Cold engine powers car with efficiency

A source of pollution free and intrinsically safe motive power becomes a practical reality 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 has raised efficiency to a point where fuel costs are comparable with petrol, but, more importantly, without the pollution. As well as solving a problem which has long plagued all Rankine cycle engines, it leads to pollution-free vehicles without the associated cost and weight penalties incurred by batteries. They will also be intrinsically safe – a matter of great interest to the oil and gas industries. The idea of providing motive power from the boiling of a liquid and the subsequent expansion of a gas has been around since the end of the eighteenth century, steam power being the most obvious proponents. However, it still suffered from the problems of heat transfer which resulted in very poor thermal efficiencies. The only answer to this problem is to have machines the size of power stations. To give some idea of efficiencies, the most efficient steam locomotives ever built, the American Union Pacific 'Big Boys', are said to have attained 14% efficiency, while the best achieved in Britain was around 8 to 9%. Similar figures pertain to another idea kicked around for more than a few years – the running of cars on liquid nitrogen which is allowed to boil and expand using heat from the ambient environment. The two liquid nitrogen powered developments most prominent on the Web are one at the University of Washington, now abandoned, and one at the University of North Texas. The best, the latter, seems to have achieved power a car for 24km using 180litres of liquid nitrogen. Where all the liquid nitrogen engines to date have fallen down is that while they make use of the expansion effect of liquid nitrogen's boiling at 77°K, they fail to make full use of the expansion of that gas from 77°K to ambient 300°K, and keep it at ambient as it expands. 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 is 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 its 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 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 and, as it does so, it absorbs more heat from the heat transfer fluid, maintaining 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 which then passes through a radiator to warm it up to ambient ready for the next cycle. The prototype 400cc single-cylinder engine has been fitted into a 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. He expects a new two cylinder engine he is building to raise maximum speed to 40 mph. Judging by the working engine model shown on his stand at the British Invention Show, held at London's Barbican Exhibition Centre, it looks relatively simple and inexpensive to make being made from aluminium alloys. This contrasts with conventional battery electric cars which tend to cost more than twice as much as their conventional IC 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 are 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 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 helped Dearman win both the Gold award in the industrial category and the Invention of the Year title. The other niche market that Dearman had already identified 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. Design Pointers •The liquid nitrogen engine is completely pollution free •It offers an end to life recycling problems •It is much more efficient than previous liquid nitrogen engines. Running costs of prototype engine being comparable to existing petrol engines •Capital cost of manufacture looks to be low, very much less than for battery electric or fuel cell alternatives