Air Cooling by Exhaust Ejector
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Interesting Proposals in a Recent Book
IN his book "Air Cooled Motor Engines (Cleaver-Hume Press, Ltd., 55s.) Julius Mackerle mentions experiments with an exhaust-gas ejector system for engine cooling. The system has been applied to a Tatra racitig-car petrol engine and if it is proved feasible to incorporate a silencer, ejector cooling could be used for normal types of road vehicle with outstanding advantage.
Mackerle is on the staff of the Research Institute for Motor Vehicles, Prague, and was formerly chief designer of the Tatra company, Czechoslovakia.
As pointed out by the author in his introductory chapters, about 50 per cent.
less air is required for direct cooling of the cylinders of an air-cooled engine than is necessary to cool the radiator of a water-cooled unit, and this offers a valuable potential saving in fan output. Although the forward movement of the vehicle at higher road speeds is generally adequate to cool a radiator located in the air stream without fan assistance, the fan must be capable of supplying the greater part of the air required when the vehicle is running in low gear on a steep gradient or in dense traffic. Unless automatic fan control is provided, considerable power is wasted under normal conditions and the excessive fan output may lead to overcooling.
The problem of adjusting fan output to engine output is also of first importance for air-cooled engines, and it is pertinent that failure of the fan drive or control mechanism is far more critical because, in contrast to the water-cooled engine, it results in immediate overheating.
Safety Margin In practice, the water-cooled engine offers a substantial margin of safety, and this probably represents the main, factor that has perpetuated water cooling over the years for the . majority ofroad vehicle engines in preference to aircooled units. If the fan drive of a watercooled engine fails when the draught of air through the radiator is insufficient to prevent boiling, some time normally elapses before the temperature of the cylinder head and walls becomes critical. Because of the high latent heat of evaporation of water, a large amount of heat is dissipated by boiling, and a dangerous condition does not develop until the water level is reduced to below that of the cooling passages, and this gives a time latitude in which the driver will take note of the excess temperature, as shown by the gauge, or of the steam produced.
If an air-cooled engine is fitted with a fan integral with the flywheel the possibility of overheating by belt failure is eliminated, but but a system based on a flywheel fan is inefficient because the output of the fan cannot be regulated to match the output of the engine. Regulation can only be achieved by by-passing some of the air or by physically obstructing air flow. In operation, fan output is higher than necessary over a large part of the load-speed range. Various methods arc available -for adjusting automatically the speed of the fan or the pitch of the blades to provide automatic temperature control of both air-cooled and watercooled units, hut these generally necessitate the use of belt drive, and fitting a control mechanism increases the risk of failure.
No Moving Parts
In the ejector system described by Mr. Mackerle, there are no moving parts, and air flow is increased automatically with an increase in engine output. Failure of the system is virtually impossible and exhaust back-pressure is not increased. Fan losses are, therefore, entirely • eliminated, without introducing compensatory disadvantages. In addition to providing freedom from maintenance problems (at the expense of a rather bulky casing) the system provides an improvement in fuel consumption and should give longer cylinder-bore rife, because of the reduced warming-up period and the elimination of overcooling.
The kinetic energy of the exhaust gas is employed to create a flow of cooling air over the cylinders, gas being discharged from a nozzle into the mouth of a mixing chamber which is formed by an extension of the cowling and acts as a venturi. A nozzle may be fed by a number of cylinders, optimum efficiency being obtained in the case of a sixcylinder engine if one nozzle (and mixing chamber) is supplied by three cylinders.
Although the silencing problem has yet to be overcome, Mr. Mackerle appears confident that a suitable finned silencer , could be evolved for fitting between the engine and mixing chamber.
In his claims for the conventional aircooled engine, Mr. Mackerle emphasizes that it can be expected to operate satisfactorily under critical tropical conditions (because of the favourable "temperature difference factor), in which water-cooled engines are liable to overheating. He also reiterates better-known claims for air cooling, including freedom from radiator maintenance, reduced cylinder wear, higher thermal efficiency and easier maintenance, the last-named being mainly derived from using separate pistoncylinder assemblies which can be dismantled individually. Control systems mentioned by Mr. Mackerle include the Scintilla electro-magnetic clutch, the hydraulic coupling and the variable-pitchfan-blade type.
It is notable that the Tatra 603 V-8 litre air-cooled petrol engine for sports cars develops 200 b.h.p. at 7,500 r.p.m. This unit has a bore of 75 mm. and a stroke of 72 mm., whilst the compression ratio is 12 to 1. Applied to this engine, an ejector cooling system is based on a separate mixing chamber for each bank of cylinders, the length of the chambers being 2 ft. 9-t in. The gas discharged from the ejector is at 160° C. at full load.
Apart from giving details of engine features directly related to problems of air cooling and comprehensive analyses of a wide variety of air-cooled engines produced in Europe and America for motorcycles, cars, commercial vehicles, aircraft and military tanks, the information provided by the author includes data covering virtually every aspect of design and basic theory. He also reviews engine layouts for commercial vehicles.
Combustion Chamber Positions
• When referring to combustion characteristics, Mr. Mackerlc emphasizes that in petrol engines, the chamber should he located in the cylinder head to reduce dissipation of heat through the piston rings, whereas incorporating the chamber in the piston of a diesel engine is advantageous because it increases the temperature of combustion. This view is of current interest because, according to a number of leading designers in this country, petrol engines should lie fitted with chambers contained in the pistons.
In reference to pre-combustion chambers, Mr. Mackerle points out that unfavour'able heat dissipation is caused by a high swirl rate, and that loss of efficiency is attributable to heat dissipation rather than the power absorbed by pumping. The value of a high-compression ratio in combination with a compact combustion chamber is, according to the author, a direct function of reducing the area of heat dissipation during a major 'part of the cycle, as distinct from increasing the expansion ratio.
nN Tuesday of .last week, Mr.
Mackerle presented a paper "Aircooled Automobile Engines" to the Automobile Division of the Institution of Mechanical Engineers in London. This lecture dealt in abbreviated form with subjects included in his book.