1999-2009 [Subscribe to Daily Digest] |
With the different grades of fuel that you tested... we changes in octane with the same brand of fuel?
Many old engines were prone to knocking because of the combusion chamber shape. The hemi head plus the right amount of squish volume helps deter knocking.
Turbo engines need strong ignition systems as the air/fuel mixtures, and air in general, have higher break down voltages. So more volts are required to punch through the plug gap. So you will find that turbo engines also have smaller plug gaps to keep the voltage requirements down. If the plugs start to foul, it is at higher boost where the resistance to the spark to travel along the fouled insulator, is lower than jumping the much shorter plug gap. So the engine can run well until the boost gets up, then it misses and the boost drops then it runs, boost goes up, misses again. And when that happens, the missing really makes the fouling go bad fast as there is all of the unburnt fuel when ignition does not happen.
The cap discharge aspect is just a method of getting a good amount of strong spark plug firing.
At light loads, the T7 might be a bit lean compared to the T5. The T7 certainly does produce better mpgs than my T5 95SET did with less weight and less drag etc. I speculate that at light loads one can go a bit lean and not generate NOX as the temperatures and pressures are lower. So the CAT can probably then be operated a bit lean without generating NOX at the tail pipe.
As for an ideal fuel/air mixture. Many old engines just ran rich to defeat knocking, and the CO was very high, and combustion chamber shape was not on the horizion. It was the two stage cats that required that the mixture be ideal. Any NOX was converted to O2 in the 1st stage of the cat, then the second stage used that O2 to convert C0 to CO2. If the mixture was too lean, there would still be NOX emissions and if rich, CO emissions. So the O2 sensor and ECU control were responsible to maintaining this AF control. This AF control point was called 'lamba' and I remember Volvo producing a 'Lamda something ?sond?' system in 1978 or earlier that managed all of this quite well. And that was with a system with a mechanical flow meter as MAF sensors were not out yet.
When the automakers tried to avoid CO by running lean, power dropped and running problems occured, and NOX got bad. The only way out was cat converters and O2 sensors. Both of these required that unleaded fuel be used. With the lead out, that was the end of abundant octane. Then to produce power, the combustion chamber became a big issue to allow for decent compression ratios with lower octane fuels.
posted by 207.43.195...
No Site Registration is Required to Post - Site Membership is optional (Member Features List), but helps to keep the site online
for all Saabers. If the site helps you, please consider helping the site by becoming a member.
 |
 |
 |
 |
 |
