Page 18 - Automotive Engineering Powertrain Chassis System and Vehicle Body

CH AP TER 1 .1 Piston-engine cycles of operation

instigation and evolution of the high-speed C.I. engine compressed cylinder air, and the burning rate then tends
burning heavy fuel oil. A brief summary of the back- to match the increasing cylinder volume as the piston
ground and achievements of these two pioneers is as moves outwards – expansion will therefore take place at
follows. something approaching constant pressure.
Herbert Akroyd-Stuart, born 1864, was trained as an A summary of the combustion processes of Akroyd-
engineer in his father’s works at Fenny Stratford, England. Stuart and Diesel is that the former inventor used a low
Between 1885 and 1890 he took out several patents for compression-ratio, employed airless liquid-fuel injection,
improvements to oil engines, and later, in conjunction and relied on the hot combustion chamber to vaporise and
with a Charles R. Binney of London, he took out patent ignite the fuel; whereas Diesel employed a relatively high
number 7146 of 1890 describing the operation of his compression-ratio, adopted air-injection to atomise the
engine. Air alone was drawn into the cylinder and com- fuel, and made the hot turbulent air initiate burning. It
pressed into a separate combustion chamber (known as may be said that the modern high-speed C.I. engine em-
the vaporiser) through a contracted passage or bottle- braces both approaches in producing sparkless automatic
neck. A liquid fuel spray was then injected into the combustion – combustion taking place with a combined
compressed air near the end of the compression stroke by process of constant volume and constant pressure known
means of a pump and a spraying nozzle. The combination as either the mixed or the dual cycle.
of the hot chamber and the rise in temperature of the
compressed air provided automatic ignition and rapid 1.1.4 Two-stroke-cycle diesel
combustion at nearly constant volume – a feature of the
C.I. engines of today. engine
These early engines were of low compression, the
explosion taking place mainly due to the heat of the The pump scavenge two-stroke-cycle engine designed by
vaporiser chamber itself so that these engines became Sir Dugald Clerk in 1879 was the first successful two-
known as ‘hot-bulb’ or ‘surface-ignition’ engines. At stroke engine; thus the two-stroke-cycle engine is
starting, the separate combustion chamber was heated sometimes called the Clerk engine. Uniflow scavenging
externally by an oil-lamp until the temperature attained took place – fresh charge entering the combustion
was sufficient to ignite a few charges by compression. chamber above the piston while the exhaust outflow
Then the chamber was maintained at a high enough occurred through ports uncovered by the piston at its
temperature by the heat retained from the explosion outermost position.
together with the heat of the compressed air. Low- and medium-speed two-stroke marine diesels
Rudolf Diesel was born in Paris in 1858, of German still use this system, but high-speed two-stroke diesels
parents, and was educated at Augsburg and Munich. His reverse the scavenging flow by blowing fresh charge
works training was with Gebru ¨-der Sulzer in Winterthur. through the bottom inlet ports, sweeping up through the
Dr Diesel’s first English patent, number 7421, was dated cylinder and out of the exhaust ports in the cylinder head
1892 and was for an engine working on the ideal Carnot (Fig. 1.1-9(a)).
cycle and burning all kinds of fuel – solid, liquid, and With the two-stroke-cycle engine, intake and exhaust
gas – but the practical difficulties of achieving this ther- phases take place during part of the compression and
modynamic cycle proved to be far too much. A reliable power stroke respectively, so that a cycle of operation is
diesel oil engine was built in 1897 after four years of completed in one crankshaft revolution or two piston
experimental work in the Mashinen-fabrik Augsburg strokes. Since there are no separate intake and exhaust
Nu ¨rnberg (MAN) workshops. strokes, a blower is necessary to pump air into the
In this engine, air was drawn into the cylinder and was cylinder for expelling the exhaust gases and to supply the
compressed to 35–40 bar. Towards the end of the com- cylinder with fresh air for combustion.
pression stroke, an air blast was introduced into the Scavenging (induction and exhaust) phase (Fig.
combustion space at a much higher pressure, about 1.1-9(a)) The piston moves away from the cylinder
68–70 bar, thus causing turbulence in the combustion head and, when it is about half-way down its stroke, the
chamber. A three-stage compressor driven by the engine exhaust valves open. This allows the burnt gases to escape
(and consuming about 10% of the engine’s gross power) into the atmosphere. Near the end of the power stroke,
supplied compressed air which was stored in a reservoir. a horizontal row of inlet air ports is uncovered by the
This compressed air served both for starting the engine piston lands (Fig. 1.1-9(a)). These ports admit pressur-
and for air-injection into the compressed air already in ised air from the blower into the cylinder. The space above
the cylinder – that is, for blasting air to atomise the oil the piston is immediately filled with air, which now blows
fuel by forcing it through perforated discs fitted around up the cylinder towards the exhaust valves in the cylinder
a fluted needle-valve injector. The resulting finely divided head. The last remaining exhaust gases will thus be forced
oil mist ignites at once when it contacts the hot out of the cylinder into the exhaust system. This process

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