AT THE HEART OF THE ROAD TRANSPORT INDUSTRY.

Call our Sales Team on 0208 912 2120

A SUCCESSFUL SMALL DIESEL ENGINE.

10th August 1926, Page 13
10th August 1926
Page 13
Page 14
Page 13, 10th August 1926 — A SUCCESSFUL SMALL DIESEL ENGINE.
Close
Noticed an error?
If you've noticed an error in this article please click here to report it so we can fix it.

Which of the following most accurately describes the problem?

Some Details of the Benz Two cylinder Power Unit Which Embodies Some Particularly Interesting Features, including a Decompression Device and Ignition Cartridges for Starting.

rrillE PROBLEM of designing satis1. factory types of power unit in which. heavy oils can be utilized is occupying the minds of many clever engineers in this country, and for those who are not already acquainted with one of the most successful types of Diesel engine on the Continent we have obtained particulars of the Benz two-cylindered unit, which has already been employed on a motor• lorry, and is now in regular use on a large number of agricultural tractors known as the Benz-Sendling.

We do not propose in this article to deal with the tractor, but only with its means of propulsion, which operates on the four-stroke-cycle principle. Dealing with one cylinder only: when the piston reaches the bottom of the suction stroke the cylinder is filled with pure air, which is compressed on the next up stroke to approximately 32 atmospheres. This causes the development of much heat. Immediately the piston reaches the uppermost portion of the stroke liquid fuel is sprayed into the combustion chamber 'through the fuel valve. It is ignited automatically, driving the Piston downwards and escaping through the exhaust valve. Any burnt gases remaining in the cylinder at the Conclusion of the working stroke are driven out during the second up stroke.

The unit has a bore of 135 mm. and a stroke of 200 mm., developing 30 b.h.p. at SOO r.p.m., but 10 per cent. greater power can be obtained if required. The cranks are set at 180 degrees and are provided with balance weights, whilst the crankshaft is supported in three bearings, having bronze shells lined with white metal. The cylinders and the upper half of the crankcase form a single casting in aluminium. Cast-iron liners are employed for the cylinder bores, these being pressed in and replaceable when worn.

Each cylinder has a detachable head, rendered gas-tight by a copper gasket. Overhead valves are employed and the push-rods for operating them are en

closed in chambers cast on to the cylinders and cylinder heads respectively, so that the moving parts are completely concealed. Oil-tight aluminium covers enclose the valve rockers and the whole motor presents a smooth and neat extenor. An unusual feature is that the chambers for the ptish-rods also function as air intake pipes, so that when removing the cylinder heads there is no need first to dismantle the intake pipe.

Where the engine is required to work in a -dust-laden atmosphere an extension pipe, reaching to about 8 ft. above the ground, is attached to the air intake, and the air is passed through a large filter to afford additional cleansing.

In the sectional view of the cylinder head, E is the air inlet valve, A the exhaust valve and V the relief valve. G is a rubber packing for the water jackets and R the copper gasket.

Spraying of the fuel is not obtained by the employment of compressed air, but by making use of a primary firing chamber (K), which is in direct communication with the combustion chamber of its particular cylinder by way of a small passage. This channel is lined by a funnel-shaped bush (B), which in turn has several very minute holes at its lower extremity. Immediately over this bush is the fuel valve (Z).

The fuel is pumped into the primary firing chamber (X) under a pressure of approximately 70 atmospheres, and the fuel valve sprays it, through this chamber in the form of a hollow cone which impinges only against the hottest portions of the funnel-shaped bush. The remainder of the firing ,chamber is thoroughly cooled by the water jacket, as is also the fuel valve. As the pressure rises in the cylinder air is forced upwards through the funnel (B) into the primary chamber (K), and when the Crankpin readies a point about 80 degrees before top dead centre the fuel is injected in the form of a fine spray in a manner already described. Directly the highly heated air meets the sprayed fuel

combustion occurs, and the pressure thus developed forces the excess of fuel in the chamber to he sprayed into the combustion chamber proper, where the rapid burning of the fuel continues.

Another cross-sectional view Shows the fuel sprayer, which, it will be noted, has an automatic spring-loaded valve. When fuel is pumped into this sprayer it lifts the valve spindle against the pressure of the spring and after passing the conical seat escapes through an _ opening between the point of the valve and a slightly smaller hole in the nozzle plate.

The third cross-sectional view shows the fuel pump and its operating gear. The pump itself is of the simple plunger type without glands and with an automatic suction valve (S) and a pressure chamber (D). Both suction and pressure valves are made of steel and have conical castings. The pressure valve is spring-loaded, but the suction valve closes under its own weight. Alteration of the 'stroke of the pump plunger is affected by a centrifugal governor ; consequently, it influences directly the amount of fuel injected.

