|Prototype II Tour Engine—a novel split-cycle engine—on the bench. The hot side is on the right. Click to enlarge.|
Tour Engine, the developer of a novel split-cycle engine (earlier post), has its Prototype II engine running and will present details on its operation at the upcoming SAE 2012 World Congress in Detroit.
In early bench-testing of the new prototype, Tour found samples of a similar work gain value to that from a conventional control engine (129 kJ for the control and 124 kJ for the Tour engine in the example depicted below), with both operated at about half throttle and using a symmetrical Tour engine with a compression ratio similar to the expansion ratio (8:1). Other examples show similar trends, according to Tour.
However, notes Oded Tour, the Tour engine can be greatly improved by having an expansion ratio of up to 3 times larger than the compression ratio and by further differential engineering of the compressor cylinder and the combustor cylinder. Thus, the company plans in the near future to modify prototype II to have an expansion ratio of 16:1 with compression ratio of 8:1.
At first cut, roughly, we have not done any worse than the baseline engine. The way forward for the Tour engine is clear substantial improvement.—Oded Tour
|Readings from control engine. Click to enlarge.|
The premise of a split-cycle engine is that segregating the intake and compression strokes in one cylinder, and the combustion and expansion strokes in another, coupled cylinder, provides a thermodynamic advantage enabling a more efficient engine. Most current split-cycle designs use a gas crossover passage or intermediate chamber to connect discrete cylinder pairs. By contrast, the Tour engine configuration directly couples the two opposing cylinders, with a single crossover valve controlling the charge flow between the two cylinders.
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. The Prototype II Tour engine, based on two 190 cc Briggs & Stratton engines, uses a mechanically actuated crossover valve.
The Tour engine 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 (before any decompression occurs)—while the crossover valve remains open—very similarly to conventional engines but retaining the split-cycle thermodynamic advantages.
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 a larger surface area acting as a heat sink at combustion initiation.) With complete charge transfer, the crossover valve closes; combustion continues in the hot cylinder.
The crossover valve is key to the success of the engine; 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, but also 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
|Crossover valve and cylinder connecting plate. The engine is designed in a modular fashion such that several different connecting plates housing different crossover valves could be tested on the same engine. Click to enlarge.|
The alpha prototype used a spring-loaded crossover valve. In addition to the current mechanically actuated crossover valve used in Prototype II, Tour is also developing several other crossover valve concepts including an electromagnetic crossover valve that is actuated by compression (open) and combustion (close) while the electromagnetic force is used to fine tune (hinge) the valve to close and open at the precise timing.
According to Oded Tour, Tour Engine is in discussions with several OEMs on establishing a joint development aiming on taking the concept to the next level by building an advanced Tour engine.
GM Vice President of Global Research and Development Dr. Alan Taub noted in his talk at the 2011 DEER conference that split-cycle engine technology looks promising and that GM was pushing 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
Dr. Chris Atkinson, Professor, Mechanical and Aerospace Engineering at West Virginia University and an advisor to Tour Engine, concurs that over the last three to four years, “a remarkable openness to contemplating such new engine architectures” has emerged among the major OEMs. As for Tour Engine, he added, leaving aside the commercialization aspects:
They have done remarkably well from a technical point of view. To have a [new] engine running legitimately with comparable efficiency to a conventional engine is very much a remarkable feat. Normally, you take several steps back and then you try to work out what you’ve done wrong, whereas here, they are close to conventional already. On a shoestring budget with a minimum of people they have done remarkable things; the quality is very good—approaching OEM quality.
The Prototype II Tour Engine was built with the aid of the Israel Ministry of Energy and Water Resources.