3.2 Connecting Rod
The connecting rod is one of the important components that have a significant effect on the achievement of development targets for the engines. While having a reduced weight the connecting rod must withstand an increase in combustion force and in inertia force following a significant increase in the number of revolutions. The following matters were estimated, based on finite Element Method (FEM) analysis and an element test when the number of revolutions increased using a conventional connecting rod:
(1) The surface pressure distribution in the diagonal serration would contain defects and an opening phenomenon would occur in parts.
(2) Excessive fluctuating stress on the internal thread for the connecting rod bolt would occur.
Accordingly, we took the following actions
(1) We created a roll-type, internal thread for the connecting rod bolt. By adding the residual compressive stress, we dealt with the increase in the fluctuating stress. Furthermore, we significantly reduced the machining roughness of the bottom holes and the tip.
(2) We used special alloy steel including nickel for the connecting rod to reduce the deviation in strength resulting from such factors as differences in times of heat-treatment. With consideration to notch sensitivity, fatigue strength was boosted while avoiding excessive hardening and shoot-peening after machined connecting rod surface.
(3) From the result of FEM analysis, we change the connecting rod big end design from slant cut to horizontal cut in order to increase the stiffness of connecting rod big end and decrease the stress of tightening bolts and the deformation of connecting rod big end. Table 2 shows the deformation of connecting rod big end by FEM analysis. ΔV and ΔH shows the direction of the deformation vertical and horizontal. It shows the ratio of the deformation of horizontal cut when the deformation of slant cut equals 1.0.
Furthermore, the connecting rod surface is machined and implemented shot peening to boost fatigue strength and created more light weight parts while maintaining the conventional safety factor.
3.3 Piston
Also, the piston is one of important components that is significant in creating a more lightweight engine with higher number of revolutions and higher output. Usually aluminum pistons are used for small size, high-speed engines.
V16FX used one piece piston of nodular cast iron (NCI) in order to cope with mechanical and thermal stress following an increase in the mean effective pressure (Pme), as well as to cope with high firing pressure (Pmax). V16FX has many experiences with good result of NCI piston. And we have an experience of one piece NCI piston for maximum firing pressure 20.0 MPa, which has been confirmed service experience.
However, V20FX used composite piston constituted steel crown to cope harder mechanical and thermal stress by increase in Pmax and Pme than 16FX. Among of all composite pistons, we used articulated piston, which constituted steel crown and aluminum skirt. To compare with normal composite piston, we could design a short size piston height and a more lightweight. It has the oil gallery structured by steel crown with bore cooling and skirt to reduce the temperature of piston by higher Pme.
Normally, in the small size engine, the positioning of piston and connecting rod has been decided connecting rod big end (crank pin) side. Also articulated piston has been decided connecting rod big end side because piston crown and piston skirt move each other in articulated piston. However, we tried to change and made possible the positioning from connecting rod big end side to piston side to make the guidance on connecting rod small end and piston skirt. Fig.5 shows above system. This system has already applied for a patent.
3.4 Cylinder Head
The cylinder head is made of high strength, highly conductive vermicular cast iron that can withstand mechanical and thermal stress resulting from high combustion pressure.
An intake port is a simplified structure and improved rigidity and flow coefficient more than Vl6FX. And in Vl6FX, an intake manifold incorporated into the cylinder head. But in V20FX, it was separated from cylinder head in order to be lightweight by it made from aluminum alloys.
3.5 Fuel Injection Pump
Generally, small size and/or high-speed engine used an in-line injection pump. However, "FX series" engines used a single cylinder injection pump in order to withstand high injection pressure and higher output
We also used a two-way delivery valve for the retraction valve in order to prevent secondary injection and cavitation. We treated the plunger with a titanium-nitrogen process to prevent sticking from high pressure and speed. We show a cross section of the fuel injection pump to Fig.6 and its main specification to Table 3.
