Something interesting that my tech told me regarding cold performance [edit]:
10-06-2005, 07:55 AM
#1
Something interesting that my tech told me regarding cold performance [edit]:
<edit> While on a drive with my tech, I asked him if he knew why our cars are so much quicker upon cold sartup than after they warm up.</edit>
He said that when you first start these cars up cold, they run very rich to help light off the cats, and as the temp increases, the mixture <i>gradually</i> leans out until it reaches the normal operating mixture at normal operating temperature.
It certainly makes sense, but if true it means that our cars (or maybe all cars these days--I don't know) normally run significantly leaner than the ideal stoichiometric ratio of 14.7:1, probably in the name of cleaner emissions. So I guess the theory is that when cold, our cars run closer (or maybe over) 14.7:1 than when running at normal operating temps.
He also mentioned that just the fact that it's cold has something to do with it (which we all know), but that most of what I was feeling was purely due to fuel mixture.
He said that when you first start these cars up cold, they run very rich to help light off the cats, and as the temp increases, the mixture <i>gradually</i> leans out until it reaches the normal operating mixture at normal operating temperature.
It certainly makes sense, but if true it means that our cars (or maybe all cars these days--I don't know) normally run significantly leaner than the ideal stoichiometric ratio of 14.7:1, probably in the name of cleaner emissions. So I guess the theory is that when cold, our cars run closer (or maybe over) 14.7:1 than when running at normal operating temps.
He also mentioned that just the fact that it's cold has something to do with it (which we all know), but that most of what I was feeling was purely due to fuel mixture.
10-06-2005, 09:15 AM
#3
No, I don't think so.
Sia's chipped car, while stronger than mine, still exhibits the same cold-start characteristics that my car does. So whatever the chipmaker has done, it doesn't seem like it changes whatever causes our cars to run so strongly at cold temps.
10-06-2005, 09:23 AM
#4
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A lot of thought must've gone into the aftermarket chip programs for our cars...
Charting the fuel/air mixture changes on the stock engine, cold to hot, trying to find the "ideal" mixture, or stoichiometric, and then trying to account for the changing air temperature as the engine and the intake components heat up and of course, the varying outside temperatures. Way beyond me...
10-06-2005, 09:26 AM
#5
In general, a cold engine requries a richer mixture to prevent stalling and hesitation vs one at
normal operating temps. A rich mixture burns at a cooler temp than a lean mixture. The B5 S4 idles very lean to keep emissions low, as the cats are more efficient at higher exhaust temps. You want the cats to light-off as quickly as possible, but this is in competition with a rich miture required by a cold engine. To get the cats lighted quicker, the mixture should be leaned out. so I think your tech either explained it not quite correctly, or you misunderstood slightly.
At WOT, all engines go richer to prevent detonation and to insure you don't burn a piston (as normal combustion temps at ideal ratios are higher than the melting point of most engine metals). The fact that exhaust gas does not have a lot of heat capacity allows engines to function normally most of the time. It's just at WOT, a slight error can burn the crown of a piston pretty rapidly, so the ECU errs on the side of safety.
The hotter the gas is after combustion, the more power the engine produces because the whole point of an internal combustion engine is to cause the gas in the cylinder to expand, and this force of expansion is what makes the power - so if you run lean, you make more power as the gas volume increases with increasing temperature (although it isn't perfectly linear). The ECU must balance emmissions, power, and engine longevity. As the engine warms up after a cold start, the ratio gets leaner as the engine can run smoothly with propotionately less fuel. It takes thermal energy to vaporize the fuel spray that comes from the injector, and this causes the intake charge temp to decrease. On cold winter days, there isn't enough heat in the ambient air to efficiently vaporize all the fuel, so you have to spray more fuel in to get the mixture to ignite properly.
You may know a lot of this already, but while I'm not trying to hammer you on your post, it seemed to me that you only partially understanding the situation and I thought my post would be helpful. Bottom line is, if you simply increased the fuel mixture to make it slightly richer across all ranges (via a chip) that would not result in power gains. The engine mapping via a chip is much more complex than that. In the old days of carburetors, if you didn't change your air filter often enough, the engine would run rough, stumble and emit black soot in the exhaust and your gas mileage would stink - that was due to no MAF. so that was an artificial way to enrich the mixture and you ended up with less power rather than more.
Hope this helps more than hurts! :-)
At WOT, all engines go richer to prevent detonation and to insure you don't burn a piston (as normal combustion temps at ideal ratios are higher than the melting point of most engine metals). The fact that exhaust gas does not have a lot of heat capacity allows engines to function normally most of the time. It's just at WOT, a slight error can burn the crown of a piston pretty rapidly, so the ECU errs on the side of safety.
The hotter the gas is after combustion, the more power the engine produces because the whole point of an internal combustion engine is to cause the gas in the cylinder to expand, and this force of expansion is what makes the power - so if you run lean, you make more power as the gas volume increases with increasing temperature (although it isn't perfectly linear). The ECU must balance emmissions, power, and engine longevity. As the engine warms up after a cold start, the ratio gets leaner as the engine can run smoothly with propotionately less fuel. It takes thermal energy to vaporize the fuel spray that comes from the injector, and this causes the intake charge temp to decrease. On cold winter days, there isn't enough heat in the ambient air to efficiently vaporize all the fuel, so you have to spray more fuel in to get the mixture to ignite properly.
You may know a lot of this already, but while I'm not trying to hammer you on your post, it seemed to me that you only partially understanding the situation and I thought my post would be helpful. Bottom line is, if you simply increased the fuel mixture to make it slightly richer across all ranges (via a chip) that would not result in power gains. The engine mapping via a chip is much more complex than that. In the old days of carburetors, if you didn't change your air filter often enough, the engine would run rough, stumble and emit black soot in the exhaust and your gas mileage would stink - that was due to no MAF. so that was an artificial way to enrich the mixture and you ended up with less power rather than more.
Hope this helps more than hurts! :-)
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10-06-2005, 10:31 AM
#9
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It isn't the leaning of the mixture that causes the "percieved" reduction in performance.
It's just the extreme heat our engines have to deal with by being trapped in such a small engine bay. The heating of the air in the manifold alone causes a loss of power. Less air in = less power.
Go strap your car on a dyno (or get a wideband O2 monitor) and you'll be able to see what kind of mixtures our cars run. At idle (and warm), they are very lean, on the order of 19:1.
During constant load driving, they attempt to maintain stoichiometric ~14.7:1. Tip-in causes a slight leaning as the system reacts, then richens back up to 10:1 or 11:1 at full throttle. It's a closed loop vs. open loop system. Constant load is closed loop. The car reacts to what the O2 sensors tell it. At full throttle, it is open loop, reverting to fuel tables and trims.
There really isn't any secret about the performance.
Go strap your car on a dyno (or get a wideband O2 monitor) and you'll be able to see what kind of mixtures our cars run. At idle (and warm), they are very lean, on the order of 19:1.
During constant load driving, they attempt to maintain stoichiometric ~14.7:1. Tip-in causes a slight leaning as the system reacts, then richens back up to 10:1 or 11:1 at full throttle. It's a closed loop vs. open loop system. Constant load is closed loop. The car reacts to what the O2 sensors tell it. At full throttle, it is open loop, reverting to fuel tables and trims.
There really isn't any secret about the performance.
