Tech Analysis 1997 Vespa 50cc ET2 - Motorcycle.com
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.
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.
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.
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.
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 ET2Type: Single-cylinder, air-cooled two-strokeBore and stroke: 40mm by 39.3mmDisplacement: 49.3ccCompression Ratio: 11:1Carburetor: Weber/Dell-Orto/Mikuni Two-PhaseTransmission: Automatic CentrifugalBody: Load-bearing, pressed sheet metalSuspension: Hydraulic Single Shock(front and rear)Front Brake: Single 200mm DiskRear Brake: 110mm DrumSeat Height: 805mmFuel Capacity: 9 litersWeight (wet): 92kg
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