TWO-STROKE IMPRESSIONS
If you're chatting with someone from Australia's Orbital Engine Co., you'd be advised to not mention the name "Trabant." You'd be better off spicing your conversation with obscenties, because to the Orbital folk, the Trabant represents not just a dirty little car, but a dirty stereotype of two-stroke automotive propulsion -- a stereotype the company is working hard to rewrite.
In the west, two-stroke engines last powered a mass-produced car back in the fifties, in the early, blue-smoking Saabs and in German-produced DKWs, the final version a copy of the '55 Thunderbird wearing the glorious four-ring Auto Union logo. Audi has come a long way since.
But locked in the communist stasis of eastern Europe, two-stroke powered cars such as the Wartburg and Trabant survived. The latter was a shoddy fiberglass chassis and body powered (after a fashion) by a small, noisy, smoking, popping, vibrating two-stroke engine; one Trabant emitted a like volume of pollutants as a hundred catalyst-equipped Volkswagen Golfs. When the Berlin wall fell, bringing an end to Eastern Germany, Trabant and Wartburg production collapsed almost as quickly. Trabants quickly became a symbol of all the East Germans wished to leave behind.
There's irony here, because Germans have been instrumental in the development of two-stroke engines, from Dr. Schnurle's loop scavenging to MZ's creation of the modern two-stroke road racing motorcycle engine. It was East Germany's MZ that introduced such basic concepts as the expansion chamber and related stinger exhaust system that made the two-stroke engine produce never before seen amounts of power, and which developed ways to have two-stroke engines seize less exuberantly. The MZ threat in GP racing lasted until their top rider, Ernest Degner collected all the classified drawings he could lay his hands on and ran to Japan, to Suzuki. Not a political refugee, Degener did that strictly for money, and by doing so put Suzuki in business. Since then, motorcycle two-stroke engines have constantly progressed in terms of power and delivery, but have never come around the old, basic problems concerning excessive fuel consumption and heavy exhaust smoke and pollution. The days of street-going two-stroke motorcycles seemed numbered, and a return to automotive two-strokes a joke.
That same Orbital engine was found in a lone Uno that Fiat has set up to investigate the Orbital concept potential for mass production and in the Pininfarina Ethos, a concept car in the form of a barchetta, intended for sporty, leisure time driving, more an alternative to motorcycling than to your everyday sedan. Fiat announces less power than Ford, while Pininfarina announces much more. Yet the engine is the same 84mm by 72mm triple supplied by Orbital. The power figure dance comes from the exhaust arrangement, an all important factor in a two-stroke engine tuning. Fiat's technicians have patched up the Orbital-Uno just to have a running prototype that would enable them to investigate the potential of the whole idea. Tuning refinement will come later, as Fiat would want to go into production with a larger displacement unit (somewhere in the neighborhood of 1.4 liters) in order to take advantage of the very peculiar taxation system burdening any motorized vehicle in Italy. The company is considering an almost square 84mm by 82mm three, but for some mysterious reason the engineers are talking of a four as well. That would throw away much of the compactness inherent to the three, and since it would feature an 180 degree crank, power strokes would come in pairs, doubling the loads on the driveline. (A 90-degree crank would create problems with vibration).
Fiat technicians are very thoroughly investigating the mechanical reliability of the two-stroke engine. Ford people reported 100,000 mile intervals between major crankshaft overhauls, but Fiat is taking nothing for granted. It has established contacts with OMC and Mercury Marine to draw all that can be possible from those companys' enormous experience with two-stroke engines featuring solid crankshafts, cap-type conrods, and related two-piece needle and ball or roller bearings. Given the relatively low engine speeds, these design features should not be any problem, but Fiat is seeking an extra hand from people who race two-stroke engines featuring the same crank designs, but capable of being pushed to somewhere near 10,000 rpm with remarkable reliability.
Yet in these days of tightening pollution control, someone pointed out that, due to a low mean effective pressure and consequent lower combustion temperatures, a two-stroke engine naturally produces far less nitric oxides (NOx) than four-stroke engines. No big deal, considering the two-stroke's bad-smelling, oil-rich exhaust smoke, its high carbon monoxide (CO) emissions, and (most important) its high hydrocarbon (HC) emissions.
But it's the control of NOx that creates most of the problems (and costs) in automotive engines. While an oxidizing catylyst can "burn" --- add oxygen to --- both CO and HC, turning them into harmless carbon dioxide (CO2) and water (H20) respectively, NOx must be reduced --- that is oxygen stripped away --- to leave nitrogen and oxygen. Only a most peculiar kind of catylytic convertor, the three-way catylyst, can reduce NOx while at the same time oxidizing CO and HC, and only under unusual conditions, with a "stochiometric" --- perfectly chemically correct --- air/fuel mixture. That stochiometric mixture is ideal for neither best fuel economy nor best power.
