Page 363 - Advanced thermodynamics for engineers

352 CHAPTER 16 RECIPROCATING INTERNAL COMBUSTION ENGINES

16.3 SPARK-IGNITION ENGINES
In spark-ignition engines, the fuel and air are usually premixed prior to admission to the engine
cylinder. This used to be done in a carburettor, but now fuel is usually injected either into the inlet port,
manifold or even the cylinder by means of a fuel injection system. The first two of these systems
prepare the charge prior to it entering the cylinder, although it is probable that the fuel enters the
cylinder with a large proportion in the liquid phase. Under fully warmed-up conditions, this fuel will
have evaporated by the time of ignition. At start-up this will not be the case, and enrichment, beyond
stoichiometric, is done to ensure that the light fractions of the fuel give a combustible mixture; the
remaining liquid fuel causes high levels of unburned hydrocarbons (uHCs). It was stated in Chapter 15
that a high level of turbulent gas motion in the cylinder will increase the flame speed, and this can be
achieved by various mechanisms. In older engines, discussion of which is worthwhile because it is
easy to see the underlying rationale for the chamber shape, the shape of the piston and cylinder head
produced a squish motion as the piston approached tdc and this enhanced the turbulence in the region
of the spark plug, and increased the flame speed (see Fig. 16.3).
Other designs were proposed, including the May ‘Fireball’ combustion chamber (Fig. 16.4(a) and
(b)) which produces a high level of turbulence by ‘squeezing’ the gas into a small combustion chamber
under either the intake or exhaust valve. While this system produces high turbulence it occurs too late
to achieve its aims. A May chamber was fitted to a Jaguar engine (Google, 2014) and produced better
fuel economy by enabling leaner mixtures to be used. More modern engines attempt to increase the
turbulence levels around the spark plug by the break-up of barrel swirl or tumble. The gas entering the
engine has a combination of swirl (vortex motion in a horizontal plane) and barrel swirl (vortex motion
in the vertical plane) (see Fig. 16.4(c) and (d)). Swirl momentum is preserved during compression, but
is not very useful for spark-ignition engine combustion. Barrel swirl cannot be preserved as ordered
motion because the shape of the vortex is destroyed as the aspect ratio of the combustion chamber

(a) (b)

FIGURE 16.3
Squish flow in ‘bath tub’ combustion chamber (a) piston at mid-stroke (b) piston approaching top dead centre
(tdc) and squishing gas from top land region.