Load vs Timing advance 1

[Rat5]

Hello,
While i understand that you need more advance as the rpm climbs, i struggle to understand why timing decreases as load increases ?

There is more mass to burn why would it burn faster ?

Flames do travel faster in hot environement ( more load, more mixture prensent, more end of comp temp)

What about the higher end of compression pressure ? Do flames travel faster in higher pressure environement ?







One last thing : A pic off wikipidiea, its been 2x that i see "at low throttle openings mixture is lean" while this may be true but with no extreme lean-ness, is it the true reason

Thanks

 

[BrianPetersen]

You said part of it yourself. "Flames travel faster in hot environment"

The other part of it is charge dilution with residual exhaust from the previous cycle. With high intake manifold pressure (high load on the engine), that's less significant than with low intake manifold pressure.

Residual exhaust from the previous cycle, and the resulting charge dilution, results in slower combustion and that means you need more timing advance.

 

[Lou Scannon]

Ignition timing in otto cycle engines is a large, complicated subject. We are just scratching the surface here. Knock, peak cylinder pressure, efficiency, NOx (and other pollutants), exhaust gas temperature and even cycle-to-cycle variability are all affected by ignition timing, and must be considered as the overall combustion recipe is developed. Sometimes even the ignition system itself becomes a limiting factor on where the timing can be set.
As to your question, knock, peak cylinder pressure and NOx are the factors that normally require that timing is retarded as load increases.

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

 

[turbomotor]

Ebuet, An interesting new(for spark ignition passenger cars, not new for F1) technology that effects timing requirements is the "prechamber" spark plug.

https://www.designnews.com/automoti...boasts-patented-pre-chamber-combustion-system

 

[gruntguru]

As to your question, knock, peak cylinder pressure and NOx are the factors that normally require that timing is retarded as load increases.

Yet none of those three explains why MBT reduces with increasing load.

The main factors are temperature and AFR, both of which affect the laminar flame speed of the charge.

je suis charlie

 

[RodRico]

I agree there are many factors affecting combustion speed. Here's a simple one I haven't seen mentioned in comments thus far...

Given:
A charge with a higher temperature ignites faster because it already has a lot of kinetic energy when the spark fires.
A charge with higher density burns faster because its molecules are more closely spaced.

Charge pressure increases by a fixed relation during compression, P2 = P1 * (V1/V2)^y
where P1 and V1 are initial pressure and volume, P2 and V2 are end pressure and volume, and y is the ratio of specific heats of air.

Charge Temperatue increases by a fixed relation during compression, T2 = T1 * (V1/V2)^(y-1)
where T1 and V1 are initial temperature and volume, T2 and V2 are end temperature and volume, and y is the ratio of specific heats of air.

Initial pressure, P1, varies with air mass, volume, and temperature, PV=mRT
where P is pressure, V is volume, m is air mass, R is the specific gas constant of air, and T is temperature.

Pressure (P) and temperature (T) follows mass (m) when outside air plus cylinder wall temperature (T) and cylinder volume (V) are held constant.

Conclusion:
Old school naturally aspirated Otto Cycle engines employ throttling to regulate load. This has the effect of reducing air mass in the cylinder which reduces initial presure (P1) of the charge. The lower initial pressure (P1) results in lower final pressure (P2) and temperature (T2) at the end of compression. The reduced mass at the end of compression also means lower density. Low charge temperature and reduced charge density result in slower combustion due to kinetics. Note a turbocharged engine manipulates air mass in the cylinder and changes conditions at the end of compression in a similar fashion.

One way to think of the impact of increasing mass in the cylinder at the start of compression is to note its similarity to increasing compression ratio; both increase pressure, temperature, and density of the charge at the end of compression. Per "A Review of Laminar Burning Velocity and Flame Speed of Gases and Liquid Fuels" page 192:

"[A]ny increasing in compression ratio will increase flame speed... A higher compression ratio increases the pressure and temperature of the working mixture before ignition which reduce the initial preparation phase of combustion and hence less ignition advance is needed. High pressures and temperatures of the compressed mixture also speed up the second phase of combustion. Increase compression ratio [also]... increases the density of the cylinder gases during burning. This increases the peak pressure and temperature and reduces combustion duration. Thus engines having higher compression ratios have higher flame speeds."

Note:
Newer Otto Cycle engines use less throttling and operate over a wider range air/fuel ratio than old-school engines in order to reduce pumping loss (intake vacuum = lost work). As many have noted, changing the air fuel ratio has significant inpact on combustion speed and hence ignigition timing. I've elected to discuss the old school Otto Cycle with fixed air fuel ratio and a lot of throttling just to illustrate the effect of changing air mass in the cyclinder at the start of compression.

 

[michaelwoodcoc]

Wow, that pre chamber spark plug engine is cool! I wonder what kindof compromises they had to do with that setup. I'm going to guess it might be a very good idea to index the "outer" spark plug. Honda found this necessary, albeit for different reasons, on the 2000 honda insight. Make the mistake of going to the dealer and you'll spend over $70 (this is the parts cost, I do my own work) on the spark plugs alone. For a 3 cylinder engine. The engine is coded from the factory as to which spark plugs go in which cylinder to be facing the intake valves in the correct orientation, and you just order spark plugs with the corresponding code.


Engineering student. Electrical or mechanical, I can't decide!
Minoring in psychology

 

[moon161]

Sort of goes back to the Honda CVCC engines of the 70's.

https://en.wikipedia.org/wiki/CVCC