InFocus
infocus: 38 Commercial Carrier Journal April 2009 COMMON RAIL INJECTION D iesel engineers know increased pressure cuts particulate matter. Now, as engine manufacturers cope with the need to use high levels of exhaust gas recirculation (EGR) for NOx control, having the best possible injection system performance is more important than ever. With unit injectors, the rpm-pressure relationship is strictly linear, says Tim Shick, director of business and product strategy at International Truck and Engine Corp. This means that as the engine slows down, the pumping rate in real time, along with the pressure, drops precipitously. If injection pressure is 32,000 psi at 1,800 rpm, it will drop to less than 20,000 psi at 1,100 rpm. Shick says that even with Navistar's decision to use enhanced EGR to meet the U.S. Environmental Protection Agency's tighter 2010 emissions standards, We are going to hold the line on soot. The biggest single contributor to that ability, he says, is high-pres-sure common-rail injection. We won't just be increasing the amount of exhaust going into the cylinder, Shick says. When we are increasing the EGR rate, we are increasing the injection pressure at the same time. Injection pressure equalizes the soot production. The present system on the MaxxForce Big Bore 11- and 13-liter engines produces in the range of 26,000 psi, but for 2010, the pressure will be more than 30,000 psi. Shick says it's important to keep in-cylinder soot from increasing so that oil change intervals won't shorten. With high-pressure common-rail injection, You get 30,000 psi right after startup, at 800 rpm, he says. Common rail substi-tutes a large-diameter high-pressure pipe which Shick refers to as an accu-mulator and a high-pressure piston pump for the barrel and plunger of the unit injector. The pump is gear-driven off the crankshaft and keeps the rail full at the desired pressure, under ECM control. The pump has sufficient capacity to keep the rail at the desired pres-sure even at low rpm. Unused fuel is bypassed, but that amount is small, Shick says. Common rail is an old idea that goes back to Rudolf Diesel's original work. In the past, the materials and science necessary to build such high-pressure common-rail systems did not exist, Shick says. One of these problems was the need for the high-pressure piping needed to carry extreme pressure all around the engine, which was not available hence the industry's development of steadily improving unit injectors over the years. Detroit Diesel says it actually side-stepped the piping problem; its ACRS Amplified Common Rail System used on the DD15 sends low-pressure fuel around the engine, then amplifies it to desired levels for injection using a double-ended piston near each injec-tor. Fuel is pumped up to 13,000 psi by the fuel pump and then injected at 32,000 psi, with up to five injec-tions per cycle, says Keith Harrington, Freightliner Trucks' product manager. These are not the engines of yester-day, Harrington says. The torque band starts at just past 1,000 rpm, and extends for 600 rpm instead of the old 300-rpm window. Detroit Diesel says it also substituted an energy-recycling turbo-compound-ing system on its DD15 for the variable geometry normally used to build back-pressure for the EGR system. Common-rail fuel systems generally have an accumulator full of fuel, like the long object on the left that runs along the side of the engine. The accumulator feeds the six injectors located to the right. A not-so-common engine design yet Tighter emissions rules lead OEMs to an old idea By John Baxter Higher EGR levels require the best possible injection system.