By Yossef
Schvetz, Contributing
Writer
I was sure that all I had to do on my trip to Rome was enjoy the great
Italian food supplied by Piaggio and sample their new 4
stroke 125cc Vespa ET4 model. But as I arrived at the press
conference it turned out that a few 50cc versions of the new model awaited
us too. "Must be Piaggio's regular 50cc two stroke engine in there..." I
mumbled to myself while chewing fresh Italian prosciuto ham from the
buffet. How wrong I was as that little tiddler turned out to be the real
ace up Piaggio's sleeve. With all due respect to the Vespa's 50th
birthday and the important new ET4 125cc model, the understated 50cc ET2
is the one that really deserved the spotlight.
Simply put, the 50cc ET2 has the most sophisticated two-stroke
engine ever mass-produced. Period. This little wonder is already able to
pass the stringent European emission regulations coming in the year 2000
without a catalytic converter! GM, Ford, Toyota, Subaru and many others
have been spending mega-bucks trying to develop clean two-stroke engines
for their cars while Honda has been persuing the same goal through the
development of their experimental (and expensive)
EXP-2. However, the EXP-2's
technologically advanced engine and all of the auto engines are yet to
reach production. It is an impressive coup indeed for a small factory
like Piaggio to produce a clean two-stroke ahead of all others.
So why are all the factories interested in two-strokes? Loads of power,
light weight, few moving parts and low bulk. But stand behind any idling
old RD Yamaha and the two stroke engine's pitfalls become clear (or rather
obscured in the smoke...) Massive amounts of pollution mean a death
sentence for two-strokes in many countries. If it only could be made to
burn cleaner we would have a powerful, light and inexpensive power
unit.
The main reason for the pollution created by a normal two-stroke engine is
the "short circuiting" that occurs during the transfer process. Fuel-air
mixture sucked into the combustion chamber rushes out the transfer ports
and helps to drive the previous cycle's burnt gases from the cylinder.
Although most of the fresh mixture remains inside, a small percentage
sneaks out to the atmosphere through the wide-open exhaust port. Without
any physical barrier between the fresh mixture and burnt gases this is
unavoidable. That small percentage of unburned fuel multiplied by
thousands of RPM gives the EPA and other green operations fits. It
also hurts the engine's already poor mileage.
Circumventing this problem is deceptively simple: Use clean air during the
transfer and clearing of the cylinder and wait until all ports close to
inject the fuel into the remaining air. Big two-stroke diesel engines
have been achieving remarkable efficiency doing just that since World War
II. No fresh mixture can escape and by the time the exhaust port opens
the burning process will be over like in any good four-stroke mill. Most
"new" two-stroke experimental engines use this approach.
However, this is easier said than done. What works for a 70" stroke
marine diesel two-stroke turning at a lazy 100 RPM doesn't necessarily
apply to a 50cc piston buzzing at 10,000 RPM. Injection time is the
limiting factor: A four stroke electronic fuel injector has more than 180
degrees of crankshaft rotation to spray the fuel into the turbulent air
stream flowing through the intake manifold. A fuel injected two-stroke
injector has about one third of the time available to deliver its load to
a rather stationary mass of air.
These problems have drawn engine developers away from the direct-injection
idea to the stratified-charge concept. By pre-mixing the fuel needed with
a small amount of air and spraying that rich mixture into the cylinder the
time problem can be solved. However, this requires a separate system
supplying pressurized air. Fuel will already be broken up into
droplets by the time it's being sprayed and the rich mixture jet can be
aimed at the spark plug, ensuring reliable ignition. The resulting flame
front can then easily burn areas of leaner mixture.
Engines using an electronic injection-stratified charge system are
available, but at a price. It's a complexity and cost toll that makes the
whole equation of emissions and performance-per-dollar dubious. The cost
of computer controlled injectors and sensors might be acceptable in
high-end products like the new Bimota 500 DueV, but this is hardly
the answer for the hordes of cheap, small two-strokes now polluting the
streets of the globe.
