Bakerian L ecture Some problems in connexion with the development of a high speed Diesel engine By H. R. Ricardo, F.R.S.* (Delivered 6 November 1947—Received 12 November 1947) [Plate 4] When invited to deliver the Bakerian Lecture, I felt very diffident about accepting, for I am a mechanic rather than a scientist, and the tale I am going to tell is mostly one of mere plumbing and ironmongery. I have chosen, as a theme, my own personal experiences over the development of high speed Diesel engines, but I would like to emphasize, at the outset, that this was no single-handed effort for, not only had I the enthusiastic support of my small but very capable staff, but also the most unstinted help and advice from the technical staffs of manufacturing firms, from such establishments as the Royal Aircraft Establishment and The National Physical Laboratory, combined with financial help and encouragement from the Air Ministry and the Shell Company; so that nobody could have had happier or more favourable circumstances than I.
Bakerian Lecture 211 was, therefore, suitable only for stationary or marine use, but by far the largest consumer of fuel was road transport, which was limited to the use of petrol. It became apparent, therefore, that there would soon be a crying need for a light high speed Diesel engine with an all-round performance comparable with that of the petrol engine. Again the Air Ministry were anxious to explore the possibilities of a Diesel engine for aircraft.
212 H. R. Ricardo inflammation then takes place, is controlled almost solely by the intensity of turbulence. There will thus be first a delay period during the building up of the initial nucleus which will be more or less constant in time, followed by a pressure rise due to the spread of flame more or less constant in terms of crank angle, see figure 1, which shows two typical indicator diagrams taken from a petrol engine cylinder.
Bakerian Lecture 213 liquid droplet. We expected, therefore, to find a condition somewhat analogous to that in a petrol engine; first a delay period while the envelope of vapour was being formed during which a substantial quantity of fuel had entered the cylinder, without any appreciable rise of pressure or temperature; this would be followed by a rapid rise in pressure when the accumulated droplets became fully inflamed and this, in turn, would be followed by a less rapid and more controllable rate of rise when the temperature was so high that the droplets would burn as they entered with very little delay indeed. We expected, therefore, to realize a pressure time diagram much as shown in figure 2, somewhere about half-way between the constant volume and the constant pressure cycle. Figure 3 shows an actual diagram taken later from a high speed sleeve valve Diesel engine.
214 H. R. Ricardo would be a nuisance, in that, when idling, the engine would be unstable and, at any fixed setting of the pump, would tend either to run away or to peter out and stop. To cope with this instability, we developed a two-speed centrifugal, and later t 450 25° TDC 25° crank angle Figure 2. Diagram showing three phases of combustion process in compression ignition engine.
216 H. R. Ricardo a vacuum governor, the functions of which were to increase automatically the delivery of the pump whenever the engine speed fell below a certain pre-determined minimum, and to cut it off altogether when the limit of safe maximum speed was reached; between these two wide limits, the engine would be operated under direct hand or foot control.
Bakerian Lecture 217 oxygen was combined; if more fuel was injected, all that happened was that the excess could not find the necessary oxygen. We concluded, therefore, that we were suffering from lack of penetration and that the spray was concentrated into a small local zone near the apex of the combustion chamber. We rang all the changes of air movement with but little effect and we varied the injection pressure and the rate of injection, also with very little effect, but, do what we would, we could not burn more than about 20 % of the available oxygen. We therefore decided to abandon the pulverizing type of injector and substitute one of the watering-can type; here we were faced with the difficulty that, in order to maintain the pressure needed for high penetration at low speeds, the holes, in a small engine, would have to be both few and very small. The smallest hole we could drill was about 0-2 mm. and the greatest number of this size we could use was about 6 or 8—these we spaced out at various angles designed to give as uniform a distribution of the fuel jets as possible.
218 H. R. Ricardo We therefore constructed a new combustion chamber of cylindrical form, the diameter being about half that of the piston and the height rather less, and we arranged our air entry vanes to give a rotational swirl about the axis of the cylinder. We also brought our piston into very close contact with the flat annular portion of the cylinder head in order both to reduce as far as possible the volume external to the main combustion chamber and at the same time to produce, by ‘squish’, a forced vortex in the chamber. In the top and to one side of this cylindrical chamber and at about the radius of gyration, we fitted our single hole injector squirting straight downwards (see figure 5). The result was an immediate and striking improvement; at once we obtained nearly double the power we had reached before and we were able to run to the maximum speed of which the engine was capable. Indicator diagrams of the pressure changes in the cylinder showed that combustion was extremely rapid and this was confirmed by the observation that the indicated thermal efficiency actually increased with increase of speed up to the highest speed of which the engine was capable—about 2300 r.p.m.
Bakerian Lecture 219 We found also that by painting the inside of the combustion chamber and the crown of the piston with a white enamel paint, allowing it nearly to dry and then running the engine for a few seconds under power, we could get clear-cut markings both of the air movement, of the impingement, if any, of the fuel jet against the crown of the piston and also some indication of the temperature distribution. Thus, we learned that even with a plain fire hose jet and a fuel injection pressure of about 300 atmospheres, the penetration in an air density of 15 atmospheres was only just sufficient to reach the piston crown, a matter of less than 2 in. It appeared also that no matter how intense the air swirl, the main core of the jet was but little deflected while the whiskers of spray were carried inwards towards the centre of the chamber. Our anemometer now proved most valuable, in that it enabled us to calibrate our external guide vanes in terms of air swirl, which latter we defined as the ratio of air to crankshaft revolutions. It proved also that for any given setting, this ratio remained practically constant over the whole speed range.