The 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 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.
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.
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
Alpha-n tuning is often referred to TPS referenced load
control, as this method uses just the data from the throttle
position sensor with relation to engine RPM and correction
factors to control fuel delivery.
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
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.
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.
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.
multi meter) electronic current/resistance/potential
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 greater power.
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 driveshaft.
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
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
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
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 operating.
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 car's computer.
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 carburetor.
Fuel starvation -
Fuel starvation occurs when fuel, for one reason or
another, is prevented from reaching the carburetor or fuel
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.
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 10-15 ohms.
IAC – Idle
Air Control. Typically a “stepper motor”.
IAT sensor -
Intake Air Temperature sensor, same as MAT, see below.
Idle circuit -
This is a special kind of circuit found in a carburetor that
only operates when the engine is at an idle.
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.
- 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 (TDC).
Ignition module -
Part of the ignition system which instructs the
ignition coil to send current to the distributor.
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 ignition condenser.
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.
- 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
Load Control -
Load is essentially a measurement of airflow since, as
discussed in our
Volumetric Efficiency article an engine is essentially a
large air pump. Since airflow determines load and is
directly correlated to volumetric efficiency, and it’s
operating parameters, including fuel and ignition
requirements, it is critical that we have an understanding
and a methodology for calculating, measuring and or
programming the load of their particular engine
configuration. Once airflow is known, fueling and other
operating parameter simply become trivial scientific
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
MAF Sensor -
Mass Air Flow sensor. Sensor, normally mounted directly
in the air intake system to extract a measurement of the
actual air flow (in units of mass/time) See
Load Control 101 for how MAF
is used in the calculation of engine load
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.
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
P&H Injectors -
Peak and hold injectors; see Low Impedance injectors.
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
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.
Speed Density -
Speed-density is one of the
most common methods of load control and airflow calculations.
This method uses an equation relating the manifold absolute
pressure (MAP) and the intake air temperature with the known
characteristics of the engine to calculate airflow, and thus
makes it possible to calculate fueling requirements.
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
Throttle Position Sensor, a voltage divider that
provides information about throttle opening, from which it
computes rate of throttle opening for acceleration
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 cycle.
Ref: Volumetric Efficiency 101
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
WUE - Warm
Up Enrichment, the enriched mixture applied when the
coolant temperature is low.