DJA3032 CHAPTER 3

CHAPTER 3: COMBUSTION AND FUEL CHARACTERISTICS

byMOHD SAHRIL BIN MOHD FOUZI, Grad. IEM (G 27763)

DEPARTMENT OF MECHANICAL ENGINEERING

© MSF @ POLITEKNIK UNGKU OMAR

(DJA3032) INTERNAL COMBUSTION ENGINE

INTRODUCTION :

This topic covers the understanding of combustion process in spark ignition as well as compression ignition engine. It also includes knocking phenomenon and fuel characteristics.

Specific objectives: At the end of this unit you should be able to: 1. describe the combustion of fuel injection.2. draw engine pressure Vs crank angle diagram.3. define the term “Knocking”.4. list the effects knocking during engine process.5. define how to reduce knocking problem during engine process.6. draw the Ricardo Diagram.7. define the Catena Number.



COMBUSTION PROCESS TERMS:

Normal Combustion A combustion process which is initiated solely by a timed spark and in which the flame front moves completely across the combustion chamber in a uniform manner at a normal velocity.

Abnormal Combustion A combustion process in which a flame front may be started by hot combustion-chamber surfaces either prior to or after spark ignition, or a process in which some part or all of the charge may be consumed at extremely high rates.1

Figure 3.1: Diagram for Normal and Abnormal Combustion



Spark Knock

A knock which is recurrent and repeatable in terms of audibility. It is controllable by the spark advance; advancing the spark increases the knock intensity and retarding the spark reduces the intensity. Knock is the name given to the noise which is transmitted through the engine structure when essentially spontaneous ignition of a portion of the end gas—the fuel, air, residual gas, mixture ahead of the propagating flame—occurs. There is an extremely rapid release of most of the chemical energy in the end-gas, causing very high local pressures and the propagation of pressure waves of substantial amplitude across the combustion chamber.



Figure 3.2: Spark knock phenomenon

Surface Ignition

Surface ignition is ignition of the fuel-air charge by any hot surface other than the spark discharge prior to the arrival of the normal flame front. It may occur before the spark ignites the charge (pre-ignition) or after normal ignition (post-ignition).

Surface Ignition is ignition of the fuel-air mixture by a hot spot on the combustion chamber walls such as an overheated valve or spark plug, or glowing combustion-chamber deposit: i.e., by any means other than the normal spark discharge.

Following surface ignition, a flame develops at each surface-ignition location and starts to propagate across the chamber in an analogous manner to what occurs with normal spark-ignition.



Figure 3.3: (a) Normally, fuel is ignited by the spark plug, and combustion spreads uniformly outward. (b) Gasoline with an octane rating that is too low for the engine can ignite prematurely, resulting in uneven burning that causes knocking and pinging.

The octane scale was established in 1927 using a standard test engine and two pure compounds: n-heptane and isooctane (2,2,4-trimethylpentane). n-Heptane, which causes a great deal of knocking on combustion, was assigned an octane rating of 0, whereas isooctane, a very smooth-burning fuel, was assigned an octane rating of 100.



Figure 3.4: The Octane Ratings of Some Hydrocarbons and Common Additives



Chemists assign octane ratings to different blends of gasoline by burning a sample of each in a test engine and comparing the observed knocking with the amount of knocking caused by specific mixtures of n-heptane and isooctane. For example, the octane rating of a blend of 89% isooctane and 11% n-heptane is simply the average of the octane ratings of the components weighted by the relative amounts of each in the blend. Converting percentages to decimals, we obtain the octane rating of the mixture:

0.89(100)+0.11(0)=89

A gasoline that performs at the same level as a blend of 89% isooctane and 11% n-heptane is assigned an octane rating of 89; this represents an intermediate grade of gasoline. Regular gasoline typically has an octane rating of 87; premium has a rating of 93 or higher.



Figure 3.5: The Distillation of Petroleum



Ignition

The ignition system is designed to ignite the air and fuel that have been mixed in the fuel system. It is important to improve this system. Each year, ignition is becoming more and more computerized. Today’s ignition systems are almost totally computer controlled for improved combustion.

