Tour Engine, the startup developing novel split-cycle engine technology (earlier post), is working on a 5 kW natural gas unit under Phase II funding from the Advanced Research Projects Agency - Energy (ARPA-E). The additional $2.59 million in funding, awarded earlier this year under ARPA-E’s GENSETS program, follows on Tour’s progress during Phase I (earlier post) in the development of a 1 kW genset engine.

Tour Engine also secured a new $2.25-million investment led by Joan and Dr. Irwin Jacobs of Qualcomm to cost-share ARPA-E’s Phase II funding and to accelerate the company’s growth. The design goal for the genset unit is 5kW @ 1800 rpm, greater than 36% BTE, and CARB2007 emissions.

Figure 1: (A) A cross-sectional view of a 1 kW inline Tour split-cycle engine developed as part of Tour’s Phase I of the ARPA-E GENSETS program. The three crankshafts driving the two pistons and the crossover transfer mechanism (CTM) are coupled together by gears (not shown). The working fluid is induced and compressed in a dedicated cold-cylinder (left), transferred and combusted via the CTM (top), and subsequently expanded and exhausted in a dedicated hot-cylinder (right). (B) The corresponding 1 kW GENSETS prototype. (C) The engine shown in B is being tested in Tour’s state-of-the-art test cell in San Diego, California (more than 100 hours of testing). Click to enlarge.

The Tour engine architecture allows for more engineering freedom to optimize each cylinder for best performance and efficiency.

In general, the split-cycle design typically divides the conventional 4-stroke cycle into a cold-cylinder (intake and compression) and a hot-cylinder (expansion and exhaust) with improved thermal management. It also allows for independent optimization of the compression and expansion ratios, allowing for the most beneficial over-expansion ratio and therefore increased thermal efficiency for any given application.

Specifically, the over-expansion increases the mechanical output of the engine and simultaneously lowers the average temperature of the working fluid thereby reducing the need for active cooling in the hot-cylinder. The 1 kW ARPA-E genset engine featured a cold cylinder displacement of 69 cc and a hot cylinder displacement of 138 cc, while the new 5 kW unit under development will move to a 350 cc cold-cylinder and 700 cc hot-cylinder, respectively.

One of the major challenges with a split-cycle design, with its theoretically higher thermal efficiency, is the mechanism used to transfer the air-fuel mixture between the hot and cold cylinders.

Tour Engine has developed and continues to optimize a family of novel Crossover Transfer Mechanisms (CTM), which enable the efficient transfer of the working fluid between the cold and hot cylinders with minimal pressure losses.

Figure 2: Measured pressure traces from the 1 kW GENSETS engine illustrate the fluid transfer between Cold- and Hot-Cylinder and combustion: The CTM alternately connects the Cold-Cylinder during phase ① with the Hot-Cylinder during phase ③. Ignition during phase ② initiates combustion at a crank angle of -6° (red arrow). During phase ①, almost the entire trapped mass in the Cold-Cylinder is compressed into the CTM volume (blue line) where it combusts during phase ② and expands into the Hot-Cylinder during phase ③. The induction and exhaust through conventional poppet valves occur during phases ③ and ①, respectively. It should be noted that there is no pressure drop during the fluid transfer to and from the CTM as the CTM pressure (dashed red line) coincides with both Cold- and Hot-Cylinder pressures during phases ① and ③. Click to enlarge.

With Tour’s novel crossover transfer mechanism, one consideration is minimizing the dead-volume to transferred-volume ratio, which correlates directly with an increase of volumetric efficiency, says Dr. Oded Tour, CEO of Tour Engine.

Moving to a larger engine, along with optimizing the crossover transfer mechanism design, should help that, by significantly improving volumetric efficiency and reducing blow-by.

The technology can be scaled well beyond the current project parameters; the company is considering a 30 kW design, but for the moment is focused on the 5 kW Phase II ARPA-E work. The company has been issued 18 patents (US and International) with several more pending.

Dr. Tour adds that in his view the Tour engine represents a platform technology that has the potential to disrupt the global ICE market. Once the technology matures, picking the optimal ‘out of the gate’ markets will be a key decision. The right market will have a relatively rapid time-to-market introduction, with applications that favor higher duty-cycles (e.g., prime vs. backup generation), mid-sized or larger engines (5-10kW to 100 kW), and the engine will be fueled by natural gas or gasoline.

He added that a phased market entry approach starting with markets such as stationary gensets and distributed power generation and over time move into other market segments (transportation, consumer).