Quote:
Originally Posted by robertwb70
I have to take issue with all the references to "burn speed" in there, exactly how do you measure "burn speed"??
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Well, if you've been tuning engines for a long time you will get a feel for it.
Using test equipment you follow the burn speed in the chamber by watching a combination of things: AFR, EGT, KR (knock retard), power output, CTS (coolant temp), MAP, and several others that are more indirect, or a direct determiner of.
I'll try to give you a few examples now, and maybe more later if necessary.
The first thing to get a handle on is to understand spark advance - why you need it and how the engine reacts to it.
At certain times the burn speed in the chamber is very slow. When you first start your engine cold and you're at an idle (throttle closed), the burn is very slow. Your ECM/PCM calibrations are set to make up for both a slow and an incomplete burn in the chamber. You will see: increased spark advance, extra fuel enrichment (cranking time-out or after-start, and warm-up enrichment), and the ECM/PCM waiting to go into closed-loop mode. The spark is started sooner in the cycle which increases the dynamic compression or cylinder pressure, which creates more heat when compressed near TDC, and results in a quicker burn of the mixture. The extra fuel is to make up for poor fuel vaporization when cold.
The slow or late burn results in less fuel burning in the chamber and more fuel burning in the exhaust port. The overall effect would be a lean burn in the chamber, if not for warm-up enrichment.
Try the cold start without the extra spark advance and observe the effects of a slow burn speed.
Once the engine is warm the ECM/PCM will reduce the spark advance (SA) down to the normal level for whatever load cell you happen to be operating in. At that point try reducing your SA by 10* and observe your datalogging readout from the sensors and/or your test equipment. You will see MAP decrease, and your CTS and EGT both increase. The slow burn speed causes a condition called late burning. That is a significant amount of fuel is still trying to burn during the exhaust cycle. Late burning results in less of the fuel's energy being transferred into piston motion, and more being turned to heat either late-in or after the power stroke of the piston. More heat is rejected to the metal parts, namely the exhaust valve and exhaust port. You will see this as increased cylinder head and exhaust temperature when measured near the exhaust port.
The retarded SA produces a lower cylinder pressure and therefore less heat around TDC of the compression stroke. This lower pressure and heat results in a slower burn speed. The slower burn speed results in less engine power at the crank and more heat in the exhaust.
There is a point where a slow or late-burn will become a misfire. At that point the exhaust temp will drop sharply, as well as engine power. When you run so lean as to get into lean-misfire, you will observe this.
Test: Let your car idle with 1/8 tank 87 octane fuel for several minutes until your EGT levels out. Next add a couple of gallons of pure Ethanol to your tank and watch your EGT. Next add another 5 gallons of Ethanol to your tank and watch your EGT. By at first adding some alcohol you will cause some late burning that will show up as increased EGT. After a point of adding so much Ethanol you will probably start experiencing misfires, or burning so late that the EGT then drops sharply. Your catalytic converter would then start to heat up dramatically.
Here's another example:
Starting from stoich 14.6:1 AFR,
if you start leaning the AFR a little, in most cases the burn-speed will increase at first, giving greater efficiency. At a point, often around 15.7:1 AFR (depending on the set-up particulars), the burn speed will start to slow back down. Once you get over 16.0:1 you will notice the burn speed decreasing significantly and depending on conditions, you might start seeing some lean misfires.
The lean-limit of an engine depends on load, engine speed, and also a lot on design. A high load and low engine speed will misfire before a condition of low load and higher engine speed.
This is why an across the board leaning of the AFR is not very effective at increasing fuel mileage. At one end you will have late burning which is very inefficient, and at the other end you will have lean misfires causing severe power loss under load. The right way is to tune across the entire load range. A speed-density system using a VE table which makes this relatively easy to tune for AFR. A MAF system is also tunable, but its not as easy to cover all load conditions.
Trying to run stoich over a wide range of load conditions like the closed-loop system does is only good for maintaining a given emissions level over than range, and is not good at all for fuel efficiency.
Once you start working with it, you will see that increasing burn speed by manipulation of the AFR and SA will get you increased fuel efficiency, though not necessary the lowest emissions.
I like to tune to the edge of lean misfire with a wide spark plug gap of 0.040" to 0.045", then tighten the gaps down to 0.030" or less to add a buffer and decrease the number of misfires.
One easy way to see the dramatic effect of burn speed is to go to the drag strip. Get there with 1/8 tank of 87 octane fuel and make a couple of passes watching your power output (ET & trap speed), and also watch your EGT as measured at the exhaust port.
Next put some high octane fuel in your tank. Add 1/8 tank of CAM2 114 octane, or even 100 octane unleaded will do if you're worried about killing your O2 sensor or CAT. A large amount of added octane booster or alcohol for that matter will also do the trick. The result will be greatly reduced power and a very hot exhaust manifold. You will see slower ETs and your manifolds will probably be glowing orange due to the slow and late burn of the fuel.
I see people do this at the track all the time thinking they will run faster with the race gas with their street car's tune.
Some people also think you can make up for the slower burn by simply advancing the SA and gain better power and efficiency, but that's not true. All things equal, any time you find yourself advancing the spark to make up for a slow burn, you are giving up efficiency when compared to less advance with a faster burning fuel. The only exception is if the faster burning fuel is putting you into a knock or detonation condition, but then all things aren't equal anymore.
Give it a try.