Science / Medicine : Different Strokes : Technology: Auto firms work on new versions of the old two-stroke engine as an improvement over current four-stroke design.
Your car has four, five, six, eight or 12 cylinders and unless it is a rotary engine--or your jalopy was made, in say, East Germany--chances are the engine rumbling underneath your hood is a four-stroke. So named because a piston slides up-down-up-down in a cylinder--four strokes in all--before a complete combustion cycle fires. On the road, thousands of these explosions occur every minute to keep the wheels turning.
Four-stroke motors have dominated automotive design this century, but that might change. Teams of engineers on both sides of the Atlantic and Pacific, including those from General Motors, Ford, Chrysler, Honda, Toyota, Subaru, Fiat and Peugeot, are racing to finish new versions of the ancient two-stroke engine that would overhaul basic automobile design.
First developed a century ago, the two-stroke’s lure has always been tremendous power in a small package. In most engines the combustion cycle starts when a spark plug ignites a mixture of gasoline and air in a cylinder and the explosion forces the piston down. While a bigger, four-stroke engine takes a full, almost leisurely, piston stroke to draw fresh air in and another stroke to push burnt gases out, a two-stroke combines the air intake and exhaust in a single piston stroke. By working harder, a two-stroke engine can match a four-stroke’s horsepower in half the piston strokes.
Think of two boats in a race. One boat has four oarsmen. The other boat has only two oarsmen, but it keeps pace because the two are stroking, or rowing, twice as fast. Two-stroke engines remain widely used today, in snowmobiles, chain saws, outboard boat engines and racing motorcycles. The price for all this power is a noisy, gas guzzling, polluting motor, which is why most car companies discarded two-strokes as primitive technology nearly 25 years ago.
Then a few years ago an obscure Australian company, applying shrewd use of compressed air and electronic fuel injection, revived the two-stroke. The firm licensed its design to major car companies, and in early tests their reworked engine appears to offer plenty of advantages: A two-stroke is half the size and 40% lighter than a four-stroke, and figures to be $200 to $300 cheaper to build per motor because it needs fewer parts.
The two-strokes also produce more power for their size and it is hoped they will be less polluting than four-strokes. The two-stroke’s compactness should lead to dramatically aerodynamic cars that are roomier in the passenger area but smaller and sleeker outside, boosting acceleration and producing 25% better gas mileage.
“If everything works and it meets emission standards, the car companies recognize the two-stroke could really be a breakthrough,” said Csaba Csere, Car & Driver magazine’s technical director. In May, Chrysler said it hoped to have its first two-stroke engine ready for sale by mid decade.
Before the two-stroke engine moves from the laboratory to the showroom floor, it must pass tough durability and emission standards, and even Chrysler’s engineers concede that is far from certain. Ian Macpherson, Ford’s director of powertrain research who is working on his firm’s two-stroke project, said: “We’re committed to try and make the . . . thing work. But we’re pushing technology in a region it has never been.”
Despite the uncertainty, nearly every major car company has entered the two-stroke derby. Although Chrysler seems to have the early lead, other two-stroke projects are close behind. Ford expects to have a two-stroke car ready for Europe this decade, and Macpherson’s team is trying to finish up a two-stroke design for the American market.
GM is pushing hard on its two-stroke model because “there is a lot of potential here,” said Roger Heimbuch, GM’s engineer for powertrain systems. “No one wants to be 10 years behind a major engine technology development.”
Car companies are testing other experimental engines, including models powered by methanol, natural gas, electricity--even hydrogen. These designs are radically different from internal-combustion gas engines and may make up only a fraction of total car fleets.
Twenty years ago, most of the auto world was breathless about the Wankel, or rotary, engine that was to revolutionize the business--except the Wankel was a dud.
On paper the Wankel looked impressive, but on the road the early models needed a spare Wankel more than a spare tire. The engines burned oil, overheated, the seals had to be replaced after 15,000 miles, and the rotary power plant recorded poor gas mileage. When OPEC quadrupled oil prices in the early 1970s, it essentially killed off the Wankel, and most major car companies abandoned the engine.
Today the Mazda RX-7 is the only car sold in the United States with a rotary engine, and while auto magazines have long given the car good reviews, last year fewer than 10,000 RX-7s were sold.
The two-stroke’s comeback is all the more remarkable because when the Wankel was introduced here, two-strokes were already forgotten. In the late 1960s, Saab sold the last of the two-stroke cars in the United States. Two-strokes of this vintage were distinctive for popcorn-like blasts, smoky exhausts and poor fuel economy. The engines ran unevenly because carburetors (this before fuel injection) imprecisely pumped air, gas and oil into the cylinders and invariably raw fuel blew out the exhaust.
Ralph Sarich, an Australian inventor, is responsible for the new interest in two-stroke engines. Sarich was working on a version of a rotary engine in the 1970s when his team devised a sophisticated injection system that used compressed air to spray fuel in a fine mist into the combustion chamber. Later, he tried the technique on a two-stroke engine, and soon had eye-popping results.
