Aerodynamic Analysis of Cooling Airflow for Different Front-End Designs of a Heavy-Duty Cab-Over-Engine Truck
Artikel i vetenskaplig tidskrift, 2018
Improving the aerodynamics of heavy trucks is an important consideration in the strive for more energy-efficient vehicles. Cooling drag is one part of the total aerodynamic resistance acting on a vehicle, which arises as a consequence of air flowing through the grille area, the heat exchangers, and the irregular under-hood area. Today cooling packages of heavy trucks are dimensioned for a critical cooling case, typically when the vehicle is driving fully laden, at low speed up a steep hill. However, for long-haul trucks, mostly operating at highway speeds on mostly level roads, it may not be necessary to have all the cooling airflow from an open-grille configuration. It can therefore be desirable for fuel consumption purposes, to shut off the entire cooling airflow, or a portion of it, under certain driving conditions dictated by the cooling demands. In Europe, most trucks operating on the roads are of cab-over-engine type, as a consequence of the length legislations present. However, there are new directions from the European Union, which would permit slightly longer vehicles to improve aerodynamics and also to allow for a safer, more environmentally friendly vehicle. The truck design, where a cab-over-engine cab has an elongated front, is often referred to as a Soft Nose, where, as the name implies, the nose should be “soft” to improve the safety for pedestrians and also for car occupants in the event of a collision. This paper deals with the analysis of cooling airflow for two different front-end designs of a heavy truck. The first design is a cab-over-engine cab; the second is a Soft Nose cab, which in this case is basically an elongation of the grille area of the cab-over-engine cab to obtain a smoother shape of the cab. The Soft Nose model used in this investigation was extended 200 mm from the cab-over-engine front. Computational Fluid Dynamics was used as the tool for examining the aerodynamic properties of the vehicle models. A steady RANS-based approach was conducted, based on the method evaluated in previous work performed by the authors. The cab-over-engine and Soft Nose models were evaluated in an open-road environment. The configurations were evaluated both with inactive and active heat exchangers, in order to examine the effect of heating the air on the drag co-efficient and also to determine the cooling capacity of the different models. A sub- study was performed where different opening percentages of the grille area was investigated to determine the minimum percentage opening that would be needed to achieve a radiator Top Tank Temperature value below a target limit of 100 °C. The results show that there was potential for drag reductions for the Soft Nose model used. The cooling airflow was different for the cab-over-engine and Soft Nose models; as a consequence of the longer distance between the grille and cooling package, less air entered the cooling module for the Soft Nose model. A large portion of the airflow entering the grille leaked around the cooling module for the Soft Nose model. Also, following on from the reduced airflow through the cooling package, the radiator Top Tank Temperature values were considerably increased with the Soft Nose model. It was also shown that for the specific driving condition simulated here, an opening of 17.5% of the grille area was required to ensure sufficient cooling capacity. An interesting continuation of the cooling airflow analysis of the Soft Nose model would be to add ducts, guiding the air from the grille to the cooling module, to investigate if leakage could be reduced and cooling capacity increased.