Then, too, an oxidizing catalytic convertor is a lot easier to manufacture, easier to operate, and consequently cheaper than a three-way catalyst; it would not even need the closed-loop control of air/fuel mixture that mandates an expensive oxygen sensor in the exhaust of engines with three-way catalysts. Controlling emissions from an engine that could get by with only an oxidizing catylyst could be done at about half the cost of controlling emissions via a three-way catalytic converter, a good enough reason to spur further research into a clean two-stroke engine. Add that the two-stroke engine is inherently simpler to build, has no valve train, no crankcase negative pressure to work against, less frictional losses, now, if it could be broken of its nasty habbit of spewing unburned air/fuel mixture out its exhaust --- dimming its fuel economy and vastly increasing its hydrocarbon emissions, to the point of thermally overloading an oxidizing catalyst --- if a two-stroke could be broken of that habit, it would be in the running, because in this single, negative aspect of the two-stroke cycle, almost all its negative features are created.
The problem comes with cylinder scavenging, because the intake charge must also push the burnt charge out of the cylinder. If the cylinder were scavenged with air alone, and fuel sprayed into the combustion chamber only after the piston had closed the exhaust port, the two-stroke would be cured of its major source of pollution. Easier to tell than to accomplish, since intuitively a number of other two-stroke specialists had previously come to that same, logical conclusion. But The Orbital Engine Co. engineers have achieved this, with the help of a highly efficient direct injection system.
Orbital actually came out with a direct fuel-injection system that would get the job done. The main problem of a direct injection system for a two-stroke is that it must be timed so that it will get the injection phase shoe-horned between the closing of the exhaust port(s) and the build-up of pressure inside the combustion chamber while the piston approaches TDC --- very little time indeed. To speed up things and fight the pressure which is already soaring, the Orbital technicians have used compressed air to assist their injection system, which is fully digitally controlled and consequently is unable to achieve particularly high fuel pressures. Fuel pressure is 6.3 bar, air pressue is 5.5 bar. The two are injected into a pre-mixing chamber, then they are squirted into the combustion chamber by the actual injector unit. The final injection pressure into the combustion chamber has not been disclosed, but should not be particularly high, since engine speed is limited by the inability of the fuel system to keep pace past 6000 to 6500 rpm. Once perfected, the Orbital fuel injection system has been applied to Orbital's own 1.2 liter, three cylinder engine that Orbital plans to manufacture in their own production premises of Cass City, Michigan.
Orbital's functional injection system (and related improvements to the two-stroke combustion chamber design) has attracted great attention by the auto industry world wide. Just about all major manufacturers have reached for their corporate check book to get a closer look at the patented project. The scenario is quite similar to that we have already gone through in Wankel days. But this time we are talking of a better-known entity, the two-stroke cycle, with known flaws and a sensible solution to iron them out, without much fuss and none of the frivolous complexities seen in some recent "improved" two-stroke projects from Japan and France. The Orbital concept applies equally functionally to crankcase-scavenged (motorcycle- and snowmobile-type two-strokes) and blower-scavenged engines, though Orbital's current engine follows the straighter, simpler crankcase-scavenged route.
In any case, Fiat rates its current Orbital engine at 75 horsepower at 5200 rpm with peak torque at 3800mm rpm, in the amount of 12 kgm. The minor differences did not matter much: the very tall geared Uno sprinted to 100 kph in just 10 second, as the Fiesta had done, and flexibility remained absurd compared to what we would expect from a four-stroke engine. The long-legged Uno proved that the Orbital three is a very flexible engine indeed, that can be used as both a very torquey low-end puller and a smartly free-spinning high-revver, and that is not only proving the great flexibility of the unit, but it is also fun.
Talking of fun, we come to the Ethos. The car is powered by a Orbital-owned and cared-for engine. So it comes as no surprise that the unit is capable of higher power than produced by the other two: 95 horsepower at 5500 rpm with 12.7 kgm peak torque at 3500 rpm. The Ethos must be regarded as a concept research prototype, so the choice of the Orbital power unit relates to the overall vehicle design rather than to a specific interest from the Pininfarina company about two-stroke engines. After all, Pininfarina is not going to mass produce the Ethos, they only hope to sell the whole project to a manufacturer who will.
In addition to the Orbital unit, the Ethos sports a space frame fabricated from aluminum extruded beams, wrapped in a very sexy plastic skin. Even the transmission is unusual, in the fact that it is planned to be a F1-style semi-automatic. So it is natural that the Orbital power unit appeared just like a tile inserted in a very innovative mosaic work as a whole. Associated to a very light, very slippery (Cd 0.34) little zapper, the Orbital shines in terms of peak perfromance. The top speed has been clocked at a blistering 190 kph, accelerating very briskly and putting down the customary show of great flexibility and torque spread. In the Orbital set-up unit powering the Ethos, revs could be pushed to 7000 without feeling the power drop, and that was the main difference in feel that I could detect, in addition to a higher pitched exhaust note. Would that be the next generation of cars' sound of power?