rate of change in velocity with respect to time. According to Newton's second
law of motion, acceleration is equal to the force, divided by mass (A=F/M).
Accelerator pump- Accelerator pumps
are found in cars equipped with carburetors. When you accelerate, the
accelerator pump delivers extra fuel through the accelerator pump circuit to
allow the engine to deliver more power.
electrical mechanism for moving or controlling something indirectly instead of
by hand, such as a door lock. Output device the PCM controls such as solenoids,
relays, fuel injectors and stepper motors.
AE - Acceleration Enrichment, the enriched
mixture provided when the throttle position sensor signal changes at various
AFR - Air Fuel Ratio, the mass ratio of air to fuel in the
combustion chamber. See NB- and WB-EGO sensors, below.
Air filter- This device
filters the air that goes into your engine. Without an air filter, harmful
particles would enter your car's engine and cause internal wear and damage.
Air pump - Many emissions systems include an air pump,
which pumps fresh air into a vehicle's exhaust to help complete the combustion
process and reduce emissions. To get
accurate lambda measurements with the LM-1, air pumps should be temporarily
ASE - After Start Enrichment, the enriched
mixture provided for a number of engine cycles when an ECU detects that the
engine has transitioned from cranking to running.
Carburetor - A mechanism
which mixes fuel with air in the proper proportions to provide a desired power
output from a spark-ignition internal combustion engine.
Carburetor jet - A
fitting inside a carburetor that meters fuel into a metering circuit where it
is mixed with air.
Catalyst - A substance that
can increase or decrease the rate of a chemical reaction between substances
without being physically consumed in the process. A catalyst, which reduces
engine emissions, is used in a catalytic converter.
Catalytic converter - An
in-line, exhaust system device, containing a catalyst, which reduces engine
exhaust emissions. Converters are located near the exhaust manifolds or headers
for maximum efficiency.
Closed loop - refers to those times when an EFI computer is using the
feedback on the mixture provided by the oxygen sensor to effectively control
the injected amounts.
Combustion - The process by
which the air/fuel mixture burns within an engine to create power.
Computer (PCM) - Many modern
cars have a central computer called an engine control unit (ECU) or power train
control module (PCM). This controls the car's fuel and ignition systems by
taking information from various sensors to determine how to run the engine with
the most efficiency and power.
Converter (Torque) - A fluid coupling device which multiplies torque between an engine and
automatic transmission/transaxle. When a vehicle is stopped, a converter allows enough fluid slippage, so
the engine can idle without stalling.
CTS - Coolant Temperature Sensor. Usually the CTS is an NTC (Negative Temperature Coefficient) thermistor, or a resistor whose resistance varies with
temperature (NTC means the resistance goes down as the temperature goes up.
DMM (digital multi meter) electronic
current/resistance/potential measuring tool.
Double overhead cam (DOHC) - A
DOHC engine has two camshafts in the cylinder head - one for the exhaust
valves, and one for the intake valves. This allows greater efficiency and
Driveline - The system of
components that connects the transmission to the wheels. The driveline consists
of axles, differentials, constant velocity (CV) or universal joints, and a
Driver - A switched
electronic device housed in a computer that controls output state. For example,
a driver controls how long a fuel injector remains open.
Duty Cycle (DC) A number indicating the
amount of time that some signal is at full power. In the context of an ECU,
duty cycle is used to describe the amount of time that the injectors are on, and to describe the hold part of the peak and hold
injector drivers (see Low Impedance Injectors, below).
Early Fuel Evaporation - Used
on carburetor-equipped engines only, a system where heat is used to help
increase early fuel evaporation of the cold-start air/fuel mixture to achieve
more efficient combustion and lower emissions. GM used an electric grid system.
EGO Sensor - Exhaust Gas Oxygen sensor, used to describe the
sensor in the exhaust that measures the lean/rich state of the AFR. Used to
control the via a feedback algorithm called "closed loop".
Emissions - Emissions are the
byproducts of combustion. After combustion is complete, water, gases, and
carbon are released through the car's exhaust system as emissions.
Emissions equipment - Emissions
equipment is equipment required by the government to keep a car's exhaust
emissions to a minimum. Emissions equipment includes catalytic converter, air
pump, and oxygen sensor.
Engine - A machine designed
to convert thermal energy into mechanical energy to produce force or motion.
Connected to a drivetrain, an engine's mechanical
energy, or torque, moves a vehicle. An engine can run by using gas, diesel
fuel, steam or other fuel sources.
Engine accessory - An engine
accessory is a peripheral piece of equipment that runs directly off of the
engine's power to supply energy or a fluid to another part of the car. Engine
accessories include the alternator, power steering pump, air pump, air
conditioning compressor, as well as many others.
Engine block - The engine
block is where the cylinders and pistons reside. The block is the strongest
part of the engine and withstands tremendous pressures while the engine is
Engine temperature sender - The
engine temperature switch and sending unit measure the temperature of the
engine's coolant. They send this information to the engine temperature warning
light and engine temperature gauge, respectively. Compare to coolant
temperature sensor (CTS) which transmits the coolant temperature to the
computer, and the radiator fan switch which engages the radiator's cooling fan.
Fuel injection - Fuel
injection is a system by which fuel is directly sprayed into the intake
manifold or intake port at high pressure. Fuel injection is often controlled by
a computer, allowing precise monitoring of efficiency and performance by the
Fuel injector - A device for
delivering metered, pressurized fuel to the intake
system or individual cylinders. An injector sprays fuel, which helps
atomization for a more dense mixture, when combined with incoming air.
Fuel pump - The fuel pump
moves gas from the gas tank and delivers it to the fuel injection system or
Fuel starvation - Fuel
starvation occurs when fuel, for one reason or another, is prevented from
reaching the carburetor or fuel injectors.