Pre- Ignition

This Is one process where the spark is heated up before the ignition begins. It causes rough running and in extreme cases, can do damage to the engine.

Causes of pre-ignition include the following:1. Carbon deposits form a heat barrier and can be a contributing factor to pre-ignition. 2. Glowing carbon deposits on a hot exhaust3. A sharp edge in the combustion chamber or on top of a piston4. Sharp edges on valves that were reground improperly 5. An engine that is running hotter than normal due to a cooling system problem6. Auto-ignition of engine oil droplets.



Ignition Delay

Ignition delay is defined as the time (or crank angle interval) from when the fuel injection starts to the onset of combustion.

Figure 3.6: Pressure – Crank Angle diagram for a four-stroke cycle

Delay period is the commencement of injection and it is indicated by the dot on the compression line 15° before dead centre. The period 1 is the delay period during which ignition is being initiated, but without any measurable departure of the pressure from the air compression curve which is continued as a broken line in the diagram as it would be recorded if there were no injection and combustion.



Figure 3.7: Pressure vs crankshaft position diagram



Combustion Process Based On The Pressure Vs Crankshaft Position

0 ° – 180 ° = Intake valve open (IVO), intake process occurred and piston moving from Top Dead Center (TDC) to Bottom Dead Center (BDC). The mixture of fuel and O₂ is entering the cylinder. Exhaust valve close (EVC) during this stroke (intake stroke) and the pressure remain constant.

180 ° – 360 ° = Intake valve close (IVC), piston climb up from Bottom Dead Center (BDC) to Top Dead Center (TDC). The mixture of fuel and O₂ is compressed and the pressure is raising up. Spark plug will ignite before TDC (Ignition Delay).

360 ° – 540 ° = Intake valve close (IVC) and exhaust valve close (EVC) during power stroke. Piston moving downward from Top Dead Center (TDC) to Bottom Dead Center (BDC). At the end of this process the exhaust valve open (EVO). The pressure is decreasing.

540 ° – 720 ° = Piston moving upward from Bottom Dead Center (BDC) to Top Dead Center (TDC). The product of combustion is expelled out from the cylinder. At the of this process, intake valve open (IVO) and continue for next cycle.



The Effect of Engine Speed on Ignition Timing

Ignition timing, in a spark ignition internal combustion engine (ICE), is the process of setting the angle relative to piston position and crankshaft angular velocity that a spark will occur in the combustion chamber near the end of the compression stroke.

"Timing advance" refers to the number of degrees before top dead center (BTDC) that the spark will ignite the air-fuel mixture in the combustion chamber during the compression stroke. Igniting the mixture before the piston reaches TDC will allow the mixture to fully burn soon after the piston reaches TDC.

“Retarded timing” can be defined as changing the timing so that fuel ignition happens later than the manufacturer's specified time. For example, if the timing specified by the manufacturer was set at 12 degrees BTDC initially and adjusted to 11 degrees BTDC, it would be referred to as retarded.

If the air-fuel mixture is ignited at the correct time, maximum pressure in the cylinder will occur sometime after the piston reaches TDC allowing the ignited mixture to push the piston down the cylinder with the greatest force. Ideally, the time at which the mixture should be fully burnt is about 20 degrees ATDC.




If the ignition spark occurs at a position that is too advanced relative to piston position, the rapidly expanding air-fuel mixture can actually push against the piston still moving up, causing knocking (pinging) and possible engine damage.

If the spark occurs too retarded relative to the piston position, maximum cylinder pressure will occur after the piston is already traveling too far down the cylinder. This results in lost power, overheating tendencies, high emissions, and unburned fuel.

The ignition timing will need to become increasingly advanced (relative to TDC) as the engine speed increases so that the air-fuel mixture has the correct amount of time to fully burn.

As the engine speed (RPM) increases, the time available to burn the mixture decreases but the burning itself proceeds at the same speed, it needs to be started increasingly earlier to complete in time.

Poor volumetric efficiency at lower engine speeds also requires increased advancement of ignition timing.




