Tech Analysis 1997 Vespa 50cc ET2
Can Vespa's Clean-Burning ET2 Pave the Way for Larger Two-Stroke Streetbikes?
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.
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