Full-vehicle CFD verification of interior climate on the Yutong HTC12

Situation

Buscotec GmbH was a compact German design consultancy — around twenty engineers, based in Bavaria — specialising in complete bus development for third-party manufacturers.

The firm delivered vehicle concepts to bus builders across Europe and Asia. In 2015 Buscotec was engaged by Yutong to design the HTC12 luxury coach — a ground-up new programme built for the European tourist-coach market. Alongside the load-carrying superstructure and its UNECE Regulation 66 rollover verification, the HVAC design was scoped for CFD evaluation ahead of the first physical prototype.

With the vehicle design in place, the interior climate had to be verified against a specific quality demand: temperature variation below 2 °C across the passenger cabin at both extreme-cold and extreme-hot ambient conditions. Buscotec engaged eCon to develop and run the CFD verification.

Barrier

At Buscotec, no simulation software and no simulation engineer sat in-house.

Every analytical workstream, structural or thermal, was bought in. eCon had already been running the HTC12 structural verification, including its UNECE Regulation 66 rollover and transient-durability analyses. Extending the collaboration to interior-climate CFD was the natural next step.

Solution

The heaviest engineering effort was getting a computationally usable model out of the vehicle CAD.

Model preparation.

The CAD as delivered described the coach as it would be built: manufacturing-grade tolerances, small gaps between adjacent components, features irrelevant to airflow but expensive to mesh. eCon rebuilt this into a CFD-ready geometry at approximately one-centimetre resolution. The bus interior — seats, dashboard, driving wheel, overhead compartments — was modelled in detail.

Duct-level pre-studies.

The passenger-cabin main duct and the driver-cabin ductwork were each analysed separately before integration into the full coach model. Air-supply services were represented as porous-media zones with directional flow — toward passenger heads for cabin services, toward the side glass for the driver-cabin — with pressure losses calibrated in dedicated loss studies.

Full-vehicle scenario matrix.

Four scenarios covered the boundary envelope: -15 °C and +35 °C ambient, each at 5,600 m³/h and 1,700 m³/h main A/C flow. Supply temperature was 25 °C in the cold cases, 17 °C in the hot; recirculation was held at 70 %. Solar radiation of 158.4 W/m² was applied to windscreen and right-side glazing in the hot cases — 800 W/m² at 45° incidence through Thermocontrol Venus light-grey glass at 28 % energy transmission.

Once the CFD model was in place, the scenario matrix ran without further modelling surprises.

The verification chain built for the HTC12 — CAD-derived vehicle geometry, porous-media service modelling, calibrated pressure losses, envelope-scenario evaluation — is directly reusable for rail HVAC verification under EN 14750-1 and EN 14750-2 for urban and suburban rolling stock, and EN 13129 for main-line rolling stock. The physics is the same; only the ambient envelope and the passenger-comfort thresholds change with the vehicle standard.

Result

Under steady-state conditions (Cases 616 and 626) the maximum temperature difference across the passenger cabin was approximately 1 °C — comfortably inside the 2 °C design target.

At maximum airflow (Cases 610 and 620) cabin air velocity remained below 1.2 m/s.

The airflow distribution across the passenger services flagged localised imbalances — excess flow through the mid-cabin outlets and elevated velocities around rows 7 to 9 — feeding back into duct-blockage recommendations for Buscotec's design.

1 °C max cabin temperature spread — 2 °C design target

1.2 m/s peak cabin air velocity at maximum airflow

4 boundary scenarios, −15 °C to +35 °C ambient