ON MODERN DIESEL ENGINE DESIGN

ON MODERN DIESEL ENGINE

DESIGN

1-low the Heavy Oil Power-Unit is being Developed. IN the early days of Diesel engines there seemed to be little chance that they would ever be a serious rival for light transport work to the highly successful petrol engine ; but so great has been the progress made in the devel opment of high speed editions of this type, that it is far from unlikely that sports car engines using heavy oil will

soon be with us, and therefore it will not be out of place to consider the advantages of this type and also the difficulties at present confronting designers. To most readers of MOTOR SPORT, the .functioning of the petrol engine is no mystery, at any rate in its general prin ciples, but although many people make a habit of referring to the heavy oil engine as the car engine of the future, they are distressingly vague as to the exact work ing of the same. It is therefore probably ea viest when discussing Diesel engines,

especially of the high-speed type, to draw parallels. where possible between it and its petrol-driven brother. In general mechanical characteristics, of a

there is, of course, a great similarity between the two types, and when working at one time on an engine which had spent its youth as the motive power of a German submarine, the writer was particularly struck by its resemblance, otherwise than in point of size, to a high efficiency car engine. Six cylinders, overhead valves operated by an overhead camshaft, pistons with very narrow rings, all these things spoke of speed and efficiency in terms known to all car enthusiasts.

It is, however, in its actual methods of turning fuel into power that the great difference is seen, and it is with the difficulties which arise, and the recent work which has been done to overcome them, that this article proposes to deal.

The Difficulties.

That these difficulties are well worth overcoming will be obvious to everyone who gives the matter a moment’s thought. The first reason is the fact that fuel

son is the fact that fuel ‘oil costs something in the region of 4d. per gallon in bulk, and even if this could only be retailed to the motoring public at a slightly higher price; the saving would still be enormous. The second reason is the higher thermodynamic efficiency of the Diesel engine.

The expression for the efficiency of a Diesel engine differs from that ‘ of a petrol engine owing to the difference in the cycle employed. This expression, however, only applies to a slow-speed engine. and is, even then, only approximate. It is found that, as the speed of a Diesel engine rises, the thermodynamic conditions break further away from the constant pressure cycle, and become more nearly similar to that of the petrol engine, which works on a constant volume cycle. It is this tendency for the high speed Diesel to work under similar conditions to the petrol engine, that makes it easier to draw parallels between the two types,

and also to see how the Diesel will have a greater efficiency. The efficiency of the otto cycle is given by the expression 1-1 —) where r is the compression ratio, and y is the ratio

of the specific heat at constant pressure to the specific heat at constant volume of the working fluid, and for air is_approximately 1.4. As the working fluid in all internal

combustion engines consists mainly of air, no appreciable error is introduced by taking the figures for air as applying in all cases. Prom the expression given, and the fact that in high speed Diesels this expression is nearly correct, it will be seen that in both classes of engine the efficiency increases with an increase in the compression ratio. This immediately brings us to the out standing difference between the petrol engine and the Diesel. In the former we cannot employ a compression ratio higher than that at which detonation takes place, which, with the normal fuels available to the public, gives a limit of about 6. In the latter, which depends on the adiabatic compression of air to such a high temperature that it ignites the liquid fuel injected into the cylinder, we cannot use a compression ratio lower than that at which self-ignition will

occur. This is about 12 in a normal design. Many sports car engines use a compression ratio higher than 6, and there are on the market to-day several special fuels, suitable for general use, such as Racing Shell, which permit of somewhat higher ratios ; while in special racing engines using very special fuels, containing alcohol and various antidetonating chemicals, compressions as high as

12 are used. These however, although useful in research work, are not and probably never will be, except in much modified form, available to the public.

