EUREKA DECEMBER 2002 COVER FEATURE STORY

Cold engine powers car with efficiency

A source of pollution free and intrinsically safe motive power becomes practicable after much effort. Tom Shelley reports

A car powered by liquid nitrogen may be seen cruising the streets of Bishops Stortford.

Cylinder injection of a heat transfer fluid followed by liquefied gas raises efficiency to a point where fuel costs are comparable with petrol, but with no pollution.

As well as solving a problem which has long bugged all Rankine cycle engines, it leads to vehicles which are totally pollution free, without the cost and weight penalties incurred by batteries, and are also intrinsically safe, a matter of great interest to the oil and gas industries.

The idea of providing forward motion from the boiling of a liquid and the subsequent expansion of a gas has been around since the end of the eighteenth century. While much used in the age of steam, the problems of heat transfer result in very poor thermal efficiencies unless the machines are of power station size. The most efficient steam locomotives ever built, the American Union Pacific 'Big Boys' are said to have attained 14%, while the best achieved in Britain was around 8 to 9%. Much the same pertains to another idea kicked around for more than a few years - running cars on liquid nitrogen, allowed to boil and expand using heat from the ambient environment.

The two liquid nitrogen powered developments most prominent on the World Wide Web are one at the University of Washington, now abandoned, and one at the University of North Texas. The best the latter team seems to have achieved is to be able to power a car for 24km using 180litres of liquid nitrogen.

Where all the liquid nitrogen engines until now have fallen down is that while they make use of the expansion effect of liquid nitrogen boiling at 77 deg K, they fail to make full use of the expansion of that gas from 77 deg K to ambient 300 K, and keep it at ambient as it is expanded. The efficiency of a heat engine depends on the difference between source and sink temperatures being as far apart as possible. Failure to keep the temperature of the gas up during expansion results in a heat engine which very much less than optimally efficient. A particular difficulty with nitrogen is that typical of gases in general, it is a good thermal insulator, making it difficult to transfer heat into the gas unless it is turbulent.

One solution suggested by the University of Washington was to make an engine using a lot of small cylinders, each only 10mm across but with a 100mm stroke. Another of their ideas required building a radiator into the cylinder head, and another, a 110kg external heat exchanger. The University of North Texas suggests injecting a 'hydraulic fluid' into the cylinder along with the nitrogen, in order to provide an internal source of heat and also lubrication. The University does not seem to have ever either tried or developed this, but in making the suggestion in a paper published in November 2001, the team put its fingers on the breakthrough which Peter Dearman has been exploiting .

His engine is two stroke. The induction stroke starts by drawing in the heat exchange fluid, which in his case is a conventional mix of ethylene glycol based car anti-freeze and water. Liquid nitrogen is then injected subsequently from a separate nozzle. (If it was injected simultaneously, the liquid nitrogen would freeze the heat transfer fluid as it entered, blocking the injection port). The heat transfer fluid possesses sufficient heat capacity to both boil the liquid nitrogen and heat it all the way up to ambient temperature. The pressure pushes the piston down the cylinder, and as it does so, it absorbs more heat from the heat transfer fluid to maintain its temperature at ambient. At bottom dead centre, the exhaust valve opens, and the expanded nitrogen and heat transfer fluid are allowed to escape. Before reaching the atmosphere, the mixture passes through a separator to recover the heat transfer fluid. The latter passes through a radiator to warm it up fully to ambient on its way back to the cylinder.

The prototype 400cc single cylinder engine has been fitted into an 'A' registration Ford Orion. Dearman says that it allows the car to be driven at up to 20mph and achieves a mileage of 1mile/litre. At a cost from Air Products of 10p/litre, this allows the car to achieve a similar fuel cost per mile to that achieved using petrol. A new two cylinder engine with twice the power ouptut is now undergoing tests.

Judging by the working engine model shown on his stand at the British Invention Show, held at London's Barbican Exhibition Centre, it is made of aluminium alloys, and looks relatively simple and inexpensive to make. This contrasts with conventional battery electric cars, which tend to cost more than twice as much as their conventional equivalents, and require additional expenditure of around 100 per month in order to lease the batteries. Furthermore, should the World suddenly turn to battery electric cars in a big way, there is insufficient nickel to give them all either nickel cadmium or nickel hydride batteries. Alternative lead acid batteries are too heavy, and lithium based batteries too expensive.

The other big advantage of liquid hydrogen powered engines over other alternatives is that they are not only totally clean, but intrinsically safe. This inspired great enthusiasm from one of the Award judges at the show, who happens to work for a company with heavy commitments in the oil and gas industry. It was probably this factor, which allowed Dearman to win both the Gold award in the industrial category, and also the Invention of the Year title. The other niche market that Dearman had already identified as a possibility, is the underground mining industry, which currently uses either electric or diesel motive power, as appropriate. Liquid nitrogen would offer considerable operating advantages over both.

And for those environmentally oriented, liquid nitrogen engines, being made of aluminium, are easy to recycle, unlike nickel cadmium batteries.

For more information: www.pollutionfreecar.co.uk
peter@pdearman.freeserve.co.uk
www.aa.washington.edu/aerp/Cryocar/Cryocar.htm
www.mtsc.unt.edu/CoolN2Car.html

Pointers

Liquid nitrogen engine is completely pollution free

It offers no end of life recycling problems

It is much more efficient than previous liquid nitrogen engines. Running costs of prototype engine are closely comparable with existing petrol engines

Capital cost of manufacture looks to be low, very much less than for battery electric or fuel cell alternatives.

For more technical developments see www.eurekamagazine.co.uk

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