Why Not Blowers or 2onamercial-vehicle Engines ?
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By the Technical Editor WHILST there have, m the past, been several instances of engine makers toying with the idea of supercharging their units, there has also been a number of good reasons why such a system has not been developed to the production stage.
First of all, we may consider why it should be necessary to boost an engine, and what are the advantages from so doing. In a unit having a normally inspired air-gas mixture, there is a definite limit to the maxi
mum volume which can be got into the cylinder, and it is this factor which, generally speaking, restricts the power output of the unit.
Although the need for such boosting becomes an absolute necessity if a vehicle be operating at a high altitude, such as in Johannesburg, which is 6,000 ft. above sea-level, there are some good claims which can be made for a moderate degree of boost under normal conditions of operation.
In the first place, it is probably the most economical way of obtaining increased power from an engine of given size, which means that units can be of smaller dimensions, or a maker could meet the demand for a wider power range, without increasing the number of his basic types.
Contrary to the general belief, a boosted engine shows less cylinder and piston wear, which is explained by the fact that the mixture distribution is improved, and this, in turn, results in more even combustion pressures as between the respective cylinders.
It must be admitted that, in the past, blowers have been responsible for a lot of trouble, and, in consequence, the extra complication has not brought the expected compensa tory results. A high-speed, screaming blower is not conducive to one's ease of mind, and the fact that gears may strip at any moment, or the whole unit decide to seize up, has not assisted in inducing makers of commercial-vehicle engines to investigate the development to any serious extent.
It is almost safe to say that all this is now behind us. The extensive research carried out during the war on blowers for high-altitude fighting aeroplanes has resulted in the engine builder of to-day having at his disposal a well-tried instrument, which, when once installed, should prove as reliable as any other engine auxiliary.
The concern responsible for the development is Sir George Godfrey and Partners, Ltd., Hampton Road, Hanworth, Middlesex. This company states that most of the vehicle makers in this country, who build their own machines, have a Marshall K 200-type blower on test.
Taking a particular instance, I am informed that the b.h.p. of one make
of engine, unblown, is 95. and blown the b.h.p. rises to 150. It is estimated that a power increase of from 30 per cent. to 50 per cent. can be expected in most cases. As the price of the component is moderate, and the installation straightforward, the claim that it is the most economical way to obtain increased power would seem to be borne out.
The Marshall blower, of which there arc many variations, is of the Roots positive-displacement type, and in the K 200 there are two lobes, of involute form, to each rotor. Whilst these rotors mesh, it is vitally important that they do not make rubbing contact, hence the need for the highest degree of precision, not only in the machining and mounting of the rotors, but in the making of the gears.
The latter are of the straight-tooth pinion type, which have been found
to be quieter running than bevels. They are hardened, ground and lapped, the permissible degree of backlash being 0.0025 in. The extent of the clearance, between the lobes when cold is from 0.006 in to 0.008 in., and the end clearance is from 0.01 in. to 0.012 in The rotors are mounted on special ball races which are kept lubricated automatically, as the pinion wheels on the rotors act as distributors. Oil is fed from the engine supply directly to the gear sump, whence it passes through ducts in the blower case to the bearings at both ends, and is finally returned to the engine sump. There is thus a, continuous circulation of oil.
It is of the utmost importance that the correct involute formation of the lobes be obtained, and this is assured by the use of a form milling tool. The nominal design capacity of the K 200 blower is 200 cubic ft. of flee air per min. at 3,000 r.p.m., and its most effective speed range is between 2,000 r.p.m. and 5,000 r.p.m. At 3,000 r.p.m. it will deliver 170 cubic ft. per min. at a pressure of 5 lb. per sq. in.
Belt Driven in Tandem As used on an oil engine producing a normal 90 b.h.p. to 100 b.h.p., two K 200 blowers are mounded in tandem and may be driven by belts from the engine crankshaft. There are, of course, several types of drive which may be used, such as from a serrated shaft, by a coupling, or by chain in lieu of a belt. Methods of mounting the blower on an engine are also subject to variation within certain limits. The instrument con. cerned in this article is a type whict is designed for face mounting on tc part of the air-induction manifold ol an oil engine.
The point I would stress in connection with this particular blower is that the maker has got down to the problem of producing in quantity a component which calls for a high degree of precision work, and which, moreover, sells at a reasonable price. For instance, the K 200 is marketed at £30, a figure which is subject to a large reduction on quantities.
It is probable that certairi vehicle makers will, in the not-too-distant future, be in a position to make some interesting announcements, which, no doubt, will be in the form of offering blown units as an alternative to the standard type.