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      03-21-2009, 12:45 AM   #84
oneginee
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Drives: GT3 RS
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There is some misconception in this thread that direct injection can "largely" counter turbo lag.

Quote:
Originally Posted by swamp2
"direct injection will largely mitigate turbo lag, when the two are carefully integrated and matched by BMW M "
direct injection allows precise control of the fuel/air ratio and more even mix which reduces pinging or engine detonation and allows an engine to be designed to higher compression ratio. Higher compression ratio means more possible air mass at lower RPM compared to a non DI engine. This translates to more exhaust pressure which keep the turbo spinning at lower RPM than a non DI engine. However the turbo spining energy is derived from both pressure AND temperature of the exhaust gas. The lean mode from the DI engine result in excess oxygen that will cool DOWN the exhaust gaz which works against keeping that turbine spining (but helps with efficiency). The result is only limited improvement to turbo lag from DI. The effective boost RPM (engine) will only be pushed to a slightly lower threshold for the DI + turbo. Lag will still be present below that RPM.

Quote:
Originally Posted by swamp2
"Before a large throttle depression the DI system will be operating in a lean burn mode, thus the exhaust gas mass will be higher than that of a port injected engine turbo charged engine. The rpm of the fans can be maintained 2-3x higher! Yes not 50% higher 2-3x!"
you are getting carried away here. Let's check the numbers. DI vaporizes fuel which has a cooling effect allowing a higher compression ratio reasonable numbers would be 10.3:1 versus 9.1:1 for no DI. The stoichiometric air-fuel ratio for petrol (gasoline) is 14.7:1. For lean combustion let's say it is 30:1. Using these numbers in the thermodynamic equation for internal combustion engine gives a pressure after combustion of 158 atmospheres and temperature of 9411°R for the engine without FI and 101 atmospheres and 5344°R for the FI engine running lean . The Result is not better but worse because running lean kills the exhaust pressure and cools exhaust gas down both bad for turbo lag. Let's see what happens if the FI engine is run with stoichiometric air-fuel ratio instead: 180 atmospheres and 9477°R. ! That is +15% improvement only. To get 50% higher rpm in the turbo you would need at least 13:1 compression ratio. This is possible with a 'ping' or 'knock' sensor and an electronic control unit both easily available in a performance car. But going higher is only seen in racing engines burning methanol and ethanol which can exceed a CR of 15:1. that gives 75% higher rpm in the turbo. But the true pot of gold is with Diesel FI engine which can go to CR of 20:1. NOW that is 2.43x higher turbo speed compared to the non FI turbo engine I started with.

To conclude in my opinion there are only two routes possible for better efficiency, low emissions and high hp:

- Diesel turbocharged DI engine (and maybe also supercharged to even further reduce turbo lag at low rpm) See Audi doing that already. I doubt BMW will do that for the M.

http://www.vwvortex.com/cgi-bin/artm...1496&printer=1

OR

-Gasoline turbocharged engine with variable Turbine geometry (VTG). Basically the turbine is made to mimic a smaller turbo at lower rpm. At higher speeds the profile is increased. This is what Porsche is doing on the 911 Turbo.

Last edited by oneginee; 03-21-2009 at 01:31 AM..
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