MaxxForce Heavy Diesels... Smooth, Quiet, Gutsy

Sept. 28, 2010

The demo tractor was a TranStar 8600 with a strong and quiet MaxxForce 13 rated at 410 hp and 1,450 lbs.-ft. The 13's other ratings are 430 hp/1,550 lbs.-ft. and 475 hp/1,750 lbs.ft. MaxxForce 11's ratings are 330/1,250; 370/1,350 and 390/1,400.

Cutaway (left) shows the 12.4-liter MaxxForce 13's internal parts, including its single overhead camshaft, Bosch high-pressure common-rail fuel system (which operates at 26,000 psi), and rear-mounted gear train. Basic design comes from MAN of Germany, adapted to North America with 12-volt electronics (instead of Europe's 24 volts).

The cylinder block's ribs actas baffles to reduce noise. Compacted graphite iron is stronger and lighter than regular grey iron. A MaxxForce 11 or 13 weighs 366 pounds less than a Cat C13, but 38 more than a Cummins ISM.

Eco-Therm coolers dominate the scene under the 8600's short hood (above, right), but Inter-national says they come off fairly easily for access to other accessories and the engine itself. In longer-nose WorkStar (above), more of the engine is visible.

Eco-Therm Air Coolers

International's MaxxForce 11 and 13 share a basic design and many internal parts: cooled exhaust-gas recirculation, an overhead camshaft, high-pressure common-rail fuel system, double turbocharging, an integral retarder, and rear-mounted gear train. While some are elegant, other modern diesels have them, too.

But here's something we've not seen before: a so-called Eco-Therm liquid heat management system using twin water-to-air coolers and double radiators to chill inlet air. Lower-temperature engine coolant draws heat from the hot, compressed inlet air and carries it to the radiators, where heat is shrugged off. This eliminates the common, but sometimes fragile, air-to-air aftercooler mounted ahead of the radiator.

With Eco-Therm, inlet air goes through the cleaner and is compressed by the primary low-pressure turbo, then cooled by the first "interstage" heat exchanger. Air then is pushed into the secondary high-pressure turbo where it's compressed further, then cooled again by the second E-T heat exchanger. Each of those is plumbed to a separate radiator whose cores are stacked one ahead of the other behind the truck's grille. The two turbos are simple, not variable geometry, so are less costly to install and replace, but still provide the boost needed to support clean combustion and develop usable power at all speeds.

Eco-Therm includes a water control valve operated by the engine's electronic control unit. This alters coolant flow to speed warm-up of a cold engine and keeps exhaust hot to encourage passive regeneration of the diesel particulate filter. Active regens are done by injecting small amounts of fuel into the exhaust just beyond the secondary turbo; atomized fuel flows with exhaust to a catalyst which reacts by heating up, and the heat then goes into the adjacent DPF where it burns out soot.

As with most other truck diesels in North America, the new MaxxForce engines use exhaust-gas recirculation to displace oxygen and lower combustion temperatures. This limits the formation of NOx, a major pollutant that helps form smog. A water-to-gas cooler brings down the temperature of exhaust as it's piped to the intake manifold, and an EGR valve modulates the amount of gas sent to the cylinders.

It sounds complex, and it is. But International engineers have been testing the EGR and Eco-Therm systems — as well as everything else on the engines — trying to get them to break,and not much has, so far, anyway.

Double turbos and water-to-air coolersare intertwined in MaxxForce's Eco-Therm system. Inlet air flows through the primary turbo to the first "interstage" cooler, then to the secondary turbo and into the second cooler, and then goes into the cylinders. Each cooler has its own radiator behind the truck's grille.

It's interesting, but will it work? That describes the design of International Truck and Engine's new MaxxForce "big-bore" engine series, and poses a question that could come from any truck operator pondering the purchase of one. The short answer to that question is yes, at least so far. And motorheads can marvel at what's been engineered into the new diesels.

International's people are proud of the engines and more than willing to explain and demonstrate them, and did so during the recent World of Concrete show in Nevada. At the Las Vegas Speedway north of the city, trade-press reporters saw running examples of the new diesels and talked with customers who testified as to the solid performance of test engines in their hands.

The new MaxxForce 11 and 13, with displacements of 10.5 and 12.4 liters (641 and 757 cubic inches), respectively, have different bores and strokes but share an inline 6-cylinder block and most working features (see sidebar). The block is made of "compacted graphite iron" that claims high strength and low weight. Ratings go from 330 horsepower and 1,250 lbs.-ft. to 475 horsepower and 1,750 lbs.-ft.

While the engines are new to North America, they are based on a proven design from International's German partner, MAN Nutzfahrzeuge, briefers told us. Company executives had previously explained that they were pursuing their own Class 8 diesels to take more control of product development and pricing. This is a logical step, especially in view of an industry trend toward vertical integration in heavy trucks and International's long history of designing and manufacturing medium- and light-duty diesels. However, as of now, it will still offer Caterpillar and Cummins diesels in many of its Class 8 trucks.

