The early GM Duramax 6600 Diesel.

During the late 1980s and through the 1990s, even the most die-hard GM truck aficionado had to admit that when it came to diesel power, the competition from Ford and Chrysler had the General beat hands down.

Though GM was the first domestic manufacturer to factory install a diesel engine in a full-size pick-up (For a number of years the aftermarket sold diesel conversions and the International Harvester Corporation (IHC) offered a Nissan diesel in its Scout II in both SUV and pick-up versions in the early 1970s.), it was the Oldsmobile sourced 350 (5.7 liter) design that proved not so worthy. To the thinking of many, this engine is still responsible for the public’s rejection of compression ignition for passenger cars.

The 5.7-liter V-8 was followed by a Detroit Diesel influenced (not designed) normally aspirated V-8, first in 6.2-liter form and then enlarged to 6.5 liters. Eventually, the 6.5 liter version was fitted with a turbocharger in some applications. That made it a much worthier competitor. These power plants were light years ahead of the Oldsmobile design and much better suited for the rigorous existence an engine finds under the hood of a truck. But they too were not recognized as the most efficient or reliable engines. A stark contrast to what conventional wisdom would attach to a fuel burner. Cracked and warped cylinder heads, connecting rod failure, and general fuel system issues topped the list of customer complaints.

The ante was raised when Ford working in conjunction with IHC developed the turbocharged and then intercooled, electronically controlled Power Stroke. Dodge not to be left behind, worked with Cummins to refine the B-series engine with first an intercooler, and then a four-valve cylinder head and electronic engine controls. Sadly, GM missed this opportunity and sat on the sidelines trying to pedal its uninspiring engine.

As in any free economy, the loss of market share and thus, profits to the competition is often the best impetus for change. General Motors recognized that it wanted a larger share of not only the commercial truck market, but also the burgeoning personal use segment. For this a new and state- of- the- art diesel engine would be required.

The problem was that GM no longer had any real in-house diesel expertise since during the restructuring of the 1980s, sold its world renown diesel engine division to Roger Penske (Detroit Diesel was eventually purchased by Damiler Chrysler shortly after the acquisition of Freightliner, Sterling, which was Ford heavy-duty trucks, and Western Star). The famous Detroit Diesel division was no longer theirs. The largest automaker in the world did not have a true diesel engineering arm. But there were some resources still at hand.

A relationship with Isuzu Motors Ltd. of Japan, was formed in the late 1960s when GM took over partial ownership of the company. The compact Chevy LUV truck (A bit of trivia: this stood for light utility vehicle) was the first product of the venture. Originally sold in North America with only a gasoline engine, the fuel crisis of 1979 brought with it a small, normally aspirated diesel engine option. The LUV was badged as a Chevy but was built by Isuzu in Japan.

During that era Ford made a similar investment with Toyo Kogyo (Mazda) and Chrysler with Mitsubishi Motors. Vehicles developed and marketed here from the partnerships were qualified as Captive Imports. The LUV, Ford Courier, and Dodge Ram 50 eventually were replaced by the higher quality and more durable Chevy S-10, Ford Ranger, and Dodge Dakota.

Fortunately for GM, Isuzu was much better at designing and manufacturing diesel engines than they were at making vehicles. The Japanese company’s cars and light trucks were never accepted too well in any Western market, but the engine division sold powertrains to equipment manufacturers around the world. Isuzu had compression ignition engines ranging from fifteen horsepower all the way up to 30 liters.

In 1997, Isuzu Motors Ltd. and General Motors created DMAX, Ltd., a joint venture for producing diesel engines. The Duramax 6600 was the first fruits of that relationship.

Stating from a a blank sheet of paper, the Duramax is recognized to date as the fastest diesel development program in the world. From concept to initial production in only 37 months. This included the construction of a brand new “green field” manufacturing plant in Moraine, Ohio. The 600,000 square foot facility resides on 40 acres and includes four machining lines, a heat treat facility and engine assembly. In addition, machining of the cylinder blocks, cylinder heads, crankshafts and connecting rods are accomplished in Moraine. The factory currently has capacity to produce 105,000 engines per year. Duramax production began in the summer of 2000, and the project was a collaboration of engineering talent from GM and Isuzu. Each taking the lead in their respective area of expertise.