Recently his company, Orbital Engine, tested a two-stroke, three-cylinder engine in a Honda CRX and saw the car’s zero-to-60 m.p.h. acceleration jump 30% while fuel economy soared 25% to 63 miles per gallon. Now Sarich’s company is completing a two-stroke, six cylinder, 200 horsepower engine for bigger cars. Because Sarich’s engine does not have conventional valves, valve springs or camshafts (which open and close the valves), it needs about 200 fewer parts than a comparably powered four-stroke.
In Sarich’s design, as the piston descends, an exhaust hole is uncovered so the spent gases escape as a reed in the cylinder wall sucks in new air. As the piston moves up the air is compressed, through a separate channel, into the cylinder top; there, fuel and air are injected and the spark plug fires, restarting the cycle. His engines burn cleaner than previous two-strokes because a minimal dose of fuel is sprayed into the cylinder top only after the exhaust and intake holes are closed, preventing unburnt fuel from shooting out. “Conventional wisdom was the two-stroke was unacceptable. Sarich is one of those guys too dumb to know it wouldn’t work,” Csere said.
Something new in Sarich’s engine is that it doesn’t need any oil change or filter changes. Instead of pumping oil throughout the engine, to minimize pollution a nozzle releases only a wisp of oil stored in a cartridge and moved by air pressure to lubricate the cylinder walls, bearings and other key parts. Most of that film of oil is burned--it’s called a total-loss system--so there’s no messy oil left to change.
Ford’s Macpherson envisions future drivers of two-strokes watching for a warning light on the dashboard, and maybe every 5,000 miles having to buy an oil cartridge at a gas station and insert it into the engine.
However, Chrysler says it does not want its customers to be scared off by noticing anything different in a two-stroke engine, including not having to change the oil and filter. So Chrysler engineers have mounted a supercharger, or external air blower, on their two-stroke engine to route air around the crankcase, enabling them to use a conventional oil pump, which means regular oil changes. Roger Schaum, executive engineer at Chrysler, said he hopes that this blower will squeeze out extra power, but with a quieter engine.
At auto shows in the past year GM and Chrysler have showed off their two-stroke concept cars--essentially fantasy models their engineers would design if they had total freedom. Chrysler’s Dodge Neon grabbed plenty of attention because of its futuristic, short, slope-angled hood, light weight and, thanks to a compact engine, 20% more interior space than a conventional car of the same exterior length. “There is definitely a market for something like that,” Csere said.
Before a car company gives the green light for mass production of two-stroke engines their engineers must be certain the motor will perform reliably. For one thing, if car designers build aerodynamic cars specifically around smaller two-stroke engines--as Chrysler hopes to--should it not catch on, car makers will not be able to plug in a four-stroke engine as a substitute because the sleek, compact cars won’t have enough room to hold them. There’s also the task of satisfying the tough new emission standards. “It doesn’t seem prudent to spend several hundred million dollars to bring a new engine out if you don’t know whether you can sell it in 2001,” GM’s Heimbuch said.
Now, the EPA requires new car engines to pass a 50,000-mile durability test that repeatedly checks to see if it meets emission limits. That test jumps to 100,000 miles for the 1994 model year, and some states, such as California, have their own stringent pollution standards set to become law. If two-strokes do not pass emission limits, Csere said, “they are out of business.”
At the moment, GM’s engineers are concerned because the two-stroke prototypes do not meet future emission limits for nitrogen oxide, a key auto pollutant. Relying on compressed air, Sarich’s two-stroke design has an unusually rich oxygen mix in the engine, which also means that current catalytic converters can’t remove enough nitrogen oxide gas to meet pollution limits.
Even Chrysler’s slightly different engine design has yet to solve this emission problem. But a possible solution, said Chrysler’s Schaum, is a new molecular sieve design that separates molecules so the gas turns into harmless water vapor. That sieve has yet to be tested on a two-stroke, but Schaum remains upbeat.
At Ford, Macpherson said, the next step is to put two-strokes into 20 cars or more and see how the engines do over several months of hard driving. “If we find one that’s still running at 50,000 miles, we’ll say ‘thank goodness, let’s keep going.’ ”
The New and the Old
Gasoline and oil-burning two-stroke engines were considered too noisy, fuel inefficient and polluting for use in automobiles. But recent innovations make these simpler engines potentially more attractive to auto makers. Here is a comparison of a two-stroke engine with the conventional four-stroke engine.
Two-Stroke Engine
1. Downward Power Stroke: The spark plug fires, forcing down the piston. Ports in the cylinder wall are uncovered, allowing burned gases out and fresh air in. The piston also uncovers a transfer port, and forces air from; the crankcase into the combustion chamber.
2. Upward Compression Stroke: The piston rises, closing off the transfer and exhaust ports, while compressing the air and fuel mixture to ready it for the next spark plug firing.
Four-Stroke Engine
1. Intake stroke: Downward-moving piston draws fuel-air mixture into cylinder through open intake valve.
2. Compression stroke: Intake valve closes and upward-moving piston compresses mixture.
3. Power stroke: Mixture is ignited by spark plug, driving the piston downward, providing power.
4. Exhaust stroke: Exhaust valve opens, allowing burned gases to escape as the piston moves upward.
