Belts & Hoses
More heat under the hood raises concerns, but testing allays fears
Tom Berg
Equipment Editor
It's always been hot under the hood of a big truck, but it can be more so with aerodynamic designs and, more recently, post-October diesels that run hotter than before. 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 more sensible.
Prior to the new engines' availability, some truck makers were alarmed about the prospects of additional underhood heat. People at Peterbilt Motors were among the more vocal. But tests showed it's not as bad as they had feared, and engineers concluded that no changes were needed in hoses or belts. Other manufacturers agreed, largely because rubber products had already improved to cope with higher temps under the sloping, sometimes narrower hoods of aero models.
Also helping on 2003-04 truck and tractor models is higher underhood 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 underhood temperatures of 110 degrees Fahrenheit, versus 93 degrees before. So the company's alternators now have tougher varnish and wiring insulation and more efficient fans to carry heat away, according to Dan Bartman, newproducts development engineer.
Testing by Flexfab LLC shows that double turbochargers on Caterpillar's upcoming ACERT diesels compress air to where its temperature goes as high as 450 degrees F — 75 to 100 degrees hotter than before — says Rod Ward, Flexfab's director of sales engineering. Higher turbo boost could strain the hoses leading into and out of the charge air cooler, but tests by Freightliner LLC showed it wasn't a problem. Al Pierson, Freightliner's director of vehicle testing, says that pre-ACERT hoses stand up to the higher pressures and heat.
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 Series 60s, Pierson says.
These things introduce more heat than what used to be in that area of the engine compartment, so wiring and hoses must be shielded or wrapped with reflective materials. While coolant carries away more heat, engine running temps remain at 210 to 220 degrees as before.
Hoses carrying exhaust gas to the jacketwater aftercoolers can see temperatures as high as 500 degrees 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 OK as provided by the truck builder. These are usually so-called EPDM (ethylene propylene dienemonomer) hoses, which have been popular for about 20 years because they cost less than silicone hoses and 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 are 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 — 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.
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 are kept in proper tension by sprung pulleys, and thus offer superior life. Thus there is little reason to spec something other than the builder's standard belt system. You may find there are no options anyway.
