Tech Talk Archives 6

[Neweller]

Q
700 7/26/2002 4:43:54 AM 7/26/2002 Tech Talk Staff TechTalk@newellcoach.com Transmissions 4,5,6 speeds? Transmissions allow the vehicle operator to run the vehicle engine either at higher rpm, for more power and acceleration, or at lower rpm, for better fuel economy but at less power. A transmission must have enough ratios so that it can match the desired range of engine rpm to road speed from initial acceleration from a stop to top speed. With more gear ratios, a transmission can keep engine rpm within a narrower band to a higher top road speed. With fewer (and therefore wider split) ratios to the same top speed, the change in engine speed between shifts becomes greater. However, an engine with a wider, flatter power curve from low rpm to high, doesn't require as many gears as closely spaced as an engine with a steeper power curve that needs to operate at a higher rpm to develop the desired torque. In addition, a transmission with few, more widely split gears does not have to shift as often, so the passengers perceive a smoother flow of power. When Newell produced coaches with Detroit Diesel 6V92 engines, they used an Allison 600 series 5-speed transmission. When Newell upgraded in 1984 to the 8V92 engine with more power, they determined that the heavier duty Allison 700 series 5-speed transmission required by the more powerful engine had relatively rough shifting characteristics. Therefore, they chose the smoother shifting Allison 700 series 4-speed transmission. The logic was that the additional low rpm torque from the 8V92 would allow for somewhat wider gear splits, the transmission would shift fewer times over the same duty cycle without an unacceptable loss of performance, and the passengers would experience smoother operation. The situation changed again in the mid-1990s with the introduction of the new Allison World Transmission. The shifting characteristics of this transmission are exceptionally smooth, with shifts almost imperceptible at times. Therefore, the transmission, which incorporates 6 speeds, allows the engine to operate with a narrow, more optimal rpm range by shifting more often, but with no loss in smoothness.