Engine Spotlight

By John Baxter | January 03, 2011

15 liters of efficiency

Cummins’ ISX has been around for more than a decade, but recent changes turn heads for durability and fuel-mileage enhancement.


The Cummins ISX was introduced with much fanfare in 1998. It was designed on a clean sheet of paper, following years of the reliable N14, derived from the H and NH engines that helped build Cummins Engine Co.’s success. The new engine featured dual overhead cams with a separate injector camshaft, making the engine brake more powerful and preparing the engine for the extreme fuel system pressures of the future.

The high-pressure fuel line passes through the cylinder head and connects to the injector of a 2010 15-liter ISX. Since pressure is supplied by the XPI fuel system’s common rail, the injector is no longer camshaft-operated, but just a valve opened and closed by the ECM.

A number of significant changes have been implemented since the engine’s introduction to effectively adapt it to the stresses of exhaust gas recirculation. These changes also enhance the engine’s potential for long life, meaning smart owner-operators will want to take a fresh look.

After EGR was introduced in 2003, Cummins hardened the ISX’s cylinder liners, producing greater wear resistance. Engineers also redesigned the connecting rods to provide pressure lubrication of the piston pins, which helps eliminate wear of the pin and bushings when exhaust is added to the air charge, increasing the load the piston needs to carry.

For 2007, as some makers needed to beef their engines to adapt to higher levels of EGR, Cummins held the line on heat and pressure stresses, says Steve Charlton, chief technical officer of engine engineering. This was done by first redesigning the combustion chamber so the engine could keep particulate levels under control with less air. Another change improved the EGR cooler, lowering the temperature of recycled exhaust.

The 2010 engine – available in Freightliner, Volvo, Peterbilt and Kenworth trucks – continues with the same combustion chamber and also sports an improved EGR cooler. In addition, the EGR valve was moved to the downstream side of the cooler. Since the gases it handles are in the range of only 200 degrees Fahrenheit rather than well over 1,000, life and reliability are enhanced.

For 2010, after finding that copper-zeolite catalysts convert oxides of nitrogen (NOx) with a minimum of diesel exhaust fluid (DEF), Cummins decided to adapt selective catalytic reduction to meet the 2010 NOx standard of 0.2 grams/hp-hour, according to Charlton.

In a joint venture with Scania, the company had already developed a new common-rail injection system, named XPI for its ability to produce extreme pressures. This advanced fuel system produces pressures well over 30,000 psi even at operating rpm below the torque peak and offers designers fuel system flexibility never before seen. The availability of extreme pressures at low rpm enhances the engine’s torque and horsepower characteristics and also reduces emissions.

“The system allows the engine to start really well,” says Customer Support Technical Director Zack Ellison. “They start right up without aids even in central Canada in winter.”

XPI also allows each injection cycle to be broken into separate shots of fuel, a subtlety not possible with the ISX’s original simple and serviceable injection system. Ellison reports this reduces combustion noise by allowing the ECM to change the number of injection pulses for each combustion cycle, depending on load and rpm.

The XPI crankshaft-driven pump also eliminates the larger injection camshaft and the unit injectors of the original ISX, which means a significant reduction in mechanical noise and engine weight.

“The transition was a significant improvement over 2007,” says Ellison.

Injection timing, too, has been advanced and EGR rates lowered. As a result, “the engine really likes running almost the way it was back in the ’90s,” Ellison says. With SCR allowing more engine-out NOx, there is the advantage that “NOx helps keep the soot cleaned out of the DPF, which means it is normally kept clear without the need to add fuel for active regeneration.”