Four Stroke Engine
The four stroke engine was first demonstrated by Nikolaus
Otto in 1876, hence
it is also known as the Otto cycle. The technically
correct term is actually four stroke cycle. The four
stroke engine is probably the most common engine type nowadays.
It powers almost all cars and trucks.
The four strokes of the cycle are intake, compression, power,
and exhaust. Each corresponds to one full stroke of the piston;
therefore, the complete cycle requires two revolutions of the
crankshaft to complete.
Intake
During the intake stroke, the piston moves downward, drawinga fresh charge of vaporized fuel/air mixture. The illustrated
engine features a poppet intake valve which is drawn
open by the vacuum produced by the intake stroke. Some early
engines worked this way; however, most modern engines
incorporate an extra cam/lifter arrangement as seen on the
exhaust valve. The exhaust valve is held shut by a spring (not
illustrated here).
Compression
As the piston rises, the poppet valve is forced shut by the
increased cylinder pressure. Flywheel momentum drives the piston
upward, compressing the fuel/air mixture.
Power
At the top of the compression stroke, the spark plug fire,igniting the compressed fuel. As the fuel burns it expands, driving the piston downward.
increased cylinder pressure. Flywheel momentum drives the piston
upward, compressing the fuel/air mixture.
Power
At the top of the compression stroke, the spark plug fire,igniting the compressed fuel. As the fuel burns it expands, driving the piston downward.
At the bottom of the power stroke, the exhaust valve is
opened by the cam/lifter mechanism. The upward stroke of the
piston drives the exhausted fuel out of the cylinder.
opened by the cam/lifter mechanism. The upward stroke of the
piston drives the exhausted fuel out of the cylinder.
This animation also illustrates a simple ignition system
using breaker points, coil, condenser, and battery.
A number of visitors have written to point out a problem with
the breaker points in my illustration. In this style ignition
circuit, the spark plug will fire just as the breaker points
open. The illustration appears to have this
backwards.
In fact, the illustration is correct; it just moves so fast
it's difficult to see! Here's a close-up of the frames just at
the point the plug fires:
My original intent was to accurately show that the points
need to remain closed for only a fraction of a second, called
the dwell. By illustrating this, I inadvertently
obscured the overall operation of the circuit. Perhaps someday
I'll prepare a more detailed illustration of the ignition
system alone.
cylinder, have various arrangements for the camshaft (dual,
overhead, etc.), sometimes feature fuel injection,
turbochargers, multiple valves, etc. None of these enhancements
changes the basic operation of the engine.
overhead, etc.), sometimes feature fuel injection,
turbochargers, multiple valves, etc. None of these enhancements
changes the basic operation of the engine.
Here is an illustration of both 4-stroke and a multiple cylinder engine in animated form:
