Westerbeke 33a Owner's Manual download pdf (Page 4)

DIESEL ENGINE THEORY

Diesel engines differ from other internal combustion engines in a number of ways.

Compression ratios are higher than in spark-ignited engines. The charge taken into the

combustion chamber through the intake consists of air only, with no fuel mixture.

Injectors receive fuel under pressure from the fuel pump and deliver it into the

combustion chambers at the right time in equal quantities and at the proper condition to

burn. The heat of compressed air in the combustion chamber causes the fuel mixture to

ignite. The most modern diesel engine has a four-stroke cycle. The four strokes and the

order in which they occur are: intake stroke, compression stroke, power stroke, and

exhaust stroke.

INTAKE STROKE

During the intake stroke, the piston travels downward. The intake valve is open, and the

exhaust valve is closed. The downstroke of the piston draws air in from outside to enter

the cylinder through the open intake valve ports.

Some engines use turbochargers to increase air pressure in the engine intake manifold,

which forces the air into the cylinder. The intake charge consists of air only with no fuel

mixture.

COMPRESSION STROKE

At the end of the intake stroke, the intake valve closes and the piston starts upward on the

compression stroke. The exhaust valve remains closed. The air that was drawn into the

cylinder is squeezed into the combustion chamber at the top of the cylinder.

At the end of the compression stroke, the air in the combustion chamber has been forced

by the piston to occupy a much smaller space than the space at the beginning of the

stroke. The compression ratio is a comparison, or ratio, of the volume of air in a cylinder

before compression with its volume after compression. For example, a 16:1 compression

ratio means air is squeezed into one sixteenth of the space at the top of the stroke that it

occupied at the bottom of the stroke.

Compressing the air into a small space causes the temperature of that air to rise. Near the

end of the compression stroke, the pressure of the air above the piston is approximately

400 to 500psi, and the temperature of that air is approximately 540°C.

During the last part of the compression stroke, and the early part of the power stroke, a

small, metered charge of fuel is injected into the combustion chamber. Almost

immediately after the fuel charge is injected into the combustion chamber, the fuel is

ignited by the hot air and starts to burn.

1 2 3 4 5 6 7 8 9 ... 32 33

No comments

Westerbeke 33a Owner's Manual Page 4