In the previous issue I covered spark ignition combustion, and showed that it takes place in a homogeneous mixture of fuel and air. The amount of mixture is usually determined by the position of the throttle pedal, which controls a butterfly valve in the intake manifold. By contrast, diesel combustion takes place in unthrottled air, meaning that the amount of air entering the engine depends only on the engine speed, and not on the position of a butterfly valve. The energy delivered is controlled by the amount of fuel being added, as determined by the accelerator pedal position. The compression ratio is high enough for the heat to cause spontaneous combustion as soon as the fuel is injected.
The previous article also showed that combustion is a process that causes a hydrocarbon fuel, such as petrol or diesel, to combine with oxygen to form carbon dioxide and water. This happens when such a fuel is mixed with air in the AF ratio (air to fuel ratio) of close to 14,7:1 by mass, and then exposed to a spark or heat. If only carbon dioxide and water are formed, the combustion is known as stoichiometric (ie chemically correct), but if carbon monoxide and unburnt hydrocarbons also result, the combustion is imperfect. The latter condition prevails in all engines because of imperfect mixing and non-ideal AF ratios. Impurities in the fuel also lead to the formation of other compounds, but that is not part of the combustion process under discussion.
The mixture is often described by the equivalence ratio (lambda ratio), which is defined as the prevailing AF ratio divided by the stoichiometric AF ratio. By this measure, lambda = 1 will imply that the actual ratio is stoichiometric, lambda < 1 will denote a rich mixture, and lambda > 1 will mean a lean mixture. Diesel combustion is considerably slower than petrol combustion, with the result that very few diesels are able to rev much beyond 4 000 r/min. One of the reasons is the lack of any pre-mixing of fuel and air, meaning that every drop of fuel has to find sufficient oxygen to combust. The process can be broken down into the following number of steps.
Injection
The fuel is injected just before TDC (top dead centre) as the piston rises on the compression stroke. The injection duration is usually about 20 crankshaft degrees, lasting from about 15 degrees before to about five degrees after TDC. Injection hole diameters range from 0,1 to 1,0 mm, delivering fuel at initial speeds of between 100 and 200 m/sec (360 to 720 km/h), but this is quickly lowered by drag, swirl and evaporation. In the ideal situation, the vapour jet, which is longer than the liquid jet, should extend all the way to the far wall of the combustion chamber.
Atomisation
The fuel drops should be as small as possible, to aid vaporisation. Diesel engines have been operating with fairly large drops for close to 100 years, because most injectors were fed by mechanical plungers, (one for each injector), that were not able to pressurise the fuel to a level that would enable it to pass through very small holes.
However, the common-rail revolution that started about 12 years ago changed the drop size dramatically. A common-rail is just a high-pressure manifold, mounted near the injectors, that is able to keep each injector supplied with fuel at high-pressure. The manifold is kept at a pressure of at least 1 500 bar by an electric pump. The injectors are opened by an electric impulse, sent by a control unit, that activates a solenoid or a piezo-electric device. This change has enabled the introduction of very small hole sizes, multiple holes, as well as multiple injection spurts. These innovations have increased combustion efficiency and reduced diesel noise.
By the way, piezo-electric crystals have the property that they change size when a voltage is applied to them. They also send out a current when under stress. The latter property is used to create a knock sensor, such as is used on many petrol engines to sense the start of combustion knock.Vaporisation
Neither diesel fuel nor petrol will burn in liquid form, so the fuel droplets have to vaporise, and this is where the drop size becomes important. The required heat comes from the air that has been raised to a high temperature as a result of the compression ratio (see graph on following page). Diesel compression ratios can be as high as 22:1, but there is a trend towards lower ratios. Higher ratios tend to improve cold-starting, but increase side thrust between the piston and cylinder bore, resulting in increased internal friction. Interestingly enough, some marine diesels manage to start on a ratio of 12:1, but in this case the fuel has to be heated, because they run on thick syrup-like crude oil.
About 90 per cent of the injected fuel vaporises within 0,001 seconds after injection. This action cools the immediate surroundings by draining some of the heat away, resulting in a slowing down of the rate of evaporation. The process may even stop near the core of the fuel jet, and only start again after additional mixing and heating has occurred.
Self-ignition Before it can combust, every fuel molecule must make contact with oxygen in the intake air. This starts a reaction that breaks down the large hydrocarbon molecules into smaller molecules that combust easier. The process is aided to a large extent by the high fuel injection speed, and the high temperature of the intake air, as well as the swirl and turbulence inside the combustion chamber. The net result is that more heat is liberated, so that the mixture temperature is raised to a level that can sustain continuous combustion. This starts about six to eight crankshaft degrees after initial injection.
The time from the beginning of injection to the start of combustion, called the ignition delay time (IDT), is an important diesel combustion criterion, because it not only affects the maximum attainable engine speed, but also the noise level of the combustion process. In addition, it does not change significantly with an increase in engine speed, so that injection initiation must occur earlier as the engine speeds up. IDT is decreased by an increase in temperature, pressure, engine speed and compression ratio, but, strangely enough, not by a reduction in fuel droplet size.
On the other hand, IDT is increased if injection occurs too early in the compression stroke, when the air temperature and pressure are lower, and also if the injection is late, because then the temperature and pressure have started to drop due to the piston being on its way down.
In both the above cases, abnormal IDT will result in a larger-than-normal amount of fuel being injected before the start of combustion, leading to an abnormal and sudden rise in pressure when combustion does occur. This leads to a rough combustion cycle and an increase in the characteristic diesel noise. On some of the latest engines, the IDT has been dramatically reduced by utilising three or more injection spurts, so that some pre-heating of the air takes place before the main injection spurts occur.
