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Tuesday November 21st 2017

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Combustion in IC SI Engines

All this article considers traditional SI engines were the air/fuel mixture is induced and compressed. GDI (Gasoline Direct Injection) will be discussed in another article.

 

The figures underneath show a typical cylinder pressure plot in an SI engine combustion chamber. The fisrt figure plots pressure against cylinder volume and the second figure plots against crank angle.

Induction

From inlet valve opening (point A) to inlet valve closure (point C).

 

Compression

From the start of the compression (point C) till the spark ignites the charge (point D), typically 10-50° BTDC (Before Top Dead Centre).

The charge temperature and pressure increase during the compression stroke with such increases being determined by factors that include the state of the charge at inlet valve closure, the engine compression ratio and the magnitude of the small amount of heat transfer that occurs with the surroundings at this time.

Between the inlet valve opening and ignition points, mixing is continually taking place between the three major constituents of the unburnt gas mixture comprising the charge that ultimately burns (fuel, air and residual exhaust gases). How homogeneous (how well mixed) the mixture is depends on the turbulence levels in the engine during induction and compression and also on the degree of fuel vaporization.

 

Combustion

From combustion is started (at the end of compression stroke) (point D) until the combustion is completed (point E), typically 20° ATDC (After Top Dead Centre).

The turbulence levels present during combustion ensure the persistence of the mixing process in the unburnt charge in this phase of the engine cycle.

During combustion the unburned portion is further compressed by the expansion of the burned products. The area marked ‘b’ is the fully burned zone, while the area marked ‘u’ is the unburned reactants zone.

 

Expansion

Some expansion took place already during combustion. From the end of combustion (point E) until the exhaust valve opens (point F).

 

Exhaust

From the exhaust valve opening (point F) until the exhaust valve closes (point B).

By this time the inlet valve has already opened (point A) to begin a new cycle. The time interval or crankangle period during which both inlet and exhaust valves are open (point A to B) is called the “valve overlap period”.

 

Note: Thermodynamic analysis shows that the thermal efficiency of the SI Engine ( Otto cycle) increases with increasing compression ratio. However, the compression ratio of SI engines is limited by “abnormal” combustion. This phenomenon invariably generates “knocking” in the engine. Knock causes reduced efficiency, increased heat transfer and if severe, engine damage. Octane number reduces knocking (in the past lead was used, now prohibited).

 

 

 

Combustion process

The combustion process of SI engines can be divided into three phases:

  1. Ignition and flame kernel evolution
  2. Flame propagation
  3. Flame termination

 

Ignition and flame kernel evolution

Ignition occurs and the combustion process starts, 5-10% of the air/fuel mixture is consumed.

 

Flame propagation

The bulk of the fuel and air burns as a result of the flame propagation. During this stage, pressure in the cylinder is greatly increased, and this provides the force to produce work in the expansion stroke.

The speed of the propagating flame front inside an SI engine is greatly increased by induced turbulence, swirl and squish within the cylinder. The right combination of fuel and operating conditions is such that knock is avoided.

The actual flame front inside the cylinder of an SI engine is distorted from smooth spherical due bulk motion, induced turbulence, swirl, squish etc. Both theory and experiment indicated that the flame structure is a highly wrinkled, thin reaction sheet.

 

The flame propagation rate, UT , (the rate at which the flame front moves into the unburned mixture) under turbulent conditions is proportional to the laminar flame speed, UL . The laminar speed is a property of the mixture and is a function of fuel type, oxidant, air/fuel ratio (equivalence ratio or ?), temperature, pressure etc. It is increased as the temperature is increased and is maximum at about ? = 1.1 as shown in the following figure.

The rate at which the flame front moves relative to a co-ordinate system fixed on the combustion chamber wall called the “measured flame speed”, Uf. The measured flame speed is equal to UT plus any movement of the flame front due to bulk motion (e.g. as caused by expansion of the hot burned gas).

A flame speed ratio (FSR) is defined as the turbulent (flame propagation rate) flame speed, UT, divided by the laminar burning velocity relative to the unburned mixture: FSR = UT/UL . Where UT is a function of UL and turbulence intensity

For a given engine design, the flame speed ratio FSR = UT/UL is directly proportional to N. Hence FSR = (1+ KT · N) . Where KT is a constant for a particular engine design.

 

The FSR is also a function of flame radius.

The flame speed during the three phases of flame propagation is:

  • The initial propagation is at the laminar speed
  • Main part is at turbulent flame speed
  • Reduced flame speed as flame approaches cylinder walls

Variation of flame speeds during combustion in an SI engine (?s : crankangle at ignition)

 

Flame termination

The final 5-10% of the mixture burns during the flame termination phase. Pressure quickly decreases near the walls due to heat transfer and combustion stops.

 


Suggest reading:

Cycle to cycle variation

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