The name is self explanatory. By measuring the rate at which a key part of
the engine revolves, the RPM can be calculated directly. The easiest item
to observe is the flywheel, which rotates once for every revolution of the
engine. (This applies to all internal combustion engines).
Direct measurement can be done mechanically, although the Vire 7 is not set
up to use such systems on a permanent basis. Jean-Francois describes his
simple tachometer:- "I have this small counter (fits in the hand) that
increments by one every turn it goes, and by simply pressing it's rubber
end right in the middle of the flywheel and holding it there for one minute
and then checking the value on the counter gives you the precise quantity
of turns it went, thus giving you the RPM.
More permanent solutions use either a contact or an optical system. These
require a specialised tachometer, such as those supplied by
SenDEC Corporation. However, this
is not necessarily the cheapest alternative. Read on...
This involves measuring some parameter that is directly related to the RPM,
although the parameter measured may need to be multiplied or divided by a
conversion factor to calculate the RPM. The principle is very suited to gasoline
/ petrol engines, where the ignition cycle can be measured easily, with
relatively inexpensive equipment.
For the Vire 7, at least three methods are applicable:-
Using a ready-made customised electronic tachometer, designed for use with
a single-cylinder 2-stroke engine. One such is a
SenDEC Engine Maintenance Meter which
has four functions including Tachometer up to 6,000 RPMs, Hour Meter, Job
Timer, and Service Alarm. It works on the principle of electrical induction,
using a wire wrapped around the spark plug cable.
Tiny Tach provide a choice of very
similar meters. In Europe, the RevCo
MTL1-P was another suitable meter, but may no longer be available. (March 2006)
If you want to build your own tachometer from a kit,
Velleman of Belgium do
a suitable digital rev counter in kit form. To make it you just need to have
a reasonable amount of experience at soldering and be able to follow 'pictogram'
instuctions. The kit number is K2625, and you can look at its specification
on the Velleman website.
It costs £25 (40 Euro / $40US - Price at August 2002) from
Maplin in the UK (Order
Code VE01B), and they do mail order delivery. It runs from a 10-15 volt supply,
and draws just 200mA. It can be configured for 2 or 4 stroke engines of 1-12
cylinders, reads to 100 RPM accuracy, and connects across the contact breaker
points.
Using a hand-held Tachometer / Dwell Tester. These are available for as little
as 25 Euro ($22US) for an unsophisticated version (Price at September 2001).
A digital display is always going to be accurate, which can't necessarily
be said of analogue-display meters. Most models are designed for use on 4-stroke
engines with four or more cylinders, but if your computational abilities
extend to being able to multiply by two, then this does not present a problem
(see below).
The meter comes with a cable and two crocodile clips:-
Attach one of the crocodile clips to the terminal at one end of the stop
cable - this is the black cable that leads to the engine stop switch from
the four-terminal connection rail on top of the flywheel housing
Attach the other crocodile clip to earth
In practice, the easiest way to make the above connections is to connect
the clips to either side of the Ignition Stop/ Run Switch.
If the meter has a setting for a single-cylinder 2-stroke (or a 2-cylinder
4-stroke) engine, select that. The meter will then give a true reading for
the RPM of your Vire 7.
If the meter doesn't have a suitable setting, select the setting
for a 4-cylinder 4-stroke engine - the meter reading will then be half of
the true RPM, e.g. 500 RPM on the meter indicates that the engine
is revolving at 1000 RPM. Click here for
the arithmetic underlying this conversion.
