GM Duramax Diesel
by Jim Kerr
Its here! General Motor’s long awaited new diesel truck engine is in production and being shipped to dealers. This 6.6 litre engine, available in GM’s heavy-duty 2500 and 3500 series trucks, is built in Moraine, Ohio, as a joint venture between GM and Isuzu. Trying to
recapture some of the diesel truck market from Ford and Chrysler, the Duramax engine sets new standards for the light-duty truck market. Here is how it achieves that.
The V8 Duramax engine uses a direct-injected, common rail fuel system. Direct injection is a type of diesel injection where the fuel is sprayed directly into the cylinder. GM’s last diesel engine design used pre-cup injection, where the fuel is sprayed into a small chamber that leads to the cylinder. Pre-cup design engines tend to be much quieter in operation than direct injection, because all ignition of the fuel takes place in the small pre-cup chamber. The direct injection design of the Duramax is noisier but GM is using pilot injection control to reduce engine noise.
Pilot injection is where a small amount of fuel is injected into the cylinder several degrees before the main fuel injection occurs. Pilot injection creates slight pressure in the cylinder to help the rings seal against the cylinder wall and push the piston toward the thrust or power side of the cylinder. This makes the engine operate much quieter and more efficiently. Stand beside a running Duramax engine and you will agree. It still sounds like a diesel, but it runs very quiet for a diesel.
The low rpm redline on the tachometer is a sure giveaway
this is a Duramax diesel. The engine develops so much torque at low rpm
it doesn’t need high rpm’s.
Two computers control the injectors. The engine control module (ECM) receives all the sensor inputs and calculates the amount of fuel to be injected. The fuel injection control module, mounted on the passenger valve cover, receives the injection data from the ECM and triggers the electric solenoids in the injectors. The injector electrical circuits are high current, with 93 volts and 20 amps of current to open the injectors. This is more than enough to cause serious bodily harm, so indiscriminate checks with a test light are no-no! The high current flow through the fuel injection control module generates a lot of heat, so to
keep it cool, fuel passes through passages in the housing on the way to the pump.
A camshaft driven pump is used for diesel fuel supply and typically provides 5000 to 6000 psi fuel pressure at idle and up to 23,000 psi at higher engine speeds. A fuel pressure sensor sends fuel pressure information to the ECM so the injection time of the electronically operated injectors can be modified as fuel supply pressure varies.
A couple more interesting aspects of the Duramax fuel system are the inclusion of a hand primer pump on the fuel filter housing to bleed air out of the system when filters are changed, and the addition of a fuel cooler just ahead of the fuel tank. The fuel cooler is a small radiator that cools the fuel as it returns to the tank.
The engine fills the engine compartment but appears easy to work on.
Performance is a must for any work truck and the Duramax has tons of it: 300 horses and 520 ft lbs. of torque. Maximum torque is reached at only 1800 rpm, so the engine pulls strong right from the start. Strong is perhaps putting it too mildly. This engine feels like it could pull a mountain and it accelerates fast too! Four valves per cylinder and an
intercooled turbocharger make sure the engine receives all the air it can use. This helps make all that power at low rpm’s.
Some of the engine’s features are aimed at ensuring durability. The waterpump is gear driven so a broken drive belt will not stop engine cooling. The engine oil cooler is mounted on the side of the engine and uses engine coolant to cool the oil. This is similar to many industrial diesels. The main bearing caps that hold the crankshaft in place are cross-bolted. Two bolts go in from the bottom and two more are threaded in from the side of the block to ensure the caps are held rigidly in place and crankshaft bearing longevity is achieved.
Starting a diesel engine requires the engine to turn over quickly or the air to be preheated. This is because the heat of the compressed air in the cylinders is the only way the fuel is ignited. An engine that cranks slowly doesn’t create enough compression to heat the air. The Duramax diesel uses glow plugs in the cylinders and electric heaters in the air intake to warm the intake air. The results were instant start-ups with no wait time before cranking the engine.
Diesel engines have been used for most of the last decade. Only in the last few years have electronic controls really enabled the diesel to excel in both power and fuel economy without producing lots of smoke or emissions. The Duramax diesel’s big advantage is in its electronically controlled direct fuel injection system. It sets a new level of performance that others will soon be targeting.