We had a conversation with a high level Porsche® 918 Engineer, we were discussing engine control strategies and a lot of interesting things were learned and validated. Take a look at our study of the 991 RS on the dyno to understand a bit more about this. Historically the engine was controlled reactively as in a sensor reads something abnormal and the car reacts to it after the event (knock sensor detects unstable combustion and the car pulls ignition timing for example). With increased emissions control required, Porsche® had to change their strategy for the 2014 and up vehicles. Porsche® has built engine models into the engine controller so that they can predict outcomes before they occur and look at sensor readings before the combustion events (like intake air tempearature, air pressure, ambient conditions, coolant temps etc...) and predict what would likely occur and change the engine parameters based on this.
So incase your dynoing your car with uncontrolled, hot intake air temps or at the track on a hot day and notice the power is down, now you know why...
Below is what the engineer told us...
Porsche's® Engine Management strategies are now proactive not re active. As soon as one parameter starts to go out of the designed limits the engine power is immediately pulled back in a sequential fashion so the driver does not feel any immediate loss of power. This is done by Cam timing, throttle plate position and Ignition timing. The ignition is done per cylinder up to a certain stage then per bank,(in the case of the 918) if large changes are required. Along with cam timing and throttle plate changes, this degradation of power is not felt as a sudden loss of power but a more gradual loss, so car performance does not suffer in the minds of the driver.
Air temp is a huge factor. With smaller packaging and lighter cars to meet fuel mileage etc, the engine inlet temps suffer quickly due to air duct entry’s etc. As soon as the air temp rises, the cam timing is changed along with the throttle plate position. This is to limit the air mass as fueling cannot be added to help lower air temps. This is because the fueling is already pulsed to keep the cat working correctly, so additional fuel added to control the air temp is an emissions problem. In the case of air temp, cam timing is changed per camshaft per bank, the throttle is closed and the fuel timing is adjusted to suit the new overlap period to ensure the Cat still gets fueled to stay alive.
They cannot see any detonation as the CEL is a major issue for servicing etc, so strategies are in place to limit the possibility of any detonation. If the engine does see major detonation, then it goes into limp mode.
In the case of the 918, the shift point in the trans are also adjusted to suit the engines drop in power output.
So air temp is a huge factor in performance now. Because they cannot add fuel to cool the charge as the fuel is in the cylinder after the hot air has entered, the other parts of the engine are adjusted to control any detonation. With so many changes happening at once and continuing to change, the driver doesn’t feel a sudden loss in power. This is done on purpose, otherwise no one would buy a Porsche®.
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