Split-cycle internal combustion (IC) engine designs have thermodynamic advantages over conventional 4-stroke engines as they allow for dissimilar compression and expansion ratios and for the differential optimisation of both the compression and expansion strokes. The desired outcome is significant reduction in the two major heat losses of conventional engines: namely exhaust losses and cooling system losses.
Since the beginning of the 20th century, several split-cycle designs have been proposed, but none has matured. The major obstacle, in our view, that has prevented their maturation is the physical separation of the compression cylinder from the combustion cylinder through the installation of a connecting tube or intermediate chambers. This 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 (50:1 and higher) 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. Perhaps the most significant drawback is that these split-cycle designs deviate from the execution of a continuous cycle since these connecting tubes also add a restriction to the dynamic flow of the working fluid. Unlike regular IC engines that intake, compress, ignite at the end of compression, and expand uninterruptedly a specific volume of working fluid, previously proposed split-cycle concepts are designed to expand working fluid that was inducted and compressed several cycles preceding the current expansion stroke. Therefore, they split the machine itself into a compressor that is coupled via a connecting tube to a combustor with a piston that ‘runs away’ from the incoming compressed charge.
Unlike regular IC engines that intake, compress, ignite at the end of compression, and expand uninterruptedly a specific volume of working fluid, previously proposed split-cycle concepts are designed to expand working fluid that was inducted and compressed several cycles preceding the current expansion stroke.
The TourEngine opposed-cylinder configuration is the first split-cycle to avoid a connecting tube or intermediate chamber and directly couple the two 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. Notably, this 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’. Moreover, the TourEngine is designed to operate using conventional realistic compression ratios (8:1 to 20:1 depending on fuel type and the use of SI or CI cycles) and is designed to fire at the end of the compression process (while the crossover valve remains open) very similarly to conventional engines but while retaining the split-cycle thermodynamic advantages.
Tour Engine Inc. has developed an operating prototype that proves the engine’s design and mechanical feasibility. In addition, two independent simulations (the first done by Nick Killingsworth (PhD) a consultant to Tour Engine, and the second by a leading OEM) validate the expected gain in fuel efficiency, both indicating the potential of greater than 56% BTE (brake thermal energy) at 500 K engine wall temperatures. Notably, the TourEngine is based on existing cylinder and piston technologies thereby increasing reliability, while reducing development and ownership costs and reducing barriers to its adoption.
As is the case with most computer models, we expect somewhat lower BTE values to be achieved in practice. However, we believe that 45% BTE is achievable using gasoline or natural gas as the fuel with even higher fuel efficiency possible with compression ignition fuels. These efficiencies imply that a significant leap in MPG is possible that will offer a competitive edge to any OEM that adopts this technology.
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.
Several engine and vehicle OEMs are in discussions with TourEngine and there is a growing recognition that the key concept – separating the compressor from the combustor while preserving the fundamental thermodynamic cycle – is practical. This key concept has been rarely implemented in the past: all current 4-stroke and 2-stroke engines compress and combust in the same cylinder with compromised efficiency. This is also true for the Junker-based designs proposed by several high-profile start-ups, in which the two opposing pistons jointly compress the charge and execute the power stroke.
In our opinion, the fastest way to realise the potential of our technology is by focusing first on power generation, in particular, replacing large standard slider-crank piston engines in generators that could benefit from another key advantage of the TourEngine: rejection of high-grade waste-energy as a secondary source for a CHP (combined heat and power) system.
Once the technology matures in the power generation application, there will be more incentive and reduced risks for its adoption by the transportation industry, in locomotives, marine applications, large series hybrids and ultimately passenger cars.
The opinions expressed here are those of the author and do not necessarily reflect the positions of Automotive World Ltd.
Oded Tour is CEO, Tour Engine, Inc.
To learn more about Tour Engine, visit www.tourengine.com
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