d by fleet managers wanting to run their trucks longer and still avoid maintenance, one of the objects of the short trade cycles that became popular in the ’90s.
However, Bridges notes that silicone hoses are sometimes prone to ‘co-coolant’ leaks and seepage where the hose is attached to a metal stem due to the two materials cooling off and contracting at different rates. That’s another reason why EPDM hoses are popular, and remain entirely adequate for most uses, he thinks. EPDM hoses are standard in most applications. Like Flexfab, Gates makes both types of hose and happily supplies either.
Behr America’s Tony Ackerman says, on average, heavy-duty big-bore engines will be rejecting 15% to 40% more heat than their predecessors, creating increased cooling needs at low engine rpm and increased pressures in charge air coolers. Many engines will have higher pressure radiator caps than before, and top-tank coolant temps will be up too. Higher pressures promote leaks in cores, hoses and fittings, so PM services and daily checks should focus on leaks, he said.
Drive belts have evolved in recent years from the narrow deep-V type to flat, ribbed types wrapped serpentine-style around two or more accessories. A typical flat belt has six to 14 grooves to handle high power and dissipate heat, Bridges explains. They’re kept in proper tension by sprung pulleys and thus offer superior life. But they won’t live forever without a little care and attention.
Among the factors that will affect belt life are more frequent cycling of fan clutches and the higher power demands of the large 34-in., 11-blade fans. To avoid premature belt failure under the dramatic load when the fan kicks in at 1500 rpm, operators should pay particular attention to belt condition and tension.
Bridges says a good belt-drive system, properly maintained, should give 60,000-mile V-belt life and 150,000 miles with the new V-ribbed belts. Automatic belt tensioners should go for 350,000 miles. Maintaining factory-recommended belt tension is critical, as is proper belt alignment, especially on ribbed drives. Belt tensioners should be checked regularly to see if they spin free and are wobble-free. A properly maintained belt drive system should be noise and vibration-free. H
A Case of Clamps
When you run through your cooling system checks, take an especially close look at your hose connections for evidence of ‘cold leaks’. These happen when hoses contract slightly as the engine cools, leading to coolant loss. Here’s a strategy to help you tighten up your clamp inspections:
1. Clamps must be of the correct size, which means both circumference and width.
2. Clamps that are too long or short can lose their grip and allow the hose to blow off.
3. A clamp that’s too narrow can cause the hose material to split under pressure.
4. A clamp that’s too wide can extend over the raised bead near the hose’s end, causing it to lose its grip as the hose heats and cools.
That raised bead must also be avoided when positioning the clamp on the hose. The clamp should be close to or against the edge of the bead, but not on top of it. Nor should the clamp be very close to the hose’s end, because it can let the hose slip off and coolant blow out.
Band-type clamps, including the screw kind – not wire-type – should be used with silicone hoses. Clamps should be of stainless steel, and any rust means the clamps are cheapies. Replace ’em.
The screw on a metal clamp should be torqued to a certain value. Manufacturers publish torque specifications in lb-in., usually ranging from about 4 up to 75 lb-in. depending on the type and size of clamp and hose. Find out what torque value should be used and follow it. Then re-torque it, or at least tighten it, after a short time to make up for ‘setting’ of the new hose and clamp.
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