That lightweight cylinder block gives a MaxxForce 11 or 13 a leg up on some competitor diesels. A MaxxForce 11 or 13 weighs the same — 2,244 pounds dry — and that's 366 pounds less than the 12.5-liter C13 (but 38 more than a 10.8-liter ISM). This was cited as an advantage by managers of two bulk-hauling test fleets who've been using C13s, where the MF 13's lower tare weight converts to more payload.

A third fleet, which runs heavy "Michigan train" dump trucks, is testing a 475-horsepower MF 13, which weighs 600 to 800 pounds less than a Cummins ISX and Cat 3406E in the fleet. At about 30,000 miles, the MaxxForce was not yet broken in, but was performing well. The engine is hobbled because it's mated to a 13-speed transmission and really needs an 18-speed to get the rig started, especially in reverse. The 11-axle truck and trailer grosses 148,000 pounds hauling salt and sand for road departments. All the test fleets, which include a beer distributor in Texas and less-than-truckload freight carriers based in Virginia and Illinois, reported that so far, the MaxxForces have also been reliable and economical with fuel. Drivers said they liked the engines' power, smoothness and quietness; one told us he liked his engine so much that he considered delaying his retirement.

Well, that made me really want to drive one, and I didn't have to be told twice to head for the demonstrator vehicles. All were highway types, including a ProStar sleepercab and a TranStar 8600 regional-delivery tractor. I chose the TranStar daycab, hitched to a 48-foot flatbed loaded with concrete beams, because it was closer to a vocational rig, and it had the larger MaxxForce 13, rated at 410 horsepower and 1,450 lbs.-ft.

The prescribed route included some of nearby Interstate 15, but there isn't much of an upgrade on this stretch. So I instead used Las Vegas Boulevard, which bisects the speedway complex and Nellis Air Force Base. Out here, it's a two- and three-lane highway that intersects with I-15 a few miles to the north. About a mile and a quarter of it is a moderate upgrade that gives any loaded rig a bit of a workout.

This rig's total weight was only about 59,000 pounds, so the engine didn't need to work too hard, but still showed how well it can pull. In top gear, we accelerated on the upgrade; I started at 50 mph at the bottom and topped the crest at 60. All the while the diesel remained rather quiet, with a purposeful hum suggesting but not proclaiming the work it was doing. And there was no vibration, even at lugging speeds. Briefers said that engineers spent a lot of time reducing noise, vibration and harshness in the engine, and it showed.

I turned around in a wide gravel and dirt area just short of I-15 and headed back down the hill, this time testing the engine's brake. It hydraulically operates the valves to make the pistons pump air and drag on the crankshaft and driveline. In two- and four-cylinder mode, it easily held road speed in check; and with all six cylinders retarding, it smoothly slowed the rig. The brake is barely audible inside the cab and probably the same outside, and there's certainly none of the loud throbbing or rapping that has caused municipalities to outlaw engine brakes. So it's likely that a driver could use it in No-zones without getting a ticket, but don't quote me on that.

Down at the speedway area I did a wide U-turn and went back up the hill, again testing the engine while watching the tach a little more closely. The engine ran smoothly at almost any rpm and didn't seem to care whether I "short shifted" to a higher gear at 1,500 or revved it a few hundred rpm higher. I didn't push it to its 1,900-rpm red line because it wasn't necessary. Vocational engines are governed at 2,100 to give them more flexibility, product planners said.

After the second climb up the boulevard I went under I-15's overpasses and turned south onto the freeway. The engine easily propelled us to cruising speed. At 65 mph, the tach read a tad over 1,500 rpm, indicating this engine was a "balanced" rating — one of three types available — or that the tractor was geared a little short for maximum economy. To get that, transmission and axle gearing should be calculated to let the engine cruise between 1,300 and 1,400 rpm. Vocational ratings put cruising rpm at 1,600 to 1,700.

Strong diagonal headwinds buffeted the rig on the I-road, and they howled as they whirled around the mirror head and the left-rear corner of the cab. But the engine remained quiet. A few miles south, I exited onto the access street that led past Carroll Shelby's Mustang modification works and the many motor-racing shops, and thence to the speedway.

After slowing for a speed hump, I somehow found myself in a higher gear than I wanted, but stayed there to see how the engine would lug. It did so without complaint, from just about the 700-rpm idle up to 1,500. It doesn't want to be lugged that low, but didn't shake or buck; at 900 I could put my foot into it, peak torque comes at an advertised 1,000, and the engine was gutsy from there up.

Workers at International's new plant in Alabama have just started on pre-production versions of the MaxxForce 11 and 13 diesels, and when they get the assembly procedures down pat, they'll go into regular production. So by summer, the MaxxForce heavies should begin appearing in new ProStar and TranStar highway tractors and WorkStar vocational trucks and tractors, which all have Diamond Logic multiplexed electrical systems that the new engines need to function. The 5000i severe-service models and 9000i road tractors lack multiplexing so won't get the new engines 'til later, if at all (they might be replaced by new models). Besides, those vehicles are usually ordered with bigger Cat C15s and Cummins ISXs.

International says it's ready to take orders for the new engines, but what'll they cost? "It's a premium engine, but it will be priced competitively," said one engine executive. OK, will a MaxxForce cost less than an ISM or C13? He danced a little, then allowed that International's truck side would likely price a MaxxForce somewhere between those two vendor products. Sounds about right to me.