A team from Universita degli Studi di Pisa (Italy) and Rolf Reitz at the University of Wisconsin-Madison are proposing a novel combustion concept—Homogenous Charge Progressive Combustion (HCPC)—based on a split-cycle principle to control HCCI combustion in diesel-fueled engines. Ettore Musu from the University of Pisa presented a CFD study of concept at the SAE 2010 Powertrains, Fuels and Lubricants Meeting in San Diego.

The HCPC concept consists of forming a precompressed homogeneous mixture outside the cylinder and then gradually admitting the charge into the cylinder during the combustion process. The turbocharged concept allows reaching engine speeds of 6,000 rpm, with a high indicated efficiency of 45% along with clean combustion; power density at 6,000 rpm is 64 kW, with 300 kPa intake pressure.

The basic idea of the new Diesel combustion illustrated in this paper is to control the heat release rate by a gradual supply of an almost homogeneous charge, without relying on exhaust gas recirculation to moderate the reaction. Thus the HCPC range can extend to all engine operating conditions, including very high loads.
—Musu et al., SAE 2010-01-2107

The intake and compression phases are performed in a reciprocating external compressor, which drives the air into the combustor cylinder during the combustion process, through a transfer duct. A transfer valve is positioned between the compressor cylinder and the transfer duct.

The compressor piston has a fixed delay, in terms of crank-angle degrees, with respect to the combustor piston. The combustion takes place after combustor TDC; during the combustion process, the combustor piston moves downwards whereas the compressor piston moves upwards. As a consequence, the air moves from the compressor cylinder to the combustor cylinder. Contemporary with the air transfer, fuel is injected into the transfer duct, evaporating and mixing with the air, thereby bringing about the conditions needed for homogeneous combustion.
HCPC Combustor and Compressor
Combustor Compressor
Displacement 598 cm3 250 cm3
Bore 86 mm 68.3 mm
Stroke 103 mm 68.3 mm
Geometric compression ratio 85:1 79:1
Squish height 0.5 mm 0.5 mm

In the study presented at SAE PFL, a turbocharging system with overall efficiency of 50% was considered for the inlet and outlet pressure. A pilot injection was used to achieve stable ignition timing, and the injector was a 7-hole injector.

Increasing engine speed from 2000 to 6000 rpm, indicated thermal efficiency decreases from 47% to 45% for the turbocharged case and from 46% to 44% for the naturally aspirated one due to higher pressure losses in the transfer phase. These results are very meaningful and represent an important step forward in the HCPC engine development. As a matter of fact, the new HCPC engine with smaller compressor is able to run at the even better ISFC of diesel engines at speeds that are typical of SI engines for passenger cars.

...Unlike in conventional diesel engines, the speed of the combustion process increases along with the engine speed, keeping almost constant combustion quality and duration in terms of crank angle, like in SI engines.

Pollutant emission behavior vs. engine speed...confirms the validity of the HCPC combustion concept. Emissions are almost independent of the engine speed. Soot and HC are orders of magnitude lower than the ones of a diesel engine. The low CO emission trend confirms the high combustion efficiency even at high speeds. NOx emissions are almost constant and similar to those of a diesel engine.

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