Symposium on International Automotive Technology 2009 1Robert D. Leopold Allison Transmission, Inc., Indianapolis, USAA New Definition of “Fuel Efficiency” in Commercial VehiclesSAE Paper No. 2009-26-026 Copyright © 2009 The Automotive Research Association of India, Pune, India ABSTRACT This paper will demonstrate that, contrary to the usual expectations, commercial vehicles with Automatic Transmissions (ATs) provide dramatically better acceleration performance than commercial vehicles withManual Transmissions (MTs) or Automated Manual Transmissions (AMTs). It will be shown yet further that, in dynamic conditions of acceleration and deceleration,Automatic Transmissions can provide significantly higher fuel efficiency than MTs or AMTs. Finally, it will be shown that the traditional metric of “fuel consumption”,i.e., liter / 100 km, as it is usually applied to commercial vehicles, is neither an appropriate nor a realistic measure of fuel efficiency. INTRODUCTION It is our observation that most people expect an Automatic Transmission (AT) to be less efficient than a Manual Transmission (MT) or Automated ManualTransmission (AMT). Technically, they would likely cite inefficiency of the torque converter, higher parasitic losses of the planetary gearsets, and the “drag” of unapplied wet clutches; all this contrasted with the low steady-state mechanical losses of MTs & AMTs, with their countershaft (layshaft) gear arrangement, closed clutch, synchronizers or dog clutches for shifting, andminimal oil pumping requirements [1]. In many areas of the world, these expectations are reinforced by experiences with passenger cars, where manufacturers are compelled to publish their fuel consumption and, often, performance data. From thesedata, people are accustomed to seeing that automatic transmissions in passenger cars provide somewhat lower performance and somewhat higher fuelconsumption than manual transmissions or AMTs. Thus, the Automatic Transmission (AT) is generally perceived as a device which provides convenience andsimplicity of operation, but which has the inherent drawback of lower mechanical efficiency in taking the engine’s power and translating it into vehicle motion. Note: In this paper, an Automatic Transmission (AT) is defined in the customary way as a transmission systemconsisting of a torque converter (with lock-up clutch) and a stepped-ratio transmission which is shifted automatically, electro-hydraulically via wet clutches,without interruption of power to the drive wheels (i.e., power-shifting). A Manual Transmission (MT) is taken to mean a transmission system of the customary architecture of dry launch clutch and stepped-ratio transmission, shiftedwith an interruption of power to the drive wheels. An Automated Manual Transmission (AMT) uses the same mechanical architecture as the MT, but shifting (andusually clutching) is done by means of an automating system (usually electro-pneumatic or electro-hydraulic, electronically controlled) instead of a human driver. Inshort, exactly as its name purports, an AMT is a MT which is “driven” in a consistent and tireless manner by automated controls. Thus, for the purposes of this paper, MTs & AMTs will be treated alike, as they are essentially equivalent, thermodynamically. PERCEPTIONS BASED ON PASSENGER-CARS The Table shown in Fig. 1 is an example of what people expect from automatic transmissions in passenger cars. The table was taken directly from information published by Opel for its Zafira minivan [2]. Highlighted data which reflect the vehicle performanceand fuel consumption with the same engine, in one case with an MT and in the other case with a conventional 4-speed AT. It is evident that the automatictransmission produces a lower top speed, slower acceleration, and higher fuel consumption, both in city and in highway test cycles.Symposium on International Automotive Technology 2009 2Although ZF, Mercedes-Benz, and others have introduced more modern ATs for passenger cars,