Tour Engine moves its opposed-cylinder split-cycle engine to beta prototype; coupling a compression ratio of 8:1 with expansion ratio of 16:1 for increased efficiency
|Drawing of the split-cycle TourEngine beta prototype. Click to enlarge.|
Tour Engine Inc., the developer of a novel opposed-cylinder split-cycle engine, presented details of its new beta prototype at last week’s DOE Directions in Engine-Efficiency and Emissions Research (DEER) conference in Detroit. At the 2010 DEER event, Dr. Oded Tour, CEO and co-founder (and son of Hugo Tour, inventor and co-founder and former Lt. Col. in the Israeli Air Force) had reported on the design and mechanical feasibility of its operating alpha prototype.
The new beta prototype, developed with the support of the Israeli Ministry of National Infrastructures, advances the engine design. The first stage of the new Tour engine is a symmetrical design (compression and expansion cylinders of the same size). The second stage exploits an asymmetric design: the compression cylinder (cold cylinder) has a volume of 95 cm3 and compression ratio of 8:1, while the expansion (hot cylinder) has an expansion volume of 190 cm3 and an expansion ratio of 16:1.
A simulation by an independent consultant (verified separately by a major OEM) suggests that the TourEngine with such a 2:1 volume ratio between expansion and compression could yield brake thermal efficiency (BTE) of 50.6%, while increasing the volumetric ratio to 3:1 could push that up to 55.6%.
Tour also described new crossover valve designs with increased actuation speed and control.
Background. The premise of a split-cycle engine is that the segregation of the intake and compression stroke in one cylinder, and the combustion and expansion stroke in another, coupled cylinder, provides a thermodynamic advantage enabling engines to be designed to be more efficient than an engine that combines all four strokes in the same cylinder.
In his talk at the 2011 DEER conference, GM Vice President of Global Research and Development Dr. Alan Taub noted that split-cycle engine technology “really looks promising”, and said that his company is putting on a “major thrust” in its R&D laboratory to see if it can get split-cycle technology “moving”. (Earlier post.)
...we may finally be entering the era where the demand for fuel efficiency will be allowing us to break away from what has become the standard architecture of our engines, and in particular the idea of separating the compression and the combustion (expansion) cylinders, into a dual stage engine. People have talked about it, a lot of people are starting to build prototypes in this, and the driving force is clear: we can see very dramatic improvements in efficiency by going to the DCDE [Discrete Compression Discrete Expansion] architecture. It takes mass, it takes cost, it takes complexity, but giving those kind of efficiency improvements, it is definitely something we need to explore further.—Alan Taub at DEER 2011
Tour Engine contends that such a configuration—discrete cylinders linked by a connecting mechanism—introduces several drawbacks that outweigh the potential benefits: significant energy losses are incurred during the transfer of the compressed working fluid from the compression cylinder to the combustion cylinder; a higher compression ratio (e.g., >20:1) is required to allow the efficient multi-stage transfer of the working fluid resulting in elevated friction losses; and a disadvantageous “dead volume” is introduced into the design that holds compressed working fluid within the connecting tube that does not participate in the combustion stage.
Unlike regular IC engines that intake, compress, ignite at the end of compression, and expand a specific volume of working fluid without interruption, other split-cycles expand working fluid that was inducted and compressed several cycles preceding the current expansion stroke. Therefore, Tour argues, they essentially split the engine into a compressor that is coupled via a connecting tube to a combustor with a piston that runs away from the incoming compressed charge.
|Cartoon of the TourEngine. Click to enlarge.|
The TourEngine design concept. The TourEngine configuration directly couples the two opposing cylinders. A single crossover valve controls the charge flow between the two cylinders—a valve that is large enough in cross-section not to be a bottleneck and thin enough in profile to ensure minimal dead volume.
