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PROSPECTS
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of the
OIL ENGINE
THE compression-ignition oil engine, or Diesel, as it is generally, but incorrectly, called, is now being developed for use in road motor vehicles and is attracting a great deal of public attention.
This type of engine has been developed, as a prime mover for industrial purposes, to a high degree of efficiency, until it is now the most efficient heat engine available. The petrol engine, on the other hand, has been so well developed in the field of mechanical transport that, where initial cost and weight are concerned, it has no equal. Now, as a further step forward, the combustion efficiency of the compression-ignition motor is to be combined with the mechanical excellence of the petrol engine.
The question may be viewed from two points, first that of the manufacturer or engine designer, and secondly, and not less important, that of the user ; it is but natural to compare the relative merits of the two types of engine in each case.'
The main reason for the higher efficiency of the oil engine lies in its higher expansion ratio, since it employs a compression ratio of about 14 to 1, against the 5 to 1 of the petrol engine. Whilst the petrol engine takes into the working cylinder only sufficient air to deal with the fuel charge, and uses the whole of its oxygen content, the oil engine always takes in a full charge of air.
Unfortunately it cannot use more than some 80 per cent. of the oxygen available, but as there is always an excess of air, the gas temperatures on the working stroke are lower, a factor tending to improve the combustion efficiency still more. This effect is accentuated at the lighter loads and so it is important in the case of a road-vehicle engine which runs for the most part at loads well below the maximum.
The designer's main problem is that of bringing together in the working cylinder the fuel and air a26 charges in such a way that the maximum amount of oxygen will be found by the fuel. It has to be done in the very short time available—while the crankshaft turns through some 40 crankshaft degrees—as against the whole of the suction and compression strokes in the petrol engine, or roughly 1/9th part of the time. The method of controlling the air charge is by suitably shaping the combustion chamber and inlet port, whilst the fuel pump and injector deal with the fuel charge. Perhaps the design of the combustion chamber is the more important and there are some four types in general use.
Combustion-chamber Types.
In the open or non-turbulent type of combustion chamber, the air is compressed into a fairly compact space and the fuel is introduced in a number of small, finely atomized jets. It is made to penetrate into the air and search out the necessary oxygen, so that much depends on the fuel-injection system. High fuel pressures are used and accuracy in the aiming of the jets Is essential. The holes are extremely small, and great care must be taken to exclude all particles of grit which might clog them. The pre-combustion-chamber pattern has a small auxiliary chamber separated from the main combustion space by a number of small holes and into this the fuel charge is introduced at a comparatively low pressure. Partial combustion takes place here, so raising the pressure and causing the partly burnt fuel to be blown out through the small holes into the remainder. of the air charge.
This method appears to be a temporary solution, but owing to the heat losses in the passages between the auxiliary chamber and main cylinder and the difficulty of obtaining a high degree of distribution, it does not give as good results as the other-types. It is more suitable for use with a constant-speed engine.
Then there is that type in which the combustion chamber is separated from the working cylinder by a neck or restriction. Through this neck the air is forced on the compression stroke, thus setting up a high degree of indiscriminate turbulence. The fuel is injected into this chamber and, in the general mix-up which ensues, a fair amount of the oxygen present is found by the fuel.
The Principle of Rotational Swirl.
Lastly there is the method of rotational swirl which is best employed in conjunction with a sleeve valve. The air is admitted tangentially to the cylinder and rotates around the cylinder. This rotation persists throughout the compression stroke and the fuel is admitted in the form of a single jet at right angles to the direction of air flow, and at a comparatively low injection pressure.
This method differs from the others in that the air is made to search for the fuel. The speed of rotation of the air charge is directly proportional to the engine speed, so that the combustion is automatically speeded up as the engine speed increases. This results in a more flexible engine, and, in fact, combustion is perfectly satisfactory up to the highest speeds at which it has been mechanically possible to run the engines.
A separate fuel pump which has to meter and time the charge is used to, supply each cylinder. Many ingenious devices are employed, but, in general, there are two distinct classes—the ported type and the valve type.
By ported type is meant that in which the start and finish of injection are controlled by the plump plunger, whilst in the valve type, the start and finish of injection are controlled by closing and opening poppet Valves.
As regards performance there appears to be little to choose between the two; each can be made to work satisfactorily up to speeds of 1,500 r.p.m. or 3,000 r.p.m. of the engine. The ported type can be by far the lightest and most compact form of pump and so is very suitable as a vehicle-engine accessory. On the other hand, it calls for extreme care in manufacture and when worn it cannot be adjusted in any way.
The valve type of pump is heavier and more bulky, but it can be made undbr ordinary workshop conditions and should therefore be cheaper. Any loss of tune due to wear can soon be made good by grinding in the valves to their seats.
