Browse Publications Technical Papers 2004-01-1617
2004-03-08

Pseudo-Empirical Efficiency Model of a Gearbox for Passenger Cars, to Optimise Vehicle Performance and Fuel Consumption Simulation 2004-01-1617

A gearbox model is proposed, to be used within vehicle performance and fuel consumption simulation computer programs. With this approach a better correlation with experimental data can be obtained.
In the first part of the activity, an experimental evaluation of transmission efficiency of a given gearbox was carried out, on a powertrain test bed, measuring the actual engine and drive-shaft torque with three torque meters in the whole engine working range. The result was a complete efficiency map versus primary and secondary shaft speed and load.
In a second step a pseudo-empirical model was found, which calculates the transmission efficiency starting from primary and secondary shaft speed, load, oil temperature and rheological characteristics, gear geometry and material. A good correlation between this model and the experimental map was found.
This model was finally introduced as an input for a vehicle performance and fuel consumption simulation computer program. Such a program can use for gearbox efficiency either a constant value for each gear ratio or a map vs. engine speed and load. In both situations no correction can be applied for gearbox oil temperature variation, which instead has an important influence on vehicle efficiency especially during warm-up. Not taking into account this effect, a significant error is introduced into the vehicle fuel consumption calculation. In order to fit the vehicle model results with the experimental fuel consumption values, a number of empirical corrections must be made which could have less physical meaning and which can affect the simulated results when changing vehicle and/or engine parameters.
The results obtained with this refined vehicle model, in terms of fuel consumption on a given driving cycle (NEDC) are quite good. Calculated values of vehicle fuel consumption are very similar to the experimental data, not only when considering the overall values. Moreover, the instantaneous behaviour of fuel consumption vs. cycle time is in close accordance with the results obtained from a modal exhaust gases analysis performed on vehicle at chassis dyno.

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