The crossover valve enables the execution of an integrated cycle: the inducted working fluid is compressed and combusted as part of a single cycle, thereby avoiding piston runaway.
|Theoretical p-V representation of TourEngine split cycle. The blue line represents the cold (intake/compression) cylinder; red, the hot (combustion and expansion). Click to enlarge.|
The TourEngine is designed to operate using conventional realistic compression ratios (8:1 to 20:1 depending upon fuel type and the use of SI or CI cycle), and is designed to fire at the end of the compression process—while the crossover valve remains open—very similarly to conventional engines but retaining the split-cycle thermodynamic advantages:
As intake and compression occur in a separate cylinder that is relatively cold, less cooling is needed. Simulations indicate a 15% efficiency increase.
Increasing the combustion cylinder volume to enable the combusted gas to expand further, and to convert the heat energy to kinetic energy more fully, reduces exhaust loss. Simulations indicate a 33% efficiency increase.
As the cold cylinder reaches half compression, the crossover valve is closed and the hot cylinder is near the end of exhaust. The two crankshafts are mechanically coupled, with the expansion piston leading with a 40 ° phase lag.
Tour Engine is making an effort to minimize dead space (e.g., completely venting the exhaust), as exemplified in the cartoons below by the cone-shaped protuberances in the pistons. (In advanced implementations of the engine, the design will be such that there will be no need for such cones, Oded Tour remarked.)
At 3/4 compression, and the end of exhaust, the valve begins to open; there is no clearance in the TourEngine design. At 4/5 compression, the valve is open and the charge is being transferred to the combustion cylinder. Fuel injection begins, and experiences robust turbulence and swirling.
At maximum compression (minimum volume) the crossover valve is open, the charge is half transferred, and combustion begins. Firing with an open crossover valve allows the TourEngine to follow the conventional 4-stroke cycle thermodynamics, but on a split-cycle platform. (The disadvantage is a small efficiency penalty associated with pressure acting on the compression piston.) With complete charge transfer, the crossover valve closes; combustion continues in the hot cylinder.
|Maximum compression, half transfer, combustion begins. Click to enlarge.||Complete transfer, 1/4 of expansion. Click to enlarge.|
The crossover valve. The crossover valve is a critical enabler for the Tour design; it must be able to open to allow the compressed charge transfer and then immediately close (on the order of 30–50 crankshaft degrees) and transfer the charge with minimal resistance. In other words, the valve needs to be large enough in cross-section not to be a bottleneck.
It also must be thin enough in profile to ensure minimal dead volume; dead volume on the compression side prevents full transfer of the compressed working fluid, while dead volume on the expansion side reduces volumetric efficiency and decreases the phase lag for a given compression ratio, which will require even faster valve actuation and therefore will be more challenging. And, it must be able to withstand a high-temperature environment.
|Measured p-V with electromagnetic crossover valve and a 1:1 expansion to compression volumetric ratio. Click to enlarge.|
For the alpha prototype, Tour used a spring-loaded crossover valve. The company is now working with an electromagnetic crossover valve that is actuated by compression (open) and combustion (close). The electromagnetic force serves only to hinge the crossover valve at the close position. At the right timing, charge forces open or close the valve.
Tour says that the valve—which is of low mass design (and low inertia) features ultrafast actuation (<1 ms) and at 2000 rpm can open fully over 2–6 ° crankshaft angle.
The company is also working on several mechanically actuated crossover valves. These mechanical crossover valves will mainly be actuated by engine forces: compression will open the valve and combustion will close the valve. The mechanical elements will time and control the actuation.
Emissions. Tour projects a significant reduction in CO2 emissions due to higher engine efficiency, along with a substantial reduction in criteria pollutants (NOx, HC, CO). A higher piston velocity at peak combustion reduces peak temperature, leading to a projected reduction in NOx. With the hot combustor burning all the fuel and inducing complete oxidation of CO to CO2, Tour says, there will be reductions in HC and CO, although the company has no estimates as yet.
The company is in discussions with several leading OEMs who are investing internal resources and considering joint development, according to Oded Tour.