Easy Starting for Big Engines
Page 36
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ANEW type of engine starter, which should be of interest to commercial-vehicle users, is now in production in France. The chief advantages claimed for it are a big saving in weight over the normal electric starting equipment, and the high rotational speed imparted to the engine crankshaft.
Known as the Berger, this device is extremely simple in design, and has only three moving parts. Two pistondriven racks, with helical teeth, are mounted in horizontally opposed hydraulic cylinders, above and beneath a pinion. A ratchet-type clutch, giving instant release, is interposed between this pinion and the nose of the engine crankshaft.
Oil at high pressure is admitted to the hydraulic cylinders from a reservoir which is in the form of an air bottle. A separator is, naturally, provided between oil and air, and the air acts as a spring, driving the oil into the cylinders. Coil springs return the racks to their normal positions at the end of their power stroke.
Smooth Engagement An important feature of this design is that, in order to avoid any violent shock to the engine-starting mechanism (the racks, pinion and ratchet clutch), oil is admitted, first, at a relatively low pressure, and then at the pressure needed for the power stroke, which is in the neighbourhood of 4,000 lb. per sq. in. This change takes place automatically.
When low-pressure oil is admitted, the pinion turns slowly under thrust from the helical teeth of the racks, and when the ratchet clutch is fully engaged, the pinion can turn no farther, because of engine resistance,
Pressure then builds up in • the hydraulic cylinder heads until a certain point is reached, at which the oil valve opens wide, admitting oil at full pressure and maximum flow. The racks thereupon shoot forward with tremendous energy, turning the pinion, and with it the engine crankshaft, at a much higher speed than that obtainable from any normal form of electric starter motor.
The actual crankshaft speed imparted to an engine by this hydraulic device, in the first three-quarters of a revolution, represents 180 r.p.m. Unlike an electric starter, which turns an engine at a comparatively low speed for a number of revolutions, the Berger delivers its full power during a very short period. Its action might be compared with that of a sharp and tremendously powerful pull-up on the starting handle.
As an example of what the Berger can do, it may be mentioned that this appliance has been. mounted on a French Arsenal aero-engine—an H-type 24-cylindered unit of 4,000 b.h.p. From cold, and with the ignition switched off, the appliance gets this great engine turning over at 179 r.p.m. in one second, and in three-quarters of a revolution.
Pressure in the oil reservoir can be built up either by a hand-operated oil pump or by means of a small electrically operated one. Using a hand pump, full oil pressure can be regained in 30-80 seconds. Whether or not a small power-operated pump be fitted, the hand pump forms a most useful piece
of emergency equipment. • The Berger starter is not exhausted after its first effort, as three or four starting impulses can be given before oil pressure in the reservoir needs building up.
Light Weight The designer, M. Maurice Berger, has sent us comparative weight tables for the electric and hydro-pneumatic starting equipment of a 7-ton vehicle fitted with a 120 b.h.p. six-cylindered oil engine. These show a total weight of 50 lb. for the complete hydraulic installation, as against 456 lb. in the case of electric starting.
Much of this 456 lb. represents additional accumulator weight, over and above that required for lighting, direction indicators, etc., to give the extra power needed by an electric starter motor. In the example given, useful load on the vehicle is increased by 406 lb. by employing the Berger system.
Because of its small dimensions, the Berger can usually be mounted • to operate directly on the nose of the crankshaft, but should this not be possible, it can operate on any driven shaft turning at a speed lower than that of the engine crankshaft.
The starter is at present being produced by an important French concern making nearly all the fuel injector pumps used in France. It may, however, be produced under licence in this country in the near future.