Schematic diagram of compound HPE used in Meng et.al. Compound HPE is divided into a compressor cylinder and a power cylinder, combined with a compressed air storage tank. In moderate operating conditions, the compressor cylinder has the intake and compression strokes. It compresses the pressurized gas produced by exhaust gas turbocharger, comprising fresh air and exhaust gas introduced from the EGR valve. Then it charges the compressed air storage tank. The power cylinder works as a fully four-stroke cycle with naturally aspirated or turbocharged intake. Fuel injection occurs in the early intake stroke or compression stroke to form a well-mixed mixture; meanwhile, suitable valve timing is used to achieve low effective compression ratio and ensure that the autoignition of the premixture cannot occur without any additional heat source. High-pressure air supplied from a compressed air storage tank is jetted into the power cylinder through a high-pressure air jet system near TDC. It compresses the in-cylinder premixture, leading to the autoignition of the local region. Credit: ACS. Click to enlarge.

Premixed charge compression ignition (PCCI) combustion can reduce both NO_x and PM simultaneously while maintaining high thermal efficiency. In PCCI combustion mode, the air−fuel mixture is well mixed before combustion occurs; formation of NO_x and PM can be suppressed because of the reduction of rich region in the combustion chamber. However, the control of combustion phasing is one of the crucial issues in determining whether the diesel PCCI engine could be commercialized or not.

The hybrid pneumatic engine (HPE) concept, which combines a conventional internal combustion engine with a pneumatic storage system, instead of using an expensive battery in an electric hybrid engine, has become an interesting method to realize both low fuel consumption and emissions. Its flexible operation modes, like air compression mode, air power mode, firing and charging mode, and normal engine firing mode, enable the engine to run in a range of high-efficiency loads all the time. The utilization of compressed air power on engine cool starts and low-load conditions could significantly reduce the overall emissions.

… To directly control the combustion phasing of diesel premixed compression ignition, a novel method called high-pressure air JCCI, which is implemented in HPE, was investigated in this paper.

—Meng et al.

High-pressure air JCCI combustion is implemented in HPE engine with the compound thermodynamic cycle of the engine—a cycle with low-compression ratio, high pressure-rise ratio, and high-expansion ratio—reducing the specific requirements of fuel properties. Earlier work showed that the process of high-pressure air jetted into the constant combustion chamber resulted in a rapid increase in mixture temperature and pressure in the local region of the chamber.

Combustion chamber section. Credit: ACS, Meng et al. Click to enlarge.

The maximum temperature rise was more than 150 K—sufficient to allow the mixture at the critical conditions to meet autoignition conditions. Using this approach, the combustion phasing can be controlled directly by the high-pressure air jet.

For the current study, the engine was derived from a single-cylinder, naturally aspirated, high-speed direct-injection (HSDI) diesel engine with a displacement of 0.418 L. A major modification was the reduction of the geometric compression ratio from 19 to 12, ensuring that no autoignition occurs in the chamber without any additional heat source. High-pressure air was jetted into the combustion chamber through a check valve, which was added in the central position of the cylinder head to prevent backfire.

The team used a 3D CFD model coupled with reduced n-heptane chemical kinetics to study the effects of high-pressure air jet pressure and temperature on combustion characteristics in the high-pressure air JCCI combustion mode. Among their conclusions:

• The in-cylinder mixture with low temperature and pressure is ignited by the high-pressure air jet compression. Thus, the combustion phasing can be controlled directly and precisely.

• High-pressure air jet leads to an intensified low-temperature reaction due to rapid temperature-rising rate and a two-stage, high-temperature reaction due to the autoignition in the local region and the subsequent combustion in the chamber.

• Using a lower air jet pressure can produce a higher and retarded peak pressure, a higher heat release peak value, and a shorter burning duration.

• Considering the thermal efficiency and combustion efficiency, the air jet pressure should be designed as low as possible in the circumstance that the autoignition can occur.

• Higher air jet temperature can obtain a higher and advanced peak pressure. The higher air jet temperature significantly improves the high-temperature reaction in both stages, leading to a shorter combustion duration.

Low jet pressure and high jet temperature have the potential of obtaining high efficiency due to the rapid combustion.

—Meng et al.

Resources

• Xiangyu Meng, Mingqi Zuo, Wuqiang Long, Jiangping Tian, and Hua Tian (2015) “Investigation of Effects of Air Jet Pressure and Temperature on High-Pressure Air Jet Controlled Compression Ignition Combustion Based on a Novel Thermodynamic Cycle” Energy & Fuels doi: 10.1021/acs.energyfuels.5b01842

• Schechter, M. M. (1999) “New cycles for automobile engines,” SAE Technical Paper 1999-01-0623 doi: 10.4271/1999-01-0623