Equipment World Digital Magazine
Issue link: http://read.dmtmag.com/i/548484
EquipmentWorld.com | August 2015 69 maintain emissions parameters, fuel economy and power output. DIESEL DYNAMICS A cylinder in a gasoline engine will consume one injection of fuel within 40 to 60 degrees of crank- shaft rotation. A diesel engine burn lasts much longer, from 90 to 120 degrees, says Serra. This slow, expanding explosion is what gives diesel engines their stump-pulling torque. Shaping and maximizing the effi ciency of this combustion plume is of paramount importance. Valve placement, piston bowl shape and the design of the injec- tor tip all infl uence how the plume circulates within the cylinder, Serra says. But injector timing and fre- quency are two elements that can change as demands on the engine change. In a typical low-power HPCR combustion scenario you might have three injection events, in sequence as follows: It starts with a small, quick pilot injection to get things going. Dur- ing light and medium engine load- ing early pilot injections also help control the formation of NOx (a pollutant regulated by Tier 4 Final) and reduce noise – that unmis- takable diesel engine "knocking" sound at idle. Next comes a full load, main injection for power. Six to eight events are possible to modify combustion or assist emissions aftertreatment. Finally you get a small, post injection to fl are off any unburned fuel remaining in the cylinder. Post injections also control particulates in the exhaust, provide extra en- ergy for aftertreatment systems and reduce turbocharger lag. When the application demands high power, the ECM will usually order one long injection. SPEED FREAKS Engineers measure the speed of these injection events in micro- seconds, which is 1/1,000th of a second. There is a window of approximately 7,000 microseconds for all the injections to occur, dur- ing which time: The injector solenoid or piezo crystal actuator begins to open within 100 to 150 microseconds of being energized. With a three-injection event, each injection delivers a measured quantity of fuel at approximately 1,225 times per minute at idle (750 rpm) and up to 3,300 times per minute at rated speed (2,200 rpm). In a six-injection event each injector can deliver bursts of fuel at up to 6,600 times a minute at 2,200 rpm. After the injection event, it takes another 50 to 100 microseconds for the solenoid or piezo crystal actuator to return to rest and dis- sipate any electrical charge. COMPUTER CONTROL "The on-engine electronic control module manages all aspects of the fuel system control," says Fier. "The ECM not only contains the electronics needed to actuate con- trol valves and injectors, but also contains the engine calibration and diagnostics. It is basically the brain Freed from having to pressurize its own fuel, the electronically-controlled injector can fi re as fast as you can turn an electric current on and off. HIGH PRESSURE FITTING VALVE GROUP ARMATURE GROUP MAGNET GROUP