Setting The Valve Timing (Aug 2016)
The book states that both the Fwd and Rev eccentrics need to be set (90+) 7.5 degs in advance of the crank position, I’m not planning to go into depth of why this is, as this is discussed in detail on the web, however the principle reason is that the advanced valve timing is required to stop steam flow to the cylinder a little way before the Top Dead Centre (TDC) to utilise the expansive nature of the steam to continue the piston to the end of the stroke and to open the steam flow to the other end of the piston before TDP to cushion its travel and make the change in direction more mechanically sympatric on the linkages.
Normally there is also a consideration for the difference in the central piston position with the effect of crank angle from the axis of the motion. I.e. the when the crank is at 90 degs the effective length from the crank pin to the piston is reduced as only the inline component of the conrod length can be included. Thus, the crank has to be just past 90degs (towards the cylinder) for the piston to be central, (in-fact the crank pin needs to be on the loci from the little end for the piston to be central), however the effect is that the ‘in’ phase is different to the ‘out’ phase as the effective number of crank degrees before top dead centre is altered as the central piston position is displaced from 90degs as a function of the crank throw. The actual eccentric setting would therefore needs to be different Fwd and Rev to account for this.
In addition, there are many thermofluid effects which would need to be taken into account, such as port positions, sizes, prevention of wiredrawing, difference between running on steam or compressed air etc etc which would all effect the perfect eccentric position. The issue with all this is that without creating a detailed mathematical model of the as built motion it would be very difficult to calculate the optimum position with any more confidence than setting the eccentrics to the suggested starting position! Thus, that is what will be done! Followed by testing on air and then steam.
The issue for the builder at this stage is then how to effectively set the eccentrics to the suggested correct position. The book suggests setting the crank to horizontal and aligning the eccentric using a small steel template cut at 82.5 degs. I can’t confess to using this method however, in theory it sounds fraught with errors, especially as the actual TDP position is not marked on each eccentric. Thus, a better way was devised and my newly made CNC 4th axis tail stock (LINK) put to use.
Having the experience of setting motorcycle cam timings using a degree wheel and dial gauge (for use with adjustable cam sprockets on tuned engines), the idea came to me to use the 4th Axis on the mill to rotate the crank shaft to the required angular position and then use a dial gauge to rotate the eccentrics to suit. The process followed was a series of similar steps based on the procedure for identifying the upper most position of cylindrical object under a dial gauge which, like most things on this build isn’t as straight forward as one would imagine.
The concept is that very near the top of a cylinder the surface can be approximated to a flat surface as the vertical distance from the centre line to the surface varies only a tiny amount with respect to the angle it is measured from, thus when using a dial gauge at the very top, moving the gauge in and out (perpendicular to the cylinder) will result in measurements less than can be accurately detected using a normal workshop dial gauge and thus preventing the operator from accurately finding the centre. What can be measured however is the vertical distance either side of the true centre point, where due to the nature of the curve the vertical distance increases as the angle it is measured from increases. Therefore a split the different process is employed.
To do this in practice, the dial gauge base was set on the Z axis of the milling machine, so it could be moved up and down and the tip set to rest on what appeared (by eye) to be the top of the cylinder , which we call the estimated centre. Then the cylinder was moved fwd a known distance and the dial gauge read, then the cylinder was moved back through the estimated centre to the same distance backwards and again the dial read. The side with the lowest reading indicates that the estimated centre is offset from the true centre in that direction. The operator then corrects the position of the estimated centre (either by doing the trigonometry on the figures or by trial and error) and two readings are again taken. If the true centre is found (to an acceptable tolerance) then the measured vertical distance will be exactly the same fwd and back from the centre.
Before starting to set the eccentric timing an angular reference point is needed, thus the angle of zero degrees is when the crank is horizontal and away from the operator (i.e. towards the cylinder block when mounted on the engine), and the angle read clockwise when looking from left to right (i.e. through the gears to the cranked portion.
Setting the centre
Once the crank was set in the 4th Axis on the mill and supported by the tail stock it was corrected for general alignment and the centre position (Y axis in relation to the tip of the dial gauge) was found using the procedure described above. To caution, once the centre was found the operator needs to take great care not to knock the gauge else this step will need to be repeated. Plus once identified the Y axis was locked and not released until the full adjustment was completed.
