New-York tribune., 1917-11-04
Enoine's $ Value as a Brake 4
Is Handy Thing to Know
1t May Save Everything
in an Emer-
gency
BY William II. Stewart, Jr.

ficient braking system they have in
the gear box, combined with the en-
gine. A thoroughly reliable braking
effect may be obtained which will save
the brake Innings and may even save
life in case the brakes fail.
A case comes to mind Of driver IN
Maine who took two women down
mile or more Of mountain road and
killed both, including himself, at the
bottom. The brakes had failed and he
did not know what else to do but steer
until death overlook them.
However, such knowledge is more
general now, the only difficulty being
that people do not know what gears to
use. Fortunately, the problem is
very simple one. II the clutch is en-
gagged and the gears in speed It is
evident that there is a positive drive
from the engine right through to the
rear wheels. 1t must be equally evil
dent if the car is running down hill
and the clutch and gears engaged, the
engine being dead, that the rear wheels
will drive the engine. This requires
work to be done, and the effect on the
wheels is to slow them down. 1t be-
comes then simply a question Of choos-
ing the gears which will cause the rea-
wheels to do the most work.
It will readily be seen that this must
be the low speed. On the direct drive
(Which is the high speed in three
speed gear set), the engine revolves
once to turn the drive shaft once. Blu.
on low gear the engine may revolvc
three or four times, depending on the
size of the gears. If the drive is re-
versed the engine must revolve three
or four times for every revolution Of
the drive shaft. As this gives three
or four times the drag that would be
given by the high gear it is eviden.
that it should be used.
This may not seem to be very hard
work, but that is because we have not
yet considered the reduction Of gear
ing that takes place at the differential.
This varies with different cars, but one
may as we'll call it four to one. That
is, the drive shaft turns four times to
drive the differential and axle once. II
We reverse this the wheels turning
once will drive the drive shaft four
times.
II the low gear is in mesh the engine
will revolve four times for every turn
Of the drive shaft, making sixteen
revolutions Of the engine to one of the
rear wheels. II high gear were used
the engine would only turn over four
times. BY using the low gear we get
four times the braking effect. If we
have a four-cylinder engine We have
two times the braking effect. If we
have a four-cylinder engine we have
two strokes Of the pistons for each
revolution, making thirty-two strokes
Of the piston to each revolution Of
the rear wheels
The question arises as to whether
suction or compression is used. Most
people say compression, as they know
how difficult it is TO crank an engine
over compression. But it is not diff
cult to spin an engine once it is
started. as the release of compression
helps the fywheel along. If in doubt
try spinning an engine by hand with
the compression cocks open and again
with them closed. It will be found
much harder if cocks are closed. The
condition of suction above the pistons
is opposed by atmospheric pressure be-
low the pistcns. making a heavy re-
sistance.. This is not only reasonable.
but may be readily tested out in prac-
tice.
10 try 1t out on a car, choose q
moderately steep hill and 90 down on
low gear, clutch in and ignition of.
Try varying the speed with the throt-
tie. II will be found that the car slows
down when throttle is closed and
speeds ur when throttle is opened.
The practice of using the engine as
brake is advisable, especially on steep
mountain roads. 1t nsures safety. saves
the brakes and cools the engine, which
should be reason enough for doing II.
The usual procedure is to use III. foot