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Cutaway shows the Nautilus Cycle engine's defining feature: a piston with a top protrusion that creates the engines "primary" combustion chamber as the piston approaches the top of the stroke (Nautilus Engineering).

Nautilus claims radical advance in HCCI engine development

Claiming its novel base-engine design solves most of the intrinsic drawbacks that have slowed auto-industry development of homogenous-charge compression-ignition (HCCI) engines, Nautilus Engineering used the 2016 SAE World Congress to introduce its Nautilus Cycle engine, saying the unique platform offers an opportunity to introduce efficient, low-emissions HCCI technology to a variety of industries.

Although Nautilus said in a news release the company “believes it now holds patents to make controlled HCCI combustion possible for a wide range of engines and applications” company CEO and chief research scientist Matthew Riley said he started the Nautilus effort with a somewhat humbler horizon: “I just wanted to clean up lawn mower engines.”

Low-temperature HCCI combustion has been a lingering Holy Grail of automotive powertrain development, promising to impart on gasoline engines the efficiency of diesel—without diesel’s emissions and cost baggage. But a decade or more of serious development has proved HCCI an elusive target, its need for intensive combustion control, issues with cold-start operation and difficulty in addressing the entire automotive rpm range suppressing introduction for production-vehicle application.

Nautilus, however, said its approach to the issue was not to layer on yet more levels of costly engine software control. Instead the engine’s defining feature—a novel piston design that creates a small “primary” combustion chamber that propagates the air/fuel detonation to the larger secondary cylinder for full and controlled expansion—and an equally unique approach to dealing with the intake and exhaust paths of the two-stroke cycle create an architecture conducive to relatively simple control of full-range HCCI operation. Nautilus' video of the operation can be seen here.

“Everybody’s always tried to take a spark-ignition engine and make it HCCI,” Riley said at the 2016 SAE World Congress. “Bad idea.”

On its website, Nautilus said, “With this new technology, we've achieved full control over the HCCI cycle in all conventional rpm ranges, loads and temperatures. This may be rapidly implemented into existing platforms and will meet and even exceed 2025 EPA (emissions) regs.”

It's all about that small combustion chamber

Air and fuel are mixed outside the cylinder in the intake manifold via a fuel injector ideally operating at something approaching 160-200 psi. A couple of potential design options for the Nautilus engine’s small primary combustion chamber mean compression ratio could be fixed (say for small single-cylinder engines) or enable a variable compression ratio of perhaps 1.5 to 2.5 times the large secondary combustion chamber’s fixed compression ratio of somewhere around 10.5:1. Twin intake and exhaust ports are valved with conventional-looking poppets envisioned to be driven by digital hydraulic (or perhaps even electric) actuators. The valves also could be operated by a more conventional camshaft arrangement.

The engine design uses a single check valve, the company said, to minimize the typical two-stroke engine’s exposure of the intake and exhaust streams to engine lubricating oil.

The Nautilus Cycle engine will run air/fuel ratios approximately ranging from 24:1 to 31:1, as the engine design means the air/fuel ratio is determined strictly by engine load. As for a projected noise, vibration and harshness signature for the engine, “We’re not sure yet,” Riley admits. He seemed to indicate an ideal configuration for the Nautilus engine could be an opposed-cylinder “boxer” layout. And he would not rule out the likelihood that an automotive application might require some kind of supercharging to aid cylinder filling and scavenging—particularly if the Nautilus Cycle engine is adapted to operate under Miller or Atkinson cycles.

Riley said combustion temperatures are expected to be around 1,200º F for an engine that for the moment exists only in computer design: no running prototype yet exists. He said the company plans to have a running version in eight to ten weeks and a prototype engine “up and functional” within six months. After a year of prototype development, he sees a window of two to three years “for major streamlining of automotive applications.”

Drone Duty?

Given some past experiences, the timeline seems uncharacteristically cautious for a new-engine promoter—but the Nautilus Engineering CEO is openly bullish about the engine design’s intrinsic appeal: Somebody’s probably going to come to us by the end of the week with a development contract,” Riley predicted.

He said the immediate goal for Kansas-based Nautilus Engineering is to work with academic partner Kansas State university to develop a version of the engine suitable for an unmanned aerial vehicle (UAV) drone.

Riley also said the Nautilus engine also could easily run a conventional four-stroke cycle. One of its chief attributes, he insisted, is scalability to accompany its easy adaptability for a range of mobility applications.

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