Automotive evolution - Appunti

Exhaust Gas Recirculation System

Xis added. It features a connection 4 inducing an exhaust flow in percentage close to 10-15% of the complete exhaust stream 10 into the intake manifold. The correct exhaust gas recirculation percentage is determined by a specific valve 5 able to deliver a suitable exhaust gas amount, based upon the requested NO reduction. XThe exhaust gases, mixed with the incoming air flow 7, reduce the temperature peak at the end of combustion.

An excess of recirculated flow leads to an hydrocarbon emission increase due to the worsening of the combustion quality. On the contrary, the exhaust flow leads to an improvement of the volumetric efficiency due to a wider opening of the throttle valve 6 required to compensate for the volume occupied by exhaust gases downstream of the throttle. The consequent de-throttling reduces fuel consumption by 2-3%.

Increasing the valve overlap, as made possible by the updated valve timing control devices, it is also possible to increase the residual gas quantity 9, from a typical

10% in part load condition, to somewhat larger value. The mixture of residuals gases with the fresh homogeneous charge 8 determines the so-called internal exhaust gas recirculation and provides the desired temperature limitation. In addition to the above, from Euro 1 onwards, all gasoline vehicles have been provided with the evaporative emission control system. The hose connection 1, through the control valve 2, introduces the gasoline vapors 3 released by an active carbon canister, to the engine intake to regenerate the carbon.

Unlike gasoline engines, the exhaust gas recirculation application with a flow up to 35% in low condition, is mandatory on direct injection diesel engines, always provided with sophisticated recirculated gas percentage lowers with increasing load, considering that the engine needs a suitable charge.

The layout applied to these engines since Euro 2 include a Diesel Particulate Filter. In the early applications, the exhaust gas recirculation valve was located on the

exhaust manifold. Today, considering that the valve actuation is electric, with electronic drive system, the valve is located close to the intake manifold. Exhaust gases from the exhaust manifold 1 are directed to the valve 2 and finally to the intake manifold 3. To increase the recirculation flow, the exhaust gases must be cooled with engine cooling fluid using a suitable heat exchanger 4 which can be bypassed when the engine is cold by activating the heat exchanger bypass valve 7. The cold recirculation flow must be prevented from causing a sharp increase of CO and hydrocarbons. To this end, the after-treatment efficiency 5 must be increased to meet the final emission standards.

The variable geometry turbocharger 6 promotes a higher recirculation flow, where in spark ignition engines the exhaust gas recirculation quantity is activated by the differential pressure available between the exhaust and the intake manifold. The negative pressure, available in the diesel intake manifold, is limited.

multipoint port fuel injection systems both naturally aspirated and turbocharged. Each spark plug is provided with its own ignition coil (in the figure in a 4 valves pent-roof combustion chamber configuration). The throttle switch provides the idle position. The ECU transmits the driving signals to the ignition coil, in the correct instant, thus providing a spark advance which is a function of many parameters, like engine speed, intake manifold absolute pressure, engine temperature, throttle position and conditions leading to knocking. The engine temperature is controlled by sensor 5 and knocking intensity by a sensor 6. The engine speed sensor 7 detects both the speed and the reference mark, shown as a tooth gap on the engine flywheel 8, thus triggering the ignition process. The voltage of the battery 9 is monitored and the electronic control unit performs the voltage correction by extending or reducing the coil charging time. The ignition key 10 completes the system. The ignition system

is now integrated with the injection system, both being driven by the engine control unit.

The so-called Minimum spark advance for the Best Torque (MBT) is the value of the advance allowing the best performance with minimum fuel consumption.

Detonation is a type of abnormal combustion, due to auto-ignition or spontaneous ignition, typical of spark ignition engines. To avoid detonation, the fuel formulation must assure the correct octane number to allow using the required compression ratio value.

The complete layout of the fuel injection and ignition system used on present day is composed by:

• Evaporative emission control system with the activated carbon canister 1 and its duty-cycle valve 4 for canister regulation
• Fuel injection system, delivering a suitable air to fuel ratio already quite close to the final one. When exhaust gas recirculation is used the air-mass meter 2 senses the mass of air entering into the engine and the absolute pressure sensor 5 is used to evaluate the amount

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• Fuel rail 6, usually made with reinforced polyamide, to distribute pressure to injectors
• Actuators and sensors 8 for variable valve timing and camshaft phase sensor 10
• On Board Diagnostic (OBD) system with the lambda sensor downstream the pre-catalyst 18, the speed sensor 14 and the malfunction indicator lamp 20 located on the driver's instrument panel
• Interface with the Controlled Area Network (CAN) 19 and diagnostic tool (21), used off-board in the repair shop, able to identify the malfunctioning component causing the lamp to switch on
• Interface with the immobilizer 22 linked with the ignition key
• Cut-off function to reduce the fuel consumption. It stops the fuel delivery when the accelerator pedal is fully released
• Intake duct, shaped in its final part close to the port, so that the mixture flow entering the combustion chamber provides the required turbulence level

In the air meter the air stream, entering the engine, flows through the tubular

Meter housing in which a bracket 2 supports an element, the hot-wire 3, electrically heated at a temperature of about 120°C. The incoming air flow cools the hot wire, so that by measuring the current needed to keep the temperature of the wire constant, a measurement of the air mass introduced into the engine is obtained. This electric signal is transmitted to the electronic control unit through the connector 1.

The task of the injector is supplying the correct fuel quantity to the engine and atomizing it. The fuel is filtered when entering the injector 1. The fuel injector takes place thanks to the motion of the needle 5 mechanically connected with the magnetic nucleus 4, which slides back driven by the solenoid winding 3. The latter receives the current pulses though the connector 2. The needle is kept in closed position by the spring 8 which pulses it in contact with the needle seat and its stroke can be controlled by the calibrated disc 7. The pintle shape 6 produces the final configuration.

of a tapered spray typically used in engines with one valve per cylinder. The tapered spray is directed into the opening between the intake valve head and the valve seat. Dual-spray formation is used in engines with two intake valves per cylinder.

Historically the application of the Gasoline Direct Injection (GDI) configuration was determined by the need to achieve fuel consumption and performance improvement in aero and racing engines.

The main differences in the components of the injection and ignition systems between port fuel injection and gasoline direct injection engines are here listed:

• The intake duct profile and piston design are shaped to produce an air motion which can widely vary according to the combustion process, stratified or homogeneous
• The two-steps lambda sensor is no longer sufficient and the new, more complex, broad-band lambda sensor is required
• The injector shape must reach the combustion chamber. Cross section A-A outlines the swirl type injector