Size Matters

Gasoline engines had dominated the full-size pick-up class. General Motors’ goal was to design a diesel engine that would take up no more space than the gasoline engine it would replace as an option. Because a diesel requires additional equipment such as a turbocharger and injection pump, this needed to be acknowledged during the early stages of development.

Due to this requirement the architecture of the engine would include the following:

The turbocharger, fuel injection pump and fuel piping would be installed in the V-bank for a smaller engine profile.

The oil cooler was deemed to be directly attached to the left side of the cylinder block. Coolant would be distributed to the left and right banks through a water passage in the flywheel housing. This simplified the coolant piping and would also increase reliability by facilitating even temperatures for both sides of the engine.

Auxiliary equipment, such as the air conditioner compressor, the power steering pump, and the alternator, would be driven by one serpentine belt. The mounting brackets for the accessories would be located on the front surface of the engine for ease of in-field service and assembly plant installation.

The criteria for the physical engine size would be a daunting task to meet due to the performance goals that were established for the Duramax.

The 6600 is a direct injected, turbocharged, intercooled, four-valve per cylinder V-8 that produces 300 horsepower and 560 ft-lbs of torque. All of the components that were required to meet the power and durability goals if not designed properly, would end up commanding a space premium that would mean the engine would not fit under the hood of a GM pick-up truck.

Through a team effort that included not only the engineers from GM and Isuzu but the factory management and hourly workers, all goals were met with little if any compromise.

Engine block and cylinder heads

The rigidity of the cylinder block was a concern not only for durability, but for noise suppression as well. This was accomplished with a deep skirt structure and by connecting the bearing caps to the lower part of the block with side bolts, in addition to the two main bolts. The Duramax block is manufactured form a special gray iron alloy.

The upper portion of the block is a closed-type and has a structure designed for minimal deformation at the high combustion pressure of a diesel engine. Another goal was to decrease the width of the lower portion of the engine.

Minimizing bore distortion was important for maintaining the sealing performance of the piston rings and to ward off engine seizure. This was done by evenly positioning six, cylinder head bolts around each bore, by evenly distributing coolant around each independent cylinder, and by arranging cooling water holes on the cylinder head deck surface. The six head bolts around each cylinder also help the gasket withstand the high in-cylinder pressure of a direct injection turbocharged engine. The upper portion of the linerless cylinder bore is induction hardened to reduce wear.

The cylinder head is made of gravity-casted aluminum and has a four-valve design for good volumetric efficiency and blowdown during the exhaust cycle. The fuel injector is located in the center of the combustion chamber along the cylinder line. A stainless steel sleeve is pressed into the cylinder head and the fuel injector is inserted in it. The arrangement of the valves is categorized as a “twist type”. This describes the two independent intake ports that are designed to maximize tangential flow for optimum intake swirl.

Adequate and even cooling of the valve seats greatly contributes to minimizing any change in the valve lash. There is a large space between the valve seats that uses the cooling ability of aluminum to reduce the heat deformation. An aluminum cylinder head has higher heat conductivity, so it has a lower temperature on the surface of the combustion chamber when compared to a cast iron design. With a cast iron test cylinder head, the exhaust valve seat temperature was 320 to 350 degrees C. With the aluminum cylinder head, which also benefited from a larger space between the valves, the temperature was maintained at 220 degrees C or less. The lower temperature enhances durability of the valve seats and increases reliability of the engine as a whole.

The head gasket has a steel laminate structure composed of three stainless steel plates. Each plate is flourine coated. Steel laminate deteriorates very little over time and is highly heat resistant, so it can withstand high in-cylinder pressure. Six M14 head bolts are used per cylinder. These provide adequate surface pressure for both the bore seal and coolant seal portions. The optimum torque for the bolts was determined by extensive testing.

The head bolts are a torque-to-yeild design that can not be reused once removed from the engine. This method is common with many newer engines. It places the bolt in what is referred to as the plastic range. The theory has the bolt tightented to a desired specification and then with a torque angle gauge installed, turned a prescribed number of degrees.  This eliminates the large variation of axial force that occurs with a traditional torque to specification head bolt, and will guarantee good sealing and little possibility of in-field service issues.

