The Two-Stroke Engine's Expected Revival

December 16, 2009
Conventional wisdom suggests that our faithful internal combustion (IC) engines are heading the way of the buggy whip. The lithium-ion battery has arrived, emissions standards are tightening, and machinery which converts hydrocarbons to horsepower will have no role in the future some pundits say.
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This point of view assumes that the 148-year old IC engine is tapped out and lacks potential for improvement. Actually, conventional engines and the cars they drive are recording notable gains in efficiency and cleanliness on a daily basis. The rise of two-stroke engines from their near-dead status is the latest evidence that internal combustion's best days lie ahead.
The four-stroke engine that powers all of today's cars and trucks (except for the Tesla Roadster) was the brilliant invention of German Nikolas Otto who wisely realized that compressing the fuel and air mixture before igniting it was the way to go. This idea came to him in 1861; three years later he and his backers were building and selling four-stroke engines that were vastly superior--mainly more fuel efficient--than alternative designs missing the compression stroke.
Naturally there were competitors with alternative designs and hopes of skirting Otto's patents not issued until 1877. Karl Benz and others had two-stroke engines running successfully by 1870 which delivered one power pulse per crankshaft revolution versus the two turns required in four-stroke engines.
The most common form of two-stroke requires no valvetrain resulting in major cost, weight, and complexity savings. That leaves the piston with a laundry list of responsibilities: pumping air and fuel into the combustion chamber, metering the lubricating oil, compressing the mixture, delivering power to the crankshaft, and ushering the spent gasses out of the cylinder. All of this occurs every 360-degree rotation of the crankshaft. The inevitable result is that some of the unprocessed fuel and all of the lubricant is swept out the exhaust pipe.
While the blue smoke trailing sixties-era Saabs, the last two-stroke cars in America, was tolerable in the day, noxious emissions are no longer socially acceptable.
Nevertheless, the compelling advantages inherent in two-strokes kept them going all these years. Two-stroke Trabants manufactured in the former East Germany lasted until 1991. And, thanks to the implementation of effective oil metering systems, improved piston and port designs, and direct electronically controlled fuel injection, two strokes are still powering many marine outboards, off-road motorcycles, and snowmobiles. Simple versions are the engine of choice for chainsaws, weed trimmers, some lawn mowers, and nearly all radio-controlled models.
The global quest for a silver bullet solution to our power and propulsion needs has prompted many engineers to dust off old text books and investigate moribund ideas in search of inspiration. The two-stroke engine recently caught their attention.
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In August 2008, a British consortium - Lotus Engineering, Jaguar Cars, and the Queen's University in Belfast - announced plans to investigate an unusual two-stroke engine with the hope it could run efficiently on a variety of gasoline and alcohol fuel. Called Omnivore for obvious reasons, this engine has direct fuel injection and the possibility of running with compression ratios ranging from 8:1 to 40:1 by means of a moveable 'puck' located above the piston which varies the combustion chamber's clearance volume. Auto-ignition (HCCI) operation has also been discussed. This Lotus-designed two-stroke has monobloc construction with the head integral to the cylinder block.
The second notable two-stroke effort resides in Chapman, Kansas, where an ambitious team of 16 scientists, engineers, and investors began more than a year ago developing an engine they call Grail, as in Holy Grail. The most unusual feature in this design is an intake valve located in the piston's crown that admits the fresh charge of air. In addition, there is one exhaust valve, one fuel injector, and three spark plugs located at the top of the combustion chamber. Goals are 100+mpg, more than 100 horsepower, and clean exhaust from a single-cylinder 1.0-liter engine.
The Omnivore and the Grail will have to hustle to keep up with a two-stroke project underway near Detroit by EcoMotors. This engine is an OPOC - opposed piston, opposed cylinder - design which bears minimal resemblance to any engine - two- or four-stroke - on the road today.
Peter Hofbauer, the founder and chairman of EcoMotors, is the brains behind this business. He graduated with engineering degrees from the Technical University in Vienna, Austria, in 1966, following studies under famous Auto-Union genius and V1/V2 rocket developer Eberan von Eberhorst. Hofbauer then spent two decades at VW, rising to guide the development of all VW and Audi engines in charge of a staff of more than 1000 employees. The first-generation VW diesel engines and the complex VR inline-V technology - still in use for the Bugatti Veyron - were Hofbauer brainstorms.
While working on what became the wasser boxer or water-cooled Beetle engine for the VW Vanagon, Hofbauer decided it might be wise to replace the cylinder heads in an opposed-piston (OP) engine with...extra pistons. That concept has merit primarily because it conveniently doubles the area for combustion forces to press against each crankshaft throw. Previous opposed-piston applications include the six-cylinder/12-piston Junkers Jumo WWII aircraft engine, various tank and naval vessel engines, and as the power source for air compressors and the like. The British firm Comer used them to power buses. The only notable car use of opposed pistons was by French maker Gobron-Brillie from 1900 through 1922.
Most of these OP engines used two geared-together crankshafts. To avoid that complication, Hofbauer designed tension rods connecting the outboard pistons directly to one crankshaft. This is an excellent approach - also used by Gobron-Brillie - because the pull loads from the outboard pistons are nearly equal but opposite to the push loads applied to the crank by the inboard pistons. That balance promotes smoothness and facilitates use of a lighter crankcase construction.
