Page 33 - Mechatronics with Experiments

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INTRODUCTION 19
incorporate a mechanically controlled lever which adjusts the phase of the camshaft sections
in order to vary the timing of the valves. Similar phase adjustment mechanisms are designed
into individual fuel injectors as well. In electronically controlled engines some or all
of these components (fuel injectors, intake and exhaust valves) are each controlled by
electrical actuators (i.e. solenoid actuated valves). In today’s diesel engines, the injectors
are electronically controlled, while the intake valves and exhaust valves are controlled by
the mechanical camshaft. In fully electronically controlled engines, also called camless
engines, the intake and exhaust valves are also electronically controlled.
Turbo chargers (also called super chargers) and charge coolers (also called inter
coolers,or after coolers) are passive mechanical devices that assist in the efficiency and
maximum power output of the engine. The turbo charger increases the amount of air
pumped (“charged”) into the cylinders. It gets the necessary energy to perform the pumping
function from the exhaust gas. The turbo charger has two main components: a turbine and
compressor, which are connected to the same shaft. Exhaust gas rotates the turbine, and it
in turn rotates the compressor which performs the pumping action. By making partial use
of the otherwise wasted energy in the exhaust gas, the turbo charger pumps more air, which
in turn means more fuel can be injected for a given cylinder size. Therefore, an engine can
generate more power from a given cylinder size using a turbo charger. An engine without
turbo charger is called a naturally aspirated engine. The turbo charger gain is a function
of the turbine speed, which is related to the engine speed. Therefore, some turbo chargers
have variable blade orientation or a moving nozzle (called variable geometry turbochargers
(VGT)) to increase the turbine gain at low speed and reduce it at high speed (Figure 1.18).
While the main purpose of the turbo charger is to increase the amount of inlet air
pumped into the cylinders, it is not desirable to increase the inlet boost pressure beyond
a maximum value. Some turbo charger designs incorporate a waste-gate valve for that
purpose. When the boost pressure sensor indicates that the pressure is above a certain level,
the electronic control unit opens a solenoid actuated butterfly type valve at the waste-gate.
This routes the exhaust gas to bypass the turbine to the exhaust line. Hence the name
“waste-gate” since it wastes the exhaust gas energy. This reduces the speed of the turbine
and the compressor. When the boost pressure drops below a certain value, the waste-gate
valve is closed again and the turbo charger operates in its normal mode.
Another feature of some turbo chargers is the exhaust back pressure device.Using a
butterfly type valve, the exhaust gas flow is restricted and hence the exhaust back pressure
is increased. As a result, the engine experiences larger exhaust pressure resistance. This
leads to faster heating of the engine block. This is used for rapid warming of the engine
under cold starting conditions.
In some designs of turbo chargers, in order to reduce the cylinder temperature, a
charge cooler (also called inter cooler) is used between the turbo charger’s compressor
output and the intake manifold. The turbo charger’s compressor outputs air with temper-
◦
atures as high as 150 C. The ideal temperature for inlet air for a diesel engine is around
◦
35–40 C. The charge cooler performs the cooling function of the intake air so that air
density can be increased. A high air temperature reduces the density of the air (hence the
air–fuel ratio) as well as increases the wear in the combustion chamber components.
The exhaust gas recirculation (EGR) mixes the intake air with a controlled amount
of exhaust gas for combustion. The main advantage of the EGR is the reduction of NO
x
content in the emission. However, EGR results in more engine wear, and increases smoke
and particulate content in the emission.
Fuel is injected in to the cylinder by cam-actuated (mechanically controlled) or
solenoid actuated (electrically controlled) injectors. The solenoid actuation force is ampli-
fied by hydraulic means in order to provide the necessary force for the injectors. Figure 1.19
shows an electrically controlled fuel injector system where a hydraulic oil pressure line is