rich w/ advanced timing vs. lean w/ retarded timing
rich w/ advanced timing vs. lean w/ retarded timing
I posted this in the Single Turbo forum but I'm also posting it here to see if I can get some additional input
The author of these two articles essentially argues that, given enough octane it seems, you are both safer and making more power by retarding timing presumably during peak load/combustion pressure. The safety of the retarded timing should allow you to run a leaner mixture, which in turn yields more power. Running richer is safer not so much because it cools combustion, but because it slows the flame-front.
These two articles written by one of the main guys at Innovate Motorsports, which of course sells all sorts of tuning products including widebands:
Some quotes:
http://www.innovatemotorsports.com/resources/rich.php
http://www.innovatemotorsports.com/resources/myths.php
anyone have thoughts on this? Also I'm still trying to figure out how octane fits into all this idea... and obviously he's not talking about a rotary but similar principles should apply right? I am going to be tuning a race gas setup myself (still working on driveability before I boost it) so I'd like some input.
The author of these two articles essentially argues that, given enough octane it seems, you are both safer and making more power by retarding timing presumably during peak load/combustion pressure. The safety of the retarded timing should allow you to run a leaner mixture, which in turn yields more power. Running richer is safer not so much because it cools combustion, but because it slows the flame-front.
These two articles written by one of the main guys at Innovate Motorsports, which of course sells all sorts of tuning products including widebands:
Some quotes:
http://www.innovatemotorsports.com/resources/rich.php
Because air has a specific heat of about 1 kJ/kg*deg K, the air charge is only 3.8 C (or K) degrees cooler for the rich mixture compared to the optimum power mixture. This small difference has very little effect on knock or power output.
Richening up the mixture results in a slower burn, moving the pressure peak later where there is more leverage, hence more torque. Also the pressure peak is lower at a later crank angle and the knock probability is reduced. The same effect can be achieved with an optimum power mixture and more ignition retard.
If the mixture is ignited to early, the piston is still moving up towards TDC as the pressure from the burning mixture builds. This has several effects:
The pressure buildup before TDC tries to turn the engine backward, costing power.
The point where the pressure in the cylinder peaks is much closer to TDC, with the result of less mechanical leverage on the crankshaft (less power) and also causes MUCH higher pressure peaks and temperatures, leading to knock.
Many people with aftermarket turbos don't change the spark advance very much, believing that earlier spark creates more power. To combat knock they make the mixture richer. All that happens really then is that the mixture burns slower and therefore hits the peak pressure closer to the right point. This of course reaffirms the belief that the richer mixture creates more power. In reality the flame front speed was adjusted to get the right peak pressure point. The same result (with more power, less emissions and less fuel consumption) could be achieved by leaving the mixture at the leaner optimum and retarding the ignition more instead.
The pressure buildup before TDC tries to turn the engine backward, costing power.
The point where the pressure in the cylinder peaks is much closer to TDC, with the result of less mechanical leverage on the crankshaft (less power) and also causes MUCH higher pressure peaks and temperatures, leading to knock.
Many people with aftermarket turbos don't change the spark advance very much, believing that earlier spark creates more power. To combat knock they make the mixture richer. All that happens really then is that the mixture burns slower and therefore hits the peak pressure closer to the right point. This of course reaffirms the belief that the richer mixture creates more power. In reality the flame front speed was adjusted to get the right peak pressure point. The same result (with more power, less emissions and less fuel consumption) could be achieved by leaving the mixture at the leaner optimum and retarding the ignition more instead.
Well..... pistons usually alow some margin of error during tunning (knock). Some engines more, others less, but they do...
Rotarys on the other hand....
runing lean(er) may be a bit scarry and something you constantly remember everytime you'll hear a strange sound coming for the engine :p
Rotarys on the other hand....
runing lean(er) may be a bit scarry and something you constantly remember everytime you'll hear a strange sound coming for the engine :p
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If you're going to be tuning an engine, I'd take a look at Internal Combustion Engine Fundamentals by John Heywood. It's pretty much considered the holly grail of ICE engines by industry. It'll give you tons of insight on the theory behind combustion and especially on how timing effects ignition properties. The person who wrote that article is pretty much correct. I would say that richening the mixture does cool it a bit more between stoichiometric and rich, (I've looked at this through analytical calculations) but still, it does not effect knock much. The key thing he describes is the flame front. A rich F/A mixture has a slower flame prorogation speed. Thus the chamber will take longer to reach peak combustion pressure. This limits the potential for knocking but causes more negative work on the rotor because combustion occurs before TDC. However, be careful! Reducing the ignition retard and leaning the mixture out is highly dangerous! The only useful thing running a leaner mixture does is reduce HC and NOx output. Thus all modern diesels run lean. Unless you need your race gas powered FD to pass smog testing, why bother?
