EngineerMiltonKeynes wrote:Many thanks for your replies. This all came about from me looking at graphs of crankshaft torque and cylinder pressure. I noticed that the crankshaft torque is a at max at 90 degrees ATDC (as you'd expect withe the longest effective lever) in a sort of inverted bell curve, but the graph of cylinder pressure , if overlaid, would give a totally different output curve, certainly higher up to 50 degrees ATDC, or so. It was the area up to 50 degrees ATDC that I was looking at. There are two fairly easy ways to build an engine which takes the torque output from the piston in a linear fashion (there are other big advantages which I won't go into here). Maybe I'll have to take a couple of little same model single cylinder engines and modify one so I can compare outputs. That's easy to do if you drive a 240v alternator from them and apply load and compare output curves.
Maybe you've considered this maybe not, however maybe you should think that piston, rod & crankshaft motion is not symmetrical throughout its movement in the 180° from TDC to BDC and then the 180° back from BDC to TDC.
For a 2.4Lt V8 F1 engine, if you assume max stroke allowable under the regs of 39.8mm (given max bore allowable is 98mm), a rod length of 110mm (for a rod/stroke ratio of 2.76:1), then the movement from TDC to 90° after TDC results in movement of the piston of 21.72mm or 54.57% of the stroke as a result of the sideways movement of the rod on the crank pin. So for the remaining 90° to BDC the piston moves only 45.43% or 18.08mm. For the piston to move 50% of the stroke or 19.8mm, the crank only rotates 84.53° and so is 5.47° short of the half stroke distance.
The proportional cosine effect of the crank angle effectively shortens the length of the rod which moves the piston the extra 4.57% over the 50% rotation angle. My engine builder calls that “dynamic shortening of the rod”. As the crank rotates the 90° back to BDC, the crankpin moves back to the centerline of the cylinder and completes the complete length of the rod. He calls that "dynamic lengthening" of the rod.
So as the piston moves away from or toward TDC or BDC, the apparent length of the rod is lengthened or shortened with regard to the motion of the piston/rod assembly by calculating the center-to-center rod length x cosine of XX° between the crankpin and the bore centerline. As such, piston movement is asymmetrical and not a simple sinusoidal motion.
This asymmetrical motion affects many engine variables.
strad wrote:You adjust the rod length to stroke ratio to achieve close to 1.5:1 . Pretty sure thats the ratio,,,been a long time Then the crank pin is at about 90º when ignition takes place and less power is lost.
Rod/stroke ratio's can have a large impact on the profile of the piston/rod velocity curve as well as where max velocity occurs during stroke event. A few thoughts and general observations on rod/stroke ratio's are below..
A longer rod engine with a larger rod:stroke ratio can change an engines profile markedly. With a higher rod ratio, the piston will have a longer dwell time around TDC due to a slower acceleration rate towards and away from TDC. Max piston velocity generally occurs closer to the middle of the stroke event. Longer rods also reduce the side loading on the piston and piston skirt which can reduce friction and wear. Longer rod ratio engines with a longer dwell time @ TDC can make better use of the combustion pressures converting this into torque on the crank pin. Lower piston acceleration aids VE at higher RPM's however can negatively affect VE at lower RPM's due to a reduced intake pulse from slower piston acceleration speeds away from TDC.
The reverse is true for engines with shorter rods (smaller rod:stroke ratios). Shorter rod ratio engines see the maximum piston velocity moving closer to TDC (both before and after TDC) resulting in a reduction in dwell time. A shorter ratio will create a stronger intake pulse as the piston accelerates away from TDC faster which can effect intake valve opening durations.
Rod;stroke ratios also affect vibration and balance characteristics of engines due to the alterations of the piston accelerations, speed and crankpin angles.
Never approach a Bull from the front, a Horse from the back, or an Idiot from any direction