[Mgs] Engine maths...and spare time

[Barney Gaylord]

Rick, ---- Okay, time to spare, so I'll bite.

You do a good job of calculating time from spark 
event to half stroke (about 6-ms at road speed), 
but I think you were asking a different 
question.  I thought you were asking how much 
time to complete combustion to get to maximum 
pressure.  That is, how much time for the flame 
front to propagate all the way across the combustion chamber?

And you also said. "we do know that maximum work 
occurs when the piston is half way down the 
cylinder", which is not true.  Most of the work 
has already been done before the piston gets half 
way down, and maximum torque on the crankshaft 
happens significantly higher in the stroke before 
pressure is lost to expansion.

For best power and efficiency, combustion should 
be completed at or slightly after TDC.  But since 
there is very little motion of the piston 
immediately after TDC, It works just about as 
well if max pressure comes just a little later, 
like maybe 10 to 20d ATDC.  That little delay can 
allow use of higher compression ratio for better power and efficiency.

I like to use 3600 rpm for road speed, because it 
divides evenly into 360 degrees rotation for nice 
round numbers.  And 900 rpm idle speed will be 
exactly 1/4 of road speed.  If you make spark at 
32d BTDC at road speed, it takes 1.5-ms to reach 
TDC.  10d ATDC is at 2-ms, and 30d ATDC would be 
3-ms (after spark).  So the flame front 
propagation to complete combustion is in the 2 to 
3-ms range.  I suppose this is the answer to your 
question, "how much time" for the flame front to cross the combustion chamber.

Distance from the spark plug to far side of the 
combustion chamber is about 2-1/2 inches, which 
it does in about 2-1/2 ms, so flame front speed 
is about 1 inch per ms, or 1000 inches per 
second, which is fairly close to 60-mph.  And you 
night notice that I did not use "MEP" in that entire discussion.

Barney


At 08:54 AM 3/28/2020, you wrote:
>Hello friends,
>
>When one is a geek, one thinks of geeky things. 
>I am a geek and this house-bound morning I woke 
>up thinking about ignition timing. Here are the details.
>
>We know that the charge (fuel plus air) in a 
>cylinder doesn't burn instantly, despite our 
>perception to the contrary. Rather, it takes a 
>finite length of time from the occurance of the 
>'spark', the flame front to cross the combustion 
>chamber, and to raise the MEP (Mean Effective 
>Pressure) to a maximum - the point where it does 
>the most work. But how much time?
>
>Physics problems always start by listing the 
>'known' and the property to 'find'. So in this case,
>
>KNOWN:
>Idle speed: 900rpm
>Idle timing advance: 4° BTDC
>Speed at maximum advance: 3500rpm
>Maximum timing advance: 32° BTDC
>
>FIND:
>Time from spark to MEP
>
>The first thing one might know is that the goal 
>at idle is not to produce maximum power. In 
>fact, at idle 100% of the available power is 
>used to overcome the friction and other forces 
>that exist at idle speed. Stated another way: 
>Idle speed is the fastest the engine can achieve 
>given the available charge. That fact is evident 
>(with carbureted engines) when one notices that 
>engine speed gradually increases, even for a 
>fixed throttle setting, as the engine warms and 
>friction forces decrease. But back to the problem.
>
>Because the goal at idle is smooth running and 
>progression off of idle (e.g. speeding up), not 
>maximum power, the calculated wavefront speed 
>may not be correct at idle. But let's see.
>
>At idle speed, 900rpm in this MG TD example, the 
>XPAG engine is turning 900rpm or 900rpm / 60mps 
>= 15rps (revolutions per second).
>
>Distributor speed is 1/2 engine speed so at idle 
>the distributor is turning only 7.5 revolutions 
>per second. But timing numbers are specified in 
>degrees of crank rotations so we will stick with 15rps.
>
>We don't know how fast the flame front travels 
>across the combustion chamber but we do know 
>that maximum work occurs when the piston is half 
>way down the cylinder. And we also know that 
>work isn't an instantaneous parameter so it must 
>begin before the half way point and last past 
>that point. Lots of unknowns and theory doesn't 
>always work in practice. But if we use the 
>average piston position at half-way down the 
>bore, where most work is most effective, and the 
>MEP (Mean Effective Pressure), since Mean is average, calculations begin.
>
>single revolution is 360° so half-way down the 
>power stroke is 90°. Add the idle spark timing 
>of 4° BTDC (Before Top Dead Center) and we get 
>94° of crank rotation from spark to MEP at 
>half-way down. That's 94/360 or about 0.26 of an 
>engine revolution. And the engine is turning 15 
>revolutions per second or 67ms (milliseconds) 
>per revolution. So 67 x 0.26 = 17ms from spark 
>to MEP at half-way down the power stroke, at idle.
>
>If we repeat the calculations for operating 
>engine speed and at maximum advance, we get 
>3500rpm / 60mps = 58rps (revolutions per 
>second). Maximum advance is 32° BTDC so 90° + 
>32° = 122°, spark to MEP or 122°/360° = 0.34 of a revolution
>
>58rps is 17ms/r so 17ms/r x 0.34r = 5.78ms from 
>spark to MEP at half-way down the power stroke. 
>This is a more representative number than the 
>17ms at idle. One might even divide the idle 
>elapsed time minus the optimal time across the 
>strike's midpoint. Doing so would mean at idle, 
>the pressure at idle becomes most effective 
>5.6ms before half-way and for another 5.6ms 
>after midpoint. Interesting that the idle 
>pressure application time is about the same as 
>the maximum pressure application time, or is that circular logic?
>
>Yes everything above is ripe with assumptions 
>and perhaps even apocryphal and resplendent with 
>errors, but it is only 7am after all.
>
>Anyone with extra house-bound time on their 
>hands, please check my maths and share your 
>corrections, including the logic of the whole 
>experiment...or perhaps even why geeks think these ways!
>
>Rick