Ricardo Engineering has been around since 1915, which makes it one of the oldest powertrain consultancies in the world. So, stopping by the company's Michigan offices to talk about the state of valvetrain design is a no-brainer, and the discussion of mechanical ver-sus camless valve actuation proceeds smoothly down a straight road. Until a side road appears in the form of a combined two-stroke/four-stroke engine Ricardo has under development. Suddenly, the straight, smooth road disappears as the mind wrestles with what the ears have just heard. "A two-stroke and a four-stroke in a single four-cylinder engine? How is that possible?"
"Actually, it's not as odd as you might think," says Russell Wakeman, technical director at Ricardo's Detroit Technology Campus (Van Buren Twp, MI). The engine in question isn't your typical weed whip's two-stroke, but a high-value design that mimics a typical automotive four-stroke engine design with overhead cams, poppet valves, and cylinder scavenging via a supercharger. Although this engine currently exists only in virtual form, if it was not, the average customer opening the hood of a vehicle equipped with this motor wouldn't be able to tell it was other than a run-of-the-mill engine. Which might be the first clue something is different: the Ricardo engine has the potential to replace V8s in many applications, or could be used in small cars as a modern-day equivalent of Ford's flathead V8. "It has the potential to change the way OEMs and customers look at powertrain choices," says Wakeman.
Doubling the number of combustion events per crankshaft revolution effectively doubles the power density, which allows a four-cylinder to provide almost the same power as a V8. "Why make an eight, then shut down four of the cylinders in order to get a fuel economy benefit?," asks Wakeman. Especially when you have to overcome the frictional losses of dragging four dormant cylinders. Says Wakeman: "We saw the benefit of making a light, small four-cylinder that you'd run really hard to get good fuel economy, then fire it twice as often to get power when you needed it. It's cylinder reactivation, not cylinder deactivation."
One of the problems Ricardo engineers faced is the different combustion needs of two-stroke and four-stroke engines. The combustion chamber of a four-stroke engine is designed to induce turbulence through the addition of "squish" areas in which the top of the piston is the mirror image of a corresponding area of the cylinder head. As the piston approaches top-dead-center, the mixture is forced into an area surrounding the spark plug, but with a highly turbulent flow. Since two-stroke engines have a high degree of valve overlap, there is a period in which both intake and exhaust valves are open. This is known as "short circuiting," where the incoming air-fuel mixture flows out the exhaust valve, and the exhaust gasses stay in the cylinder. Adding a supercharger as a scavenging device alleviated the scavenging problem, but does nothing to eliminate short circuiting. As a result, Ricardo engineers reevaluated cylinder head design with an eye toward eliminating this problem without harming four-stroke performance.
"The combustion system we've developed is perfectly happy whether used under two-stroke or four-stroke conditions," says Wakeman, "though it took a number of years of hard work and research to reach that point." A patented design (U.S. patent number 5,065,711), the Ricardo combustion chamber creates a compact, highly turbulent flow that moves toward the piston along, or adjacent, to the axis of the cylinder, which purges the center of the cylinder. Placing the intake and exhaust valves in opposing pairs, with the intake valves inclined approximately 45º to a line parallel to the cylinder axis creates intense turbulence when the piston is at top-dead-center and reduces short circuiting. This turbulent flow also occurs in four-stroke mode, improving combustion in this mode. As you might expect, the design also can be applied to diesel engines, which also can be either two- or four-strokes.
Designing the valvetrain, Wakeman contends, was much easier than creating the high-compression cylinder head. "Once you get an engine that runs as a two-stroke, you change the cam pulley so that it runs as a four-stroke," he says. "In essence, you delete the two-stroke valve events to turn it into a four-stroke." The difficult part, the part that is still being researched, is seamlessly stepping from one combustion mode to another. "The things you have to do during the transition are the hardest parts of this concept," he says. "The rest of the technology exists somewhere else in mature form, so we're not pushing back the frontiers of science with this too much–except for the controls."
The key to Ricardo's variable cycle engine is its cylinder head design, specifically the inclination and location of the intake and exhaust valves. Preventing the air-fuel mixture from exiting through the exhaust valves without creating a design that adversely affected combustion in four-stroke mode took years of research.