saviour stivala wrote: ↑
Wed Nov 21, 2018 10:54 am
gruntguru wrote: ↑
Tue Nov 20, 2018 11:19 pm
saviour stivala wrote: ↑
Tue Nov 20, 2018 5:23 pm
In my opinion rapid or slow combustion does not change the need for ATDC optimum (PCP - PPP) point (greatest possible burn pressure force on crankshaft angle and for maximum possible power stroke duration). what will need change will be the ignition point (ignition point advance). when the engine achieves (PCP - PPP) at 14 degrees ATDC then maximum brake torque (MBT) is produced, shift the (PCP -PPP) position and less (MBT) is produced. the faster the engine is rotating, the shorter the time for crankshaft angle to reach (PCPN - PPP).
You might have missed the point. 14* is not a magic number. Different engine designs will result in a different value for the optimum value for PCP.
With the rapid combustion obtained with TJI it is likely that PCP for the F1 engines occurs at less than 14*.
I did not miss your point and I did not call the 14 degree ATDC a magic number. Agree that different engine designs could result in a different value for the optimum PCP/PPP + MBT, but any number above that will come at the cost of a shorter power stroke duration. But I do not agree that a rapid or slower burn will move or effect the moving of the 14 degree ATDC optimum point. The engine designs items that can move that optimum point is as Godlameroso has explained (con-rod center to center length to crank radius ratio). The 14 degree ATDC number seems to me to be the number vastly agreed upon by most people that matter, some people also that matter, sometimes adds (+) 1 to 2 degrees to the number 14 for some con-rod length/crank radius ratios but admits that will result in a shorter power stroke duration. Also I never read anybody deducting (-) any degrees from the number 14.
The number 14 means nothing. Leaving aside detonation and pressure rise rate (which must be limited for mechanical reasons - typically to less than 10 bar/degree), ignition timing will be set at the value which produces best torque at the given operating point (MBT). This will result in the pressure peak occurring at some point ATDC.
Lets take an extreme case. An engine has extremely rapid combustion (again at some particular operating speed and load). The entire burn takes only 10* of crankshaft rotation. If the pressure peak is to occur at 14* ATDC, the spark timing (ignoring ignition delay) will need to be set at about 9* ATDC. This is ridiculous because the optimum spark timing in this case would be very close to TDC and the pressure peak would be at about 5* ATDC.
"Agreed apon by most people that matter"
Well now you must be talking about "performance engine tuners" and such rules of thumb (magic numbers) only apply to the engines that we are all familiar with and constitute 99% of the work that this group do.
The first image below gives some idea of the different burn rates for conventional ignition vs TJI. All these data points are for optimised timing (MBT). If you look at the right-hand red dots you can see at lambda 1.4, the conventional system fires the spark at 40* BTDC, 10% of the fuel mass has burned by 5* BTDC, 50% by 5* ATDC and 90% by 25* ATDC. Peak pressure would occur somewhere between the last two numbers (5* - 25* ATDC) so perhaps close to 14* ATDC. At the same AFR the light green TJI angles are 20* BTDC, 1* BTDC, 7* ATDC and 18* ATDC. Peak pressure at perhaps 12* ATDC.
Notice the 50% burn angle (a number often quoted by researchers and called "CA50") ends up being very similar (about 7* ATDC) at different AFRs, ignition systems and burn rates. Likewise CA50 stays about the same for different engine speeds (bottom image). CA50 is clearly a preferred metric for "people who really know" and is obviously one that is more reliable than PCP as a predictor of MBT.
@SS and @godlameroso.
Any talk of "cylinder pressure occurring at optimum crank angle/geometry/mechanical advantage" is rubbish. Work done during the power stroke is the integral of PdV. It doesn't matter what the mechanism below the piston does. If a more favourable mechanical advantage is able to extract more work early in the stroke, there will be less work extracted later in the stroke - period!
CITATION: Attard, W. and Blaxill, H., "A Gasoline Fueled Pre-Chamber Jet Ignition Combustion System at Unthrottled
Conditions," SAE Int. J. Engines 5(2):2012, doi:10.4271/2012-01-0386.