Tuesday, March 3, 2020

Finalizing Engine Geometry

When starting with a clean sheet engine design it can be easy to get lost in the permutations of the various main parameters.  Since I am not going fully clean sheet and instead using a complete cylinder head from an existing engine, some of the important parameters can be reused from the donor engine's values:  in this case a bore of 116mm and a stroke of 60.8mm.  Just a bit oversquare.  This leaves a couple of other major parameters open for interpretation: conrod length and, less common, cylinder offset.

How connecting rod length affects engine dynamics is pretty straightforward:  the longer it is, the less secondary vibration you have at the expense of a taller overall engine. With my linkage front suspension, the chassis area near where a traditional headstock would be is pretty open, so a taller engine is not a packaging problem.

Using a non-zero cylinder offset, a Désaxé engine, has a more complex affect on an engine's dynamics as introduces a significant asymmetry into the piston motion, which can have far-reaching consequences.

This image shows how cylinder offset can affect piston lateral forces which generate significant internal friction.  My design choices of a long rod and cylinder offset should provide a low friction, smooth running engine.

Setting up a dynamic simulation, validating it, and sifting through the results would be a very time consuming proposition, so I did the next best thing, turned to a subject mater expert.  Actually, since I got a good answer with minimal effort, this was the best choice!

The subject matter expert in this case is Tony Foale, most likely one of the top 10 experts on motorcycle dynamics worldwide.  Unlike the other 9 experts, Tony is not sequestered in someone's factory, he is open with his knowledge and willing to answer any questions.  His website, https://motochassis.com/, is a great source of information on characterizing motorcycle dynamic behavior.  His book, Motorcycle Handling and Chassis Design, (check out page 9-23) is required reading for anyone wanting to understand or improve motorcycle operation and performance.  His thorough analysis of the motorcycle turn-in process is second to none.  He also has several software packages available for overall chassis and suspension setup that are in use by race teams worldwide.

A quick email to Tony explained my project, and as always he was willing to help.  He 'just happened to have' some software already written to do force and vibration analysis of singles.  Well, some people crochet for a hobby.

A few simulations and back-and-forths and the results were 'an offset of close to 10 mm will reduce piston frictional losses by a useful amount over the planned rev range.'  The following graphs were provided to support this conclusion.  The absolute values of the graphs are less important than the difference between the zero offset and offset cases.

Graph showing axial piston pressure load.  As expected, not much difference between the two cases.
Graph showing axial inertia loading for the two cases.  Note how the curves are not symmetrical around TDC and BDC.  This is purely a geometric effect of the offset cylinder centerline.  It also has the benefit of increasing the intake/power cycles to about 182 degrees of crank rotation and the exhaust/compression cycles to 178 degrees.  This introduces a slight amount of Atkinson cycle efficiencies without dilution of the intake charge, which is great.

The combination of the two top graphs which shows a small reduction in the maximum axial piston loading, which is nice.

Graph showing the piston lateral loading, the main reason for this analysis.  In this case the max loading is not as critical (as long as it is not excessive) but the 'area under the curve', which indicates the total frictional loss, is.  In this case the area is noticeably reduced from the non-offset to the offset, which was the goal of the analysis.  So, as previously said 'an offset of close to 10 mm will reduce piston frictional losses by a useful amount over the planned rev range.'  Excellent!

To get more detailed information would have required an order of magnitude more effort in simulating the piston motion and secondary effects and still be subject to experimental verification.  As it is, this information was extremely helpful and allows me to move on to detail design of the engine support components. Thanks, Tony!

Sunday, March 1, 2020

A Good Season Opener at Roebling Road!

Its been a little while since the last post but I have not been idle.   A little client work and some volunteer machining for the Cooper Union FASE team has kept me a little busy.  I did manage to get a race weekend in and am getting back on Hypermono design ASAP.

After the past couple of events with the bike not running, I was a little reluctant to pack up and ship out to Roebling Road, near Savannah, Georgia, for an early season race weekend.  The bike has been troublesome lately.......  Since Peter's Dynojet dyno was back in working condition, a little dyno work on the injection maps was little more than wheeling the bike 20 feet to the dyno room.  For the first few runs it was running bretty badly.  The AF sensor was out of range and the bike was not cooperating.  A little imagination, some internet searching, and some excel cut and paste got me to a decently running map.

The bike still breaks up here and there but in general it was a big improvement on the previous map with partial mapping not being a light switch anymore.

The long ride from NYC to Savannah was pretty uneventful with Jamie taking driving duties while Kerry and I swapped out co-piloting and pointing out Cracker Barrel locations.

Thursday morning we were greeted by a cold rainy day.  I usually don't mind wet riding but when temps are in the 40s I have to draw the line.  We all rescheduled our practice day to Friday, I did a little fiberglass patchwork to repair transport damage, and they we hit the road back to our AirBnB.  Had a good diner followed by some drinks at the Pinky Masters with some of Kerry's friends, who made another appearance at the end of the weekend.

Friday was also cold, but no rain so we waited out the cold and did a few afternoon sessions to shake the cobwebs out.  Unfortunately, I had lots of cobwebs and would spend the rest of the weekend trying to get out from under them.  Jamie's mother arrived midway through the day and was a welcome paddock guest.

Saturday greeted us with a layer of frost on everything in the pits.
Morning practice was again written off as we huddled in the sprinter with the heat on.  Lunch was provided by Jamie's mom, which was greatly appreciated.  The afternoon warmed up a little and as I lined up on the race was ruing my lack of practice.  The race was uneventful but as the laps wound down felt better and better on the bike.  Roebling is a weird track, there is not much braking, and none of it hard.  I was coming to the end of the main straight at about 125mph or so and hitting the brakes, then realizing that I didn't have to.  Lap after lap this went on as I struggled to get into a decent groove.  All too soon the race was over and I was left wanting more.

Sunday was pretty much a carbon copy of Saturday.  Cold, cold morning and slightly better afternoon.  Did some more laps feeling slow, then a couple that were maybe not slow, then the checkered flag flew and the weekend was done.

Overall, I was satisfied to finally get the bike on the track for some trouble-free running.  If not for the cold weather, I would have had a lot more track time and had a better chance of getting back into the racing groove.  As it is, I was satisfied to have done both races with no bike problems.

I was so happy with how the season opened that as we met up with Kerry's friends at Riverside Tattoo Parlor I decided a tattoo was in order.  There was no peer pressure involved.  Lauren did a great job slightly adapting my logo to a tattoo and inking it on my skin forever.

Now to think of what tattoo #2 will be.

Will have some more posts on the Hypermono coming soon, finalizing some main engine parameters and ordering long lead-time parts.