Fuel system - The fuel system
is the system by which fuel is stored and delivered to each cylinder. The fuel
system includes the fuel tank, fuel tank level sending unit, the fuel pump, the
fuel filter, and fuel lines. For carbureted cars, the fuel system also includes
the carburetor. For fuel injected cars, the fuel system also includes
injectors, fuel pressure regulator and often a main computer.
G-Force - Unit of measurement
used to describe lateral acceleration generated while the vehicle is driven in
a steady state turn on a skid pad circle. An average sedan generates 0.60 G of
lateral acceleration. Measured in "gravities", one G equals the
earth's gravity at sea level.
Ground - An electrical
conductor used as a common return for completing an electric circuit(s). Car
batteries contain a ground terminal, usually the negative terminal.
Head gasket - The head gasket
seals the cylinder head to the engine block. It is subject to tremendous
pressures, and often fails if and when an engine overheats.
Headers - Constructed from
steel tubing, headers provide a smooth and efficient exhaust flow path from the
exhaust port to the exhaust system. Headers are frequently used in performance
engine applications and are generally less restrictive than the stock exhaust
manifold, resulting in increased power.
High Impedance Injectors - (a.k.a. hi-Z) Fuel injectors designed to
work with a simple switch in a 12 volt circuit, no special signal conditioning
is required to drive them. The resistance of a high impedance injector is about
IAC Idle Air Control. Typically a "stepper motor".
IAT sensor - Intake Air Temperature sensor, same as MAT, see
Idle circuit - This is a
special kind of circuit found in a carburetor that only operates when the
engine is at an idle.
Ignition - Complete system
used to step up battery voltage to a higher voltage and deliver it to the spark
plug to complete the combustion process. When the key is turned on, the
ignition system is energized.
Ignition Advance/Retard - The advancing or retarding (in crank degrees) of ignition spark relative
to the piston location in the cylinder. In performance applications, the goal is to set ignition timing such
that peak cylinder pressure occurs at 16-18 degrees after top dead center
Ignition module - Part
of the ignition system which instructs the ignition coil to send current to the
Ignition system - The
ignition system contains the components that supply spark to the vehicle's
spark plugs. These include the battery, the ignition coil, the distributor
(including the cap and rotor), the spark plug wires, the ignition module, and
the spark plugs themselves. Older cars also have ignition points and an
Knock (Engine) - The sharp,
metallic sound produced when two pressure, or flame fronts collide in the
combustion chamber. This could be the result of incorrect ignition timing,
incorrect air/fuel mixtures, or the wrong grade (octane rating) of gas. Also known as Detonation.
kPa (kiloPascals) - the measurement of air pressure used
in some ECU computations. Average pressure at sea level is 101.3 kPa.
Lambda the ratio between actual
air/fuel ratio and stoichiometric ratio. Lambda of less than 1 is rich, and greater than 1 is lean.
Low Impedance Injectors - (a.k.a
low-Z) Fuel injectors that are designed to run at a much lower current than
would be supplied by a direct 12 volt connection. They require a special
signal that is initially at full current (4-6 amps, a.k.a. "peak current") for
about 1.0-1.5 ms, but then drops down to about 1 amp ("hold current") for the
rest of the opening pulse. The resistance of a low-impedance injector is
typically 1-3 ohms.
MAP sensor - Manifold Absolute Pressure sensor. Measure the
absolute pressure in the intake manifold (related to the engine vacuum), to determine
the load on the engine and the consequent fueling requirements.
MAT Sensor - Manifold Air Temperature sensor, the same as IAT.
The MAT circuit is identical to the CTS circuit, see CTS, above.
NB-EGO Sensor - Narrow Band EGO sensor, gives a switch at the
stoichiometric ratio (the chemically correct mixture of air and fuel), but
unreliable for AFR other than stoichiometric.
OEM (original equipment manufacturer) - refers to parts
produced for initial assembly of a new vehicle.
Open Loop - refers to those times when ECU ignores the feedback from
the oxygen sensor.
P&H Injectors - Peak and hold injectors; see Low Impedance
Pulse Width Modulation (PWM) - A signal with a fixed pulse
width (frequency), which is turned on for part of the pulse. The percent of
time that the signal is on is called its duty cycle. PWM is used to control
voltage (and consequently current) to fuel injectors.
Required Fuel - For some ECUs and EFI
systems, the injector pulse width, in milliseconds, required to supply the fuel
for a single injection event at stoichiometric combustion, 100% volumetric
efficiency and standard temperature.
Stoichiometric Ratio- The ratio at which all available fuel is combined with oxygen during the
combustion process. This theoretically ideal ratio produces
minimum emissions, however maximum power is achieved at an AFR 10-15% richer
than stoichiometric, while maximum efficiency is achieved at an AFR 3-5% leaner
than stoichiometric (depending on many engine variables).
TPS - Throttle Position Sensor, a voltage divider that provides
information about throttle opening, from which it computes rate of throttle
opening for acceleration enrichment.
VE - Volumetric Efficiency. The actual amount of air being pumped by the engine as compared to
its theoretical maximum. A 200 cubic inch motor will theoretically move
200 cubic inches of air in one cycle at 100% efficiency. If the engine is
actually running at 75% VE, then it will move 150 cubic inches of air on each
WB-EGO Sensor - Wide Band EGO sensor, can be used to derive
real AFR data with mixtures from 10:1 to 20:1, i.e. anything you are likely to
be interested in.
WOT - Wide open throttle.
WUE - Warm Up Enrichment, the
enriched mixture applied when the coolant temperature is low.