Monday, 11 June 2012

Comparisons of cross head and trunk piston engines,

Comparisons of cross head and trunk piston engines,
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    There are two basic types of connecting a piston to a crankshaft;
        Crosshead construction-used by all slow speed two stroke engine manufactures
        Trunk piston construction- used in smaller four stroke engines

Crosshead construction
The piston is rigidly fixed to the piston rod. The rod passes through a gland to a cross head to which it is attached via a flange , or shoulder through bolt and nut. The crosshead consists of an rotating element which is attached to the connecting rod. It is through this bearing that the vertical load is transferred from the combustion space and converted into the rotary motion of the crankshaft via the connecting rod and big end bearing. Horizontal thrust generated at the crosshead are absorbed by white metal surfaced shoes which run up vertical athwartships mounted guides.

    The advantages of the crosshead design are
        guide faces take side thrust, this is easily lubricated, wears little and takes side forces off the piston and liner running surfaces.
        Uniform clearance around piston allows for better lubricating oil distribution reducing wear
        simplified piston construction designed for maximum strength and cooling. Extended load bearing skirts found on trunk pistons unnecessary
        due to gland lubricating oil may be optimised for crankcase and cylinder. High alkalinity oils used in cylinder allow poorer quality fuels to be burnt.

Trunk piston construction
The piston is directly attached to the connecting rod by a small end rotating bearing. Side thrust is absorbed by extended skirts on piston.

The main advantage is reduced engine height
Opposed piston engines
Mainly built by doxford and consisted of two opposing piston moving in a common liner. Fuel injection occurred at the centre where the piston met. Construction is of the crosshead design with the upper piston connected to the crankshaft via two side rods and transverse beam. Timing was approximately 180oC except for a small angle of advance for exhaust timing.

    Advantages are;
        Perfect primary balance by balancing
            upper reciprocating masses and lower velocity side cranks against
            lower reciprocating mass and the higher velocity centre crank
        No gas loading transvered to bed plate (normally via head and tie rods) on engine meaning that construction could be lighter

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