C420/84 SAE 841304 ereedes-Benz automatic transmissions for passenger cars H J FORSTER, Dr-lng, FSAE Formerly with Daimler Benz AG, Stuttgart, West Germany SYNOPSIS Transmissions have a great influence upon the operating mode of the engine and by this also upon car performance and fuel consumption. The determining factors are analysed and reassessed and recommendations for the transmission design are given. INTRODUCTION The continuously increasing demand for automa­ tic transmissions and the necessity to provide adequate production facilities gave us in the early 70's the chance to develop a complete new range of automatic transmissions for our pas­ senger cars. Because such an opportunity does not arise too often, we started with a new evaluation of the transmission's influence on power, performance and economy, the results of which, together with the description of the new transmissions MB-W4A 040 and MB-W4A 020, are presented. ANALYSIS The new transmissions had to cover the full mo­ del range with the data of table 1: - Power range from 50 kW to 200 kW - Engine torque range from 120 Nm to 450 Nm - Engine speed range up to 7000 RPM - Vehicle mass fully loaded from 1600 kg to 2200 kg, trailer 1200 kg to 1500 kg - Standard arrangement of the power train: engine in front, driven rear axle - Lower weight, smaller dimension, lower cost Table 1, layout data for automatic transmission Since we built the first automatic transmission in the 60's, we hold in our company a basic philosophy regarding transmissions which, once again, we had to adhere to: - The engine power has to be made available to the greatest possible extent as propulsion power for the vehicle. - Special operating conditions, even if rarely encountered, have to be taken into conside­ ration. - Good performance and fuel economy have to be combined in such a way that the result will be accepted by the driver and thus be effec­ tive in real traffic. - Foolproof automatic control with the possibil­ ity to be overriden by the driver. Because the new compact Mercedes, model 190, was already under development, high emphasis was put on low weight and small cross-section dimensions to narrow the distance between the two front seats. The necessity, for 2 sizes of transmis­ sion has been acknowledged at an early stage of the investigation. The 2 versions, however, should be interrelated in production as far as possible. Of course we again began with a study about the ability of CVTs to meet the various demands. . After a thorough detailed investigation of CVTs available, their possibilities and their re­ strictions, we decided, however, finally again in favour of the 'classical' version, that is converter plus multistep gearbox. Besides the highly valued absence of gearshifting in the CVT, no other advantages over the stepped gear­ box were found, but many drawbacks regarding efficiency, adaptability to a large power -range, weight, bulk volume and cost became evident'. - In my opinion, neither car function nor fuel saving demand a CVT. There may exist design concepts like the front drive with transversally mounted engine and transmission where a large distance between two shafts has to be bridged, where special CVTs offer a favourable design. With the standard arrangement of engine, trans­ mission in the front and the rear axle driven, that is to say our concept, the greatest objec­ tion against CVTs, however, is the negative answer to the question: Is it reasonable to take a transmission type which has to work hard all the time when there do exist other transmission types, which do not have to operate at all for more than 90 % of the time? (The CVT we fa­ voured in this study was of the chaindrive type because of its rather good efficiency}. The basic decision in favour of the converter- multiratio-transmission taken, we concentrated the further analysis upon its potentiality of further improvement.