The ongoing trends to reduce emissions and improve fuel economy are driving research to improve the efficiency of internal combustion engines. Several alternative engine designs are discussed in this article, including the rotating liner engine (RLE).

The two major thermodynamic cycles for internal combustion engines are spark ignition and diesel. The diesel thermodynamic cycle has a higher thermodynamic efficiency due to the much higher cylinder pressures and, therefore, is preferred in heavy-duty applications, but these high pressures also generate more friction losses. Dr. Dimitrios Dardalis, director of technology for Rotating Sleeve Engine Technologies, Inc. in Austin, Texas, says, “The main source of friction in a conventional diesel engine is found in the piston assembly. The RLE improves the efficiency of an engine by enabling the cylinder liner to rotate, which minimizes metallic contact of the piston assembly components, which, in turn, reduces localized friction coefficient by a factor of 100 and minimizes wear. The cylinder liner rotates at a surface speed of 2-4 meters per second. Improved performance is realized when the cylinder pressure is high as the piston is near the top of its stroke during the compression and early expansion transition, at a stage known as top dead center (TDC).”

The RLE concept was initially evaluated in a firing diesel engine. Dardalis says, “The diesel engine is ideal for the RLE for commercial applications because the higher cylinder pressure generated due to contact between the piston ring and the cylinder leads to higher friction. This benefit will persist under low loads due to the high compression ratio and unthrottled operation.”

The next step in the testing regime is to evaluate the RLE concept in a diesel engine. Dardalis says, “Our initial work to prove the viability of the RLE was easier to accomplish with the gasoline engine. The diesel engine is ideal for the RLE because the higher cylinder pressure generated due to contact between the piston ring and cylinder leads to higher friction. This benefit will persist even under low loads due to the high compression ratio and unthrottled operation.”

Idle operating conditions
The researchers worked with a four-cylinder Cummins 4BT diesel engine where only cylinder #2 was active. The rest of the pistons were removed. Dardalis says, “Running the diesel engine under idle conditions makes it easier to measure friction because all of the fuel energy is used to fight internal friction.”

A conventional baseline diesel engine was configured in the same manner as the RLE and run under identical idle operating conditions. Both engines were maintained at 70 C and used a 15W-40 lubricant. The study was conducted at the University of Texas Engine Lab with the support of professors Matt Hall and Ron Matthews in the Walker Department of Mechanical Engineering.

The engines were evaluated by comparing the difference between the peak cylinder pressure (PCP) and the peak motoring pressure (PMP). Dardalis says, “This difference is a measure of how much fuel an engine requires to sustain itself, and is directly linked to the heat released per cycle. We are assuming that the RLE and the baseline diesel engines have identical indicated thermal efficiencies, which means that since the only load on both engines is internal friction, any reduction in fuel burnt per cycle is directly linked to friction reduction.”

Another term used to categorize friction is fMEP, which is the friction mean effective pressure. Dardalis says, “fMEP is a measurement of the friction torque and is a figure that is independent of engine size. This parameter is used because the complex nature of an internal combustion engine makes it impossible to derive a single value for coefficient of friction.”

The researchers found that the RLE achieved a 32% reduction in fuel consumption, which can add up to a significant improvement in fuel economy. In addition, simultaneous carbon dioxide measurements corroborated the above results.

When the engines were disassembled and examined after testing (see Figure 1), the carbon soot trace was used to evaluate the sealing performance of the rotating liner sealing device, which is the primary technical challenge of the conversion. The conclusion was that the leakage of the face seal was negligible, and, therefore, the technical problem is resolved.


Figure 1. The rotating liner sealing device was taken from a disassembled RLE after testing to show that leakage of the face seal is negligible due to the presence of only a trace of carbon soot. Figure courtesy of Rotating Sleeve Engine Technologies, Inc.

The researchers then extrapolated the data generated with testing the single cylinder engines to complete four-cylinder engines. Dardalis says, “The single cylinder engine drives a 5 bearing crankshaft, as opposed to 1-1/4 bearings for a complete engine. Similar adjustments were done to compensate for the parasitic power of the oil, water and the injection pump, which also would be shared among four cylinders rather than just one.”

The calculations showed that a four-cylinder RLE produces a 40%+ reduction in friction compared to the baseline engine. This figure is superior to the less than 10% reduction found when just engine oil viscosity is reduced in conventional diesel engines. Dardalis says, “We predict that extrapolation of our test results to the API heavy-duty engine tests will lead to a greater than 10% fuel economy benefit plus reductions in carbon dioxide, NOx and particulate emissions.”

Future work will involve connecting the single cylinder RLE to a dynamometer to compare friction under high load conditions (the engine has already been operated under load with no signs of distress). The friction reduction under high load will increase, but the accurate measurement of this reduction will be more challenging. Additional plans include the development of a four-cylinder RLE that can be evaluated versus a baseline engine. Dardalis says, “We hope to offer a program to overhaul diesel engines by converting them to RLE in the future, in order to bring the technology into the market. We also will be able to evaluate lower viscosity engine oils in the RLE to determine if additional fuel economy benefits can be achieved.”

Additional information can be found in a recent article¹ or by contacting Dardalis at dardal@rotatingliner.com.

REFERENCE
1. Dardalis, D., Hall, M., Matthews, R., Basu, A. and Ching, Z. (2021), “Testing the rotating liner engine: Over 30% reduction in diesel engine fuel consumption at idle conditions,” SAE Technical Paper, 2021-01-0448, DOI:10.4271/2021-01-0448.