Seminar On Variable Compression Ratio Engine
What is Variable Compression Ratio Engine
The variable compression engine means that the compression ratio of the engine can be controlled at each engine operation condition. When the more power of engine is needed during high load, the compression ratio is decreased, and when the higher efficiency is needed during low load, the compression ratio is increased. Under full load conditions, the performance and efficiency of an engine with a compression ratio that is adapted to load demands is capable of reducing knock susceptibility. In addition, the risk of pre-ignition, mega-knocking effects and engine jerking, as the result of retarded combustion phases, can be reduced.
The VCR also provides further potential to control the exhaust gas temperature, contributing to protecting component temperatures. A VCR Engine performance, efficiency, engine can continuously varies the compression ratio by changing the combustion chamber volume.
- The Variable Compression Ratio Engine can operate at different compression ratios, depending on the particular performance needs of the vehicle.
- The VCR technology was introduced to increase fuel efficiency under varying loads since conventional gasoline engines due to their fixed compression ratios did not achieve fuel efficiency at low power operations.
- VCR engine operates at high compression for low power levels to enable fuel efficiency benefits and operates at low compression for high power levels to prevent knock.
- VCR can be obtained by altering the clearance volume or both the clearance and swept volume.
- The usual size of VCR engine is small, about one-third the displacement volume of a conventional gasoline engine to further improve the engine’s fuel economy.
NECESSITY OF VCR:-
The present challenge in automotive engine technology is the improvement of thermal efficiency and hence the fuel economy and lower emission levels. Compression ratio is the key features which affect thermal efficiency of engine, the formula for air standard cycle efficiency is
η=1− (1/r) k−1
Higher compression ratio results in higher thermal efficiency and improved fuel economy in the internal combustion engine. Generally, the operating conditions vary widely, such as stop and go city traffic, highway motoring at constant speed, or high speed freeway driving. In a conventional SI, the maximum compression ratio is set by the conditions in the cylinder at high load, when the fuel when the fuel and air consumption are at maximum levels. If the compression ratio is higher than the designed limit, the fuel will pre-ignite causing knocking, which could damage the engine. Unfortunately, most of the time SI engines in city driving conditions operate at relatively low power levels under slow accelerations, low speeds, or light loads, which lead to low thermal efficiency and hence higher fuel consumption. As the engine load decreases, the temperature in the end on thermal efficiency is compression ratio and air–fuel mixture strength. The fuel–air cycle Efficiency gas drops, so that high compression ratio could be employed without the risk of knocking in naturally aspirated or boosted engines. Raising the compression ratio from 8 to 14 produces an efficiency gain from 50 to 65 per cent (15%), whereas going from 16 to 20 produces a gain from 67 to 70 %.
Various Methods used for VCR Concepts :
- A very new and efficient method is slidable piston head and cylinder.
- Variation Of Combustion chamber volume.
- Variation of piston deck height.
- Modification Of Connecting rod Geometry
- Moving the crankpin within the crankshaft.
- Moving the crankshaft axis.
- Traverse type Mechanism.
MOVING THE CRANKSHAFT AXIS
In this method a crankshaft bearings are carried in an eccentrically mounted carrier that can rotate to raise or lower the top dead centre (TDC) positions of the pistons in the cylinders. The compression ratio is adjustable by varying the rotation of the eccentric carrier.
- High efficiency for lower levels of engine power
- Favourable burn rate which permits use of lean burn technology
- High compression ratio at low loads gives good starting and idling performance
- VCR engine is compact and has good multifuel capability
- Higher power to weight ratio
- Lower thermal and structural loads
- Produces very less low frequency noise
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