Of injectors there are also two main types. The open type and the closed, or differential-area type. Naturally the first is the simpler of the two, the only moving part being a small non-return valve, but its use depends much on the other components of the system. The closed type has a spring-loaded valve seated directly above the spraying orifice which is opened and closed by the fuel pressures alone. It needs considerably more care both in manufacture and when in use, but is in general ouite reliable. The two-stroke engine has not been discussed. It indoubtedly has great possibilities and is considered yy many people to be the ultimate solution, but its resent stage of development is hardly sufficient to ,varrant its serious consideration for road-vehicles. Likewise the question of supercharging has not been Jrought in. The oil engine lends itself to supercharging far more readily, but the supercharger can be applied to the petrol engine and so its use hardly comes into the argument.
In considering thZ application of the oil engine to a commercial vehicle, the ultimate view must be that of the user. His first consideration is that of running costs, and so it seems advisable to consider the problem on these lines and compare the costs of running oil and petrol engines.
The charges to be met are:—(1) Fuel. (2) Lubricating oil. (3) Maintenance. (4) Tyres. (5) Tax. (6) Insurance. (7) Wages. (8) First cost and depreciation.
Taking full-load bench tests the oil engine uses .42 pint of fuel per brake-horse-power per hour, against .55 pint for the petrol engine. Assuming that the engine generally runs round about half-load, the relative figures are 38 per cent, in favour of the oil engine. The present costs of fuel, taking in each case bulk supply of the same amount, are petrol lid, per gallon, oil fuel 31d. per gallon.
Why the Oil Engine Must Save on Fuel.
That is to say fuel costs favour the oil engine in the ratio of 4 to 1. The relative figures obtained from road tests seem to be about 8 miles per gallon for petrol against 12 miles per gallon for the newer type.
The argument may be advanced that as the demand for oil fuel increases so will its cost increase, and, further, there is a tax of 4d. per gallon on petrol as against nothing on the other fuel. Taking, then, the most pessimistic view, and assuming the demand for the heavier fuel to equal that for petrol (it is hardly likely to exceed it), the cost of the two fuels may eventually be equal. In this case, fuel costs favour oil engines in the ratio of 1.33 to 1.
There seems to be no reason why costs of lubricating oil should differ in the two cases.
The maintenance item rather appears to be the crux of the problem and one which can only be settled as the result of experience. No hard-and-fast rules can be laid down at the present stage. Taking mechanical considerations, the gas pressures in the cylinders are higher, but the designer will take this into considera: tion so that stresses and loadings will be equivalent to those of the petrol engine. It is probable that cylinder wear will be higher due to higher gas loading of the piston rings.
A more important factor is the time between top
overhauls, determined mainly, by the behaviour of the exhaust valve. This has to work under rather different conditions from the petrol-engine valve. The temperature of the exhaust gases is lower, but, on the other hand, in many types of oil engine combustion is often less complete and there is more possibility of particles of carbon sticking an to the valve seat and holding it up. On the whole there is little to choose between the two. Then there are the fuel pump and fuel valves to be pitted against the carburetter, sparking plugs and magneto.
Wear on tyres may be heavier owing to increased weight, but the difference should not be much. Tax may he heavier on the oil-engined vehicle if the weight be much in excess of the equivalent petrol-engined machine.
Insurance must favour the compression-ignition unit on account of the reduced fire risks. Driver's wages are the same in each case.
As already pointed out, the oil engine cannot make use of the whole of the oxygen of the air charge, and so in spite of Its higher expansion ratio it cannot develop the same power per unit of cylinder capacity as the petrol engine. Also, the gas pressures in the cylinder are higher roughly in the ratio of 750 to 500. On both of these points the engine must have a higher -weight/power ratio, therefore the cost of manufacture will be higher. Taking the cost of the -fuel-injection system as being about equal to the carburetter and ignition apparatus, the oil engine can never be made as cheaply as the petrel engine.
Low Poison Content of Exhaust Gas.
Apart from these there are a few details which may be of greater importance in particular cases. Owing to the excess air-fuel ratio there is seldom any appreciable amount of carbon monoxide in the exhaust, so that it is less poisonous than that of the petrol engine. As it is cooler, it may appear more smoky when discharged into the atmosphere.
Where starting is concerned, the compression-ignition motor is at a distinct disadvantage. A multi-cylinder engine of, say, 40 h.p. -cannot be swung by hand against a compression ratio of 14 to 1, and some artificial means has to be provided. It is usual to decompress all but one cylinder and cut in the remainder one at a time. Also an electrically heated coil may be fitted inside the combustion chamber, which is used at starting only.
It is certain that for road transport there is great scope for the new type of prime mover. The field Of the motorcar and the smaller commercial vehicles must always be left to petrol, whilst the oil engine may he expected to predominate in the larger sizes, for all heavy goods traffic, and where fuel costs form a large proportion of the total cost.