The correct timing advance for a given engine speed will allow for maximum cylinder pressure to be achieved at the correct crankshaft angular position. When setting the timing for an automobile engine, the factory timing setting can usually be found on a sticker in the engine bay.

The ignition timing is also dependent on the load of the engine with more load (larger throttle opening and therefore air:fuel ratio) requiring less advance (the mixture burns faster). Also it is dependent on the temperature of the engine with lower temperature allowing for more advance. The speed with which the mixture burns depends also on the octane rating of the fuel and on the air-fuel ratio.



Figure 3.8: Example of Timing Map Diagram



Figure 3.9: Example of Mapping Graph for Tuning



Figure 3.10: Example of Software for Tuning/Mapping The Engine



Knocking or Detonation Process in Spark Ignition Engine

Term of Knocking or Detonation

This is one process that happens within the combustion chamber. It sounds like a small ticking or rattling noise within the engine. In long term, the piston and ring can be damaged as well as the spark plug and valve.

Term of Surface Ignition and Knocking Phenomenon in Spark Ignition Engine

“Refer slide 3 until 6”

Factors That Contribute Knocking

Detonation occurs when several conditions / factors inside the combustion chamber exist at the same time:-

Increased compression high temperatures lean fuel/air mixture

advanced ignition timing lower octane fuels are all factors that

PROMOTE detonation conditions



Effects of knocking during engine process

The effects of knocking during engine process are :-1. a drop in engine performance. 2. pollution of gases from the combustion is incomplete.3. high consumption of fuel.

Reduce knocking problem during engine process

In this case we have three options to reduce knocking during engine process :-1. Increase the ignition combustion engine.2. Reduce the heat the final combustion; enriching the air-fuel ratio which alters the

chemical reactions during combustion, reduces the combustion temperature and increases the margin above detonation.

3. Use high quality fuel, the use of a fuel with high octane rating, which increases the combustion temperature of the fuel and reduces the proclivity to detonate.



Figure 3.11: Ricardo Diagram With Important Points

Ricardo Diagram



According to Ricardo, There are three stages of combustion in SI Engine as shown :-1. Ignition lag stage2. Flame propagation stage3. After burning stage

Ignition lag stage:

There is a certain time interval between instant of spark and instant where there is a noticeable rise in pressure due to combustion. This time lag is called IGNITION LAG. Ignition lag is the time interval in the process of chemical reaction during which molecules get heated up to self ignition temperature , get ignited and produce a self propagating nucleus of flame. The ignition lag is generally expressed in terms of crank angle (q1). The period of ignition lag is shown by path ab. Ignition lag is very small and lies between 0.00015 to 0.0002 seconds. An ignition lag of 0.002 seconds corresponds to 35 deg crank rotation when the engine is running at 3000 RPM. Angle of advance increase with the speed. This is a chemical process depending upon the nature of fuel, temperature and pressure, proportions of exhaust gas and rate of oxidation or burning.



Flame propagation stage:

Once the flame is formed at “b”, it should be self sustained and must be able to propagate through the mixture. This is possible when the rate of heat generation by burning is greater than heat lost by flame to surrounding. After the point “b”, the flame propagation is abnormally low at the beginning as heat lost is more than heat generated. Therefore pressure rise is also slow as mass of mixture burned is small. Therefore it is necessary to provide angle of advance 30 to 35 deg, if the peak pressure to be attained 5-10 deg after TDC. The time required for crank to rotate through an angle q2 is known as combustion period during which propagation of flame takes place.

After burning:

Combustion will not stop at point “c” but continue after attaining peak pressure and this combustion is known as after burning. This generally happens when the rich mixture is supplied to engine.



Pressure ( P ) Line 2

Line 1

Line 3

Crank angle ( TCD )

Figure 3.12: Ricardo Diagram

In Figure 3.12 in the Ricardo diagram Line 1 explains the good condition of combustion. Line 2 explains the overhead and the curve in Line 3 explains late ignition.



