Page 333 - Mechanic Diesel - TT

MECHANIC DIESEL - CITS

Abrupt contours in an intake manifold result in pressure drops and some of the fuel droplets in spark-ignition
engines form pools in the interior part of intake manifolds surface which results in the uneven air-fuel mixture
to enter for combustion. High-performance cars use manifolds with smooth contours for increased volumetric
efficiency.
The design of manifold uses Helmholtz Resonance in which first, the air flows at considerable speed through
the open valve. When the valve closes, the air that has not entered the valve yet still has a lot of momentum and
compresses against the valve, creating an area of high pressure. This high-pressure air begins to equalize with
lower-pressure air in the manifold. Due to air’s inertia, the equalization will tend to oscillate, at first, the air in the
runner will be at a lower pressure than the manifold. The air in the manifold then tries to equalize back into the
runner, and the oscillation repeats. This process occurs at the speed of sound, and in most manifolds travels up
and down the runner many times before the valve opens again.
Data link connecter, On board diagnosis system
The OBD connector, also known as the On-Board Diagnostics connector, is a standardized port used in vehicles
to communicate with the onboard computer systems. It’s typically located under the dashboard, and it allows
mechanics and technicians to access diagnostic information and perform various tests on the vehicle’s systems.
The OBD port is a universal connector that mechanics can use to tap into a vehicle’s computer for running all sorts
of tests and diagnostics. The history of the device goes as far back as 1968, when Volkswagen introduced its first
form of on-board computer system.
OBD stands for On-Board Diagnostics and is a computer system inside of a vehicle that tracks and regulates a
car’s performance. This on-board computer system collects information from the network of sensors inside the
vehicle, which the system can then use to regulate car systems or alert the user to problems. A technician can
then simply plug into the OBD system to collect vehicle data and diagnose the problem. OBD systems have been
a great help in helping users better understand vehicle diagnostics.
The history of OBD begins in the 1980s. During this time, vehicle monitoring systems were developed in response
to several factors, including:
Emissions control: One of the biggest reasons for developing OBD was to help reduce vehicle emissions. OBD
systems help in this area by monitoring the performance of major engine components for any system failures that
could result in increased emissions. OBD is so helpful in this area that it is incorporated into EPA literature on the
implementation of the Clean Air Act.

Electronic fuel injection: In the 1980s, automakers began the widespread production of vehicles with electronic
fuel injection. Unlike mechanical fuel injection systems, electronic fuel injection works via computer control, with
the computer system monitoring and determining the fuel flow into the engine.

Electronic components: As electronic fuel injection gained popularity, more electronics became commonplace
in cars, increasing the need for more sophisticated monitoring systems to help identify problems more accurately.
Since its initial development, vehicle monitoring systems have undergone several iterations. Today, OBD serves
as a standardized system that dictates the connectors and trouble codes used, making it easy for technicians to
service a wide range of vehicles quickly and accurately.
How Does OBD Work?
A basic OBD system consists of a central system, a network of sensors, a connection point and indicators,
creating a complete monitoring system with standardized access and readability. The OBD system consists of the
following components:
ECU: The central part of the OBD system is the Electronic Control Unit, or ECU. The ECU collects input from
various sensors throughout the vehicle. The ECU then uses this data to either control parts of the vehicle, like fuel
injectors, or monitor for issues.
Sensors: There are sensors throughout vehicles covering every area from the engine and chassis to the electronic
system itself. Each one of these systems sends codes to the ECU, specifying the source and the parameters of
the signal. The ECU then “reads” and interprets this signal.

DTC: If a sensor sends information to the ECU that falls outside of the normal range, the ECU saves the
information as a code called a Diagnostic Trouble Code, or DTC. The DTC code essentially is a list of letters and
numbers, which indicate the source and nature of the problem. DTC codes are usually standardized but may

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CITS : Automotive - Mechanic Diesel - Lesson 98 - 103