Effect of Gasoline Fuel Additives on Combustion and Engine Performance

2.4.2
 
Optical Combustion Characterisation
Several optically accessible rigs can be utilised to enable the investigation of 
combustion characteristics under engine-like conditions. The rigs can be divided into 
piston and non-piston based rigs. The benefit of optical access arises from the 
capability of imaging of flames and the application of laser diagnostic methods that 
can reveal and explain fundamental processes during combustion. 
2.4.2.1
 
Optical research Engine
The most commonly used piston-based optical rig is the Optical Research 
Engine (ORE) which, due to its operational characteristics, are closest to normal IC 
engines. Optical access to the combustion chamber can be achieved through several 
techniques. Most commonly a window in the piston crown is employed [165, 166, 
167, 168], although optical access can also be gained through cylinder head or cylinder 
walls/liner [165, 169]. Advantages include analysis of combustion in engine 
conditions that might not be possible in other rigs. However, the optical windows and 
cylinder liners can be very expensive and fragile, thus, limiting operational conditions. 
2.4.2.2
 
Rapid Compression Machine
A further piston based rig used is the Rapid Compression Machine (RCM). 
The RCM utilise a single rapid compression stroke to achieve test conditions. Due to 
the lack of valve train requirement, very good optical access can be gained to the 
combustion chamber [170]. However, Mittal and Sung [171] discuss that a very rapid 
pressure rise is necessary in order to avoid significant heat losses to the vessel walls 
and combustion reactions from occurring before the compression process has been 
completed. The timescales available are in the order of 10s of ms and as a result very 
complex designs using hydraulic and pneumatic actuators need to be used [171, 172] 
. Although RCMs closely replicate turbulence seen in IC engines, the rapid piston 
movement experienced often causes vibrations that have been reported to limit the use 
of laser techniques [114, 173].