Time

Cylinder Pressure

Figure 3.13: Normal Combustion with no Knocking

Time

CylinderPressure

Figure 3.14: Combustion with light Knocking

Time

CylinderPressure

Figure 3.15: Combustion with Heavy Knocking © MSF @ POLITEKNIK UNGKU

OMAR


Effect of engine operating variables on the engine knocking (detonation):

The various engine variable affecting knocking can be classified as : Temperature factors Density factors Time factors Composition factors

Temperature factors

Increasing the temperature of the unburned mixture increase the possibility of knock in the SI engine. We shall now discuss the effect of following engine parameters on the temperature of the unburned mixture: RAISING THE COMPRESSION RATIO. Increasing the compression ratio increases

both the temperature and pressure (density of the unburned mixture). Increase in temperature reduces the delay period of the end gas which in turn increases the tendency to knock.

SUPERCHARGING. It also increases both temperature and density, which increase the knocking tendency of engine



Temperature factors

COOLANT TEMPERATURE Delay period decreases with increase of coolant temperature, decreased delay period increase the tendency to knock.

TEMPERATURE OF THE CYLINDER AND COMBUSTION CHAMBER WALLS: The temperature of the end gas depends on the design of combustion chamber. Sparking plug and exhaust valve are two hottest parts in the combustion chamber and uneven temperature leads to pre-ignition and hence the knocking.

Density factors

Increasing the density of unburnt mixture will increase the possibility of knock in the engine. The engine parameters which affect the density are as follows: Increased compression ratio increase the density. Increasing the load opens the throttle valve more and thus the density. Supercharging increase the density of the mixture. Increasing the inlet pressure increases the overall pressure during the cycle. The high

pressure end gas decreases the delay period which increase the tendency of knocking.



Density factors

Advanced spark timing: quantity of fuel burnt per cycle before and after TDC position depends on spark timing. The temperature of charge increases by increasing the spark advance and it increases with rate of burning and does not allow sufficient time to the end mixture to dissipate the heat and increase the knocking tendency.

Time factors

Increasing the time of exposure of the unburned mixture to auto-ignition conditions increase the possibility of knock in SI engines. Flame travel distance: If the distance of flame travel is more, then possibility of

knocking is also more. This problem can be solved by combustion chamber design, spark plug location and engine size.

Compact combustion chamber will have better anti-knock characteristics, since the flame travel and combustion time will be shorter. Further, if the combustion chamber is highly turbulent, the combustion rate is high and consequently combustion time is further reduced; this further reduces the tendency to knock.



Time factors

Location of sparkplug: A spark plug which is centrally located in the combustion chamber has minimum tendency to knock as the flame travel is minimum. The flame travel can be reduced by using two or more spark plugs.

Location of exhaust valve: The exhaust valve should be located close to the spark plug so that it is not in the end gas region; otherwise there will be a tendency to knock.

Engine size: Large engines have a greater knocking tendency because flame requires a longer time to travel across the combustion chamber. In SI engine therefore , generally limited to 100mm.

Turbulence of mixture: decreasing the turbulence of the mixture decreases the flame speed and hence increases the tendency to knock. Turbulence depends on the design of combustion chamber and one engine speed.



Composition

The properties of fuel and A/F ratio are primary means to control knock :(a) Molecular Structure: The knocking tendency is markedly affected by the type of

the fuel used. Petroleum fuels usually consist of many hydro-carbons of different molecular structure. The structure of the fuel molecule has enormous effect on knocking tendency. Increasing the carbon-chain increases the knocking tendency and centralizing the carbon atoms decreases the knocking tendency. Unsaturated hydrocarbons have less knocking tendency than saturated hydrocarbons.

Paraffins: Increasing the length of carbon chain increases the knocking tendency. Centralising the carbon atoms decreases the knocking tendency. Adding methyl group (CH to the side of the carbon chain in the centre position

decreases the knocking tendency.

Olefins:Introduction of one double bond has little effect on anti-knock quality but two or three

double bond results less knocking tendency except C and C. © MSF @ POLITEKNIK UNGKU

OMAR


Composition

Napthenes and Aromatic: Napthenes have greater knocking tendency than corresponding aromatics. With increasing double-bonds, the knocking tendency is reduced.