This shows that the normal Diesel engine begins, in this particular characteristic, some way beyond where the petrol engine leaves off, and the only thing that has so far kept it from more general use, is the fact that it has been only a comparatively low speed engine. Recent researches however by the Royal Aircraft Establishment, and experiments by H. R. Ricardo, whose name is well known to all who have followed the development of the internal combustion engine, have shown that it is not only possible to build heavy oil engines with a piston spe. d of 2,000 ft. per minute and over, but that the overall efficiency is higher than with any

low speed heavy oil engine of the past. Naturally, the most important factor limiting engine speed is the rate of combustion of the fuel in the cylinder and the L iesel has so far been at a disadvantage in this respect for various reasons. Firstly, the fuel and air in a petrol engine are thoroughly mixed before entry to the cylinder, while the process corresponding to carburation does not take place

in a Diesel engine till just before the top of the compression stroke, since only air is drawn in on the suction stroke. The exact time and rate of injection of fuel corresponds to the ignition timing of a petrol engine, except that in the Diesel carburation and ignition occur simultaneously. According to Ricardo, the ignition process of a petrol engine takes place in two stages, and that of a Diesel in three. The first stage is similar in both types, and cons sts of a delay period in which a nucleus of flame is built up from which the whole is. to be ignited. During this period there is no appreciable pressure rise, and the process itself takes a certain time independent of engine speed, and is nearly constant for any given set of conditions of fuel, mixture strength, pressure, etc , in the petrol engine, and on these factors and also the fineness of the initial jet of fuel in the Diesel engine. From this can be seen the reason for advancing the ignition timing in

proportion to the engine speed.

The second, and with the petrol engine, the final stage consists of a rapid spread of the flame through the cylinder in conseqtlence of the inevitable turbulence which occurs in any engine, and which alone makes it possible for a high speed engine to function at all. The comparatively recent introduction of so-called ‘ turbulent heads” has given rise to a general idea that turbulence is something new. It has always occurred however, in varying degrees though imperfectly understood, and what has now happened is that recent knowledge on this subject has enabled designers to arrange the nature and degree of turbulence required for certain conditions, and so get greatly improved results. This turbulence is dependent on engine speed, and not on time, and thus the second stage of combustion in each case, consisting of the pressure rise, is nearly constant in relation to the position of the piston at all engine speeds.

In the Diesel engine however, only a proportion of the fuel is actually in the cylinder during this stage, as it is still entering, and this brings us to the final stage where, owing to the very high temperature and pressure now prevailing the fuel burns completely as it enters, giving the power of mechanically controlling further rise of pressure by the rate of Injection. But ,n the petrol engine there is no further control, and when the ignition advance, and degree of turbulence, are sufficient to ensure reasonably complete combustion, the maximum pressure at high speeds will be extremely high, and will give rise to violent detonation. If the compression ratio is kept high, the only way to avoid this excessive pressure rise is by the use of ingredients in the fuel which will slow down combustion. This is done in most ordinary fuels to a certain extent, but it cannot be done to

Inlet ports admit air tangentially so giving a rotational swirl and additional turbulence near the top of the stroke is obtained by the air being “squirted” into the head from the cylinder.

excess or, in controlling the final stage of combustion, the first stage will be delayed.

This shows at once the limitations of the petrol engine in the matter of compression ratio, but in the Diesel this excessive maximum pressure is avoided by there only being a small portion of the fuel in the cylinder at the beginning of combustion, and the remainder, which is admitted during the final stage, gives the opportunity of maintaining the required pressure by varying the rate of injection.

Combustion Stages.

The points which have so far been considered are applicable whatever the engine speed, but when the speed is raised considerably, designers are confronted with the problem of ensuring that the various stages in the combustion take place snfficiently rapidly to be completed each cycle.

The first stage, or building up of a flame nucleus, being dependent on time and not on engine speed, cannot be* speeded up by alteration in combustion head deign, and this must be achieved by increasing the tendency of the fuel to self-ignite.