Combustion
The fuel leaves the injector as a liquid, but, within less than a millisecond, the evaporation and mixing process results in the layers of mixture strengths shown schematically in the figure at the top of the page. When a drop breaks off from the stream, it carries similar layers of mixture strengths with it. The result is that combustion starts simultaneously in various parts of the combustion chamber in the fuel-rich zones labelled B and C in the figure (lambda ratio between 1 and 1,5). Between 70 and 90 per cent of the fuel is now in a gaseous state, so that multiple flame fronts are able to spread rapidly into these areas, and consume any fuel/air mixture that is in a combustible state. The result is a rapid rise in temperature and pressure, causing a reduction in vaporisation time and ignition delay time for the remainder of the fuel still being injected. The further course of the combustion process is controlled by the rate at which fuel can be injected, atomised, vaporised, and mixed into a state where a combustible air-fuel ratio can be reached.
The last of the fuel particles take a long time to combust, so the complete process lasts for about 40 to 50 degrees of engine rotation, which is almost twice as long as the fuel injection period. The result is that the pressure remains high until the piston is nearly 30 to 40 crankshaft degrees down the bore.
Smoke production
Diesel engines are often blamed for producing an unpleasant amount of smoke, more correctly known as soot, which consists of small particles of carbon. It’s fascinating to note that carbon particles are always being produced partway through normal diesel combustion, but that most of these get changed to carbon dioxide before the combustion process is completed.
Every fuel stream and, consequently, most fuel droplets, contain areas of varying mixture strength, and normal combustion can only take place in areas where the mixture strength is not too far from the ideal. In the borderline areas, the combustion is imperfect, leading to the formation of carbon monoxide and carbon particles. If there is enough free air, turbulence causes many of these particles, as well as the carbon monoxide, to find more oxygen and form carbon dioxide. The result is that very few carbon particles survive to appear in the exhaust gas.
Experience has shown that as long as there is about 20 per cent excess air in the combustion chamber, the soot production will be minimal, but when this limit is overstepped, the engine starts to smoke. The carbon becomes visible in the exhaust as soon as the percentage of carbon particles in the smoke approaches 0,5 per cent. A visibly clean diesel exhaust is thus not entirely free of carbon, and some of the latest modern diesels have special particle traps that practically eliminate carbon in the exhaust.
It follows that smoking is usually caused by the engine getting too much fuel, whether due to injector wear or deliberate overfuelling. This anti-social practice is due to the fact that a diesel engine will produce more power when it is overfuelled, by contrast with a petrol engine, which can only increase its output to a limited extent when supplied with an over-rich mixture. The effect of overfuelling can be seen most dramatically when viewing the many videos on YouTube of the diesel-engined dragsters that have become the latest craze in the USA. There is often so much black smoke that the dragster can hardly be seen.Diesel fuel
Diesel fuel also has a rating system similar to a petrol’s octane rating, but instead of being a measure of a fuel’s ability to resist spontaneous combustion, it is a measure of the opposite effect, ie a fuel’s ability to combust without a spark. It is called the cetane number, and fuels with a low number tend to increase the IDT, whereas fuels with a high cetane number promote a shorter IDT. Cetane numbers normally range from 40 to 60, with South African diesel being in the 47 to 48 range.
The final say on diesel combustion belongs to Sir Harry Ricardo, one of the pioneers of combustion research, who in one of his talks explained it as follows:
“I am going to take the rather unconventional course, in a technical lecture, of asking you to accompany me in my imagination. Let us imagine ourselves seated comfortably on top of the piston, at or about the end of the compression stroke. We are in complete darkness, the atmosphere is a trifle oppressive, for the shade temperature is well over 500°C – almost a dull red heat – and the density of the air is such that the contents of the average sitting-room would weigh about a ton; also it is very draughty, in fact, the draught is such that, in reality, we should be blown off our perch, and hurled about like leaves in a gale. Suddenly, above our heads a valve is opened and a rainstorm of fuel begins to descend. I have called it a rainstorm, but the velocity of the droplets approaches much more nearly that of rifle bullets than of raindrops. For a while nothing startling happens, the rain continues to fall, the darkness remains intense. Then, suddenly, away to our right, perhaps, a brilliant gleam of light appears, moving swiftly and purposefully; in an instant this is followed by a myriad others all around us, some large and some small, until on all sides of us the space is filled with a merry blaze of moving lights; from time to time the smaller lights wink and go out, while the larger ones develop fiery tails like comets; occasionally these strike the wall, but, being surrounded by an envelope of burning vapour, they merely bounce off like drops of water spilt on a red-hot plate. Right overhead, all is darkness still, the rainstorm continues, and the heat is becoming intense; and now we shall notice that a strange change is taking place. Many of the smaller lights around us have gone out, but new ones are beginning to appear, move overhead, and to form themselves into definite streams, shooting rapidly downwards or outwards from the direction of the injector nozzles. Looking round again, we see that the lights around are growing yellowed; they no longer move in definite directions, but appear to be drifting listlessly hither and thither; here and there they are crowding together in dense nebulae, and these are burning now with a sickly smokiness, half suffocated for want of oxygen. Now we are attracted by a dazzle overhead, and, looking up, we see that what at first was cold rain falling through utter darkness, has given place to a cascade of fire, as from a rocket. For a little while this continues, then ceases abruptly as the fuel valve closes. Above and all around us are still some lingering fireballs, now trailing long tails of sparks and smoke and wandering aimlessly in search of the last dregs of oxygen, which will consume them finally and set their souls at rest. If so, well and good; if not, some unromantic engineer outside will merely grumble that the exhaust is dirty and will set the fuel valve to close a trifle earlier. So ends the scene, or rather my conception of the scene, and I will ask you to realise that what has taken me nearly five minutes to describe may all be enacted in one five hundredth of a second, or even less.”
