It has now been shown that the combustion process in the Diesel engine should be controlled to avoid both excessive maximum cylinder pressure and an excessive rate of pressure rise, in terms of crank angle. At the same time, the process should be so rapid that substantially all the fuel is burned
early in the expansion stroke.
In discussing methods used for attaining these objectives, it is convenient to divide Diesel engines into two types :
open-chamber engines and divided chamber engines. These two types will here be defined as follows:
Open Chamber Engine
An open combustion chamber is one in which the combustion space incorporates no restrictions that are sufficiently small to cause large differences in pressure between different parts of the chamber during the combustion process. The chambers illustrated in Fig.1 come under this classification.
Fig.1 Open combustion chambers. Chambers a-g use air swirl from tangential inlet ports or shrouded inlet valves; chamber h is NACA type using air motion from displacer on piston. Squish from small clearance over piston increases from a to f. Dashed lines indicate direction of sprays.
A divided combustion chamber is one in which the combustion space is divided into two or more distinct compartments, between which there are restrictions, or throats, small enough so that considerable pressure differences occur between them during the combustion process. The chambers illustrated in Fig.2 are divided combustion chambers. When the burning starts in a chamber separated from the piston by a throat, divided chamber engines are often called pre chamber engines.
Fig.2 Typical divided combustion chambers. Black areas indicate high-temperature inserts, usually of stainless steel. Dashed lines indicate direction of spray; arrows indicate direction of air swirl.
Open-Chamber Engines. In the open-chamber type, the mixing of fuel and air depends entirely on spray characteristics and on air motion, and it is not vitally affected by the combustion process itself. In this type, once the compression ratio, maximum operating speed, and operating temperatures are selected, the delay angle is determined chiefly by fuel characteristics.
Engines of this type are very sensitive to spray characteristics, which must be carefully worked out to secure rapid mixing. Subdivision of the spray and the use of high injection pressures are usually required. In the case of high-speed (small-cylinder) engines, mixing is usually assisted by swirl, induced by directing the inlet air tangentially, or by squish, which is air motion caused by a small clearance space over part of the piston. Chambers d to h in Fig.1 have notably large squish areas. All except h employ swirl induced by a shrouded inlet valve or a tangential inlet port.
The effects of air motion on the Diesel combustion process have already been mentioned in the discussion of the MIT rapid-compression machine.
It is evident from Fig. 3 that air motion speeds up the combustion process in the second and third stages of combustion. This relation is also confirmed by experience with other engines. Reference 3.191 is an excellent study based on photographs of injection and combustion in typical open and divided-chamber cylinders, and includes a very complete discussion of squish and swirl in engines of types d, e, and f of Fig.1. This work shows evidence of much smaller radial velocities due to squish than have been assumed; this leads to the question whether the observed beneficial effects of squish-type chambers are due to the squish motion itself or to the increasing swirl velocity in the piston cavity as its diameter is made smaller.
Fig.3 Effect of air swirl on pressure-crank-angle diagram; sleeve-valve cylinder, type g, Fig. 1 N = air-swirl rpm, n = engine rpm
The latter increase follows from the conservation of angular momentum in the charge. Figure 3 shows the influence of the rate of swirl on the pressure crank- angle diagram for an engine of type g of Fig.1 The swirl rate was varied at constant rpm by changing the angle of some guide vanes that were placed in the inlet ports. It is interesting to note that the delay period is unaffected by the swirl rate, although the effect on the rate of pressure rise and on maximum pressure is large. The improvement in mep and efficiency due to increase in swirl is very marked in this case because the fuel spray was designed in the first place to depend on swirl for proper mixing. However, a considerable gain in performance is usually obtainable
by increasing the swirl, even when the optimum spray for use without swirl is used. The beneficial effects of increasing swirl seem to be attributable, to a great extent at least, to faster mixing during injection on account of the increased velocity of the air with respect to the spray. Increasing swirl is accompanied, in nearly all cases, with increased small scale turbulence, which may assist the mixing process during all stages of combustion.
The M.A.N. “Meurer” Combustion System, Fig.4, shows a combustion system developed about 1954 by the Maschinenfabrik Augsburg- Nurnberg A.G. of Germany for small, high-speed engines. It differs from other open-chamber engines in being designed so that the fuel spray impinges tangentially on, and spreads over, the surface of a spherical cavity in the piston.
Fig.4 M.A.N. type M combustion chamber. Fuel jets impinge tangentially on surface of spherical cavity in piston. Shrouded inlet valve is used to give rapid air swirl in direction of arrow.
It is known that, except perhaps at light loads, there is some impingement of the spray on the combustion-chamber walls in all successful Diesel engines. However, until the advent of the Meurer design it had generally been assumed that fuel-spray impingement was undesirable. The theory behind this system is that enough of the spray will ignite before impingement so that the delay period will be normal, while the bulk of the spray will have to evaporate from the cavity walls prior to combustion. Thus, the second stage of the combustion process is slowed down, avoiding excessive rates of pressure rise.
In practice this engine gives good performance even with fuels of exceedingly poor ignition quality, such as motor gasoline. Its fuel economy appears to be extremely good for an engine of small size. From this fact it may be concluded that the third stage of combustion is not unduly extended, and that spray impingement can be employed with useful results. Evidently the advantages of this design are not great compared
with those of more conventional designs, since it has been used in only limited quantities although it has been available both in the U.S.A. and in Europe for many years.
Divided Combustion Chambers. Divided types of chamber (Fig. 2) have been developed chiefly for use in high-speed (small) engines, in an attempt to overcome some of the limitations of the open-chamber type. In the case of divided combustion chambers the following possibilities may be realized:
1. Extremely high air velocity through the throat during the compression stroke, with resultant intense turbulence and also, in most cases, swirl in the pre chamber.
2. The first and second stages of combustion can be forced to take place within a space whose structure is so strong that much higher pressures and higher rates of pressure rise can be tolerated in this space than can be allowed in the space over the piston (Fig.5).
Fig.5 Pressure-crank-angle curves from the combustion chamber and the air-storage chamber of a Lanova type cylinder, type a, Fig.2.
3. The mixing process may be greatly accelerated by the early stages of the combustion process itself. At high fuel-air ratios, combustion is incomplete in the pre chamber because of insufficient air, and the high pressure developed by the early part of combustion projects the unburned fuel, together with the early combustion products, into the other part of the chamber with very high velocities, thus causing rapid mixing with the air in the space over the piston.
4. It is usually possible to allow all or some part of the walls of the pre chamber to run at very high temperatures, thereby tending to reduce the delay as compared with that with open-chamber engines using the same fuel. The sections shown in black in Fig.2 are designed to operate at high temperature. These parts usually take the form of stainless steel inserts arranged to have a rather poor thermal connection with the cylinder walls, owing to a loose fit. Such inserts run very hot after the engine is properly warmed up. They do not, of course, assist in the cold-starting process.