Lengthening the side chains increases the knocking tendency whereas branching of the side chain decreases the knocking tendency.

(b) Fuel-air ratio. The most important effect of fuel-aft ratio is on the reaction time or ignition delay. When the mixture is nearly 10% richer than stoichiomiric (fuel-air ratio = 0.08) ignition lag of the end gas is minimum and the velocity of flame propagation is maximum.

By making the mixture leaner or richer (than F/A 0.08) the tendency to knock is decreased. A too rich mixture is especially effective in decreasing or eliminating the knock due to longer delay and lower temperature of compression.

(c) Humidity of air. Increasing atmospheric humidity decreases the tendency to knock by decreasing the reaction time of the fuel.



Research Octane Number (RON)

The most common type of octane rating worldwide is the Research Octane Number (RON). RON is determined by running the fuel in a test engine with a variable compression ratio under controlled conditions, and comparing the results with those for mixtures of iso-octane and n-heptane.

Motor Octane Number (MON)

There is another type of octane rating, called Motor Octane Number (MON), is determined at 900 rpm engine speed instead of the 600 rpm for RON. MON testing uses a similar test engine to that used in RON testing, but with a preheated fuel mixture, higher engine speed, and variable ignition timing to further stress the fuel's knock resistance.

Depending on the composition of the fuel, the MON of a modern pump gasoline will be about 8 to 12 octane lower than the RON, but there is no direct link between RON and MON. Pump gasoline specifications typically require both a minimum RON and a minimum MON.



Catane Number

There is a delay between the time that fuel is injected into the cylinder and the time that the hot gases ignite. This time period or delay is expressed as a catane number. Catane number ranges from 30 to 60 on diesel fuel. Catane number is an indication of the ignition quality of the diesel fuel.

The higher the catane number, the better the ignition quality of the fuel. High catane numbers should be used to start an engine in cold weather. A catane number of 85 to 96 is often used for starting diesel engines in cold weather.

If a low catane number is used in diesel engine, some of the fuel may not ignite. The fuel will then accumulate within the cylinder. When combustion finally does occur, this excess fuel will explode suddenly. This may result in a knocking sound as in gasoline.



Fuel Additives

Additives - Chemicals are added to gasoline in very small quantities to improve and maintain gasoline/fuel quality.

Effects of fuel additives: to improve combustion and pollutant emissions, to ensure reduced wear and limit deposit formation during the engine life cycle of

several hundreds of thousands of miles.

Example of Fuel Additives:

1. Oxygenates Alcohols: Methanol (MeOH)

Ethers: Methyl tert-butyl ether (MTBE), now outlawed in many states of the U.S. for road use, mostly because of water contamination.

2. Antioxidants, stabilizers Butylated hydroxytoluene (BHT)

3. Antiknock agents Toluene



Stoichiometric Ratio

The stoichiometric mixture for a gasoline engine is the ideal ratio of air to fuel that burns all fuel with no excess air. For gasoline/petrol fuel, the stoichiometric air–fuel mixture is about 14.7:1 i.e. for every one gram of fuel, 14.7 grams of air are required. The fuel oxidation reaction is:

(Balance chemical equation of air-fuel ratio for combustion process)

Figure 3.16: Ignition limits for hydrocarbons



Lean Mixture

Fuel-air mixtures with more than stoichiometric air, excess air, can burn. With excess air you get fuel lean combustion, the extra air appears in the products in unchanged form.

Rich Mixture

Fuel-air mixtures with less than stoichiometri cair can also burn. With less than stoichiometric air you get fuel rich combustion, there is insufficient oxygen to oxidize all the C and H in the fuel to CO2 and H2O. Get incomplete combustion where carbon monoxide (CO) and molecular hydrogen (H2) also appear in the products.

where for fuel lean mixture have excess air so γ> 1

where for fuel rich mixture have insufficient air ; γ< 1





DJA3032 CHAPTER 3

Automotive