why do engines burn up when run lean
12-04-2003, 09:34 AM
#3
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Being lean, in and of itself, does not burn engines up.
Chrysler intentionally used incredibly lean fuel mix back in the 80's on their larger motors and advertised them as "lean burn" engines. It was essentially a cheap way to eke out a few extra mpg on their oversmogged and choked v8's. But they didn't burn up.
Currently, there is a company who modifies AIRCRAFT engines to use a turbocharger to blow a highly lean mix thru engines at cruise speed. Exhaust gas temps drop dramatically and engine efficiency improves a significant degree.
Similarly, a lawn tractor with a dirty fuel filter can mow your 2 acres stumbling and choking on itself from running lean and it can do this for months on end.
Ever run out of gas in your Accord (or whatever you drive or have driven)? Of course you have. Did it burn the motor up?
So what's the dealio ??
The quest to make power. THAT's the dealio.
In our quest for more power, we run our engines closer and closer to their detonation thresholds. Higher compression, advanced ignition timing, more boost. All of these things increase cylinder pressure, which is what we use to create motion in our engines.
The cylinder pressure/fuel octane relationship is critical when you get close to the "edge".
To make horsepower, it has been proven time and time again, that it takes a bit more than the theoretical "chemically-optimum" ratio of fuel to air. This is the case in all gasoline engines that run at rpm higher than a lawn mower (interestingly enough, a low rpm motor makes best power AT the chemically optimum ratio). It has been surmised that the additional fuel does two things: 1) at higher speeds, the burn is incomplete so additional fuel provides a balance to allow the portion that IS burned to be closer to optimal AND 2) the additional fuel acts as a heat buffer, providing a bit more substance to the charge so that combustion chamber hotspots can transfer some of their heat to the additional fuel in the charge.
As far as actual heat of combustion, for a given amount of air, as you progress from rich to lean you will slowly increase the temperature of the burn. As you pass the "optimal" point, temperature will rise a bit more sharply. Beyond that point, however, the temperature starts falling again. Consider a cutting torch with oxygen and acetylene (fuel mix doesn't have to relate to gasoline only). There is a point in the fuel/oxidizer mix that produces the "white-hot" flame. With gasoline in a motor it is the same thing. You can have a flame with too much fuel or not enough fuel, but there is a spot where it is just right.
Anyhow, when a motor is being run closer to its detonation limit, a SLIGHTLY lean condition will cause combustion temperatures to rise sharply which will cause cylinder pressure to rise. If this causes the pressure/octane relationship to cross the borderline, you are now into light detonation, which is incredibly hard on your components from both a temperature loading standpoint (detonation sends cylinder pressures and temperatures WAY up, which is a chain-reaction) and a mechanical impact loading standpoint. Light detonation is sometimes difficult to detect, but it will turn into Heavy Detonation in short order. Heavy Detonation does not go unnoticed for very long.
Boom (or more commonly Tap Tap TAP KNOCK KNOCK KNOCK BaGaddaBaGaddaBaGaddaBaGadda ThUNK!).
Chrysler intentionally used incredibly lean fuel mix back in the 80's on their larger motors and advertised them as "lean burn" engines. It was essentially a cheap way to eke out a few extra mpg on their oversmogged and choked v8's. But they didn't burn up.
Currently, there is a company who modifies AIRCRAFT engines to use a turbocharger to blow a highly lean mix thru engines at cruise speed. Exhaust gas temps drop dramatically and engine efficiency improves a significant degree.
Similarly, a lawn tractor with a dirty fuel filter can mow your 2 acres stumbling and choking on itself from running lean and it can do this for months on end.
Ever run out of gas in your Accord (or whatever you drive or have driven)? Of course you have. Did it burn the motor up?
So what's the dealio ??
The quest to make power. THAT's the dealio.
In our quest for more power, we run our engines closer and closer to their detonation thresholds. Higher compression, advanced ignition timing, more boost. All of these things increase cylinder pressure, which is what we use to create motion in our engines.
The cylinder pressure/fuel octane relationship is critical when you get close to the "edge".
To make horsepower, it has been proven time and time again, that it takes a bit more than the theoretical "chemically-optimum" ratio of fuel to air. This is the case in all gasoline engines that run at rpm higher than a lawn mower (interestingly enough, a low rpm motor makes best power AT the chemically optimum ratio). It has been surmised that the additional fuel does two things: 1) at higher speeds, the burn is incomplete so additional fuel provides a balance to allow the portion that IS burned to be closer to optimal AND 2) the additional fuel acts as a heat buffer, providing a bit more substance to the charge so that combustion chamber hotspots can transfer some of their heat to the additional fuel in the charge.
