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A comparison of energy vectors in powering hybrid buses

Derek Charters

HORIBA MIRA’s Derek Charters, Senior Consultant – Future Transport Technologies, analyses the different energy vectors used by bus manufacturers and introduces the benefits of utilising liquid nitrogen.

Buses have come a long way since the first engine powered bus came into existence in 1895. Since then, buses have been made more efficient through modifications and improvements to the diesel engines that power them. Despite advancements, however, buses remain a noisy addition to our roads and contribute copious amounts of pollutants to our already smoggy world. Alternative energy vectors provide a great response to this challenge – but how good are they? 

What are the different technologies?

There are currently five main types of bus technologies making use of a range of alternative energy sources. These include batteries, compressed natural gas and hydrogen.  

Battery powered buses 

Battery powered buses come in a variety of guises, including diesel hybrids, electric buses and plug-in hybrids. They all have their unique benefits, however, refuelling and recharging are proving to be consistent challenges. 

Electric/diesel hybrids 

Electric/diesel hybrids produce around 40% less CO2 than a diesel powertrain and have similar performance results as standard buses. There are two main types of electric/diesel hybrids: a parallel hybrid, where the bus is driven by the battery and directly by the engine, and a series hybrid system, where there is no direct link between the engine and wheels, with the bus driven solely by electrical power. Although series hybrids are significantly quieter than parallel hybrids, they are sensitive to failure. If any of the electrical components fail the bus will come to a complete standstill.

This unreliability does mean that some resort to running purely on diesel, counteracting the benefits of the hybrid. London buses are well known to run primarily on diesel, with drivers claiming the hybrid system does not work 90% of the time.  Batteries also require changing several times over the course of a bus’ lifetime; some every four years. With battery components imported from overseas, the benefits of the hybrid electric bus become greatly reduced. 

Battery electric

Battery electric buses operate by using an electric motor powered by a lithium ion battery charged using mains electricity. Electric buses produce no tail-pipe emissions and are therefore given the status as ‘zero emission capable’. The main challenge for electric buses is the lack of charging points, because the technology is still relatively new. Furthermore, lithium batteries are sourced from overseas, many from politically unstable countries. As such, they do not provide the same benefits as locally sourced energy vectors.

Plug-in hybrids

Plug-in hybrids are similar to a traditional hybrid bus, but are equipped with a larger battery which solely powers the bus for a specific distance, giving a ‘zero emission capability’. The battery can be ‘plugged-in’ and recharged using mains electricity. They are quiet and emission-free at bus stops with 80% lower exhaust emissions. However, they do require large recharging facilities which are not widely available. 

Compressed natural gases and biomethane

Compressed natural gas and biomethane can also be used to power buses. These buses are equipped with a spark ignition engine and store compressed natural gas or biomethane in high pressure tanks on the roof of the vehicle. Natural gas is a fossil fuel with supplies originating from the North Sea and European gas fields. Biomethane is a renewable form of natural gas derived from the anaerobic digestion of organic waste products such as sewage and food waste. These vehicles have a reduced driving range, meaning they perform less well than their diesel counterparts. 

Hydrogen fuel cells 

Hydrogen buses are powered by an electric powertrain whereby fuel cells convert the chemical energy of hydrogen into electrical energy. Hydrogen is stored in high pressure tanks on the roof of the bus. Hydrogen fuel cell buses produce no tail-pipe emissions and are classed on ‘zero emission capable’. Fuel cells generate electricity from the reaction between hydrogen and oxygen to make water. Instead of carbon dioxide and other pollutants, vehicles that use them emit nothing but water vapour. The downside is that it takes a great deal of energy to make the hydrogen they need. Hydrogen can be made by electrolysis, where electricity is used to split water into oxygen and hydrogen. Therefore, overall carbon dioxide emissions depend on how that electricity is generated. This type of energy vector is only truly ‘green’ when produced in countries with an entirely renewable national grid. 

Furthermore, the infrastructure required to create a ‘Hydrogen Economy’ includes large investment in the form of large hydrogen generating plants, pipelines, trucks, storage facilities, compressors, hydrogen gas stations and more. 

With no energy vector a clear front-runner in the search for greener buses, a better solution still needs to be identified.  

Liquid nitrogen – the future? 

Engineers are now turning to liquid nitrogen for inspiration. The Dearman Engine Company has been developing a piston engine driven by the expansion of liquid nitrogen to produce clean, cold and power. Through a partnership co-funded by Innovate UK including HORIBA MIRA, Coventry University, the Dearman Engine Company, Air Products PLC, Manufacturing Technology Centre Ltd, Productive Ltd, Cenex and TRL Ltd, this technology is being applied to a power hybrid bus. The project is set to be completed in 2016. 

Liquid nitrogen has clear benefits as an energy vector – it is renewable, easy to store and refuel, and can be produced in any country. Additionally, it can be used to increase efficiency and reduce the carbon footprint of the vehicle it is powering. 

The liquid nitrogen power source will be primarily used when the bus stops and starts – when the bus picks up and drops off passengers. This portion of the bus’ drive cycle traditionally has a heavy impact on the diesel engine and can produce vast amounts of NOx and CO2 emissions. As liquid nitrogen produces none of these harmful emissions, it will enable the bus to continue frequently stopping to unload and pull away from a bus stop without expelling damaging pollutants and greenhouse gases. 

Whilst driving at 20 mph or below, the liquid nitrogen - stored in a low pressure insulated cylinder - is warmed up to the point of evaporation, at which time it creates enough pressure to drive the multi-cylinder Dearman engine. Once the bus reaches 20 mph, the diesel engine will kick in as at this speed the bus requires less effort from the engine to operate.

Liquid nitrogen is a fascinating energy source as it can be created in any part of the world from many sources of renewable energy, meaning its carbon footprint is minimal. It’s easy and quick to refuel as liquid air can be transferred between vessels at high rates. The industrial gas industry has developed filling systems capable of greater than 100 litre/min transfer rates. Furthermore, there is significant existing infrastructure and the manufacturing process is simple with the only requirements being air and electricity. 

With the UK looking to continue to reduce its reliance on carbon fuels from overseas and the search to reduce bus emissions, this project takes the UK one step closer to achieving this goal. 


Derek Charters is Senior Consultant – Future Transport Technologies at HORIBA MIRA, a world-leader in advanced engineering, research and product testing. MIRA’s work spans the automotive, aerospace, rail and defence industries and includes the development of low carbon and autonomous vehicle technologies.


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