Page 108 - 05. Subyek Teknik Mesin - Automobile Mechanical and Electrical Systems Automotive Technology Vehicle Maintenance and Repair (Vehicle Maintenance Repr Nv2) by Tom Denton

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92 Automobile mechanical and electrical systems

(tdc)

Piston
(bdc)

Air charge slightly below Air charge nearly 8 to 10 times greater
atmospheric pressure than atmospheric pressure

Figure 2.19 Compression

the trapped gas is compressed and the amount of compression is known as
Key fact
the compression ratio. Compressing the charge before combustion allows more
Most spark ignition engines have a oxygen or fuel/oxygen in the cylinder than would otherwise have been available
compression ratio of 8:1 to 10:1. without compression and this improves combustion effi ciency. Most spark ignition
engines have a compression ratio of 8:1 to 10:1. This means that the cylinder
volume reduces by eight or ten times during the compression stroke ( Fig. 2.19 ).

2.1.2.1 Spark ignition (SI)
During compression of the fuel/air mixture in a petrol engine, heat energy and
kinetic energy (due to gas movement) are imparted into the mixture owing to
the reducing volume and rising pressure. This creates a signifi cant temperature
increase and the magnitude of this increase depends upon the speed of the
compression process and the amount of heat rejected to the surroundings (via
the cylinder combustion space, walls, head, etc.). The temperature rise elevates
to a point just below the self-ignition temperature of the fuel/air charge, which
will combust at or above the fl ashpoint when ignited via an external source
(i.e. the spark plug). Note that if the temperature of the mixture was too high,
spontaneous self-ignition could occur and this would be a limiting factor for the
maximum compression ratio in a petrol engine.
After compression of the inlet charge, combustion of the fuel creates heat and
pressure energy, which is imparted on the piston to generate mechanical work. In
a petrol engine, this process is initiated by the high-voltage arc at the spark plug
electrodes in the cylinder.
Combustion in the cylinder of an engine is a chemical reaction process between
carbon and hydrogen in the fuel and oxygen present in the induced air. The
), and the hydrogen
carbon and oxygen combine to form carbon dioxide (CO 2
combines with oxygen to form water (H O). Nitrogen passes through the engine
2
as long as the combustion chamber temperatures remain below critical limits.