The Dearman Engine operates by the vaporization and expansion of cryogenic fluids. Ambient or low grade waste heat is used as an energy source with the cryogen providing both the working fluid and heat sink. The Dearman Engine process involves the heat being introduced to the cryogenic fluid through direct contact heat exchange with a heat exchange fluid (HEF) inside the engine.

Prior cryogenic expansion engines have worked on an open Rankine cycle—i.e., similar to a traditional steam engine but operating across a different temperature range. In this approach, the cryogenic fluid is pumped to operating pressure and vaporized through a heat exchanger, before expansion in the engine cylinder.

This approach has a number of drawbacks, DEC suggests, as the heat exchanger must be large to cope with the heat transfer rates and heavy to withstand the high pressure. Additionally, little heat transfer occurs in the expansion stage (near adiabatic expansion) reducing the work output.

The Dearman Engine instead uses the heat exchange fluid to facilitate extremely rapid rates of heat transfer within the engine. This allows injection of the liquid cryogen directly into the engine cylinder whereupon heat transfer occurs via direct contact mixing with the HEF. The heat transfer on injection generates very rapid pressurization in the engine cylinder.

A Dearman Engine power cycle.

Direct contact heat transfer continues throughout the expansion stroke giving rise to a more efficient near-isothermal expansion. With the pressurization process taking place in the cylinder, the amount of pumping work required to reach a given peak cylinder pressure is reduced.

After each expansion cycle the heat exchange fluid is recovered from the exhaust and reheated to ambient temperature via a heat exchanger similar to a conventional radiator.

The Dearman engine is constructed almost entirely from the components of a conventional piston engine, requires little maintenance and has a light environmental impact.

The Dearman engine could be used in a number of configurations: on its own, as the prime mover of a zero emissions vehicle (ZEV); combined with an internal combustion engine (ICE) to form a ‘heat hybrid’; or as a power-and-refrigeration unit.

Vehicle manufacturers and industrial gas producers have begun to offer vehicle refrigeration for lorries and trailers based on the evaporation of liquid nitrogen, but these systems do not extract any power from the evaporation process. The Dearman engine extracts both shaft power and cold from the same unit of liquid air or nitrogen. First the cryogen is vaporized in a heat exchanger in the refrigeration compartment, so cooling it down; then the high pressure gas is used to drive the Dearman engine, the shaft power of which can be used to drive a conventional refrigeration compressor or for auxiliary power.

The engine, designed to provide the power for refrigerated trailer applications, could be in production within two years and, with a network of industrial gas plants across the UK already producing liquid nitrogen, there is no infrastructure barrier to rapid deployment, the company said.

The concept for the new technology includes a diesel hybrid application. By harnessing the low grade waste heat of the internal combustion engine cooling loop, the Dearman engine can deliver 25%+ reduction in fuel consumption for a diesel heavy duty engine. The ability to work alongside other waste heat recovery systems is an additional advantage. Further development work is underway in this area.

Preliminary findings from a new report on the use of liquid air engines in commercial applications from the Liquid Air Energy Network (LAEN) (earlier post), Centre for Low Carbon Futures (CLCF) and University of Birmingham, to be published in early March, suggest that the adoption of liquid air technologies in heavy-duty vehicles could reduce the UK’s diesel consumption by 1.3 billion liters (343 million gallons US) and its carbon emissions by over a million tonnes by 2025.

The engines could also reduce local air pollution significantly: introducing liquid air trailer refrigeration alone would cut emissions of carcinogenic particulate matter by 180 tonnes per year, equivalent to taking 367,000 modern diesel trucks off the road.

The report has also identified that the roll-out of liquid air vehicles could be fueled entirely from existing spare industrial gas plant production capacity until at least 2019.

The engine is the brainchild of archetypal British garage-inventor, Peter Dearman, and subsequently developed in partnership with top UK engineering consultancy, Ricardo, and a number of leading UK Universities including Leeds, Birmingham, Loughborough and Brighton.

Liquid air as a new zero-emission energy vector emerged into public view in May 2013 with a report from the Centre for Low Carbon Futures (CLCF) entitled “Liquid air in the energy and transport systems: Opportunities for industry and innovation in the UK,” launched at a conference hosted by the Royal Academy of Engineering. Contributors to the nine-month study included National Grid, Arup, Ricardo, Messer Group, Spiritus Consulting and academics from the Universities of Leeds, Birmingham, Strathclyde, Brighton, Queen Mary University of London and Imperial College.

The CLCF report found that liquid air could reduce diesel consumption in buses or freight vehicles by 25% using a liquid air / diesel hybrid, while using a liquid air engine would cut emissions from refrigeration on food trucks by 80%. The report also raised the possibility of zero-emission liquid air city cars filling up at road-side forecourts at a fraction of current fuel costs and with lower lifecycle vehicle emissions than either electric or hydrogen powered vehicles.

Liquid air offers significant potential benefits as a future energy vector, both for use in light duty propulsion and as an enabler for other promising low-carbon power train innovations, particularly waste heat harvesting.

—Neville Jackson, Chief Technology and Innovation Officer, Ricardo plc

Resources

• Liquid Air in the energy and transport systems: Opportunities for industry and innovation in the UK