Beijing Institute of Technology team developing new hydraulic free piston diesel engine

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Beijing Institute of Technology team developing new hydraulic free piston diesel engine

29 November 2010

The principle of the single piston hydraulic free piston diesel engine. Click to enlarge.

Researchers at the Beijing Institute of Technology have designed and built a prototype hydraulic free piston diesel engine (HPFDE). The single-piston, two-stroke compression ignition engine leaves out the crankshaft, pump axle and swash plate of a conventional diesel-engine hydraulic drive pump. The piston and the hydraulic plunger are connected directly together, resulting in a smaller and mechanically simpler device.
The main elements of the engine are the combustion cylinder; the compression element; and the flow output element. In the working cycle of the engine, the heat energy produced by fuel combustion is transformed into hydraulic energy by means of linearly moving of the piston assembly:

• The cycle begins with the piston at BDC. To start a stroke, a frequency control valve (16 in the diagram above) opens, and pressured oil from the compression accumulator (7) is admitted to the compression volume (3), pushing the piston assembly moving towards TDC.
• During the compression stroke, the low-pressure oil is sucked into the pump volume (2) through the check valve (4). At the same time, air in the cylinder is compressed.
• When the piston assembly reaches TDC, fuel is injected into the combustion chamber. The resulting expansion stroke pushes oil out of the pump volume into the high pressure rail. The oil in the compression volume (3) is pushed back into the accumulator (7), and the oil can be used in the next stroke.

The HFPDE engine is uniflow scavenged with a roots pump.

The position of the TDC is not fixed by any mechanism, which makes variable compression ratio possible in the HFPDE. The compression ratio of the HFPDE is determined by compression energy supplied by the compression accumulator if the fuel mass keeps constant every cycle. Then the delivered compression energy is determined by the plunger surface area, piston stroke length and compression accumulator pressure together. The stroke length is nearly constant and the plunger area is fixed, therefore the variation of compression energy is accomplished by varying the pressure in the compression accumulator.

—Zhenfeng et al.

Injection timing is controlled by piston position. When the piston reaches the appropriate position for fuel injection, the piston displacement sensor transmits the piston position signal to the solenoid valve, then the injector begins to inject the fuel into cylinder.
Starting the HFPDE is accomplished with the help of the compression accumulator.
From a study of the HFPDE focusing on compression and ignition published as an SAE paper, the team concluded that:

• Compression pressure is a very critical parameter that controls the HFPDE compression ratio and the piston velocity.
• Injection timing of the HFPDE has a great impact for the cylinder pressure; the peak of the cylinder pressure and the time of arrival peak changes with injection timing variation. Injection timing has an impact for the piston stroke length at expansion stroke.
• HFPDE cold-start is easier than in a traditional engine.

Next steps in the development of the engine are to optimize the scavenging system and the injection system for the operation under different frequencies. The current DDC6V53 unit pump-nozzle injector will be replaced by the Caterpillar Hydraulic Electronic Unit Injector (HEUI). The HEUI is electronically controlled and hydraulically actuated. Combustion process analysis is another important future research aspect of HFPDE.
Resources

• Zhao Zhenfeng, Ying Huang, Fujun Zhang, Changlu Zhao and Kai Han (2010) Experimental Study on Hydraulic Free Piston Diesel Engine (SAE 2010-01-2149)

November 29, 2010 in Engines | Permalink | Comments (12) | TrackBack (0)

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