Effects of Compliant Geartrains on Engine Noise and Performance

Geartrain rattle has been an issue in heavy-duty diesel engines for a number of years, affecting not only noise hut also engine performance and durability. A project at Caterpillar that completely removed the front geartrain from the engine (all ancillary devices and drove the camshaft from the rear) demonstrated a large noise reduction potential in heavy-duty diesel engines. This work prompted an effort to identify production viable devices that would reduce engine noise through geartrain compliance without affecting performance or the engine envelope. This article shows that compliant geartrains effectively reduce gear rattle (and hence noise), increase the life of geartrain components, and may even improve the performance of the engine.

Within the heavy-duty diesel engine community, gear rattle has been an issue for many years. Gear rattle is the phenomenon during which gear teeth come out of mesh and are forced back into mesh by a backside tooth impact. More complete descriptions of gear rattle, especially the modeling of the phenomenon, are described in the literature.1,2 Note, though, that while the automotive industry deals with gear rattle at certain conditions (normally low idle), mechanically injected heavy-duty diesel engines usually exhibit gear rattle at most operating conditions if not properly dealt with. This has been further exasperated by the need to reduce gaseous emissions in diesel engines, which has been partially accomplished with elevated cylinder peak pressures and injection pressures.A previous unpublished Caterpillar project demonstrated a large noise reduction by completely removing the front geartrain from a heavy-duty diesel engine (all ancillary devices and drove the camshaft from the rear). Thus, Caterpillar has focused its efforts on geartrain noise in the past few years. Traditional gear rattle literature suggests the following for gear rattle reduction: modified gear positioning, modified phasing, mass variation of gears, mechanical pre-load system, viscous dampers, changed number of teeth, modified engagement factor/helix angle, or modified backlashes.1'2 While these methods can be useful for fine-tuning geartrains, larger changes are necessary for significant noise reductions.

Several options were considered that parallel the investigations of Zhao.3 Specifically, the focus was on introducing additional compliance in the geartrain while maintaining cam to crank timing. Croker's relationship for rattle in heavy-duty diesel engine geartrains4 (Figure 1) shows a nonlinear jump where increased backlash quickly becomes beneficial. Tolerance restrictions in heavy-duty diesel engines can make it very costly to take advantage of tight backlash; therefore large backlash is the desirable region of operation. This concept of increased backlash was combined with nonlinear, frequency-dependant isolation principles to reduce rattle. While the specifics of the actual hardware remain undisclosed, this article will discuss the effects of this compliance on engine noise, gear loads and engine performance.

Geartrain Isolation

Due to the impulsive nature of reciprocating engines, crankshafts do not rotate at constant speeds, but instead often vary from 3-5% within one cycle of rotation. Additionally, the masselastic system of the flywheel-crankshaft-damper has torsional modes which, when excited, add to this speed variation as well. see Figure 2 for a typical time history of crank speed at a lug condition. Thus, for the first engine to be quieted, devices were devised to isolate the geartrain from this rattle-producing event. One such device effectively places a nonlinear, tuned torsional spring between the crankshaft and crank gear. The geartrain in Figure 3 for Engine A shows the location of the crankshaft gear, which was where this device was located.

Even though the geartrain was now isolated from the crankshaft torsionals, rattle still existed in the geartrain. The impulsive nature of mechanical fuel injection systems also causes the camshaft speed to vary 3-6%. This speed variation was causing the rest of the geartrain to rattle. To address the issue of camshaft induced geartrain rattle, another device was needed to isolate the geartrain from cam-induced excitation. This device isolated the rest of the geartrain from these cam torsionals. Note that further torsional activity comes from the pumps, but this study found that these pump torsionals were negligible in this geartrain.

The layout of the geartrain for the second engine on which a compliant device was pursued is shown in Figure 4 (Engine B). Note the significant differences between the two geartrains. Instead of separating the cam and crank by four meshes, this engine is only separated by two. Additionally, due to some size restrictions at the crankshaft, similar devices from the previous engine could not be applied. Instead, new devices were constructed that replaced both of the idler gears in mesh with the crank gear. The cam-to-crank idler gear isolated the camshaft and crankshaft from each other, as well as isolating the pumps from the crank and cam torsionals. The other idler gear isolated the pumps from the crankshaft torsionals. Although the method by which compliance was achieved for these devices differed from previous geartrain treatments, the general trends of the effects of isolation remained the same