Fiat 124 Spider - Concepts; The Otto Cycle

FIAT 124 Spider
Engine Maintenance + Modification
6
2. Concepts
This section describes some of the basic things we’ll discuss throughout this guide.
2.1 The Otto Cycle
The most common type of operation for automotive engines is the four-stroke cycle, also
known as the Otto cycle. Conceptualized in the 1870's by Nicolaus Otto (and others),
the cycle has four stages of operation, beginning with the engine at Top Dead Center
(TDC), when the piston is furthest from the crankshaft:
• Intake: The intake valve opens and fuel and air are drawn into the engine as the
piston is drawn down.
• Compression: The intake valve is closed and the piston rises, compressing the
mixture.
• Power: The compressed mixture is ignited, the resulting explosion forces the
piston down.
• Exhaust: The exhaust valve opens and the spent gases are forced out as the
piston rises.
If you want to know more about the Otto Cycle theory and all of the mechanical
engineering concepts that are involved pick up a copy of “A Practical Treatise on the
‘Otto’ Cycle Gas Engine” by William Norris, Longman, Green, and Co., London, England,
1896. At the time of this writing it is available for download on books.google.com. It is a
very old book but is wonderful at describing the basic concepts of an automobile engine.
2.2 Lambda
Fuel Injection systems and Carburetors have a common goal – to reach an air to fuel ratio
of 14.7:1 and maintain that ratio through changing engine loads. The ratio is known as
the stoichiometric air-fuel ratio for gasoline. It is commonly referred to as lambda. Any
mixture less than 14.7:1 is considered rich; any mixture above is considered lean.
The air fuel mixture is established on a carburetor by setting mechanical devices, such as
air bleeders, fuel jets, and emulsion tubes. On a fuel injection system the mixture is
constantly adjusted by a computer that varies the amount of fuel sprayed into the
cylinder. Both systems rely on a stream of measured air, regulated by a throttle, to
deliver atomized fuel into the engine.