[Mgs] Engine maths...and spare time

Richard Lindsay richardolindsay at gmail.com
Sat Mar 28 13:32:11 MDT 2020


...from Campbell's book.

On Sat, Mar 28, 2020, 2:07 PM Barney Gaylord <barneymg at mgaguru.com> wrote:

> 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
>
>
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