From the first three positions on the grid, Porsche swept to a 1–2 finish in the gruelling 24 hours of Le Mans, beating the long-dominant Audis and World Champions Toyota in the process. With 2015 being the Stuttgart marque’s second year in the modern LMP1 hybrid class, this would already be regarded an achievement. But to do so with a tiny 2-litre engine shows that the tide in motorsport is changing towards downsized power units.
So how did Porsche find such a huge advantage with its 919-hybrid over such seasoned opposition, in such a short space of time? Well, the downsized power unit has a lot to do with the result. Why it is so critical, however, is a longer story.
Le Mans, along with the FIA World Endurance Championship (WEC) that pairs with it, have always been about endurance and efficiency. Large-capacity engines have been common in order to reduce the stresses that come with 6 or 24-hour races. Audi took up the diesel option for a further boost to fuel\power efficiency – its 4-litre turbo-charged V6 being the class of the Le Mans field in previous years – while Toyota ran a petrol V8, displacing 3.8 litres and unaided by a turbo. When the regulation changes came in for 2014, allowing energy recovery systems to boost efficiency, a complicated formula was created in which fuel flow and fuel tank capacity were traded against greater energy recovery systems. The more energy you can harvest, the less fuel you had to play with – although the formula put ERS at a greater advantage over fossil fuels.
Thus, four sub categories within the top LMP1 class were created, based on the mega joules of energy you could recover over a lap, and sized incrementally in 2Mj steps (from 2Mj to 8Mj). A new sports car era was born, where cars could exploit 1000hp. With a near 50-50 split between internal combustion engine power and recovered energy, both the sport’s leading incumbents – Audi and Toyota – retained their large displacement power plants and added larger energy recovery systems. Audi opted for a lesser ERS in the 4Mj category, whilst Toyota – aiming higher – went for two high-capacity energy recovery systems in the 6Mj class.
Porsche was instrumental in working with these other teams – as well as with the FIA – in developing these rules. Yet, not being active in the category meant that it was able to develop a power train from scratch to maximise the benefit of the new rules. Whilst it was a benefit, Porsche also needed to look at the long-established competition. Despite the marque’s heritage in the endurance racing world, this is a new era and clean sheet design was required.
Under the current regulations, the car’s performance is capped through a complicated series of templates and restrictions on the aerodynamic bodywork. This particularly affects the cars diffuser and underfloor, which is tightly controlled from the front axle line rearwards. Cars therefore need to be designed with almost identical under floors, which limits rear downforce. Additionally, for safety, the cars must sport a large dorsal fin and openings in the wheel arches over the wheels. Both of these changes – along with the floor design – reduce the chance of the car flipping over when following another car, or turning sideways to the airflow.
Additionally, there is a minimum weight limit of 870kg; a hard target to meet, especially with a heavy engine and\or ERS. However, these restrictions belie a few opportunities. There is little aero restriction on the front end of the cars, and with front tyres sized equally to the rears, that minimum weight limit does not enforce a front to rear weight distribution. There is, therefore, a huge potential to gain on the front end of the car, if weight and aero can be brought forwards to maximise the relatively wide front tyres.
Porsche realised that, to beat Audi and Toyota with their larger and heavier power units, a different approach was required. With a downsized engine and greater ERS, the car could still create prodigious amounts of horsepower. Yet, with a tiny power unit, the whole car could be repackaged to maximise front aero, and improve efficiency.
Thus, Porsche went for the smallest power unit it could – a mere two litres, formed into a “v4” layout for a short and structurally stiff installation. Being turbocharged brings the power level up to ~500hp, and then two energy recovery systems are used. The first is a conventional KERS-like system, recovering energy under braking from the front wheels, storing this energy in Li-Ion batteries inside the cockpit. Then, a clever turbo waste energy recovery system is used, which takes a branch off the exhaust system to use the exhaust gas pressure to spin a turbine, which is connected to a generator. At full throttle, there is more than enough exhaust gas pressure to power the turbo engine. Rather than vent this excess pressure through a wastegate valve, the pressure spins the turbine\generator to create power when at full speed along the Mulsanne straight. Again, this energy is stored in the central battery, with the car being entered in the maximum 8Mj category. Under acceleration, the control electronics switch, and the battery sends its energy to the front motor, which is now in its generator mode to power the front axle of the car. This gives the car 4WD under ERS-boosted acceleration.
As with current F1 practice, it pushed the power unit as far forward in the car as possible. This pushed the weight distribution forwards too, which makes the most of the of the front tyres. Although still a rear-engine and rear-wheel drive car, the engine being further from the rear axle line creates a problem with the gearbox stretched in-between the two, in that it makes it heavy. So, Porsche opted for a lightweight carbon-fibre outer case, with an aluminium inner casing to house the gear cluster itself. Although the alloy gear case sits inside the carbon outer shell, the two parts are separate and the gear case can be easily removed. This design echoes a similar set up to that used by the Mercedes AMG F1 team.
Now with enough power, and enough weight towards the front, Porsche needed to make the car’s aerodynamics more front-end focussed and efficient. Rather than open bodywork formed from vanes around the front wheel arches, Porsche chose to keep much of the bodywork enclosed, with only small outlets behind the front wheel arches to vent the wake from the front splitter. This allows the front splitter to create downforce, and the unpunctured bodywork keeps drag low. The small engine then allows the rear bodywork to be very tight and low, which further aids drag reduction and sends clear air to the rear wing for more downforce.
Although design is a key part of the car’s success, the team’s experience led to a car that was also well-engineered and well-prepared. Throughout the 24 hours, the cars ran be reliable, an impressive feat for three relatively new cars. With the team winning Le Mans, and with them now chasing Audi in the full WEC series, Porsche is again a threat to all comers in the endurance racing world – The result of which leaves Audi and Toyota now having to work very hard to keep up with Porsche’s efficient new-generation downsized race car.