by JohnNicholas on 12/28/2009
NO new car marketed in the United States has been powered by a two-stroke engine since Saab phased out its hard-to-housebreak 3-cylinder in the late 1960s, when federal air pollution laws were taking hold. And with images from Eastern Europe of belching Trabants still fresh, there hasn’t been much of a clamor for a two-stroke revival, either.
Simple and inexpensive, yet powerful for their size, two-stroke engines held great appeal in the years before fuel economy and exhaust emissions became primary concerns. But now, with the technology of conventional four-stroke engines quite mature, there is renewed interest in taming the two-stroke to compete with diesels, hybrids and electrics as part of the solution to meeting future mileage standards.
Though two-strokes have largely faded from motorcycles and may be endangered even in applications like lawn trimmers, news of recent progress in research programs has emerged in Europe and the United States. For instance, EcoMotors International, based in Troy, Mich., has had encouraging results in tests of its experimental engines, which the company hopes will be used to power everything from stationary generators to tractor-trailer rigs.
Last year a collaborative effort in Britain of Lotus Engineering, Jaguar and Queen’s University Belfast began developing a two-stroke engine called Omnivore, for its ability to run on a variety of fuels. The direct-injection engine has the ability to vary its compression ratio from 8:1 to 40:1 by changing the volume of the combustion chamber. On Dec. 9, Lotus released test results that it termed successful for fuel economy and exhaust emissions.
And in Chapman, Kan., impressively credentialed scientists, engineers and investors are developing what they call the Grail Engine, a reference to their quest for a power plant beyond current levels of efficiency and greenness.
The Grail Engine differs from two-stroke convention in the use of an intake valve positioned in the top of the piston to admit the fresh charge of air for combustion. It also has an overhead exhaust valve, surrounded by three spark plugs, and a fuel injector at the top of the combustion chamber. The project’s goals are to exceed 100 miles per gallon and 100 horsepower from a clean-burning 1-liter engine. While there is no running prototype, interest in its approach has brought a visit by research engineers from Honda Motor.
In simplest terms, the strength of the two-stroke engine lies in its delivery of a power pulse with every revolution of the crankshaft; the familiar four-stroke piston engine in today’s cars fires each cylinder on every other revolution.
Both types of engines use a four-step combustion cycle that converts fuel and air to power: intake, compression, ignition and exhaust. But the two-stroke does it all with just two sweeps of the piston through the cylinder, one up and one down. The four-stroke design takes twice as many trips of the piston up and down the cylinder, allowing more time for each step and placing less stress on the engine.
Today’s gasoline piston engines trace their roots to Nikolaus Otto, a German inventor. Though patents covering his breakthrough were not issued until 1877, Otto and his partners began manufacturing and selling four-stroke engines in 1864. It was a lucrative business that immediately stirred competition.
A two-stroke alternative was championed by Karl Benz and others. Two-stroke engines have generally been lighter, cheaper and simpler because they did not need an intricate system of valves that open and close to direct air and fuel into the cylinders and to let burned gases exit. Instead, the piston takes on this job, along with a handful of other tasks.
One drawback of the familiar, simple two-stroke engine designs is that some unburned fuel and most of the lubricating oil are swept out the exhaust pipe. Decades ago, the blue clouds trailing a ’60s Saab were deemed tolerable, but no longer.
In spite of its shortcomings, the two-stroke never died. Instead, improved designs, electronic fuel injection and automatic oil-metering systems kept it practical for chainsaws, outboard motors and snowmobiles.
Donald L. Runkle, who was appointed chief executive of EcoMotors last September, spent 31 years at General Motors in top engineering posts. He can barely conceal his enthusiasm for the two-stroke his company is pursuing. “Our OPOC design is the cellphone of power sources,” he said. The two-stroke operating cycle is just one of several unusual aspects of the EcoMotors engine. It also has an opposed-piston, opposed-cylinder layout — the source of its OPOC name — and an electrically assisted turbocharger. The first-generation version runs on diesel fuel.
Peter Hofbauer, chairman and chief technical officer of EcoMotors, conceived this engine in 1997. The idea grew out of his 20-year tenure at Volkswagen, where he developed VW’s first diesel engines and the unusual narrow-angle VR6.
The opposed-cylinder layout — paired horizontal cylinders with a crankshaft down the middle — was the key to the VW Beetle engine’s small size and smoothness. In the late 1970s, while developing a water-cooled version of that engine, Mr. Hofbauer investigated the opposed-piston concept, which had been used in a variety of military, aircraft, marine and industrial engines. The opposed-piston engine has two pistons that move like clapping hands inside each cylinder. Outboard pistons are attached to the crankshaft through long tension rods while the inboard pistons have conventional connecting rods.
As the outboard piston nears the end of its motion away from the crankshaft, it uncovers ports — essentially the window opening of a passageway in the cylinder wall — that admit fresh air to the cylinder. When the twin pistons move toward each other, two injectors deliver fuel to the cylinder, initiating combustion. The rising pressure drives the pistons apart. EcoMotors calls the process inside each cylinder a Direct Gas Exchange Cycle, because the air and fuel mixture moves in only one direction.
Mr. Hofbauer says the EcoMotors engine has twice the power density — the output on a per-pound, or per-cylinder-displacement basis — of conventional engines. The production cost is 20 percent less, he said, because the engine has only half the parts of a conventional power plant. Ganging modules together and using an electric clutch to shut down one module when its power isn’t needed yields a 45 percent gain in efficiency over conventional engines. Mr. Hofbauer said a hybrid configuration, with two power modules assisted by a motor-generator, would raise efficiency an additional 10 percent.
According to Mr. Runkle, the OPOC engine has already cleared two big hurdles in its initial development: it is capable of meeting future gas and diesel emission standards while operating more efficiently — by 15 to 60 percent — than conventional engines. Mr. Runkle has his eye on a shuttered factory in Livonia, Mich., not far from the EcoMotors headquarters. His dream is to put the Motor City back on the map as the world’s pre-eminent source of clean, cost-effective and fuel-efficient engines.
This article is from the New York Times – http://www.nytimes.com20/automobiles/20STROKE.html?_r=1