The next step in this build was to finalize the design of the cylinder liners so the designs for the connecting rods and pistons could also be finalized. These are the last major components to be machined, and the interactions among them have to be carefully checked. This particular step wasn't one of my favorites because it involved sitting in front of a computer for the last week instead of making parts and progress in my shop. But it was an important step, and for anyone with a set of these castings it may save a lot of headaches and re-work later on. I've been told by Richard Maheu that there are between 50 and 100 sets of his castings out here somewhere.
The Quarter Scale's stock liners are probably faithfully scaled replicas from the full-size Merlin's top end and, of the parts remaining to be machined, these will likely be the most difficult. In addition to the features required to seal the combustion chambers and the coolant jackets around them, the stock slip-fit liners have a worrisome wall thickness of only .035". The Quarter Scale's stock bore is 1.35", and with a stroke of 1.5" the total displacement is nearly 26 cubic inches.
My concern with the Merlin's thin wall slip-in liners arises from my experience with the cast iron liners in the Howell V-4 which was my second engine build. Finishing the i.d.'s of its 1/16" liners to within two to three tenths of being truly circular for a proper fit to its cast iron rings ended up being much more difficult than I had expected. All the machining and finishing had to be done outside the engine block since the liners weren't pressed into place but were supported with o-rings. Not only were the machining steps and their order important, but work-holding was especially critical to prevent workpiece distortion during both machining and honing. Material selection was also important not only from a functional perspective but also for dimensional stability during and after machining. I machined liners from three different lots of cast iron before I finally had four acceptable parts. I'm sure that being a newbie had a lot to do with my problems, but the larger physical size of the Merlin's liners makes their thinner walls scary even with the experience I've accumulated since my Howell days. For this engine, I'm going to need to end up with at least a dozen usable parts.
In addition to wall thickness, there are other reasons to re-consider the design of the stock liners. Poor statistics are available on the Quarter Scale's ability to run without overheating, and John Ramm may be the only builder to have accomplished this. The stock cylinder design provides for less than a teaspoon of coolant around each cylinder, and the lack of space between bores prevents any enlargement of the jackets inside the cylinder blocks. The only way to increase the volume of coolant around the liners is to reduce their diameter, and this will also reduce the engine's displacement.
The liners also affect the static compression ratio. If the Quarter Scale's c.r. is calculated using the typical simplified model of a stock piston sitting at TDC in a cylindrical combustion chamber, the result is 7.9 compared with 6 for the full-scale Merlin. I did a closer calculation using measurements on my completed heads to account for volume reductions created by the valve heads and coolant tunnels that protrude into the combustion chambers. The result was a whopping 9.7.
High compression ratios create needless problems in multi-cylinder four stroke model engines intended to run as displays. High cylinder pressures aggravate marginal cooling systems, and they increase starting torque requirements. Starting torque is a particular concern in the Quarter Scale because of its unproven electrical starting system. John's last video,
http://www.homemodelenginemachinist.com/showthread.php?t=25754, shows him hand starting his engine because of starting system difficulties that arose from increased cylinder pressures after the rings and valves bedded in.
Finally, measurements made on my engine's completed head assemblies show that it is currently within .005" of becoming an interference engine if completed using the stock rods and pistons. Although not fatal, while doing engine timing adjustments the fully open valves will come very close to extending into the space in which the pistons move.
All these potential issues are a result of the interactions among the designs of the liners, rods, and pistons as well as an accumulation of the machining tolerances of many, many already machined parts. The liner design can be adjusted to compensate for all these issues, but changes to the liners will likely introduce clearance issues between them and the connecting rods. A reliable method for verifying the clearances between the rods and the stationary parts of the engine is needed before making any rod design changes. Rod clearance issues typically don't occur at TDC or BDC which are easy positions to sketch on paper. In order to continuously visualize what will be going on during an entire crankshaft revolution, a CAD assembly model was used. A partial SolidWorks assembly was created that included a pair of cylinders, liners, heads, pistons, rods, and the crankshaft. The virtual crankshaft was rotated in order to spot clearance issues throughout the entire range of complex motions of both the blade and fork rods. Simple models for the heads, cylinder blocks, and crankcase were created using actual measurements taken from my already machined castings. Since I already had a full model of the crankshaft, I had only to create additional full models for the as yet un-machined rods, pistons, and liners.
The first issue revealed by the modeling was a minimum clearance of only .005" between the stock rods and the bottom edges of the stock liners during their closest approach. This insured that any reduction in the liner i.d. would definitely require a change to the stock rod design. I've included a few cross-sectional CAD snapshots taken showing the stock rods and pistons in a few locations of interest inside the stock liners.
I began design changes on the CAD assembly model by shortening the pistons by .023" above their wrist pins in order to increase their clearances to the fully opened valves at TDC. This also had the effect of dropping the compression ratio from 9.7 to 8.6. The i.d.'s and o.d.'s of the liners were then reduced in order to increase the liners' wall thickness and the coolant volumes around them. The i.d. reduction was limited by the additional loss in compression ratio that I was willing to accept. After several iterations, I arrived at a new i.d. of 1.2" down from 1.35" and a new o.d. of 1.35" down from 1.42". This change increased the liner wall thickness from .035" to .075", and the coolant jacket volume by 80%. The new wall thickness was still a bit less than I had been hoping for, but the compression ratio had decreased to 6.2, and I wasn't willing to reduce it any further. This change resulted in the engine's displacement dropping from 27 c.i. to just over 21 c.i..
As expected, the liner i.d. reduction created an interference between the bottom edges of the liners and the stock connecting rods. Before modifying the rods, the liners were shortened by .075" which brought their bottom ends flush with the interior surfaces of the crankcase. I also added a 60 degree chamfer to the inside edge of the liners' lower ends in order to ease assembly with the ringed pistons later during final assembly. In addition to reducing the diameter of the pistons to fit the new liners, they were also shortened by .075" to match the new liners at BDC. The rods were finally modified to eliminate the interference that was created by the changes, and the minimum clearance ended up at .060" compared with .005" for the stock configuration. Removing the rod material weakened them slightly, but the mass removed from the pistons as well as the c.r. reduction most likely mitigated all the loss in strength.
Although the reductions in displacement and compression ratio may seem rather harsh, I believe they are now more in line with the expectations for a typical model engine, and hopefully they will improve its run-ability later when/if it runs. - Terry