AEM 30-6905 Universal Programmable EMS-4 - Manual (Page 269)

Page 269 of 279 EMS-4 Install and Tuning Guide_Rev 1.6
Glossary of Terms
4 Stroke Cycle
Intake - During the intake stroke, the piston is moving from top to bottom and the intake valve is
open. As the piston moves down, a vacuum is created which draws the air/fuel mixture into the
combustion chamber. The intake valve is closed after the piston reaches the bottom. This
position is normally called bottom dead center (BDC)
Compression - During the compression stroke, the piston moves up and compresses the air/fuel
mixture against the cylinder head. When the piston is near the top of the stroke (the top of the
stroke is typically called top dead center, TDC), the ignition system produces a spark at the
spark plug. The spark ignites the mixture which creates a rapid rise in cylinder pressure.
Power - During the power stroke, the high pressure gases produced by the burning of air/fuel
mixture force the piston back down the cylinder bore. This is the only stroke that actually
produces useable power from the engine.
Exhaust - During the exhaust stroke, the piston moves upward. The exhaust valve is open and
the piston forces the spent gases out the exhaust port and into the exhaust system.
5 Volt Reference
The high voltage reference for most automotive analog sensors. Typical three wire sensors will
have a high reference, a low reference and a signal output. The signal output will always fall
between the low and high reference points.
Air / Fuel Ratio
In engine testing, both the air mass flow rate and the fuel mass flow rate are normally
measured. The ratio of these flow rates is useful in defining engine operating conditions. The
normal operating range for a conventional spark ignited gasoline burning engine is between
12:1 and 18:1. Turbo/supercharged engines typically operate even richer with air/fuel ratios as
low as 11:1. The most common way to measure Air/Fuel ratio for engine calibration work is by
using an oxygen sensor in the exhaust stream.
Analog Circuits
Electrical circuits that amplify or alter a voltage signal to represent some physical quantity.
Atmospheric Pressure
Normal pressure in the surrounding atmosphere generated by the weight above us pushing
down. At sea level, in average weather conditions, atmospheric pressure is approximately 100
kPa (about 14.5 psi) above vacuum or zero absolute pressure.
Base Idle
Idle RPM when the throttle lever is at rest on the throttle stop and the idle speed control function
is disabled.