Setting the crank angle
The crank was rotated to an estimate 90 degs,(i.e. pointing upwards), and then procedure above used to set the angle to a true 90 degs, i.e. when the true TDP centre is directly below the tip of the dial gauge.
Setting the fwd eccentric
The eccentric needed to be set at 90+7.5 degs in advance of the crank, thus using our coordinate system, this is 97.5 degs clockwise. To visualise this, picture the engine moving forward, as it is a 4 shaft engine the crank will be rotating clockwise (wheels anticlockwise), thus the valve event needs to occur in front of the piston event which puts the eccentric in front of the crank in a clockwise manner. To use the splitting the difference technique the crank need to be rotated to where the timing position of 97.5 deg is vertically upwards (i.e. the position of the eccentric) which requires the crank angle to set to -7.5deg. The dial gauge was then raised (controlled on the Z axis) and moved (controlled by the X Axis) to rest on the outer most surface of the eccentric. (To note the out most surface was used as that was the only continuous surface, if the centre of the face was used then the tip of the dial gauge would spring into the eccentric locking screw hole every time it passed by!) At this point the angular reference of the 4th axis was temporarily set to 0 (using G Code G92 A0) and the eccentric locking screw set so that the eccentric would just rotate on the crank using a firm press with the fingers. Once this all done the set up was ready to move the eccentric into position!
The eccentric was then set by eye to what appeared to give TDC under the tip of the gauge and the splitting method used. In practice I used +-20degs about the estimated centre and a manual trial and error method for moving the eccentric a fraction in the required direction (normally in the right direction, although this was a bit hit and miss) until the two reading were identical. I managed to get the dial gauge to read 0.0001” each side, which I considered to be fine.
Setting the rev eccentric
Same as before however at angle of -97.5degs (anticlockwise from the crank), which requires the crank angle to be set at 187.5degs.
Setting the water pump eccentric
The book nor the drawings really detailed or discussed this setting, thus I set it 90 degrees in advance of the crank, which requires the crank angle to be set at 0degs. I’ll be happy receive comments if anyone knows better!
Final check
To ensure the measurements were accurate (and that nothing had moved), the final operation was to return the tip of the dial gauge to the crank main bearing location and check that the dial gauge read the centre in the right place – it did!
Normally there is also a consideration for the difference in the central piston position with the effect of crank angle from the axis of the motion. I.e. the when the crank is at 90 degs the effective length from the crank pin to the piston is reduced as only the inline component of the conrod length can be included. Thus, the crank has to be just past 90degs (towards the cylinder) for the piston to be central, (in-fact the crank pin needs to be on the loci from the little end for the piston to be central), however the effect is that the ‘in’ phase is different to the ‘out’ phase as the effective number of crank degrees before top dead centre is altered as the central piston position is displaced from 90degs as a function of the crank throw. The actual eccentric setting would therefore needs to be different Fwd and Rev to account for this.
In addition, there are many thermofluid effects which would need to be taken into account, such as port positions, sizes, prevention of wiredrawing, difference between running on steam or compressed air etc etc which would all effect the perfect eccentric position. The issue with all this is that without creating a detailed mathematical model of the as built motion it would be very difficult to calculate the optimum position with any more confidence than setting the eccentrics to the suggested starting position! Thus, that is what will be done! Followed by testing on air and then steam.
The issue for the builder at this stage is then how to effectively set the eccentrics to the suggested correct position. The book suggests setting the crank to horizontal and aligning the eccentric using a small steel template cut at 82.5 degs. I can’t confess to using this method however, in theory it sounds fraught with errors, especially as the actual TDP position is not marked on each eccentric. Thus, a better way was devised and my newly made CNC 4th axis tail stock (LINK) put to use.
Having the experience of setting motorcycle cam timings using a degree wheel and dial gauge (for use with adjustable cam sprockets on tuned engines), the idea came to me to use the 4th Axis on the mill to rotate the crank shaft to the required angular position and then use a dial gauge to rotate the eccentrics to suit. The process followed was a series of similar steps based on the procedure for identifying the upper most position of cylindrical object under a dial gauge which, like most things on this build isn’t as straight forward as one would imagine.
