Fuel Combustion Fundamentals in IC Engines

Fuel combustion

A hydrocarbon fuel is an organic compound consisting of hydrocarbon molecules. A molecule is a group of atoms tied together by chemical bonds. A hydrocarbon fuel basically contains only C and H atoms, mixed with O and N from air. A fuel could also contain a small amount of sulphur, oxygen, nitrogen, and impurities such as water and sand. Depending on the carbon to hydrogen ratio, petroleum fuels are divided into families as follows: Paraffin (CnH2n+2 such as Methane, Ethane, Propane, and Butane), Olefin and Naphthene (CnH2n, such as Hexene and Cyclopentane), and Aromatic (CnH2n-6 such as Benzene). An internal combustion (IC) engine is a device which transforms the chemical energy of a fuel into thermal energy and utilize this thermal energy to perform useful work. A fuel is heated up either by an electric spark or by compression (diesel engines).  A four-stroke engine completes its power cycle in four stages (180° x 4= 720°): 1-suction  2- compression (ratio 6-10) 3-exansion (power stroke) 4- exhaust. The four stokes can be expressed through a pressure-volume diagram (p-V).  There is only one power stroke for every two revolutions of the crankshaft.  Fuel combustion effectively takes place between compression and expansion. Its lifetime is about 10% of a rotation (typically between 1 and 3 ms). There is time needed for the ignition lag and for the propagation of flame.

Four-stroke IC engine cycle (from http://www.thermopedia.com/content/880/)

 

Depending on the ignition strategy engines can be divided into two categories:

– Spark ignition (SI) engines: fuel is ignited using electronic spark plug just before the TDC causing instantaneous burning and the temperature rises to about 2000 °C. The fuel is normally mixed with air outside the combustion chamber.

– Compression ignition (CI) engines: In suction stroke only air is introduced then compressed at high ratios (16-20).Fuel is injected by the end of the compression stroke and compressed with air to the auto-ignition point (diesel has a lower self-ignition temperature than gasoline). CI engines normally have higher thermal efficiency than SI engines due to the high pressure ratio.

Fuel injection

The fuel injection system is an important component of an IC engine, and it is vital in the CI engines for producing a uniform distribution of fuel droplets throughout the combustion place. Large droplets generate higher penetration, but small droplets (less than 5 microns in diameter) are required for effective mixing and combustion. The mean diameter of the fuel droplets decreases as the injection pressure and the air density increase. The mean diameter increases with the increase in fuel viscosity and the injector orifice diameter.

Fuel injection takes place toward the end of the compression stroke (say 13° bTDC). Injection timing is more important in CI engines than in SI engines.

Spark Ignition

An electronic spark takes place before the end of the compression stroke. Theoretically, a very fast spark should be adequate to start the chemical reaction, but in practice a spark duration is of the order of 0.5 ms. Since there is a lag between the occurrence of the spark and the actual burning of the mixture, the spark must take place before the top dead centre (say 10° – 40° bTDC). A too early ignition would develop a pressure opposing the piston movement during the compression stroke. On the other hand, too late spark reduces the combustion efficiency as the explosion happens while the piston is already started the expansion stroke. In both cases , an amount of the engine power is lost.

The ignition delay bTDC should be increased (for example from 10° to 20° bTDC) as the engine speed increases (for example from 1000 to 2000 rpm). This delay should be reduced when the load is increased or a richer fuel mixture is used, as well as when a fast-burn fuel grade is used.

In SI engines an electronic spark initiates the combustion towards the end of the compression stroke, then a flame propagates through the premixed charge. In this case two types of combustion  can be recognised, normal combustion and abnormal combustion. Flame velocity is normally of the order of 40 cm/s increasing as the mixture get richer and by adding turbulence to air. In homogeneous mixtures (normally used in manifold-injection SI engines) the fuel and the oxidant molecules are spread fairly uniformly in the combustion place, i.e. the air fuel ratio is almost constant over the entire mixture, unlike the heterogeneous mixtures where the ratio varies from one point to another.

