Page 102 - 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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86 Automobile mechanical and electrical systems

during the upstroke, the volume in the crankcase increases and atmospheric
pressure forces the fresh fuel/air charge into the crankcase (under the piston).
On the downstroke the piston moves towards BDC as the power stroke begins;
the expanding gases force the piston down the bore, producing torque at the
crankshaft via the connecting rod. At the same time, the crankcase volume
decreases and the fuel/air mixture is compressed under the piston. As the
piston approaches BDC, the transfer port connecting the cylinder volume to
the crankcase volume is uncovered by the piston. On the opposing side of the
cylinder, the exhaust port is also uncovered.
This allows the fresh charge in the crankcase volume to transfer and fi ll the
cylinder volume, at the same time forcing the exhaust gases out of the cylinder
via the exhaust port. The effi ciency of this scavenging process is very dependent
on the port exposure timing and the gas dynamics. Often the piston crown has
a defl ector to assist this process and to prevent losing fresh charge down the
exhaust. Note that two-stroke gasoline engines are normally lubricated via the
provision of an oil mist in the crankcase. This is provided by oil mixed in with the
fuel/air (premixed or injected); hence the oil is burnt in the combustion process,
which produces excessive hydrocarbon emissions.
Two-stroke engines are generally more powerful for a given displacement owing
to the extra power stroke compared to a four-stroke engine, but the problem is
that the expansion stroke is short and volumetric effi ciency (how easy it is to get
the gases in and out of the engine) is poor, so they are less effi cient. In addition,
exhaust emissions are higher than from a four-stroke engine.
Some large static diesel engines are often two-stroke types ( Fig. 2.10 ). Note that
all four operating processes are executed in one engine revolution (induction,
compression, expansion and exhaust). The diesel engine requires a charge of
air that is compressed to raise its temperature above the self-ignition point of
the fuel. This air charge is supplied by an air pump or pressure charging device
(turbo or supercharger). The pressurized air from this device passes into the
combustion chamber via ports in the cylinder wall. The exhaust gases leave the

Figure 2.10 Diesel two-stroke cycle