From time to time, a product hits the market that is just so perfect
that it captures the imagination and fancy of generations to come.
One such product was the 1968 Honda Z50 minibike. The Z50
was a tiny little motorcycle that was designed to be thrown into the
trunk of the family sedan and taken out into the wilderness and driven.
Granted the trunks were bigger back then, but the Z50 captured
the hearts of countless thousands of children of the late '60s and
'70s. Z50's were found strapped to the back of cars, trailers and
RV's all across the US, and apparently, the world.
If you check out ebay, you will find Z50's for sale almost
continuously. The later versions sported headlights, and tail
lights, and were even title able for use on the road (imagine!).
To this day, clones of the Z50 are made in China, and elsewhere,
and litter the streets of many countries.
Well, that's nice, but what does that have to do with useful hacks?
It seems that Honda's inspiration for the Z50, was the little
Briggs & Stratton powered minibikes made by Rupp, and others.
Honda used their motorcycle engine prowess, and whipped together
a tiny 49cc over head valve engine to compete. They spent no
effort to extract better performance out of the engine, because after
all, it was competing with a heavy noisy lawnmower engine. The
funny thing is the little engine that Honda made could have been so
much more, but it has a fixed pointless magneto ignition system, which
seriously limits the power you can extract from such a tiny cylinder.
At one point, my friend Gary, came to me and spelled out the problem.
If an ignition system that had a modern centrifugal
advance could be strapped to the little bike, the engine would
really sing at higher rpms. I thought about it, and quickly
envisioned a little PIC based, table driven, CDI unit that would plug
in in place of the Z50's ignition module. I promised Gary that at
the first free moment, I would design and build such a system for
him... Well, an embarrassingly long amount of time passed, and I
never found that free moment. So, I decided that I would push
aside other projects, and get on with the CDI unit.
The Design Phase
There are some significant problems in making a computerized CDI system
for use on a minibike. First, the minibike has no accessory
electrical system. What are you going to power the CPU with?
Second, is the minibike engine has to start from a dead stop, and
run up to its idle speed in a nice easy sort of way.
Honda put a simple magneto based CDI system on the engine. They
used a two coil magneto, and a tiny SCR based capacitor discharge
circuit board that drives a fairly conventional motorcycle ignition
coil. They have used the same basic circuit for many years.
I took apart one of their ignition modules, and here is what was
inside:
The way this works, is the magneto produces a pulse that looks like a
single cycle of a sine wave when the flywheel's magnet passes the
magneto's pole piece. The positive part of this pulse passes
through a steering diode, and charges the 0.39uf mylar capacitor
through the ignition coil's primary to ground. Some time after
the magneto's pulse [5 degrees Before Top Dead Center (BTDC)] a second
coil on the magneto, called the trigger coil produces another pulse,
just like the main magneto pulse, and turns on the SCR. When the
SCR is turned on, it short circuits the + side of the 0.39uf capacitor
to ground, causing the charge stored in the capacitor to dump into the
primary of the ignition coil, making the spark.
That's pretty cool, because it tells us a few things: first,
there is a source of power we can steal from to run our PIC
controller, and second, we have a ready source of a trigger pulse to
tell the PIC where the crank shaft is once per revolution.
That leaves us with the problem of how to get the computer started.
We could just charge up the PIC's power supply each time the
operator kicks over the engine, and when there is enough charge to run
the PIC, start firing the engine...Have you ever tried to start a balky
motorcycle engine? Got pretty tired of kicking, didn't you?
I didn't think I could sell the need to "prime the PIC" with a
couple of kicks to the anyone, let alone Gary, a genius with IC
engines. So, that got me thinking: Somehow, I have to let
this bike start "conventionally", and then after the PIC is powered up,
switch over to PIC controlled spark. Well, a quick peek at the
spark advance curves for any IC engine will show you that the most
retarded the spark ever is happens at idle. So, my PIC will need
to fire before the idle spark advance for any speed above idle.
Why not have the PIC steal the capacitor's energy before the
magneto trigger coil fires? So that is what I did. I put
FET in parallel with the SCR's trigger, and now the PIC would race the
trigger coil for the privilege of triggering the spark.