Rotating assembly

In respect to the high cylinder pressure of a 300 horsepowerdiesel engine, the pin/journal diameters of the engine are 63/80mm. The steel is forged using a 90-degree plane twist method. The entire crankshaft is also treated to the Tuftriding process for additional strength.

An eight mass couterweigth method was chosen, and a balance weight is added to the crank pulley and the flywheel in order to keep the crankshaft as compact as possible. There is an oil passage from each journal to the connecting rod pin, which lubricates the connecting rod bearing for each cylinder. The journal bearing is a Kelmet design with an overlay. The total thickness including the steel backing is 2.5 mm.

Two gears, one for driving the camshaft and another for driving the oil pump, are attached to the front end of the crankshaft. The gear used for driving the oil pump has a 57-tooth disc attached, which is used to generate a pulse for the crankshaft sensor. The flywheel is attached using eight M16 bolts on an 82 mm pitch circle diameter. An axial seal is used for the oil seal at the front and rear of the crankshaft in order to resist deterioration over time.

The piston is made of high-silicon aluminum alloy through a gravity casting process. To reduce noise, a cast-in strut is employed. The three piston rings consist of two compression rings and one oil control ring. The top ring is a barrel design. A Physical Vapor Deposit (PVD) is used to put a negative ion plating on the surface of the top ring. This coating provides excellent durability. The second ring is undercut and the oil ring has a backup spring. A ring carrier is cast into the top ring groove, to minimize any wear of the ring grooves. To accept the high thermal load, an oil cooling spray wets the underside of the piston.

The piston pin diameter is 34.5 mm, which is large enough to accommodate the loads to be placed upon it. The connecting rod is made of forged steel. A fracture division method is used on the larger end of the connecting rod to improve precision in fastening the two parts together. Because this is a structure in which the cap portion is rejoined exactly with the upper section, the shape of the bearing housing has improved accuracy.

Valvetrain

The overhead valve mechanism minimizes friction and wear by using a pair of rocker arms for each set of intake and exhaust valves. Each forged steel rocker arm uses a bridge to operate two valves. The bridge does not have a guide because this component complicates adjustment of the valve lash.

The portion of the rocker arm that touches the bridge has a sintered iron tip, and the bridge has a hardened steel cap at the contact point. This combination has good wear resistance to minimize any changes in valve adjustment.

The steel camshaft’s surface is induction hardened and the forged tappet is carburized. A roller is used at the bottom of the tappet. It rides on the camshaft and reduces internal friction and increases wear resistance. The pushrod is made of steel pipe. Because both ends of a pushrod have a tendency to wear, extra material was added to those areas. Also, lubricating oil is forced through the hollow shaft of the pushrod to help minimize the lash change over time.

Cooling system

The cooling system employs a path that first directs coolant to the back of the engine, and then around the cylinders toward the front of the engine. The coolant travels from the water pump to the oil cooler which is attached to the left side of the cylinder block. The coolant goes through a passage in the flywheel housing in the rear of the engine toward the left and right sides of the cylinder block. After coolant flows forward around the cylinders and up to the cylinder heads, it then goes through a tube at the front of each head and to the thermostat housing. Dual thermostats are employed, with opening temperatures of 82 and 85 degrees C. This method increases the accuracy of temperature control in the engine.

Fuel injection

The design team decided on a Bosch common rail system that consists of a supply pump, a function block, along with a common rail and a fuel injector for each cylinder. The pump is driven at crankshaft speed. The pump has a gear-type feed and a rail pressure control valve. An advantage of this design is that injection pressure can be raised independently of engine speed. This means the size of the nozzle hole in the injector can be reduced, which, in conjunction with the high operating pressure, makes for better atomization and faster combustion. Pilot injection is used to limit tailpipe emissions and reduce combustion noise.

Market acceptance

The Duramax quickly gained the respect and acceptance of the diesel enthusiast and agricultural market. The engine has been extremely trouble free and devoid of embarrassing recalls and service issues. In addition, it was unequivocally the quietest diesel engine of its size sold in North America when first introduced.