In normal combustion, the flame travels across the combustion chamber more or less uniformly. The abnormal combustion causes a loss in power and a potation damage to the engine. The abnormal combustion is called the phenomenon of engine knock.

Engine Knock

Combustion is ideally started by the spark plug and then grow in an accelerating uniform manner. The spark is timed so that the maximum cylinder pressure is produced a few crankshaft degrees after the TDC, pushing the piston away from the piston top. When the mixture behind the flame fronts are under the effect of high pressure and temperature for a time longer than a certain delay (the time of pre-flame reaction) determined for every type of fuel, then one or more explosive ignition may occur outside the flame front pocket , producing a shockwave which interacts with the piston surface and walls generating a metallic knock sound (of the order of 5 kHz in automobile engines). The reason of the knock phenomena could be a concentrated deposition of energy in a gas at a high rate causing an auto-ignition at various pin-points. Because of the auto-ignition, other flame fronts travel in the opposite direction of the main flame front. The blast wave generated by the detonation can cause structural damage of the engine parts due to the sudden excessive pressure.

Engine knock can be reduced by improving the following factors:

1-      The use of high-octane rating gasoline (high cetane number in the diesel case), so  that a fuel can withstand a higher compression rate before it detonates. Meaning that a fuel with a higher octane number is less likely to self-ignite. This is in fact the most important factor. Low octane/ cetane rating have long short ignition lag. Any fuel accumulated in the combustion chamber because of the combustion delay can produce a sudden release of energy. Iso-octane (C8H18) is a very anti-knock fuel, while heptanes (C7H16) is a very poor anti-knock fuel. Octane rating is the percentage ratio, in volume, of iso-octane in a mixture of the two fuels. However, iso-octane is not the most knock-resistant fuel available, so that it is possible to have an octane rating higher than 100. Also the variation in the octane rating is not linear, meaning that going from 92 to 93 rating would have much more improvement on the fuel performance than from going from 32 to 33 rating.

2-      The use of lower cylinder pressure (can be produced at higher rpm, means lower gear). High pressure causes high density of the charge. Typically all factors that decrease the temperature of the unburned fuel should reduce the possibility of knocking. For instance, reducing the inlet temperature of the air  would reduce the knocking possibility.

3-      Having the ignition spark further from the TDC (increasing the delay).

4-      Increasing mixture turbulence (this can be also increased at higher rpms) which increases the flame speed.

5-      Reducing the engine size. Large engines give more space, and hence more time, for the end gas auto-ignition (also large engines are slow engines).

6-      Reducing the engine load.

7-       Enriching the fuel/air ratio, which adds extra fuel to the mixture and increases the cooling effect when the fuel vaporizes in the cylinder.

8-      Location and number of spark plugs could play rule in reducing the engine knock, by reducing the flame travel.

Engine knock can be detected by using pizo-electric sensor to detect changes in the frequency (vibrations) from the engine noise, or pressure transducers to measure the pressure verses time.

Engine normal combustion against engine knock (from http://www.ret-monitor.com/articles/919/knock-about/)

Bio-fuels

Bio-fuel is a type of fuel extracted from biomass (living organisms). Bio-fuel is considered to be a source of renewable energy. Ethanol (ethyl alcohol C2H5OH) is the most popular bio-fuel used in automobile engines (butanol is another example of alcoholic fuel). An advantage of ethanol is that it has a high octane rating which improves the engine efficiency. Ethanol is mainly made from corn and sugarcane. Bio-diesel is also a popular bio-fuel, especially in Europe, which can be made of palm oil. Green-diesel is produced by processing vegetable oil. Bio-fuels are typically blended with other conventional petroleum fuels in various percentages. Most of the commonly used fluid bio-flues are extracted from crops, where the process is slow and expensive, beside processing the plants into fuel consumes a lot of energy.

Further Reading and References:

V. Ganesan (1996), “Internal Combustion Engines”, McGraw-Hill, USA