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354 CHAPTER 16 RECIPROCATING INTERNAL COMBUSTION ENGINES

changes: barrel swirl is broken down into smaller scale motion which enhances the flame speed. In
more modern engines, and with the introduction of manifold or in-cylinder fuel injection, and
computer-based control systems, it is possible to operate engines over a wide range of air–fuel ratios.
Horie and Nishizawa (1992) describe the operating regions of the Honda VTEC engine, which can run
with very low mixture strengths (down to l ¼ 0.67) in certain regions of its operation. This engine is
discussed in Stone (2012), where he shows a section of the combustion chamber, and the engine’s
operating regions – the latter are shown in Fig. P16.15. It is noticeable that the engine, which has a
four-valve head, runs with only one inlet valve opening at low speed, and the two valves only operate at
higher speeds, where they are necessary to achieve the gas flow through the engine. The effect of using
only one valve at low speed is to produce a ‘tumble’, or barrel-swirl vortex in the cylinder (see
Fig. 16.4(c) and (d)). This is broken down into general turbulence as the piston approaches tdc, which
enhances the flame speed factor, f f , enabling the weak mixtures to be burnt.
Further features which must be borne in mind with spark-ignition engine combustion chambers are:
1. reducing zones where combustion can be quenched (see top land region in Fig. 16.3);
2. limiting zones which might trap the end gases and cause detonation;
3. reducing crevice volumes where uHCs might be trapped, e.g. around top ring groove.
Recently a significant advance in engine combustion systems has been the gasoline direct injection
(GDI) engine in which the air–fuel ratio can be varied over the operating range of the engine: this
degree of control gives improved fuel consumption and lower emissions. In an ideal GDI engine, it
would be possible to achieve qualitative control of the engine power output, doing away with the need
for the throttle valve. This would minimise the engine pumping loss, and the efficiency of the engine
could approach that of the diesel engine. The first commercial GDI engine was produced by Mitsubishi
in 1996 (Kume et al. (1996)): most of the major manufacturers have direct injection (di) versions in
their product line now. Besides being able to introduce lean-burn operation at low speed and load, di
allows stoichiometric operation during mid-range, and rich operation at high load (to reduce thermal
load on the engine). A further benefit of di is that the liquid fuel lowers the temperature in the cylinder,
and this increases the volumetric efficiency and enables the engine to have a higher compression ratio.
These engines are described in Stone (2012).

16.4 DIESEL (COMPRESSION IGNITION) ENGINES
The design of diesel engine combustion chambers is different from that of spark-ignition engines
because of the nature of the diesel process. Fuel, in liquid form, is injected into the diesel engine
cylinder through a high-pressure injector. The difference between conventional diesel injection and
that in the GDI engine is that in the former the injection occurs very close to tdc, whereas in the GDI
injection is earlier. This fuel enters the engine as a jet, or jets, which has to entrain air to enable
evaporation of the fuel and subsequent mixing to a point where hypergolic (see Chapter 15) com-
bustion occurs. The mixing and combustion processes are similar to that shown in Fig. 15.11 for a
gaseous jet. The droplet size of the fuel varies but is of the order of 20 mm; the size depends on the hole
size of the injection nozzle and the fuel injection pressure, which might be 0.20 mm and 700 bar,
respectively, in a small high-speed di diesel engine. The prime considerations in the design of a
combustion chamber for a diesel engine are to obtain efficient mixing and preparation of the fuel and
air in the time available in the cycle.