Hybrid bus promises fewer emissions and improved efficiency

A hybrid bus running on diesel and liquid nitrogen has completed a series of trials that bring the vehicle closer to use on the road.

The hybrid bus – the world’s first to be powered by liquid nitrogen – has been built by engineers at HORIBA MIRA as part of an Innovate UK consortium.

Dubbed CE Power, the bus features a Dearman engine powered by liquid nitrogen, alongside a conventional diesel engine. The Innovate UK consortium was led by Dearman and included Air Products, Cenex, Coventry University, HORIBA MIRA, Manufacturing Technology Centre, Productiv Ltd, and TRL (Transport Research Laboratory).

The bus uses a hybrid propulsion system to reduce emissions during acceleration after stopping. This portion of the bus’ drive cycle traditionally has a heavy impact on the diesel engine and can produce large amounts of nitrogen oxide and carbon dioxide emissions.

The Dearman engine does not produce these emissions, enabling the bus to frequently stop, unload and pull away from a bus stop without expelling the same level of pollutants.

Whilst driving at 20mph or below, the liquid nitrogen – stored in a low pressure insulated cylinder – is warmed up to the point of boiling, at which time it creates enough pressure to drive the multi-cylinder Dearman engine. The diesel engine takes over once the bus reaches 20mph.

The bus trials were completed at HORIBA MIRA’s facilities in Nuneaton and included components and full system testing along with an engineered drive cycle to simulate a standard bus route with a variety of stops.

David Sanders, commercial director at Dearman said: “The Dearman engine has the potential to significantly improve the efficiency of both buses and HGVs, reducing fuel consumption and cutting pollution. Crucially it can provide a cost effective alternative to other emerging zero emission technologies, whose environmental performance if often offset by complexity and cost. This successful trial could be the first step towards rolling out a British innovation to the streets of the UK and around the world.”

“The development of an on-vehicle cryogenic system was a notable achievement to come from the project,” added Jon Trembley, head of cryogenic technology at Air Products. “The innovative system was developed by the consortium team to store cryogenic nitrogen liquid at low pressure, then pressurise and store in high pressure cylinders for use in the Dearman engine. This stored pressure is then used to provide the rapid reaction pressure responses required.”

According to Dearman, the benefits of using liquid nitrogen over an electric hybrid bus include a longer life, local production and easy refuelling.

Liquid logic: Peter Dearman, inventor of the liquid-air engine