Enter Piaggio's greatest stroke of genius since introducing the Vespa.
The engine's bottom end is a regular clean burning two-stroke that
pumps in clean air that will be transferred to the cylinder through
the ports. Incoming air is throttled by one of the bores in a special
double barreled carburetor. This half of the carb doesn't have any fuel
jets, it only flows clean air. The real action happens in the cylinder
head where a tiny crankshaft drives a small piston. This piston, of about
0.8" diameter, acts as a pump sucking an extremely rich gas-air mixture
through the other half of the double barrel carburetor. On its downward
stroke it compresses the rich mixture to about 70 PSI at which time the
rising pressure raises a spring loaded poppet valve off its seat and the
charge is squirted into the cylinder. There it is aimed at the spark plug
area and ignited. The combustion pressure immediately shuts the
spring-loaded poppet valve and from then on its just a "regular"
stratified-charge ignition process with the flame front igniting those
lean mixture areas in the cylinder.
A tiny 0.9" stroke crankshaft is driven at engine speed by a toothed
belt drive. Total displacement is about 6cc and in order to meter fuel
exactly the bore at the "mixture side" of the carb is a miniscule 4mm. (In
my model airplane days I would have given an arm and a leg for a 4mm carb
for my 0.049 cu.in Cox engine.) The pumping piston has to be timed to
work with the cylinder, though the exact moment for injection is
controlled by the poppet valve's spring preload. The simple beauty of the
beast is hard to miss. Perhaps the most important aspect of the Piaggio
engine is that it achieves what others have been trying to
accomplish with complicated electronic wizardry through relatively simple
and cheap mechanics.
An added bonus of the Piaggio engine is that the lean overall mixtures
possible with the stratified charge leave big amounts of oxygen in the
exhaust gases. This oxygen can be used later in a catalytic converter to
burn hydrocarbons and to further clean the exhaust. I would also offer
Piaggio the idea of counter-rotating the overhead crankshaft in the
opposite direction, thus getting a balancer shaft in the bargain.
Are we entering the era of OHC two-stroke engines, meaning OverHead
CrankShaft? Or is it rather OHP2 standing for OverHead Pumping Piston?
Remember you saw it here first!
The claimed performance of the ET2 on the spec sheet wasn't too promising
(more on that later), but I just had to take a spin on the bike that
holds what might be one of the most important engines of the 21st century.
The ET2 is basically an ET4 model with a smaller engine and therefore
handles as well as its bigger brother. Weight is ten pounds less than the
larger bike but still twenty pounds more than most of its competition at
230 pounds. Performance is also hampered by required horsepower limits.
The maximum output is not claimed and top speed is noted as : "by law
code". In Italy, 50cc scooters are aimed at 14 year-olds who ride
helmetless, so the laws are rather restrictive.
After riding the torquey and responsive 125cc ET4 the ET2 felt entirely
gutless. But in my experience with Italian 50cc scooters outputs grow
a healthy 6-7 bhp when the things are de-restricted for export markets. A
proper test ride will have to wait till spring 1997 when the ET2's
worldwide marketing begins. But just to make sure that it wasn't your
standard issue two-stroke I put the ET2 on its centerstand and revved the
guts out of it. Nothing, absolutely nothing came out from that exhaust
pipe. Try that on any other two-stroke engine and you will appreciate the
achievement. So its hats off to Piaggio, and I believe that some
engineers in Detroit and Japan will have to take off their hats as well.
Specifications:
Model: Vespa ET2
Type: Single-cylinder, air-cooled two-stroke
Bore and stroke: 40mm by 39.3mm
Displacement: 49.3cc
Compression Ratio: 11:1
Carburetor: Weber/Dell-Orto/Mikuni Two-Phase
Transmission: Automatic Centrifugal
Body: Load-bearing, pressed sheet metal
Suspension: Hydraulic Single Shock(front and rear)
Front Brake: Single 200mm Disk
Rear Brake: 110mm Drum
Seat Height: 805mm
Fuel Capacity: 9 liters
Weight (wet): 92kg
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