It is true that you can only combust a stoichiometric amount of fuel, the rest is wasted through the tail pipe. Leaning out the mixture will save you some power because you are wasting less energy to vaporizing fuel and to mechanical pumping. The pumping losses are minimal. Fuel takes about 0.18 MJ/kg to vaporize. This may seem like a lot but when your engine consumes far less than an kg a second, it's not sapping tremendous amounts of power. Did you know that almost all production cars will run rich when a WOT? Some will approach 11:1, a far cry above the stoich 14.5:1. This is for multiple reasons.
For one, this ensures that the maximum amount of fuel will burn in the cylinder. Maximum fuel burn = maximum power. As you increase your RPM's your ignition timing must be advanced with increasing load. This is because there is less time for the fuel to burn. If it is too close to TDC you won't burn everything. Again, this is why most cars run rich at WOT. It's better to burn all your fuel and get the power the throttle demands than to bog down.
Two, more fuel does cool the engine down. People run Methanol injection for the same reason. It takes heat to vaporize the fuel. Although, it takes about 10 times more heat to vaporize the same weight of Methanol but that's not the point.
Third, fuel acts as a lubricant. This is especially important on a rotary. If you lean out the mixture, the housings will not be lubricated, thus increasing friction. Increased friction means increased heat which means.... You get the idea. This is especially true for the 13b-REW. Those apex seals scrap along the walls and even with the oil injectors, need every bit of extra lubrication they can get!
Another point: as you decrease ignition retard, you give the fuel less time to burn, especially at high RPM's. Just because your engine is spinning faster doesn't mean that the flame front's speed increases exactly in tune. Less burn time means less fuel is actually burned. So you may actually not burn all your fuel anyways.
My last point is this, Engine tuning is an art. There are many more things involved than you can imagine. If you read that book you'll get a small idea. Personally, I know enough to not ever want to attempt it. I hold an experienced tuner in the highest respect but still, it doesn't take much for the system to crash. Bad fuel, high cornering g's causing starvation, different air temperatures, etc. Running lean can decrease your negative work but you run a high high risk of destroying your engine. I don't think that tuners of rotary engines, at least not the good ones, just add more fuel to compensate. It's a factor of safety. Those statements are true, just meant for a piston engine.
Anyways, sorry for the technical jargon, I tend to get carried away sometimes. I took a course devoted to ICE, so I kinda like this stuff...
It is true that you can only combust a stoichiometric amount of fuel, the rest is wasted through the tail pipe. Leaning out the mixture will save you some power because you are wasting less energy to vaporizing fuel and to mechanical pumping. The pumping losses are minimal. Fuel takes about 0.18 MJ/kg to vaporize. This may seem like a lot but when your engine consumes far less than an kg a second, it's not sapping tremendous amounts of power. Did you know that almost all production cars will run rich when a WOT? Some will approach 11:1, a far cry above the stoich 14.5:1. This is for multiple reasons.
For one, this ensures that the maximum amount of fuel will burn in the cylinder. Maximum fuel burn = maximum power. As you increase your RPM's your ignition timing must be advanced with increasing load. This is because there is less time for the fuel to burn. If it is too close to TDC you won't burn everything. Again, this is why most cars run rich at WOT. It's better to burn all your fuel and get the power the throttle demands than to bog down.
Two, more fuel does cool the engine down. People run Methanol injection for the same reason. It takes heat to vaporize the fuel. Although, it takes about 10 times more heat to vaporize the same weight of Methanol but that's not the point.
Third, fuel acts as a lubricant. This is especially important on a rotary. If you lean out the mixture, the housings will not be lubricated, thus increasing friction. Increased friction means increased heat which means.... You get the idea. This is especially true for the 13b-REW. Those apex seals scrap along the walls and even with the oil injectors, need every bit of extra lubrication they can get!
Another point: as you decrease ignition retard, you give the fuel less time to burn, especially at high RPM's. Just because your engine is spinning faster doesn't mean that the flame front's speed increases exactly in tune. Less burn time means less fuel is actually burned. So you may actually not burn all your fuel anyways.
My last point is this, Engine tuning is an art. There are many more things involved than you can imagine. If you read that book you'll get a small idea. Personally, I know enough to not ever want to attempt it. I hold an experienced tuner in the highest respect but still, it doesn't take much for the system to crash. Bad fuel, high cornering g's causing starvation, different air temperatures, etc. Running lean can decrease your negative work but you run a high high risk of destroying your engine. I don't think that tuners of rotary engines, at least not the good ones, just add more fuel to compensate. It's a factor of safety. Those statements are true, just meant for a piston engine.
Anyways, sorry for the technical jargon, I tend to get carried away sometimes. I took a course devoted to ICE, so I kinda like this stuff...
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