The concept is that very near the top of a cylinder the surface can be approximated to a flat surface as the vertical distance from the centre line to the surface varies only a tiny amount with respect to the angle it is measured from, thus when using a dial gauge at the very top, moving the gauge in and out (perpendicular to the cylinder) will result in measurements less than can be accurately detected using a normal workshop dial gauge and thus preventing the operator from accurately finding the centre. What can be measured however is the vertical distance either side of the true centre point, where due to the nature of the curve the vertical distance increases as the angle it is measured from increases. Therefore a split the different process is employed.
To do this in practice, the dial gauge base was set on the Z axis of the milling machine, so it could be moved up and down and the tip set to rest on what appeared (by eye) to be the top of the cylinder , which we call the estimated centre. Then the cylinder was moved fwd a known distance and the dial gauge read, then the cylinder was moved back through the estimated centre to the same distance backwards and again the dial read. The side with the lowest reading indicates that the estimated centre is offset from the true centre in that direction. The operator then corrects the position of the estimated centre (either by doing the trigonometry on the figures or by trial and error) and two readings are again taken. If the true centre is found (to an acceptable tolerance) then the measured vertical distance will be exactly the same fwd and back from the centre.
Before starting to set the eccentric timing an angular reference point is needed, thus the angle of zero degrees is when the crank is horizontal and away from the operator (i.e. towards the cylinder block when mounted on the engine), and the angle read clockwise when looking from left to right (i.e. through the gears to the cranked portion.
Setting the centre
Once the crank was set in the 4th Axis on the mill and supported by the tail stock it was corrected for general alignment and the centre position (Y axis in relation to the tip of the dial gauge) was found using the procedure described above. To caution, once the centre was found the operator needs to take great care not to knock the gauge else this step will need to be repeated. Plus once identified the Y axis was locked and not released until the full adjustment was completed.
Setting the crank angle
The crank was rotated to an estimate 90 degs,(i.e. pointing upwards), and then procedure above used to set the angle to a true 90 degs, i.e. when the true TDP centre is directly below the tip of the dial gauge.
Setting the fwd eccentric
The eccentric needed to be set at 90+7.5 degs in advance of the crank, thus using our coordinate system, this is 97.5 degs clockwise. To visualise this, picture the engine moving forward, as it is a 4 shaft engine the crank will be rotating clockwise (wheels anticlockwise), thus the valve event needs to occur in front of the piston event which puts the eccentric in front of the crank in a clockwise manner. To use the splitting the difference technique the crank need to be rotated to where the timing position of 97.5 deg is vertically upwards (i.e. the position of the eccentric) which requires the crank angle to set to -7.5deg. The dial gauge was then raised (controlled on the Z axis) and moved (controlled by the X Axis) to rest on the outer most surface of the eccentric. (To note the out most surface was used as that was the only continuous surface, if the centre of the face was used then the tip of the dial gauge would spring into the eccentric locking screw hole every time it passed by!) At this point the angular reference of the 4th axis was temporarily set to 0 (using G Code G92 A0) and the eccentric locking screw set so that the eccentric would just rotate on the crank using a firm press with the fingers. Once this all done the set up was ready to move the eccentric into position!
The eccentric was then set by eye to what appeared to give TDC under the tip of the gauge and the splitting method used. In practice I used +-20degs about the estimated centre and a manual trial and error method for moving the eccentric a fraction in the required direction (normally in the right direction, although this was a bit hit and miss) until the two reading were identical. I managed to get the dial gauge to read 0.0001” each side, which I considered to be fine.
Setting the rev eccentric
Same as before however at angle of -97.5degs (anticlockwise from the crank), which requires the crank angle to be set at 187.5degs.
Setting the water pump eccentric
The book nor the drawings really detailed or discussed this setting, thus I set it 90 degrees in advance of the crank, which requires the crank angle to be set at 0degs. I’ll be happy receive comments if anyone knows better!
Final check
To ensure the measurements were accurate (and that nothing had moved), the final operation was to return the tip of the dial gauge to the crank main bearing location and check that the dial gauge read the centre in the right place – it did!