Control of Boost Ratio and Variable Geometry Turbocharging

For the conventional turbocharger, boost pressure ratio characteristics vary nonlinearly with the airflow rate. At the low airflow rates, the boost pressure ratio is small.  The pressure ratio increases as a power law of air flow rate. As the flow rate increases the boost pressure ratio increases disproportionately. Secondly, at low engine speeds the available exhaust gas energy is low and hence, boost pressure is also low which further compounds the problem. The turbine power therefore, is matched to provide adequate airflow to the engine at low engine speeds. At high speeds, a waste gate valve is used that makes the exhaust gas to bypass the turbine. As the engine speed increases, an increasing fraction of total exhaust gas is made to bypass the turbine directly to the atmosphere to reduce power of the turbine. These are ‘fixed geometry’ turbochargers’ and their use results in a compromise between low-speed and high-speed performance of the engine.
Variable geometry turbochargers (VGT) have been developed to overcome the limitations of the fixed geometry turbochargers. The VGT are better matched with the engine operational needs. The turbine of VGT has movable vanes that can change turbine flow area or the angle at which the exhaust gas enters or leaves the turbine rotor. VGT are also designed where both the flow area as well as angle at which gas enters or leaves the rotor can be changed simultaneously. A reduction in turbine flow area increases upstream exhaust gas pressure and it results in an increase in speed of the turbocharger and higher boost pressure.  With the variable geometry turbochargers, the closed loop control of engine boost pressure by ECU is possible.
The VGT have been introduced on the automotive engines during the late 1990’s in Europe and their use is increasing. Although, the variable geometry turbochargers were primarily developed to improve low speed torque and transient response, but their use also provides   emission reduction under low load operating regime and a better optimization throughout engine operating range is possible.

CONTROL OF ENGINE OIL CONSUMPTION

Engine lubricating oil enters combustion chamber through;

• Valve guides,
• Piston ring/liner interface and
• Turbocharger seal.

Part of the engine oil gets burnt but 10 to 40% of oil entering combustion chamber may not burn at all. The unburned oil gets adsorbed on soot particles during exhaust process and forms soluble organic fraction of the particulates. The ash produced on combustion of organometallic lubricating oil additives containing Ca, Ba, Zn, P etc., may also become important as the emission limits are lowered. Depending on engine load and speed, oil consumption may vary from 0.1 to 0.4 % of fuel consumption.  The oil thus could contribute 5 to 50% of mass of PM emissions depending upon the engine operating conditions.
Due to importance of contribution of engine oil to particulates, the oil consumption in the modern engines is being reduced through improved designs of piston, piston rings, valve guides and control of surface geometry and roughness of the cylinder liners. The engine oil consumption levels are being lowered to 0.1% of fuel consumption at rated engine power.