VARIABLE COMPRESSION RATIO ENGINE

VARIABLE COMPRESSION RATIO ENGINE

INTRODUCTION:

An engine is a device which, transforms the chemical energy of a fuel into thermal energy and utilizes this thermal energy to perform useful work. This engine is classified into two broad types; External combustion engine and internal combustion engine.
Since 1860, the developments were carried out in internal combustion engine and still it is continuing. In internal combustion engine, there are various factors to improve the performance of I.C.engine which gives scope for improvement in future. Some of these factors are compression ratio, variable valve timing, turbocharging, ignition delay, dual spark-plug etc. One of the possible improvement in I.C.engine is variable compression ratio engine which changes the compression ratio according to change in load on engine to get maximum efficiency. To vary the compression ratio, we have either vary clearance volume or both clearance and swept volume.
As far as compression ratio is concerned, the real requirement is for a high compression ratio for starting and for running the engine in lower part of the load range, up to the load at which the safe limit of peak cylinder pressure is reached and thereafter progressively reduce the compression ratio to maintain peak cylinder pressure at desire value for the rest of load range. This is exactly achieved by the VCR engine. In the past development, the British Internal Combustion Engine Research Institute –BISERI developed VCR piston which can vary compression ratio according to peak cylinder pressure. This VCR piston operates on high pressure oil which causes the movement of upper crown and it gives variable compression ratio as per the cylinder pressure.
In the recent developments, Saab Automobile AB unveils the Saab Variable Compression (SVC) engine, a new engine concept that enables fuel consumption to be radically cut while increasing engine performance per liter of engine displacement. The combination of reduced engine displacement, high supercharging pressure and a unique system for varying the compression ratio enables the SVC engine to get high performance with low fuel consumption and low exhaust emissions.


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