Big Rig Basics
D iesel injection systems thrive on clean fuel. The tolerances how snugly the parts fit together are tighter in an injection system than any in medicine, the space program or any other field. Tight tolerances are necessary because the system works under enormous pres-sure and also meters the fuel, which means it measures the exact amount going in each time each cylinder fires. The injector plunger is a small cylindrical part that slides through a long, tubular hole drilled inside the body of the fuel injector. When the plunger is forced down by the camshaft and a rocker lever, it pushes the fuel in front of it and can gen-erate more than 30,000 psi. If much fuel were to leak around the plunger, it would not only take more power to generate the pressure, it would be impossible to ensure the right amount of fuel was get-ting pumped in each time. Injectors work really hard. In a trip of 1,000 miles of continuous highway cruising, each injector will fire about 692,000 times. Firing means injecting an individually metered dose of fuel with the help of the engine ECM. The plunger is not the only precision part. There's also a tight-fitting needle valve and spring at the bottom of the injector that keeps the nozzle from drib-bling fuel when the injector is supposed to be inactive. The injectors Cummins uses in the ISX don't have needle valves, but they do have a double plunger sys-tem to handle both the timing of the injection and the metering. And all injectors have extremely tiny holes in the nozzle at the bottom. These have gotten smaller and smaller in the last few years as injection pressures have risen. Research has shown, how-ever, that the tinier the holes and the higher the pressure, the less soot the engine will produce and the better it will perform. The top of the injector is lubed by engine oil. But the plunger itself and the needle valve are lubricated by the fuel they handle. And because of the tight tolerances and tiny holes, unless that fuel is absolutely clean, the moving parts will be scored or scratched and will quickly lose their snug fit because of wear, or the holes may clog. The plungers may even stick, which will stop an injector cold. So in order to protect injectors with tighter and tighter toler-ances, finer and finer fuel filtration has come to our industry. Primary and secondary filtration Trucks have a mechanical fuel transfer pump on the engine that draws the fuel from the tank to the engine, then gener-ates a pressure of about 40 psi to send the fuel to all the injectors. While the engine's fuel pump is not as vulnerable as the injectors, it still could be easily damaged by water. So all engines use a primary fuel filter, which means a filter in the line leading from the tank to the pump. The fuel flows through this filter because of the suction generated by the fuel pump. Filtration means moving fuel through a complex synthetic material, something like a very fine screen that grabs solid particles and holds them. The finer the screen, the greater the restriction it presents to the flow of fuel. So the engine and truck makers install a rela-tively coarse primary fuel filter on the suction side of the pump, where there is only suction, and add a very fine filter on the line running from the pump to the injectors, where there is plenty of pressure to work with. The primary filter removes larger particles that could dam-age even the relatively simple engine-driven fuel pump and, in many cases, is specially treated to remove water as well. Even though the very fine second-ary filter may generate some restric-tion, the positive pressure generated by the pump ensures good flow unless it becomes thoroughly clogged. Wix Filters' manager of technical ser-vices and customer training, Paul Ban-doly, likens the process to using sand 30 TRUCKERS NEWS MARCH 2009 BIG RIG BASICS Fuel Filters Maintain your fuel filter for the entire system's sake By John Baxter Change It! This technician is installing a Donaldson primary filter. Before installing any filter, thoroughly lubricate the gasket with clean engine oil and fill the filter all the way with clean fuel. Installing an empty filter will make the engine very hard to start. COUR TESY THE DONALDSON COMP ANY continued on page 82