External combustion on way in?
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The need to reduce air pollution could stimulate the development of power units having a separate burner in which the fuel and air can be burnt more efficiently than is possible in an internal combustion engine.
THE burner of a modern automotive gas turbine is a notable example of a highly efficient external-combustion system that gives virtually complete combustion of the fuel and air for conversion into hot gas to feed the turbine. Because combustion is extemal to the power unit, the burner can be designed as a burner without complication by other considerations.
In diesel and petrol engines the fuel and air are burnt internally in a chamber with a moving wall in the form of a piston that exposes an increasing area of cylinder wall as combustion progresses, and burning must be completed before the piston has moved more than a limited distance in the interests of efficiency.
If burning is protracted, expansion of the combustion chamber volume slows up the rate of combustion and additional heat is lost by conduction through the large exposed area of cylinder wall and by virtue of the relatively high temperature of the exhaust gas. More over, the combustion tends to be inhibited by quenching of the flame resulting from the low temperature of the chamber and cylinder walls. This can prevent completion of combustion, notably if the engine is running at a sub-optimum temperature.
In the first part of a recent SAE paper particular mention is made of the flame quenching disadvantage of diesel and petrol engines. The paper, "Proposed reciprocating internal combustion engine with constant pressure combustion having a modified Brayton cycle" by Glenn B. Warren, was read at the International Automotive Engineering Congress at Detroit.
The proposed engine described in the paper incorporates a gas turbine type of burner fed with air by one bank of a V8 unit that supplies the other bank with gas, both the compressor and power banks of the unit operating on the two-cycle principle. It is anticipated that the engine will provide a saving in fuel consumption of 20/30 per cent compared with a petrol engine, that it will burn a wide range of fuels (including paraffin) without producing "undesirable exhaust elements" and have steam engine characteristics. It could be equipped with a turbocharger and regenerator.
Important
While these are rather extravagant claims for a unit that has got no further than the drawing board /computerized stage, they highlight the important fact that the advanced type of gas turbine external-combustion system has a potential for applications to a piston engine. And it is possible that such a unit will be competitive with a number of types of advanced external-combustion steam engines or the equivalent.
The impetus behind the development of external-combustion engines is derived from the anti-pollution laws in America and the promise of help from the American Govern ment in the production of steam engine prototypes.
According to an article in the March 1969 issue of the American journal Business Week, pending legislation may require the Government to aportion a large part of the S 155m that it currently spends on motor vehicles to the development of steam-driven vehicles. And in the February 1969 issue of the New York Times, it was announced that the Dallas Transport System had been granted $ 309,709 to test buses driven by a freonpowered non-polluting external-combustion engine developed by Walter L. Minto, freon being used in place of water as a working medium.
Special mention was made in the Business Week article of a delta-type steam engine built by Bill Lear of the Lear Motor`Corporalion in Reno. Bill Lear has promoted many successful projects and he is investing $ 10m in the new venture.
It was appropriate to mention the Brayton engine. before referring to the Minto and Lear units because, like the gas turbine, the gas produced by combustion is the working medium. In a steam engine a liquid is converted
into a gas by the application of heat to act as the working medium and if expenditure of a large quantity of liquid is to be avoided, it is necessary to condense the steam to convert it to a liquid for re-use.
Why bother with an additional working medium? And the Brayton engine could be produced in many versions, and be supercharged over a limited period by stored compressed air to provide increased acceleration. This might well be a very useful feature of a bus engine.
Advantage
The steam or freon engine could have the advantage that the working medium could be utilized more conveniently than the gas from a gas-turbine type burner. Freon has a very low boiling point (it is widely used in refrigerators) and this might enable it to be employed efficiently at lower temperatures.
In the Minto engine the boiler containing freon is heated by propane and the gas produced is fed to a double-acting piston engine, the only component of the exhaust being carbon dioxide. Mr. Minto is the head of the Kinetics Corporation, a Sarasota research and engineering company, which has tested a freon engine in a Volkswagen bus.
A Lear 100 hp delta-type steam engine has been built and a 125 hp delta prototype is planned having an output of 125/600 hp which it is claimed, will be particularly suitable for powering trucks and buses. The higher rating in this case is that obtainable over short periods for acceleration and hill climbing and such a reserve should make it possible in my view to employ a power unit of relatively low continuous output in stage-carriage buses, stop-start vehicles and trunking vehicles that require a greatly increased power for short climbs and so on.
It is stated that the exhaust of the unit comprises water vapour, carbon dioxide, nitrogen and "immeasurably small" quantities of toxic pollutants. A paraffin burner heats the fluid contained in long lengths of stainlesssteel tubing and the steam is heated to around 1,050deg F. According to Mr. Lear the engine is more efficient than an internalcombustion engine, is powerful for its size, is quieter in operation and can be operated at a lower maintenance cost. The delta engine has six cylinders and 12 pistons.
An eight-cylinder barrel-shaped engine will be developed in due course for passenger cars. Steam-turbine versions will include an automotive type with a rating of 100/300 hp. Between 50 and 100 steam units will, it is expected, be produced this year.