When the Citaro NGT bus from commercial vehicle manufacturer EvoBus GmbH passes by, drives children to school, takes parents to work, or brings grandparents to the shops, it not only emits less CO₂ than a diesel bus, but also keeps costs low for transport companies with gas-powered fleets. Its main advantage? Low gas consumption with seamless performance.
The structure on the roof is what distinguishes it visually to other buses: this is not just its most distinctive feature, but also contains the ingredient for its sustainable mobility concept. It conceals tanks made from a stable composite material, in which the fuel required to power its petrol engine is stored: the compressed – yet still gaseous – methane, on which the bus runs to get from A to B.
From the roof to the engine bay
It goes without saying that its alternative drive also manifests itself in the engine bay: right at the heart of this eco-friendly bus is a petrol engine, modeled on a diesel engine. The natural gas engine burns fuel in a clean manner yet still achieves a high output. To comply with the low exhaust gas emissions values, two lambda probes are installed in the flow of the exhaust gas of the pre-catalyst.
They determine the residual oxygen content in the exhaust gas, thereby providing the signal for continuously perfect control of the combustion air ratio for reduced pollutant emissions. However, in this function in a natural gas bus, they can heat up quite considerably. This is because petrol-engine combustion reaches higher temperatures than in a diesel engine. So ideally they need to be kept cool – for as long as possible.
Next stop: auxiliary fan
So how do you increase the longevity of these exhaust gas probes? By providing them with additional cooling. What was needed was a small auxiliary fan that cools the lambda probes locally. It was decided that the perfect device for this task was an axial compact fan from ebm-papst. The fan had to be compact, and generate a high enough air flow and speed that provided the exhaust gas sensors with sufficient cooling.
A separate solution had to be found for the electrical actuation process, as the existing control unit was not ideal for this. It was a tricky task that required a careful approach with multiple development loops. It took just under a year of testing before all the parameters were correct.
“We now have a model that enables ambient temperature-dependent control and thus achieves a consistent effect all year round – whether at 40 degrees in summer or at low temperatures in winter.”
Oliver Baier, Business Development Manager at ebm-papst
A small device with a big impact
And they couldn’t be more correct: “Thanks to the electrical connection, we now have a model that enables ambient temperature-dependent control and thus achieves a consistent effect all year round – whether at 40 degrees in summer or at low temperatures in winter,” says Oliver Baier, Business Development Manager at ebm-papst. “The ambient temperature is used to determine how fast the fan has to rotate so that the air flow is sufficient to cool the lambda probes.”
The auxiliary fan is now installed as standard on the side flap of the engine bay to the rear, where it draws in the ambient air through slits and blows it directly at the exhaust pipe on the catalytic converter. The reliable cooling effect on the lambda probes will enables them to perform their task for a long, long time. And when it comes to noise, the fan is also bound to impress: at around 60 dB (A), it is another low-noise component of the natural gas bus. In conclusion, the green heart of this eco-friendly bus, with its high number of strokes and cooler temperatures, will be able to beat for a good while longer and save considerable costs.