OPE & VCR
Introduction
The revival of the Opposed Piston Engine (OPE) concept, by new and established engine companies, has prompted this design submission. OPE's are well known for providing the superior power density, Brake Thermal Efficiencies (BTE) and low specific fuel consumption for two and four cycle engines. These features are attractive to companies wishing to minimise fuel economy and reduced manufacturing cost.

Based on these attributes, the design submitted for this Contest is a Variable Compression Ratio (VCR) piston, to improve BHP, BTE% and sfc.

Opposed Piston Engines (OPE) have been around since 1858 and serious variable compression ratio (VCR) devices have been tried since the 1950s. It is believed that OPE and VCR piston combination is novel.

Benefits

Before exploring the workings of the VCR piston design submitted below, it is appropriate to list of the benefits of VCR pistons achieved:
 
• Limiting cylinder peak pressures to 140 bar while allowing higher boost to 40 bar BMEP
• Reducing sfc at partial load by increasing compression ratio
• Reducing wear rates and friction due to limited peak pressures
• Easier engine starting at low ambient temperatures (-25° C)
• Lower start-up emissions and smoother idling
• Limiting structure and bearing loads while increasing power
• Increases expansion ratio
• Multi-fuel capability

VCR Piston – Working Description
To provide an understanding of the working of the VCR piston, refer to Fig. 1, 2,3 and 4.
Pressurised lube oil supply is fed via a Check Valve (not shown) through a drilling in the Con-Rod. This feeds an Oil Supply Reservoir within the Gudgeon Pin. From here the pressurised oil is force through the Hydraulic Supply Valve to a Hydraulic Chamber contained within an
Inner Piston by a Valve Housing Piston. The Inner Piston itself is housed within the Piston Outer, so that its movement upwards is restrained by the Inner Piston Retainer. The Inner Piston's downward movement is restricted by the Valve Housing Piston and the Con-Rod.
The Inner Pistons upper position creates a combustion volume between the VCR Bowl Upper of the opposing piston and the VCR Bowl Lower, representing a compression ratio of 20 : 1 at TDC and part load. The reverse of this, is with the Inner Piston at its lowest position creating a compression ratio of 10 : 1 at TDC at high load. This limited movement is achieved by oil flow from the Hydraulic Chamber via the Discharge Valve to the sump. The Discharge Valve is set limit the peak cylinder pressure. see Fig.1.

Tapered slots in the crown of the Piston Outer provides access to the combustion chamber for the common rail fuel Injectors Spray Plumes. See Fig 3.

To envisage conditions within a cylinder with VCR pistons installed, the three dimensional PV diagram Fig.4, shows the limiting of peak cylinder pressure, whilst the PV diagram area increases thus creating higher BMEP at the low compression ratio of 10:1.

The VCR piston's design enables conventional volume manufacturing processes.