As sports car teams ramp up preparations for the opening round of the World Endurance Championship at Sebring in March, Porsche continues to dole out a trickle of details about its new 919 hybrid prototype. Thanks to a that launched a few days ago, we now know that the 919 will carry a lithium ion battery pack for electrical energy storage.
The last time a Volkswagen Group brand other than Audi took part in the top echelon of sports car racing, the Bentley Speed 8 that won at Le Mans was little more than a lightly modified closed-top Audi R8 in British Racing Green. This time, the Porsche racing crew at Weissach have cooked up something entirely different from the latest generation R18 e-tron.
A few weeks ago, we learned that Porsche's engineers have opted for a spark-ignition, gasoline-fueled two-liter four-cylinder as the heart of their propulsion system while Audi is sticking with a compression-ignition diesel.
Porsche's decision to use a lithium-ion battery pack rather than the same electromechanical flywheel found in the Audi might seem surprising given that to sports car racing in 2010. The flywheel system was originally devised by the Williams F1 team in 2009; limited energy storage was allowed, but quick power delivery was needed to maximize performance.
Like the ultracapacitors used by Toyota on its TS030 prototype, the flywheel can absorb and release energy more rapidly than a battery but has lower energy density. The flywheel in the 2010 911 GT3R Hybrid only had a capacity of 200 Wh.
In 2014, LMP1 cars can use from 555 Wh to 2.2 kWh of electrical energy storage. The inherent efficiency of Audi's diesel coupled with slower acceleration response makes it a good fit with a lower capacity but powerful flywheel. On the other hand, Porsche's gas four-cylinder could probably benefit from more electric boost, but it can probably get by with the somewhat slower power delivery of the battery.
If Porsche is going with up to 2.2 kWh of electrical capacity, a battery would likely be a lighter and more compact solution.
As always, engineering is about making tradeoffs on the details to get the best overall solution. It's going to be fascinating to watch who gets the upper hand this year: Porsche, Audi, or Toyota.