10-04-2017, 08:21 PM
Presented By:
R.J. Stafford, J.M. Mulloy, T.M. Yonushonis, H.G. Weber, and M.J. Patel
Cummins Engine Company
INTRODUCTION
Turbocharging of diesel engines has enhanced fuel economy and reduced diesel engine emissions. The initial applications of turbochargers to heavy duty diesel engines during the early 1970's reduced Bosch smoke (a measure of particulate matter used at the time) from 2.4 to 0.6 units. At the same time, engine specific fuel consumption was reduced from 0.36 Ib./bhp-hr to 0.35 ib./bhp-hr. Since that time, a variety of improvements including further advances in turbocharger tech nology have reduced particulates to the level that they are below 0.1 Bosch units (the bottom of the scale) while specific fuel consumption is at the 0.32 Ib./bhp-hr level. At optimum conditions, modern diesel engines can achieve in excess of 42% thermal efficiency which is competitive with stationary power plant efficiencies. One hundred percent (100%) of Cummins heavy duty diesel and medium duty diesel engines are turbocharged. Current turbochargers are optimized at one set of engine conditions and by necessity, at the off-de sign conditions or transient conditions the fuel economy and emissions performance are pena lized.
Cummins and Holset, a Cummins subsidiary, have been working together on development of an advanced variable geometry turbocharger system for heavy duty diesel engines to improve off-design conditions. Variable geometry turbocharging pro vides an additional control of the turbocharger and assists in low speed response by modifying the inlet area to the turbine rotor. Cummins has also investigated modified radial rotors to improve off-design turbocharger performance. We believe that these new rotor designs coupled with variable geometry control systems can improve turbo-charger efficiency by up to 15% at off-design points, which would result in particulate reductions of up to 40% and engine fuel economy improve ments of 2 to 4 %. These improvements repre sent major advances for state-of-the-art die sel engines.
The current variable geometry turbines uti lize existing turbine rotor or impeller designs. To be successful, the variable geometry turbine must provide acceptable efficiency over a 2:1 or 3:1 range in turbine flow at a given pressure ratio. The typical turbine has great difficulty in doing this, because the current impeller designs cannot efficiently accommodate such a wide range of flow conditions. Figure 1 shows the Dry Partic ulate versus Response curves for standard Fixed and Variable Geometry turbine wheels.
A rotor was designed and a prototype fabri cated which showed as much as a 10% efficiency improvement at off-design con ditions (1, 2). The leading edges are blunt and rounded to accept the flow from the turbine nozzles at a variety of inlet con ditions with a minimum of losses. Pho-tographs of this prototype are shown in Figure 2. The rotor efficiency is better at all conditions and the advantage improves as it operates at conditions further from the design point. Unfortunately, the conven tional materials from which this turbine rotor was constructed had inadequate strength to allow its use on engines, and had such high rotational inertia that transient response would have been severely compromised.
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http://fischer-tropschDOE/_conf_proc/DEER/970799/conf_970799_pg163.pdf