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contd...
Table 2.3 |
Typical Volume Contained in Engine Crevices, cm3
(Engine Displacement Volume/ Cylinder = 352 cm3, CR = 9:1) |
| |
Volume, cm3 |
Percent |
| Clearance volume per cylinder |
44 |
100 |
| Volume above first ring (top land) |
0.51 |
1.32 |
| Volume behind first ring |
0.32 |
0.86 |
Volume between Ist and 2nd rings (Second land) |
0.40 |
0.88 |
| Volume behind second ring |
0.32 |
0.86 |
Total ring crevice volume |
1.55 |
3.5 |
Spark plug thread crevice |
0.20 |
0.45 |
| Head gasket crevice |
0.20 |
0..45 |
| Total crevice volume |
1.95 |
4.4 |
During compression and combustion, unburned charge is pushed into these crevices and at peak pressure, maximum gas would be stored in the crevices. The gas composition into the crevices depends on the location of spark plug. In the piston-cylinder crevices mostly unburned charge would be filled in unless the flame has reached piston top in some location nearest to the spark plug before the peak pressure occurs, which would result also in small amounts of burned gas being pushed into the crevice in this location. The other crevices close to spark plug would be filled with a larger fraction of the burned gas. During expansion, the stored gases in the crevices begin to flow back into the cylinder. Part of the unburned charge from crevices that expands back into the combustion chamber is oxidized on mixing with the hot burned gases.
Amount of HC Stored in Crevices include:
Contribution of crevice volume to HC emissions may be understood as follows. The crevice gas temperatures are nearly equal to the temperature of walls which are cooled. Hence, the density of the charge stored in the crevices is higher than in the cylinder. The maximum fraction of the unburned charge stored in crevices, Es occurs at peak pressure and is given by;
where m, V, T and P are mass volume, pressure and temperature. The subscripts cr and o refer to the conditions in the crevices and at the end of intake stroke in the cylinder, respectively. Pmax is the peak pressure in the cylinder.
Typically Pmax /Po = 40, To= 300 K and Tcr = 400 K. Taking piston top land crevice volume equal to 0.9 cm3 and the engine cylinder volume of 300 cm3 for a compact car, 9% of the charge is stored in the piston ring crevice. The crevice charge would consist of 10 to 15 percent residual gases and some burned gases forced into it when flame propagates across the crevice opening.
In a production engines ring crevice region may contribute 25 % to 50% to exhaust HC emissions depending upon the operating conditions.
Increase in radial clearance between the piston and cylinder beyond two-plate quench distance would allow flame penetration in the crevice. It would result in reduction of HC as the flame would be able to penetrate in the crevice volume. However, increase in radial clearance would lead to increase in blow by gases and loss in engine power output.
Under conditions of high residual gas dilution or use of very lean mixtures, the flame may quench much before it reaches the crevice region. Thus, increase in crevice volume by increasing radial clearance can result in an increase in HC emissions under engine operation on lean mixtures or with high EGR.
Example 2.2 A SI engine has bore x stroke = 76 x 76 mm and compression ratio equal to 9.0:1. Top piston land height is 7 mm and clearance between piston and cylinder liner is 0.35 mm. At the end of intake stroke the stoichiometric mixture of gasoline (C8H18) is at 0.09 MPa and 330 K. Peak cylinder pressure during combustion reaches 3.0 MPa. The temperature of gas in the piston crevice region due to heat transfer to the cylinder walls is 400 K. Calculate the amount of charge stored in the top land crevice at the instant of maximum cylinder pressure. What fraction of the charge inducted is stored in this crevice?
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