Page 35 - Mechatronics with Experiments

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INTRODUCTION 21
Each fluid (liquid or gas) circuit has a component to store, condition (filter, heat or cool),
move (pump), and direct (valve) it within the engine.
The cooling and lubrication systems are closed circuit systems which derive their
power from the crankshaft via a gearing arrangement to the coolant pump and the oil pump.
Reservoir, filter, pump, valve, and circulation lines are very similar to other fluid circuits.
The main component of the cooling system is the radiator. It is a heat exchanger where
the heat from the coolant is removed to the air through the a series of convective tubes
or cores. The coolant is used not only to remove heat from the engine block, but also to
remove heat from the intake air at after-cooler (inter-cooler) as well as to remove heat from
the lubrication oil. Finally the heat is dissipated out to the environment at the radiator. The
radiator fan provides forced air for higher heat exhange capacity. Typically, the cooling
system includes a temperature regulator valve which directs the coolant flow path when the
engine is cold in order to help it warm up quickly.
The purpose of lubrication is to reduce the mechanical friction between two surfaces.
As the friction is reduced, the friction related heat is reduced. The lubrication oil forms a
thin film between any two moving surfaces (i.e., bearings). The oil is sucked from the oil
pan by the oil pump, passed through an oil filter and cooler, then guided to the cylinder
block, piston, connecting rod, and crankshaft bearings. The lubrication oil temperature must
◦
be kept around 105–115 C. Too high a temperature reduces the load handling capacity,
whereas too low a temperature increases viscosity and reduces lubrication capability. A
pressure regulator keeps the lube oil pressure around a nominal value (40 to 50 psi range).
The fuel pump, lubrication oil pump, cooling fan, and coolant pump all derive their
power from the crank with gear and belt couplings. The current trend in engine design is
to use electric generators to transfer power from the engine to the electric motor-driven
pumps for the sub-systems. That is, instead of using mechanical gears and belts to transmit
and distribute power, the new designs use electrical generators and motors.

Diesel Engine Operating Principles Let us consider one of the cylinders in a
four-stroke cycle diesel engine (Figure 1.20). Other cylinders go through the same sequence
of cycles except offset by a crankshaft phase angle. In a four-cylinder diesel engine, each
◦
cylinder goes through the same sequence of four-stroke cycles offset by 180 of crankshaft
◦
◦
angle. Similarly, this phase angle is 120 for a six-cylinder engine, and 90 for an eight-
◦
cylinder engine. The phase angle between cylinders is (720 )/(number of cylinders). During
the intake stroke, the intake valve opens and the exhaust valve closes. As the piston moves
down, the air is sucked into the cylinder until the piston reaches the bottom dead center
(BDC). The next stroke is the compression stroke during which the intake valve closes and,
as the piston moves up, the air is compressed. The fuel injection (and spark ignition in the
SI engine) is started at some position before the piston reaches the top dead center (TDC).
The combustion, and the resulting energy conversion to mechanical energy, are
accomplished during the expansion stroke. During that stroke, the intake valve and exhaust
valve are closed. Finally, when the piston reaches the BDC position and starts to move up,
the exhaust valve opens to evacuate the burned gas. This is called the exhaust stroke. The
cycle ends when the piston reaches the TDC position.
This four-stroke cycle repeats for each cylinder. Note that each cylinder is in one of
these strokes at any given time. For the purpose of illustrating the basic operating principle,
we stated above that the intake and exhaust valve open and close at the end or beginning of
each cycle. In an actual engine, the exact opening and closing position of these valves, as
well as the fuel injection timing and duration, are a little different than the BDC or TDC
positions of the piston.
It is indeed these intake and exhaust valve timings as well as the fuel injection
timing (start time, duration, and injection pulse shape) decisions that are made by the