As far as actual heat of combustion, for a given amount of air, as you progress from rich to lean you will slowly increase the temperature of the burn. As you pass the "optimal" point, temperature will rise a bit more sharply. Beyond that point, however, the temperature starts falling again. Consider a cutting torch with oxygen and acetylene (fuel mix doesn't have to relate to gasoline only). There is a point in the fuel/oxidizer mix that produces the "white-hot" flame. With gasoline in a motor it is the same thing. You can have a flame with too much fuel or not enough fuel, but there is a spot where it is just right.
Anyhow, when a motor is being run closer to its detonation limit, a SLIGHTLY lean condition will cause combustion temperatures to rise sharply which will cause cylinder pressure to rise. If this causes the pressure/octane relationship to cross the borderline, you are now into light detonation, which is incredibly hard on your components from both a temperature loading standpoint (detonation sends cylinder pressures and temperatures WAY up, which is a chain-reaction) and a mechanical impact loading standpoint. Light detonation is sometimes difficult to detect, but it will turn into Heavy Detonation in short order. Heavy Detonation does not go unnoticed for very long.
Boom (or more commonly Tap Tap TAP KNOCK KNOCK KNOCK BaGaddaBaGaddaBaGaddaBaGadda ThUNK!).
VIP Member
12-04-2003, 10:38 PM
#5
Toxic FORMULA
Platinum Member
And then there's DIESELS
That run chit gas...constantly lean.... 20+:1 comp...NO spark...and tack on a turbo. This defies physics
That run chit gas...constantly lean.... 20+:1 comp...NO spark...and tack on a turbo. This defies physics
Last edited by mopower; 12-05-2003 at 01:46 PM.
12-05-2003, 01:28 AM
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A specific octane of gasoline can only withstand a specific temperature or pressure during combustion. When it reaches this critical temp or press it will release the remaining energy instantaneously. Instead of a smooth burn during the entire combustion process. AKA detonation.
Also as mentioned above if the mixture is at the lean ratio that burns at a temp that exceeds the valve or piston design. it will not last long.
Also as mentioned above if the mixture is at the lean ratio that burns at a temp that exceeds the valve or piston design. it will not last long.
12-05-2003, 01:40 AM
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It's simple, peak temp in the cylinder is approx. 14.7:1 air fuel ratio, if you get leaner, temp comes down, get richer, it comes down. You can't burn anything at 17:1, not enough heat. What burns the motor is too much BTU's. Sometimes this can create detonation which increases psi almost 15x depending on the intensity, in turn dramatically increasing heat. It's very possible to burn without detonation though, thats simply too much heat for too long of a time. A piston doesn't burn in one firing, it's heat and time.
Dustin
Dustin
12-05-2003, 08:10 AM
#9
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Hi Dustin: I was thinking the same thing while reading this thread. There's only so much heat produced by compression of the air in the cylinder and only so much energy available in the fuel. So the number of BTUs is limited. If this energy is released smoothly in a nice "slow" flame front then the BTUs are spread over time and the conduction of heat by metal and cooling water has time to work and prevent hot spots.
Detonation is a very fast chemical reaction that releases all the remaining BTUs at once as noted above. Somewhere, usually on the piston, this results in the start of physical damage and then the hot gases escaping through this new hole concentrate a lot of energy in a small area with no cooling. This is a cutting torch.
It's amazing that high octane gas, with just as many BTUS as low octane gas, can avoid this simply by having additives that slow down the flame front. The other very important factor is charge air temperature. My understanding is that the temperature of the charge air is the primary initiator of detonation. Thinking back to chemistry class, there were reactions which would not proceed unless you brought the chemicals up to and over a certain activation energy, then BOOM, the reaction would start. Increasing temperature was usually the way this was initiated. Detonation sounds like that.
Detonation is a very fast chemical reaction that releases all the remaining BTUs at once as noted above. Somewhere, usually on the piston, this results in the start of physical damage and then the hot gases escaping through this new hole concentrate a lot of energy in a small area with no cooling. This is a cutting torch.
It's amazing that high octane gas, with just as many BTUS as low octane gas, can avoid this simply by having additives that slow down the flame front. The other very important factor is charge air temperature. My understanding is that the temperature of the charge air is the primary initiator of detonation. Thinking back to chemistry class, there were reactions which would not proceed unless you brought the chemicals up to and over a certain activation energy, then BOOM, the reaction would start. Increasing temperature was usually the way this was initiated. Detonation sounds like that.
12-05-2003, 09:37 AM
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With all the descriptions, I'm feeling kind of "simple".
1) Lean codition reslts in detonation
2) Detonation causes increased cylinder pressures
3) Incresed cylinder pressure causes increased temps
4) Incresed temps melt stuff
BTW, anything that increses cylinder pressures (increased CR, advanced ignition timing) can increase temps.
Simple enough?
Gary
1) Lean codition reslts in detonation
2) Detonation causes increased cylinder pressures
3) Incresed cylinder pressure causes increased temps
4) Incresed temps melt stuff
BTW, anything that increses cylinder pressures (increased CR, advanced ignition timing) can increase temps.
Simple enough?
Gary
