Flame piston

A piston Mach number may be related to a flame Mach number if, under the condition of low overpressure, the mass enclosed by the piston flow field is equated to the mass enclosed by a flame flow field ... 

The relationship to the Taylor expanding-piston solution becomes evident for small-flame Mach numbers. 

Once the piston-driven flow field is known, the flame-driven flow field is found by fitting in a steady flame front, with the condition that the medium behind it is quiescent. This may be accomplished by employing the jump conditions which relate the gas-dynamic states on either side of a flame front. The condition that the reaction products behind the flame are at rest enables the derivation of expressions for the density ratio, pressure ratio, and heat addition... 

As mentioned above, the numerical solution of exact equations breaks down for low flame speeds, where the strength of the leading shock approaches zero. To complete the entire range of flame speeds, Kuhl et al. (1973) suggested using the acoustic solutions by Taylor (1946) as presented earlier in this section. Taylor (1946) already noted that his acoustic approach is not fully compatible with the exact solution, in the sense that they do not shade into one another smoothly. In particular, the near-piston and the near-shock areas in the flow field, where nonlinear effects play a part, are poorly described by acoustic methods. In addition to these imperfections, the numerical character of Kuhl etal. (1973) method inspired various authors to design approximate solutions. These solutions are briefly reviewed. 

PISTON RINGS are normally cast-iron of the single-piece snap-ring type, although other materials may be used when conditions require. PISTON ROD is carbon-steel, flame-hardened over packing travel area. 

PISTON ROD is carbon-steel, flame-hardened over packing travel area unless otherwise specified. 

Volume changes also can be mechanically determined, as in the combustion cycle of a piston engine. If V=V(i) is an explicit function of time. Equations like (2.32) are then variable-separable and are relatively easy to integrate, either alone or simultaneously with other component balances. Note, however, that reaction rates can become dependent on pressure under extreme conditions. See Problem 5.4. Also, the results will not really apply to car engines since mixing of air and fuel is relatively slow, flame propagation is important, and the spatial distribution of the reaction must be considered. The cylinder head is not perfectly mixed. 

Good combustion is characterized by a flame burning uniformly across the piston head. Deposits and poor-quality fuel can interfere with the movement of the flame across the combustion chamber. Rough running can result. 

Knock in spark ignition engines may be defined as an abnormally rapid combustion of the unburned fuel-air mixture ahead of the normal flame front. A severe pressure unbalance due to this rapid combustion process sets up shock waves which impinge on the cylinder walls and piston and produce the characteristic metallic knocking noise (43). 

When the volume uS of the original mixture is replaced by volume nuS of the hot reaction products, the unburned mixture must move so as to free a volume equal to (n — 1 )uS. Thus, the motion of the flame from the closed end at a velocity u results in motion of the original mixture with a velocity of (n — l)ii, i.e., 4-11 times greater. The flame acts as a moving piston causing the gas before it to move. 

The flame, like a piston, moves and compresses the gas before it. To flame propagation at normal velocity in a detonation mixture of 2H2 + 02... 

In order to interpret this recently discovered, but absolutely fundamental fact, we shall consider more carefully the conditions of the gas motion. The flame functions as a piston, and the dependence written above of the gas velocity on the flame velocity, w — (n—l)u, is valid insofar as the combustion products do not cool. Therefore, for detonation to occur the ratio of the drag and heat transfer is of particular importance. It is precisely in rough tubes that conditions are most favorable the increased drag accelerates the establishment of the velocity profile, while the heat transfer remains practically unchanged by the introduction of roughness. 

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