SOME DIESEL ENGINE CONSIDERATIONS.
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Products from a British Works. The Influence of b.m.e.p. upon Exhaust Cleanliness. The Beardmore Fuel Pump.
THEgreat attention which has been paid recently to the heavy-oil engine in its application to commercial vehicles has brought into prominence products which are mainly of foreign origin, and however much we may admire the manner in which foreign manufacturers are pushing ahead in this line of development, we must not forget that comparatively small and successful Diesel engines have been built during the past few years by at least one British maker—to wit, William Beardmore and Co., Ltd., 36, Victoria Street, London, S.W.1.
Engines for Railcars and Aircraft.
Beardmore engines of the Diesel pattern have not yet actually been built for commercial vehicles, but they have been constructed in sizes down to 100 h.p. and are being utilized to a considerable extent in railcars with Diesel-electric drive; in fact, such cars 'during the three years ended May last ran a total of one and a quarter million miles. As regards larger sizes, it may be mentioned that the engines for the airship R101 are of this type and make.
During a recent visit to the Beardmore works at Parkhead, Glasgow, we saw a number of Diesel engines of small and medium sizes in course of construction and on the test bench, also a 300 h.p. unit coupled up with a portable generator, the whole being mounted on a trailer; this is for use in London by the General Post Office, the engine having a speed range of from 400 r.p.m. to 1,350 r.p.m. The four cylinders are of 6 ins, bore, whilst the stroke is 8 ins.
So far as the reliability of these engines is concerned, mention may be made of the comparatively small Beardmore railcar engines utilized in Canada. With these, sometimes 80,000 miles are run without touching the piston valves of the fuel pump; a good average can be taken, however, as 60,000 miles, when the valve and its bush may require renewing, a complete set being available at a small cost.
Supplying Two Cylinders from One Plunger.
In the Beardmore system, a four-cycle engine requires only one pump for each two cylinders. The pump plungers are driven by eccentrics and sheaves at crankshaft speed, but there is a half-speed valve, known as a switch valve, which directs the fuel supply from each pump plunger to each of its two cylinders alternately.
A combined pump-plunger and valve system closely resembling that now utilized in the Bosch pump was developed by the Beardmore Co. nine years ago, but It was decided that it was preferable to employ separate control valves. Each valve has a helical portion formed on it. The helicle controls the time period during which the bypass from the plunger cylinder back to the suction side of the pump is in operation, thereby limiting the supply passed to the cylinders.
n46 Timing of the injection is effected through the control valve .by altering the position of this valve relative to the main plunger. This is effected by mounting the valve-driving lever on an eccentric which can be rotated either by a governor or by hand. Similarly, rotation of the valve plunger, by .altering the point at which the helicle cuts off the bypass, controls the quantity of fuel injected—in other words; the power developed by the engine.
Each ejector is merely a spring-loaded needle valve resting on a nozzle in which is a number of holes, each 10 to 18 thousandths of an inch in diameter, according to the size of the engine concerned.
The pumping pressure at full load is approximately 8,000 lb. per sq. in., and to prevent troubles from breathing of the tubing solid-drawn high-tensile steel fuel pipes with very thick walls are utilized.
On the smaller Diesel engines, the compression ratio is about ni to 1; this compares with a ratio of approximately 5.25 to 1 used in a high-efficiency petrol engine. The highest pressure developed in the cylinders is about 700 lb. per sq. in. Modern petrol engines, as used for aircraft, average about 550 lb., although in racing aircraft the pressure may rise to something like 800 lb. when doped fuels are employed.
According to the Beardmore experts, there is no great difficulty in the actual making of a Diesel engine either as regards design or cost; the. extra expense usually lies in the pump and atomizers.
The Difficulty of the Foul Exhaust.
One of the difficulties which have been found is that of keeping the exhaust clean, and it has been discovered that this is bound up with the brake mean effective pressure. To obtain really good combustion, there must be a fairly big margin of air to fuel, but, unfortunately, this usually means a high weight-power ratio of the engine, for the b.m.e.p. then becomes low. It is probable that engines can be run satisfactorily at b.m.e.p.s of between 70 and 80 lb.; above these figures, on high-speed engines, the exhaust is apt to become black with unburnt carbon, particularly where the engine speed is constantly being changed.
On the 100 h.p. Beardmore engine the fuel consumption is approximately .42 lb. per b.h.p.-hour, and this with a b.m.e.p. of 75; a bigger engine can, of course, do much better than this. The actual fuel consumed is about 25 per cent. less in bulk than that used in a petrol engine and it costs about 80s. per ton. Some of our railway companies which are operating Beardmore Diesel-engined railcars are using standard Diesel oil, which is cruder than gas oil.
It will be seen that the problem is not so easy as It may have been regarded by many, but it is satisfactory to know that the British maker is not altogether neglecting the Diesel engine. We should like to see greater co-operation between commercial-vehicle builders and such a concern as the one in question.