Why
Do Old John Deeres
Sound
the Way They Do?
Dave Merchant built
this incredibly cool image,
showing why John Deere two-cylinder engines have to go "putt-putt,
miss-miss."
Like
virtually all other engines big enough
to power a tractor, these are "four stroke" engines, meaning each
piston
has to make four movements--down, up, down, up--to complete a single
combustion
cycle. Those four strokes correspond to what is happening
inside
the cylinder:
|
Piston
Moves
|
What Happens |
|
Down
|
Power, from rapid expansion of the fuel-air mixture after the spark plug ignites it |
|
Up
|
Exhaust of combustion
gases |
|
Down
|
Intake of fresh air
and fuel |
|
Up
|
Compression of fuel-air
mixture before the spark plug fires |
|
John
Deere two-cylinder engines use a
"180-degree crank": The throws of the crankshaft are offset
180 degrees,
so that when one piston is up, the other is down. That
balances the
internal dynamics of the engine--the inertia of one piston moving up is
offset by the inertia of the other piston moving down, and the volume
of
air in the crankcase beneath the pistons is kept essentially
constant.
But it also forces the engine to have that trademark "poppin' Johnny"
rhythm:
|
Piston
1
|
Piston
2
|
You
Hear
|
|
Down
|
Power |
Up
|
Compression |
Putt
|
|
Up
|
Exhaust |
Down
|
Power |
Putt
|
|
Down
|
Intake |
Up
|
Exhaust |
-
|
|
Up
|
Compression |
Down
|
Intake |
-
|
|
A
two-cylinder engine could have an even
firing pattern if it had a "0-degree crank," where both pistons
travelled
up and down together. The crankshaft would have to be
counterweighted
to offset the inertia of both pistons moving up at the same time or
down
at the same time, to keep the thing from hopping around like an
out-of-balance
washing machine, and the crankcase would need a great amount of
ventilation
to accommodate the big changes in volume beneath the pistons.
But
the engine wouldn't sound like a John Deere:
|
Piston
1
|
Piston
2
|
You
Would Hear
|
|
Down
|
Power |
Down
|
Intake |
Putt
|
|
Up
|
Exhaust |
Up
|
Compression |
-
|
|
Down
|
Intake |
Down
|
Power |
Putt
|
|
Up
|
Compression |
Up
|
Exhaust |
-
|
|
Image
copyright 1999 by Dave
Merchant, used here by permission. To see other
examples of three-dimensional
art created with "Persistence of Vision Ray Tracing", or
POV-Ray,
check out Dave's
Place.
|
