Motiv Engines, LLC introduced the second-generation of its engine concept dubbed the MkII Clarke-Brayton Engine, which it intends to develop into a heavy-duty on-highway engine fueled by liquid natural gas (LNG). The prototype is fueled by diesel, a first step in proving the technology before developing a new LNG fuel system.

The MkII Clarke-Brayton Engine is a boxer-configuration split-cycle engine implementing what Motiv calls the Clarke-Brayton cycle. The thermodynamics of the engine are virtually identical to the company’s previous CCI (Compact Compression Ignition) design, as described in a 2013 SAE paper, but are implemented in a much more conventional way.

Motiv suggests that the MkII engine will be up to twice as efficient as a conventional automobile engine and up to 20% more efficient than modern diesel engines. It achieves this by splitting the processes of an engine into three separate optimized spaces—i.e., across three optimized cylinders—rather than performing all the operations in a single “compromised” space. This allows it to implement a fundamentally more efficient cycle, achieve an extremely high 56:1 compression ratio leading to 30 MPa peak pressure, and lose much less of its energy to heat than a conventional engine due to the very small bore of the combustion chamber (the middle set of cylinders).

(The very high compression ratio also enables the engine to burn natural gas as a compression ignition fuel, without any ignition assistance.)

As illustrated in the animation above, air moves sequentially through the 3 cylinders from top to bottom. The small combustion piston area leads to lower forces on the crank than a conventional engine would have if it were able to reach similar pressures, reducing the friction compared to conventional architectures. It expands all the way to ambient pressure before the exhaust stroke.

Gas transfer from one cylinder to the next is begun at equal pressures on either side of the valve, keeping velocities low and minimizing pumping losses. The power is produced in almost a 50-50 split between the combustion (central) and exhaust (largest) pistons.

All valves are actuated by overhead cams. Piston ring sealing is completely conventional, eliminating the dynamic effects of the first-generation design. The reciprocating mass is greatly reduced.

We have taken what we learned from our first engine and applied it to the new design. This is a major step toward our goal of building the most efficient engine in the world for trucks, automobiles and power generation. The improvement in efficiency and the use of natural gas makes this engine a natural fit to meet the upcoming greenhouse gas emissions regulations while reducing costs for operators.

Motiv said that the major components of the MkII Clarke-Brayton Engine have been released for casting and everything else should be released for fabrication within a couple of weeks. The company will test the engine this summer.

Background. The technology for the MkII Clarke-Brayton Engine originated at Caterpillar, Inc. where it was invented by John Clarke, now the Chief Scientist at Motiv Engines. Motiv has advanced the technology from basic concept through the construction and testing of the first prototype to the MkII.

The Clarke-Brayton Engine uses a modified Brayton Cycle—the same cycle used by gas turbine engines. At equal pressure ratios, the Brayton Cycle is inherently more efficient, but even more so at the extremely high 56:1 or greater compression ratio of the Clarke-Brayton engine.

At the Engine Stretch Efficiency Colloquium in 2010, Clarke noted that the classic piston and crank mechanism constrains thermodynamic efficiency, while separating the cylinders allows them to be individually optimized.

There are a number of advantages to this split-cycle concept:

• The expansion cylinder can be larger than the compression cylinder, thereby realizing the expansion advantage of the Atkinson or Brayton cycles over the conventional Otto and Diesel cycles.
• Moving the compressed gas to a smaller diameter combustion cylinder with a geometric compression ratio of approximately 8 improves both the chamber shape and time available at optimum combustion conditions.
• The Clarke-Brayton engine’s three sequential chambers allow for a large bore compared to stroke enabling a smaller engine size without compromising on combustion chamber shape or friction.
• With combustion occurring in a smaller diameter cylinder, the required crank diameter is reduced.
• While the Clarke-Brayton Engine is a four-stroke engine, there is a power stroke every revolution, as with a two-stroke engine. This enables a higher power density.

Motiv suggests that the Clarke-Brayton engine design can deliver brake thermal efficiency of 52%, and the same power as a conventional engine but at 1/4 the size, as well as lower noise levels and less vibration.

Gas transfer. Fundamental to achieving the theoretical benefit, however, is exchanging gas between the cylinders without incurring losses large enough to negate the other benefits. Clarke earlier articulated four requirements for efficient gas exchange:

• The duration of the transfer process must be long enough and the flow area large enough that the pressure loss arising from the flow should be small relative to the useful pressure differences developed by the engine.
• Starting the gas exchange between two significant volumes at different pressure, leading to a blow-down event, must be avoided.
• Stopping the gas exchange should occur at a time when the flow is minimal.
• The timing of the transfer process itself should not reduce the useful compression and expansion ratios inherent in the
  engine.

In the CCI engine, gas transfer from the induction to the combustion cylinder starts when the pressures are almost equal and continues for 90 degrees of crank angle during which compression continues with both chambers reducing volume. The end of this transfer occurs when the induction cylinder reaches its minimum volume and the flow stops, so the closing corresponds to a no-flow situation.

Transfer from the combustion cylinder to the exhaust cylinder takes place when the exhaust cylinder volume is minimal. If there is any pressure difference, this can be adjusted by exhaust valve closing time; the flow needed to equalize pressures is very small and the losses accordingly are small.

Transfer occurs during 90 degrees and during this time both cylinders are expanding but because the exhaust cylinder expansion rate is higher most of the exhaust gas leaves during this process.

The end of the transfer occurs when the combustion cylinder is at maximum volume so that flow rate is again small and the loss due to closing the transfer port is minimal.