General Motors has announced an upgrade to the 6.6L V8 Duramax diesel with 12% more horsepower and 19% more torque.  The cylinder block was announced as grey iron with induction hardening. Can you comment on the engine and the choice of cylinder block material?

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At present, there are only two V-diesel engines in the market that are not based on CGI cylinder blocks: the Mercedes 3.0L V6 (aluminium) and the General Motors 6.6L Duramax V8 (grey cast iron).  CGI has effectively become the standard material for V-diesel engine cylinder blocks.  SinterCast has consistently said that it is possible to produce a cylinder block in any material: grey iron, CGI or aluminium – the ultimate difference is in the size and the weight of the engine.  If the material is weaker, the walls of the cylinder block must be thicker to ensure durability.  This impacts size, weight, packaging and crash impact.  We believe that CGI provides the best overall solution for V-diesel engines.

Engine design is a complex science with many different factors determining performance and durability.  One of the specific features of the Duramax V8 is that the stroke is relatively short.  The stroke of the Duramax V8 is 3.95 inches (100 mm) while the stroke of the Ford Power Stroke V8 is 4.25 inches (108 mm).  The shorter stroke in the Duramax allows performance to be achieved through rpm and places less emphasis on increasing the combustion pressure.  As the combustion pressure is decreased, the mechanical load on the materials is also decreased.  The shorter stroke in the Duramax engine is therefore an influencing factor in the use of grey iron for the Duramax cylinder block.

The use of induction hardening is a common technology in metallurgy that can be applied to all types of cast irons and steels.  Induction hardening heats the surface of the iron for a few seconds and then allows the surface to cool.  This rapid heating and cooling increases the hardness on the surface of the material, to a depth of approximately 0.2 mm.  Induction hardening can be included as an extra step in the manufacturing process to improve wear resistance – it does not provide any strength benefits to the material or to the component.  Induction hardening is commonly used in the valve seats of grey iron cylinder heads to minimise valve seat wear, and for many other wear components.  In the absence of detailed information, it can be assumed that the induction hardening in the Duramax engine was applied in the ring reversal area at the top of the cylinder bores (a span of approximately 25 mm, approximately 25 mm below the top surface of the cylinder bores).  CGI is harder than grey iron and also has superior wear resistance (this also explains why the machining of CGI is more difficult than grey iron).  None of the current production CGI cylinder blocks require induction hardening.

SinterCast has a good rapport with the Duramax design team.  We wish them success with their new upgrade and welcome the opportunity to work together with them at some stage in the future.