Energy sources


efficiency of the unit by up to 40% over that of the water-injected engine demonstrated in the 1990s. Dr Andrew Atkins, Ricardo’s chief


Return stroke


Warm heat exchange fluid (HEF) enters the cylinder


Top dead centre


Cryogenic liquid is injected directly into the cylinder. Heat transfer with the HEF causes rapid vaporisation


technology engineer, lists a range of other applications where the addition of a Dearman engine could improve efficiency. “It’s basically where you need to keep things or people cool, or run zero emissions,” he states. So the bus market, with its stop/start cycles, could harvest heat from the passenger cabin and primary diesel engine to help improve the efficiency of a Dearman-based hybrid system; similarly temperature-controlled transport where, say, the hot food would help power the Dearman-based chiller system.


Power stroke


The vaporised cryogenic liquid expands pushing the piston down. Direct contact heat transfer continues allowing near isothermal expansion


Source: Dearman Engine Company


combustion chamber at top-dead centre, along with a small quantity of a heat-exchange (or ‘thermal’) fluid, which is generally glycol and water at ambient temperature. This mixing of these two fluids,


with their wildly different temperatures, causes the liquid nitrogen to boil and expand in the chamber, and drive a piston in a two- stroke cycle – or “one-stroke cycle, as there’s no compression stroke”, comments Michael Ayres, an engineer with the Dearman Engine company. At the bottom of the piston’s stroke, the exhaust port is exposed and a separate overhead exhaust valve opens, and the rising piston pushes the thermal fluid and air out of the engine.


Cryogenic injection “I knew that, if you could expand the liquid air isothermally [at a constant temperature], it would be comparable to other energy sources,” said


May/June 2013


Dearman. “And I knew that, if I could build an engine that used this principle, I’d be able to say, ‘that’s the bit I can do’.” Intriguingly, the characteristics of


this ‘cold-powered’ engine mean it can be used in a hybrid system with a conventional internal combustion engine to efficiently scavenge the low- grade waste heat, such as radiator heat, which is more or less irrecoverable with existing technology. That heat warms the ‘thermal fluid’ in the Dearman engine and increases its efficiency. In a split cycle engine, such as that proposed by Ricardo, along with Brighton University, for heavy truck operations, the introduction of cryogenic injection to the engine can scavenge waste heat in the engine and improve the


Bottom dead centre


The exhaust mixture leaves the cylinder. The gas is returned to the atmosphere and the HEF is re-heated and re-used


Pilot storage plant The promoters of the fuel, Dearman and its sister company Highview Power Storage, have already had a pilot storage plant operating in Slough, UK, and are working on efficiency improvements, using scavenged low-level heat. Problems? As an alternative to


fossil fuels, such as petrol and diesel, liquid air is a non starter; its energy density is worse than that of a decent lithium-ion automotive battery. There’s also the issue of boil-off from the vacuum flask, which will eventually drain the tank and, if it’s air that’s been liquefied, the fractionated boil-off can leave you with a tank full of oxygen, rather than air. “That’s a known issue and something the industry is used to dealing with,” says Atkins. It’s an interesting idea, convincing


technology and a pleasingly mechanical alternative to the advanced batteries that are creeping across the


industry. As I left the conference, the Technology Strategy Board had issued the


requirements for a new series of projects in the area of grid energy storage. Liquid air


proponents were licking their pencils at the prospect.


www.automotivedesign.eu.com 31


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