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Hofbauer never got the chance to implement his OPOC idea in Germany. After his stint at VW, he moved to Klockner-Humboldt-Deutz, a Cologne firm with direct roots to Nikolas Otto's original engine manufacturing company. In 1997, after a decade or so employment there, Hofbauer retired and moved to America.
When retirement provided unsatisfactory, Hofbauer revived his engine concept. With development partners FEV and AVL, he got the first demonstration engines running in 2003 and attracted investment funds from DARPA. A couple of prototype units have been delivered to the US Army's tank-automotive R+D group. Green venture capitalist Vinod Khosla has provided a major portion of the $60-million investment to date.
The EcoMotors OPOC engine's claim to fame is double the power density (per pound and per cubic inch of exterior volume) of today's gasoline engines. Since less than half the parts are required, EcoMotors expects at least a 20-percent cost advantage. Projected fuel economy is up to 50-percent better than current gas and diesel engines.
Like all two strokes, the EcoMotors OPOC engine delivers one power pulse per cylinder per crankshaft rotation. In the interests of efficiency and exhaust cleanliness, there is no fuel in the incoming air stream. Instead, it's delivered by two injectors positioned at the sides of the cylinder wall. The first-generation OPOC engine runs on diesel fuel but gasoline versions are also planned for future development. Special plasma igniters are required to initiate gasoline combustion.
Hofbauer calls the one-way flow of air through the engine a "Direct Gas Exchange Cycle." This means that pressurized air enters the cylinder when ports are uncovered by the outboard pistons reaching the end of their stroke away from the crankshaft. After ignition, as both pistons are forced in opposite directions by combustion pressure, power is delivered to the crankshaft. As the inboard piston nears the bottom of its travel, the exhaust ports are exposed. Piston motion is timed and ports are located such that the exhaust opens before the intake. To avoid loss of fresh air from the cylinder, the exhaust ports also close before the intake openings are sealed.
This uniflow arrangement is crucial to a clean exhaust and making the most of every increment of fuel. It also means that the OPOC engine needs a source of pressurized air to start and run. Hofbauer came up with a clever means of satisfying that requirement: an electrically-assisted turbocharger. During start-up, a motor spins the compressor to deliver intake air. After the engine begins running and exhaust energy rises, the turbine wheel takes over the task of driving the compressor, allowing the motor to become an electric generator.
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Controlling the electric turbo with a computer module gives the OPOC engine the equivalent of variable valve timing according to Hofbauer. The electrical energy that's generated is used to recharge a storage battery.
While the OPOC engine is not yet ready for sale, excellent progress has been made. Oil consumption and exhaust emissions are on target. Hofbauer is hopeful that heavy doses of EGR will mean that urea injection won't be necessary to meet emissions requirements. Continued development is underway to prove the engine's durability, to solve any issues that arise, and to add the gasoline version to the product menu.
This engine's 15-percent fuel economy improvement is mainly attributable to its lighter weight and significantly reduced heat rejection into the cooling system (since no cylinder head is present). Using two OPOC engines with a computer controlled clutch to disable one when its contribution isn't needed doubles the advantage over conventional engines to 30-percent. Add another five percent for the vehicle weight savings attributable to this engine and 15-percent for what EcoMotors calls its Tribrid design - two OPOC engine models combined with an electric motor - and the net gain is 50-percent over today's cars.
Hofbauer and his team of 25 engineers hold 114 patents with another hundred applications in the pipeline. Don Runkle, a seasoned executive with a long career at GM and Delphi, recently joined the EcoMotors staff as CEO to help attract the $200-million in federal grant money needed to step through the next development stages towards production.
EcoMotors has aspirations to supply engines for applications ranging from microcars for emerging nations to semi-trailer trucks in the US. Auxiliary power unit and generator set applications are also on the list. Runkle reports strong interest from marine, commercial truck, and agricultural equipment makers.
According to Runkle, EcoMotors is ready to partner with any customer interested in purchasing a licensing agreement. Joint design ventures are a second possibility. The most likely first step will be to supply OPOC two-strokes to manufacturers who lack the capability to design and build their own engines.
Runkle notes, "What we have here is the cell phone of engines. Why would any new manufacturer bother reinventing a standard four, V-6, or V-8 engine when they can obtain an OPOC power source from EcoMotors that's vastly superior in terms of cost, efficiency, and environmental footprint?"
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If EcoMotors succeeds according to plan, Runkle hopes to put a shuttered GM engine plant in Livonia, Michigan, back to work building innovative engines. His goal is to put the motor back into the Motor City, thereby reviving Detroit's reputation as a global center of automotive excellence.
While the arrival of electric propulsion will surely help uplift the automobile's reputation as a resource consumer and pollution generator, major obstacles lie ahead. Advanced batteries and new electrical generating infrastructure are frightfully expensive. That leaves ample opportunity for a new generation of internal combustion engines - such as the OPOC two-stroke - to pitch in with clean and cost effective propulsion power. In all likelihood, heat engines will serve a crucial role in the global personal transportation system for decades to come.


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