How far can we take the internal combustion engine with waste heat recovery?

Back in 2015 we had Dr Peter Newton tell us about his work in a blog post. It took almost two years but today we had him come give our weekly energy seminar. He expanded on improving the internal combustion engine and harnessing waste heat recovery and follows it up with this complementary blog post. You can also download a copy of his slides [PDF].

Today, the effects of carbon dioxide emissions on global warming are clear. To halt or, ideally, reverse the effects of climate change we must reduce our emissions of CO2 drastically, and transportation plays a significant role in this. In an era when personal transportation is ubiquitous and the world population is growing, it is a difficult challenge to reduce the overall emissions from this sector.

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Some have bet on the battery electric vehicle as a clear winner. It is undoubtedly the case that the internal combustion engine will have to give up some of its share of the worldwide fleet power plant up to batteries and electric motors. Only time can really tell how much. The benefits of electric vehicles are clear in some applications, but I believe that the internal combustion engine still maintains some advantages and will continue to be a key player in this sector. Hybridisation between these two technologies will certainly be an important solution for decades to come.

For the internal combustion engine to compete in the future it must be as efficient as possible with the fuel it uses. Currently, in some areas of operation the thermal brake efficiency of a modern engine could be anything from 0% at idle, up to 40% or 50% at maximum. With the majority of the heat generated by the combustion of the fuel escaping without doing any useful work.

There are a number of ways to recover this wasted energy, some of the most common are thermoelectric generators, turbocompounding or bottoming cycles, which form the basis of my research. In particular the organic Rankine cycle, which uses the waste heat from the engine to vapourise a pressurised liquid into a gas. The high pressure gas that results then goes through an expanding device (in this case a turbine) to produce work, which can be usefully consumed. Being derived from the very well-known steam Rankine cycle, the organic Rankine cycle (ORC) holds some benefits over other forms of waste heat recovery because its main elements are based on very mature technologies. However, optimising these different components for the wide range of organic fluids and duty cycles for potential waste heat recovery applications is still a significant challenge.

The turbine expander is a crucial element in the ORC, where the thermal energy is ultimately converted into shaft work. The overall efficiency is proportional to the efficiency of this device. Due to the requirements of working on an internal combustion engine, in terms of power density, it is subject to significant demands. With relatively small mass flow rates and very high pressure ratios over a single stage and often limited by the speed of the generator, producing an optimised design is not always straightforward.

Thermo- and fluid dynamically, the operation of these devices is in areas with almost no experimental data, making it impossible to produce a well validated efficient design. Not only are there significant real gas effects, but there are also areas where the flow leads to non-classical gas dynamic behaviour and the normal relationships that we would expect an expanding gas to show are inverted.

We are currently building a test facility to investigate the expansion of these fluids in the regions of interest. This rig will operate at pressures and temperatures expected in some of the most extreme ORC expander environments and produce unique data in these regions. This can be used to test numerical simulations of the same conditions and provide much needed validation in this area.

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