Hello this is my first post on the forum, i have very little knowledge in physics and high level mathematics, but for the past few years ive been dabbling with excel on a way to compare and give rough estimates of many facets of engines. At the moment what im working on is determining the forces on a rod per degree of rotation(as if something were spinning the crank and neglecting compressive forces in the chamber). I have the complete geometry worked out for the crank, rod, and piston at the moment, and this seems to be the next step. I tried to post my sheet but it was too large. Please let me know what values would be necessary to do this and ill attempt to get them from the spreadsheet. If possible i would like to be able to account for different rod materials like steel and aluminum(different alloys if possible) Im not familure with most symbols so any formula will need a pretty thorough explanation. Thanks ahead of time
So you are finding the acceleration of the piston over tthe two cycles. I'm not sure how you can post your spreadsheet on here. You can upload it to google docs and share the link. Similar thread. https://www.physicsforums.com/showthread.php?t=343029
Thanks that isnt quite what i was going for, its actually much better. At the bottom of that thread is exactly the kind of detail im looking to do, but it has no figures with the graphs. I would imagine the gas pressure force would hold approximately the same shape but change in magnitude with other engines? Would there be a way to create those graphs with excel without importing it, or is there a more versatile program that i should be using anyway? http://spreadsheets.google.com/ccc?key=0AlR-WyVQnf6LdF9MS2JRMi15T2N5Y1FyaHpCMHcwRHc&hl=en seems that the lookup function i relied on so heavily isnt supported in google docs
What level of detail are you looking for in the simulation? Also what exactly are you trying to find the forces for? My sim will only find the forces due to the piston moving, what I did to simplify the model (and to be conservative) was I lumped the top half of the conrod mass and piston together (so basically said the piston weights more but the conrod doesnt weight anything). All of what I did was done using spreadsheets. I'm acutally looking to improve the model by using matlab and treating the conrod and piston seperately. This is hard though as the centriod of the rod doesnt stay on a linear path. The GPF changes with RPM, the problem with the one I did was I had to assume that was constant over the whole operating range as I didnt have enough data to calulate more than 1 GPF cycle. This also makes an approximation to pumping losses. It will also change with engine but the same basic shape is kept. It doesnt matter if googledocs screws it up, all the data will be there i'll just look at it offline.
I would like to be as accurate as possible within reason (does that actually help?) Im trying to find any forces that might limit the capabilities of the parts or the engine as a whole. Mostly im hoping i can get reasonable estimates of limiting factors like maximum RPM, and cylinder pressures, or flow, or to find what would be necessary to reach certain torque, power, rpm, or flow. Basically something where i can know what to expect from different engines or changing parts or capibilities. Eventually i would like to be able to estimate cylinder pressures, volumetric efficiency, and fiddle with cam timing, but at the moment that looks to be far into the future. Wouldnt the centriod(quick wiki search) stay in one fixed location between the ends of the rod? I would assume there would have to be a way to find its position given the two ends once its relative position is known