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What are the factors influencing flame speed in I.C. engines?

FACTORS INFLUENCING THE FLAME SPEED

The study of factors which, affect the velocity of flame propagation is important since the flame velocity influences the rate of pressure rise in the cylinder and it is related to certain types of abnormal combustion that occur in spark-ignition engines. There are several factors which affect the flame speed, to a varying degree, the most important being the turbulence and the fuel-air ratio. Details of various ‘factors ·that affect the flame speed are discussed below.

Turbulence:

The flame speed is quite low in non-turbulent mixtures and increases with increasing turbulence. This is mainly due to the additional physical intermingling of the burning and unburned particles at the flame front which expedites reaction by increasing’ the rate of contact.
The turbulence in the incoming mixture is generated during the admission of fuel-air mixture through comparatively narrow sections of throttle pipe, valve openings etc., in the suction stroke. Turbulence which is supposed to consist of many minute swirls appears to increase the rate of reaction and produce a higher flame speed than that made up of larger and fewer swirls. A suitable design of the combustion chamber which involves the geometry of cylinder head and piston crown increases the turbulence during the compression stroke.
Generally, turbulence increases the heat flow to the cylinder wall. It also accelerates the chemical reaction by intimate mixing of fuel and oxygen so that spark advance may be reduced. This helps in working in lean mixtures also. The increase of flame speed due to turbulence reduces the combustion duration and hence minimizes the tendency of abnormal combustion. However, excessive turbulence may extinguish the flame resulting in rough and noisy operation of the engine.

Fuel-Air Ratio:

The fuel-air ratio has a very significant influence on the flame speed. The highest flame velocities (minimum time for complete combustion) are obtained with somewhat richer mixture (point A) as shown in Fig.12.4 which shows the effect of mixture strength on the rate of burning as indicated by the time taken for complete burning in a given engine. When the mixture is made leaner or richer (see point A in Fig.12.4) the flame speed decreases. Less thermal energy is released in the case of lean mixtures resulting in lower flame temperature.
Very rich mixtures lead to incomplete combustion which results again in the release of less thermal energy.

Temperature and Pressure:

Flame speed increases with an increase in intake temperature and pressure. A higher initial pressure and temperature may help to form a better homogeneous air-vapour mixture which helps in increasing the flame speed. This is possible because of an overall increase in the density of the charge.

Compression Ratio:

A higher compression ratio increases the pressure and temperature of the working mixture which reduce the initial preparation phase of combustion and hence less ignition advance is needed. High pressures and temperatures of the compressed mixture also speed up the second phase of combustion.
Increased compression ratio reduces the clearance volume and therefore increases the density of the cylinder gases during burning. This increases the peak pressure and temperature and the total combustion duration is reduced.Thus engines having higher compression ratios have higher flame speeds.

Engine Output:

The cycle pressure increases when the engine output is increased. With the increased throttle opening the cylinder gets filled to a higher density. This results in increased flame speed.When the output is decreased by throttling, the initial and final compression pressures decrease and the dilution of the working mixture increases. The smooth development of self-propagating nucleus of flame becomes unsteady and difficult.
The main disadvantages of SI engines are the poor combustion at low loads and the necessity of mixture enrichment (¢ between 1.2 to 1.3) which causes wastage of fuel and discharge of unburnt hydrocarbon and the products of incomplete combustion like carbon monoxide etc. in the atmosphere.

Engine Speed:

The flame speed increases almost linearly with engine speed since the increase in engine speed increases the turbulence inside the cylinder. The time required for the flame to traverse the combustion space would be halved, if the engine speed is doubled.
Double the engine speed and hence half the original time would give the same number of crank degrees for flame propagation. The crank angle required for the flame propagation during the entire phase of combustion, will remain nearly constant at all speeds.

Engine Size:

The size of the engine does not have much effect on the rate of flame propagation. In large engines the time required for complete combustion is more because the flame has to travel a longer distance. This requires increased crank angle duration during the combustion. This is one of the reasons why large sized engines are designed to operate at low speeds.

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