THE AUTOMOBILE DIESEL ENGINE.
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A Survey of the Situation Arising Out of the Developments in the Year 1928. The Type of Engine on the Market and the Fuels Available.
By Dudley E. Batty, M.I.A.E.
THE automobile Diesel engine has been very much J. in the limelight recently, both in the technical Press and the daily newspapers. This is, no doubt, due to the successful road demonstrations of the Mercedes-Benz heavy-oil-engined lorry. The focus of the limelight on this chassis has been so well directed that the efforts and designs of other manufacturers have been almost blotted out. It is well, therefore, to restore the balance somewhat, by reviewing the situation and setting down the work of others in the development of the high-speed heavy-oil engine.
Those Responsible for the Development of the Engine.
The development of the automobile Diesel engine has mainly been the work of Continental firms such as Mercedes-Benz, M.A.N., Maybach, Junker, Saurer, Bosch, Deutz, 'etc., whilst in this country little has been done in the automobile line. Beardmore's, however, have, for a considerable time, been designing solid fuel injection engines for rail cars and airships. The Attendu Co., in America, have been experimenting with this type of engine for aircraft.
Looking around upon the various types that have been designed the development of the automobile Diesel engine seems to have arrived at the stage st which it can he said that the solid-fuel injection method is accepted as the best way of introducing the fuel into the engine. This remark holds good whether the engine is a four-stroke-cycle or two-stroke-cycle type. The majority of makers has adopted the fourstroke-cycle design.
The original Diesel engines were all worked with airblast injection and by this method a high degree of turbulence was set up in the combustion chamber, with the result that very effective distribution of the fuel In thecompressed air was attained. Solid-fuel injection, however, lacks this useful quality and other means have to be adopted to obtain the necessary oil diffusion.
One of the earliest methods of obtaining turbulence was the Brous system, consisting of an anti-chamber into which the fuel was first injected. It ignited there c26 under the compression temperature and then blew through small holes at the end of the chamber into the combustion space, igniting the rest of the air.
The Mercedes-Benz use this system modified, no doubt, to suit their particular designs. The M.A.N., on the other hand, originally obtained turbulence by directing two sprays of fuel tangentially under extra high pressures. This method has since been considerably modified, as only one jet per cylinder is now used instead of two. The Helios engine employs one injector per cylinder. The injector and fuel pump are combined into a unit per cylinder, the pump portion being driven from a camshaft on the opposite side of the engine to the valve camsbeft. The fuel is, therefore, injected under high pressure at the side of the cylinder, sweeping across the top of the piston. The Maybach engine, so far as the writer knows, has only been applied to rail cars and such like, entailing power units of 150 h.p. and upwards. This engine Employs the air-blast system of fuel injection.
The Bosch Co., whilst not being a manufacturer of cars or lorries, has developed the Acro system of combustion for Diesel engines in order to assist it with the sale of its fuel pumps. The Acro system is employed, for instance, amongst others, by the Saurer Co. A diagrammatic sectional sketch of the Saurer engine was shown in The Commercial Motor for June 26th, 1928. This illustrates the way in which the fuel injector sprays the fuel into a venturi opening, where it combines with the air compressed in an air bottle situated in the cylinder head. Another variation of the Acro system places the air bottle in the piston and•the injector in tile cylinder head. The Junker type of engine is fairly well known in the industrial field. It works on the two-stroke-cycle principle with opposed pistons, with a scavenging pump operating above the main pistons. It has now been successfully developed for automobile duty and is used by the Peugeot Co., of France.
The ,Deutz engine works on the principle of the pre-combustion chamber. All these designs aim at the same thing, namely, by setting up turbulence, to saturate, the compressed air with fuel, so that the latter is completely consumed.
Other designers :.obtain this turbulence by aiming the injected fuel into and against a recess, so that rapid air currents are set up. ,A large number of these engines make use of the now well-known Bosch fuel pumps and injectors, which may almost be .considered as being to the Diesel type of engine what the magneto and sparking plug are to the petrol engine, except that the ignition devices of the petrol engine are not concerned witlk the 'actual supplying of the fuel.
The Chief Drawbacks in Heavy-oil Engines.
It has been previously pointed out that the two main drawbacks in these heavy-oil engines are:—
(a) Excessive weight, (b) Limitation of engine speed,
resulting in a high weight/horse-power ratio. Much progress has been made towards rectifying these defects. Excessive weight has been reduced by careful designing and the use of aluminium alloys. By the accurate and careful manufacture of the fuel pumps it is possible to reach speeds of 1,600 r.p.m., where in the past 1,000 r.p.m. was a maximum. The Bosch pumps are said to be capable, on a four-strokecycle engine, of delivering accurately measured quantities of fuel up to a crankshaft speed of 2,000 r.p.m.
Another feature of the Diesel engine which makes it heavier than a modern petrol engine is the low mean effective pressure under which it operates. The modern petrol engine will develop a b.m.e.p. of 100 lb. per square inch, svhilst the Diesel engine gives its best results at 80 lb. per square inch. It is a fact that the greatest economy with these engines is obtained with b.m.e.p.'s ranging from 50-80 lb. per square inch. The b.m.e.p. can be increased to, say, 90 lb. per square inch with a slight• loss of fuel economy, by increasing the fuel injected, but if this be carried too far, incomplete combustion and smoke will be the result. If the exhaust gases be analysed when combustion is complete, it will be found that there will be a large excess oe oxygen. This demonstrates the fact that the whole of the air taken by the engine cannot be utilized. How the Consumption of the Diesel Engine Compares with the Petrol Type.
Before completing these notes a 'few words should be said from the reneral economic side of the situation. The Diesel engine consuming .45 lb. of fuel per b.h.p.hour has a thermal efficiency of 31 per cent. A petrol engine consuming A lb. per b.h.p.-hour has a thermal efficiency of 22 per cent. In addition to this, the Diesel maintains its high fuel economy over a bigger range of loads than is possible with a petrol engine. The Diesel also develops high torque at low speed. Both these features are invaluable. Leaving out the price of fuel, the Diesel lorry will travel 30 per cent. to 40 per cent, more miles to a gallon than will the petrol lorry.
The fuel used in these engines can be chosen from a variety of so-called heavy oils, that is, oils having a specific gravity above, say, .85 and an initial boiling point of about 200°C. The oil generally recommended for use with these automobile Diesel engines is known as gas oil, which is the fuel that distils next to paraffin (which passes off at about 200°C.). The gas oil distils at between 200° and 350°C. The remainder is known as fuel oil. An oil known as Diesel oil is marketed, being a mixture of gas oil and fuel oil. This latter oil can be purchased in large quantities at 4d. per gallon, whereas the gas oil will cost from 51d. to 61d. per gallon. It is of the utmost importance that the oil should be free from any solid matter which might block the small passages in the fuel pumps and injectors.
The analysis of the oil should show no asphalt and less than 2 per cent. sulphur content. It should be borne in mind ln comparing petrol with heavy oil that whilst the calorific value per lb. is approximately the same, a gallon of petrol will weigh 7.5 lb., whilst the heavy oil will weigh 8.5 lb. These engines will operate without a smoky exhaust after the first starting from cold, but the exhaust has a distinctive odour, more pungent, perhaps, than a petrol exhaust, but not so acrid. It remains to be seen how this feature will be taken when a large number of these vehicles is put on the road.