The Tour Engine is a patented, split-cycle, internal combustion engine that stands to
deliver substantial efficiency gains through superior thermal management.
Unlike current combustion engines, which use the same cylinder for all four strokes (intake, compression, combustion, and exhaust), Tour's patented engine design splits the conventional 4-stroke cycle between two cylinders: the cold-cylinder hosts intake and compression, and the hot-cylinder hosts combustion and exhaust. A proprietary crossover valve is used to transfer the compressed charge from the cold-cylinder to the hot-cylinder. This thermal management strategy reduces the magnitude of the two major efficiency losses in conventional 4-stroke engines - heat loss to the coolant/oil and exhaust energy loss. The end result is a thermally optimized engine that advanced computer simulations and working prototypes suggest can be as much as 45% more efficient; dramatically reducing both fuel costs and noxious emissions.
Better still, because Tour Engines employ the same “block and piston” architecture that has been at the heart of ICEs for more than a century, they will be inexpensive to manufacture, adopt and maintain.
The Tour split-cycle engine implements a superior thermal management strategy that reduces the magnitude of the two major thermal losses in conventional 4-stroke engines - heat loss to coolant/oil and exhaust energy or enthalpy loss.
The heat rejected to the cylinder walls (and absorbed by the coolant/oil) is, to first order, proportional to the temperature difference between the bulk gas temperature and the average wall temperature, where most of the heat is rejected during the expansion and exhaust part of the cycle since the temperature difference by far outweighs the temperature difference during the induction and compression part of the cycle. Since any split-cycle approach, including the Tour cycle approach, separates and assigns the cold and hot portions of the full thermodynamic cycle to separate cylinders, it is now possible to control their respective wall temperatures in order to lower the temperature differences of both the induction/compression and combustion/exhaust strokes. In particular, raising the wall temperature of the Hot-Cylinder noticeably decreases the temperature difference during the combustion/exhaust strokes. Obviously, the wall temperature of the Hot-Cylinder can only be raised within reason to safeguard the proper functioning of the piston ring lubrication and maintaining engine reliability.
The efficiency of the split-cycle approach is significantly improved by over-expanding the gas in the Hot-Cylinder (as in the Atkinson cycle for naturally aspirated engines and the Miller cycle for forced induction engines), which has the two-fold benefit of increasing the mechanical work extracted and lowering the average gas temperature at the Hot-Cylinder (Over expansion has an advantage also of lowering the temperature differential driving the heat rejection to the Hot-Cylinder. See “Superior thermal management strategy” above). The Figure below (Panel A) depicts indicated thermal efficiency (ITE) results at a fixed speed of 2400 rpm from GT-Power simulations for three engine configurations: a 2-cylinder baseline 4-stroke engine with 1000 cc (blue, solid line), an Atkinson cycle engine (green, solid line) and various Tour cycle engines (dashed lines). A symmetric Tour engine (red, dashed line) with 1000 cc (500 cc compression/500 cc expansion) has only a slight advantage over the baseline but asymmetric (over-expanded) Tour engines gain a significant advantage up to 1500 cc (500 cc compression/1000 cc expansion). Combustion strategies similar to those used in the Atkinson cycle enabling a 13:1 compression ratio would further increase the efficiency of the Tour cycle (orange, dashed line). The results at 2400 rpm suggest that overexpansion of engines with the above stated displacements can increase indicated thermal efficiency (ITE) relative to the baseline by up to 19%, and relative to the Atkinson cycle by up to 13% depending on the specific engine configuration. In order to simulate the performance maps (shown with ITE contours) for the baseline engine (Panel B) and the over-expanded Tour engine (Panel C), a throttle was added to the models. The performance maps show a higher ITE for the Tour engine and an expanded high efficiency region across a wide range of engine speed, including low engine speeds. This is advantageous for increasing engine durability while maintaining high efficiency.
Indicated Thermal Efficiency (ITE) from GT-Power simulations
The power stroke in the Hot-Cylinder and the intake stroke (of the next cycle) in the Cold-Cylinder occur concurrently. Hence in a pair of Tour engine cylinders, two 4-stroke cycles are being executed at the same time. In this respect, the Tour engine and a conventional four-stroke twin engine have exactly the same number of power strokes per crankshaft revolution. For example, a conventional twin engine will have one power stroke in each cylinder, while the Tour engine will have two power strokes in the Hot-Cylinder and none in the Cold-Cylinder. The advantages of the split cycle approach will be discussed in detail below.
As demonstrated from early prototypes and modeling, TEI has built at its R&D facility in Israel two functional prototype engines that implement the Tour cycle. The first prototype, Prototype I, became operational on 2008 and used two identical off-the-shelf 50cc Honda GXV50 engines, one for the Cold-Cylinder and the other for the Hot-Cylinder. Prototype I proved the mechanical feasibility of the Tour engine design and showed for the first time that the crossover valve can be built in such a way that there is very little energy loss due to the transfer of the working fluid from the compression cylinder to the expansion cylinder.
Tour engine Prototype I
A. Design. Two Honda GXV50 engines were connected with the crossover valve positioned at the interface between the two cylinder heads.
B. Assembled. Notice that over 85% of the prototype parts are taken from off-the-shelf engines.
The second prototype, Prototype II, became operational on 2012 and is based on two identical off-the-shelf 190cc Briggs and Stratton engines. Prototype II was designed primarily as a platform to test, in a modular fashion, various crossover valve designs. This prototype was later modified to implement an over-expanding of the gas in the Hot-Cylinder (as in the Atkinson cycle), and a movie of this prototype being tested in our R&D facility in Israel can be seen here.
A clean sheet engine has been designed, built and is currently being tested at Tour Engine, Inc.’s new test facility in San Diego. Based on detailed system simulations (GT-Power), first performed by Tour Engine, Inc. and independently confirmed by Wisconsin Engine Research Consultants (WERC, which are a subrecipient of this project), the engine in this project has been designed with an expansion ratio of nearly two times the compression ratio and each cylinder has been fitted with a dedicated cooling circuit to characterize and subsequently minimize the heat losses of the engine. The combination of these two features provides the theoretical basis for significant efficiency gains of the Tour split-cycle over conventional engines.
Several other split-cycle engines (Scuderi engine and Zajac engine, for example) have been proposed in the past but none have been successful due to detrimental compromises in the thermodynamic cycle (over-compression, charge storage, heat loss and retarded combustion) and the reliance on ultra-fast valves that pose reliability challenges. The Tour engine is the first split-cycle engine to closely emulate the thermodynamic cycle of the 4-stroke engine (integrated cycle) and offers a comprehensive set of solutions.