The pump plunger (P) is raised by means of a cam and lowered automatically by the pressure of a coil spring. The cam (N) consists of a hardened and ground cylinder pressed into a disc in such a manner that it projects about 2 mm. above the surface • of the disc ; this method of construction presents the advantage that the cam can be made most accurately without difficulty and does not involve elaborate grinding machines and formers.

• The cam (N) lifts a finger lever through the medium of a roller, and the end of the finger acts on the tappet of the pump plunger. This tappet is employed in the mechanism to avoid, so far as possible, side pressure on the plunger. The free end of the tappet is supported by an adjustable nut. The tappet continues past 'this nut into a guide. The centrifugal governor controls the position .of the tappet-operating finger by raising or lowering its fulcrum. Thus, When the fulcrum point is moved upwards the roller on the finger is lifted, and, consequently, the stroke of thepump plunger is decreased. To pieserve the governor gear from the impact blows of the cam and roller a guide plate is provided, and the pivot of the finger slides on this guide, contact being maintained between it and the guide by means of a spring ; the angle at which the guide plate is located is such that the thrust of the roller is at right angles to the path of the pivot.

The governor itself consists of a pulley shaft which revolves at crankshaft speed and carries at its right side two governor weights. These operate, by means of finger levers, a rod passing through the centre of the pulley shaft, and thus transmit the movements of the weights to a bell-crank lever, which is connected to a push rod.

The upper end of this push rod is connected to the forked end of the lever on which the pump-plunger finger is pivoted. A spring is employed to limit the action of the governor and the pressure of this spring is regulated through a double cam lever which can be adjusted from the driver's seat, either by a cable or a screw. The last-named is used only when the unit is required to run at a constant speed, as in threshing. This governor permits a variation of engine speed from 250 r.p.m. to 800 r.p.m., quite independently of the load on it. Both the fuel pump and the governor are housed in a cylindrical casing mounted on a saddle at the side of the crankcase and held by suitable straps in the same manner as is usually employed for the magneto of a petrol engine.

It is almost impossible to start an engine of this size by hand unless means be provided lor lowering the compression, but in such cases the heat produced by the reduced compression of the air is insufficient to ignite the fuel, and it is necessary to provide some auxiliary and temporary means of ignition.

In the Benz Diesel engine the compression pressure is reduced by utilizing release cams, which lift the exhaust valves slightly off their seatings during their respective compression strokes. By this means the pressure is reduced to approximately 12 atmospheres.

Much interest attaches to the temporary ignition device, which is almost the acme of simplicity. It consists of strips of blotting paper, 30 mm. by 40 mm. long, each rolled up to a diameter of 8 mm. These rolls are inserted Into special holders and are saturated

in a solution of saltpetre, which gives them the property of glowing for a long time when fire is applied to them. The cartridges thus formed are inserted in the chambers K, to which we have already made reference, being screwed in by handles (L), which protrude outside each cylinder. Each cartridge ignites the fuel in its primary combustion chamber. Directly ignition takes place full compression can be restarted and the engine is immediately ready to deal with its full load.

Further to facilitate starting a third position of the camshaft is provided, when in this position the exhaust valves

are kept wide open. This is found useful to enable the crankshaft to be rotated as it is freed from any load, so that a certain amount of energy can be stored up in the flywheel. Then by changing the cam-gear control froin'the third position to the second, where no compression is provided, the firing of the charge at the first attempt is practically assured.

We will now deal with the system of lubrication. Oil is drawn from a sump by a pinion-type pump, being passed through a filter on the way. It is then forced to the rear main bearing, from which it is fed to the other main bearings through the drilled crankshaft.

This Diesel power unit can be run on gas, oil, yellow and brown paraffin, brown coal-tar oil, Texas oil and raw

oil when its viscosity is not too great. Experiments have shown that the fuel consumption from approximately half load downwards is almost independent of the load. In practical use, with a varying load the mean fuel consumption, as compared with that of a petrol engine, is even less than is the case with the two types of engine at full load.

Owing to the absence of the carburetter and the definite supply of fuel in measured quantities, it is claimed that this engine is always provided with the correct quantity of fuel, and that it affords the greatest turning moment even with the slowest number of revolu

films. It can be run light for long periods without damage or disturbance to the ignition ; there is also no danger of oiling up or sooting..up the combustion chamber.

An important point is that the exhaust is quite invisible when the engine is under full load.

In view of the excellent figures regarding economy in fuel consumption, the fact that in the Benz engine the cooling required is about 30 per cent. lower than is the ease with a petrol engine of equal power need not occasion surprise. Cooling is effected by water, which is passed through a radiator by an ordinary centrifugal pump.

We are indebted to our German contemporary Motar for the information and illustrations in this article.

Tags

Organisations: US Federal Reserve

comments powered by Disqus