This can be done either by increasing the compression ratio with the same fuel, or adding some such ingredient as amyl nitrate to the fuel to make it detonate more easily with the same compression ratio. The second phase, or spread of flame through the combustion chamber, is dependent, as in a petrol engine, on the degree of turbulence. An organised flow is of no great assistance at this point, and. violent turbulence which will thoroughly” stir things up” is what is required. Too much turbulence will re.ult, as in a. petrol engine, in roughness of running due to the sudden pressure rise to a high maximum. It is therefore desirable to cut the second stage short and merge it into the last as soon as possible. This

entails a certain minimum temperature being attained by tha second stage, before the fuel will burn steadily from the jet.

It is in the research on this part of the cycle and the methods now used to control it in high speed engine ;, that so much is due to the work of H. R. Ricardo, who has carried out a vast number of experiments with a view to finding suitable cylinder head and valve designs for heavy oil engines.

In his later work he has chiefly employed sleeve valve engines, as with these it is easily possible to arrange, by deflectors on the inlet ports, any desired direction and rate of flow of the air, and thus find the best conditions. These can be permanently maintained in the final design by casting the ports according to the results obtained from the experiments with variable deflectors. The great essential in the third, and main, stage is a high relative velocity between the air in the cylinder, and the fuel

from the jet. Owing to the difficulties of very high pressure injection of the fuel, and the fact that the velocity of the particles is not maintained far from the jet, the method adopted has been to inject the fuel normally and arrange that the air shall move past the jet at the required velocity. The full pressure then required is in the region of 2,000 lbs. per sq. in. or more, and as this is considered fairly low, the difficulties of using a really high pressure may well be imagined.

The air speed past the jet is obtained by arranging the inlet ports so that the air is admitted tangentially and sets up a rotational swirl, of which the speed can be adjusted by deflectors as mentioned above. Tne rotational speed is further increased by making the actual combustion chamber about half the diameter of the cylinder, and therefore doubling the rotational speed for the same induction velocity. Thus by placing the jet near the outside of the combustion space, a state of affairs is obtained which is remarkably similar to that in the choke tube of a normal petrol carburettor, except that in this case combustion is taking place at the same time as carburation. As the air velocity is proportional to the engine speed this arrangement works as well at high speed as low. Experiments have shown that good results are obtained with a rotational speed of 10 to 12 times the crankshaft speed, this result being obtained by varying the rotational speed by the deflectors while under test, and actually measuring, by an internal auemo

meter, the air speed in the combustion chamber at various engine speeds.

It has, however, been shown that some degree of indiscriminate turbulence is required, especially for the second stage of combustion, and it is here that this particular cylinder head form serves a dual purpose. At the top of the stroke the air is ” squirted ” from all round the cylinder and this produces a certain degree of “rough and tumble” in the cylinder head to supplement the ordered flow caused by the suction stroke. The whole process of combustion in Diesel engines has now been made the subject of sufficient research to bring it nearly into line with the petrol engine, and the factors limiting their speed are now very similar. Considerations of

mechanical strength, and balance of moving parts have already received ample attent.on as the result of racing, and the other chief difficulty, that of getting a full charge into the cylinder at high speeds, can be solved as in the case of the petrol engine, by supercharging. Moreover, owing to the supercharger having to deal with air only, and not having to respect the feelings of a sensitive carburettor, the arrangement will be greatly simplified.

The sleeve valve is; of course, only one method for the high speed Diesel, and we have mentioned it as representing the latest research of a well-known authority. One of the most successful high speed heavy oil engines on the road to-day is,

of course, the Mercecks-Benz, which’ s now a thoroughly tried job, which has saved many far sighted transport owners vast sums in running costs, and it is very likely that this astoundingly enterprising firm will be among the first to put a private car on the market with a similar engine.

It is always hard to forecast developments, especially in the world of motor racing, but we shall not be surprised to see within a few years, a racing car, similar in outward appearance to the Grand Prix racers of today, but devoid of plugs or magneto, having a supercharger but no carburettor, and winning races on a fuel costing under sixpence per gallon !

W. S. B.