Someone tell me again why our cars run so lean....
01-19-2008, 10:11 PM
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Someone tell me again why our cars run so lean....
Passing a car again at night on the road and I look in to rearview mirror to see this huge cloud in guys headlights. It's actually kind of embarrassing with such a modern car leaving behind such a large trail of "smoke."
01-19-2008, 10:52 PM
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Smoke normally means you're running rich...
Sudden transitions between load sites in the fuel map (e.g. hard change between 1st and 2nd gear) generate lean conditions so there is typically acceleration/deceleration enrichment maps that enrich the mixture...this would explain puffs of smoke between gear changes...
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01-20-2008, 01:23 PM
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It's an issue with direct injection engines. They have higher particulates..
especially under full power and a little enrichment. There's debate right now about whether or not the higher PM from direct injection gasoline engines will be compatible with future regs for particulate matter from gasoline engines.
The carbon dust builds up on the cat, and exhaust pipes, and it you drive slow for a while, then punch it, you are literally blowing out the carbon.
As for lean...
FSI really stands for fuel stratified injection. In Europe, they run lean for fuel economy at part throttle. They run so lean that with a normal injection system, it'd be too lean to ignite. But, the direct injection system stratifies the fuel near the plug, so it can ignite.
But, you end up with lots of excess O2 in the exhaust.
A lean burning engine will produce less NOx, but with the excess air, the cat cannot reduce it to N2 and O2. A catalytic convertor does 3 main classes of reactions. It oxidizes unburnt HC to CO2 and H2O, it oxidizes CO to CO2, and it reduces NOx to N2 and O2. With excess O2, the oxidations work great, but the NOx reduction won't go. So even though the NOx produced by the engine is lower, the cat won't touch them, and the exhaust NOx is higher than allowed.
In Europe, they use a NOx trap. This is a monolith of a material that adsorbs NOx. Every so often, the engine goes into stiochiometric mode and the trap quickly releases and reduces the adsorbed NOx. This is a regeneration period. Unfortunately, the trap idea won't work here in the US because the higher sulfur in our fuel destroys the NOx trap material. The adsorption reaction uses barium carbonate to adsorb NO2 to make barium nitrate. If too much SO2 is there, it will make barium sulfate which won't regenerate at normal regen conditions.
So in the US, we don't have a trap, nor do we have stratified lean burn mode. The engine just always runs in stoichiometric mode (non-lean) with a conventional 3-way cat. The mix is the same (lambda=1) as everyone else.
Part of the VW504 oil spec is also fewer compounds that will ruin the sensitive NOx trap if they get burnt.
This stoichiometric-only mode might also be leading to higher fuel dilution since the fuel can spray-down the cylinder walls and get into the oil. I think that's why we are told to run 5W-40 instead of 5W-30, where FSI is enabled. They just compensate for a little dilution with the 40 diluting down to 30 which is what the engine is designed for.
The carbon dust builds up on the cat, and exhaust pipes, and it you drive slow for a while, then punch it, you are literally blowing out the carbon.
As for lean...
FSI really stands for fuel stratified injection. In Europe, they run lean for fuel economy at part throttle. They run so lean that with a normal injection system, it'd be too lean to ignite. But, the direct injection system stratifies the fuel near the plug, so it can ignite.
But, you end up with lots of excess O2 in the exhaust.
A lean burning engine will produce less NOx, but with the excess air, the cat cannot reduce it to N2 and O2. A catalytic convertor does 3 main classes of reactions. It oxidizes unburnt HC to CO2 and H2O, it oxidizes CO to CO2, and it reduces NOx to N2 and O2. With excess O2, the oxidations work great, but the NOx reduction won't go. So even though the NOx produced by the engine is lower, the cat won't touch them, and the exhaust NOx is higher than allowed.
In Europe, they use a NOx trap. This is a monolith of a material that adsorbs NOx. Every so often, the engine goes into stiochiometric mode and the trap quickly releases and reduces the adsorbed NOx. This is a regeneration period. Unfortunately, the trap idea won't work here in the US because the higher sulfur in our fuel destroys the NOx trap material. The adsorption reaction uses barium carbonate to adsorb NO2 to make barium nitrate. If too much SO2 is there, it will make barium sulfate which won't regenerate at normal regen conditions.
So in the US, we don't have a trap, nor do we have stratified lean burn mode. The engine just always runs in stoichiometric mode (non-lean) with a conventional 3-way cat. The mix is the same (lambda=1) as everyone else.
Part of the VW504 oil spec is also fewer compounds that will ruin the sensitive NOx trap if they get burnt.
This stoichiometric-only mode might also be leading to higher fuel dilution since the fuel can spray-down the cylinder walls and get into the oil. I think that's why we are told to run 5W-40 instead of 5W-30, where FSI is enabled. They just compensate for a little dilution with the 40 diluting down to 30 which is what the engine is designed for.
