A 24-hour Battle of Speed and Efficiency

June 1, 2016

Dan Carney

World endurance racing at Le Mans is no longer only about who finishes first after a grueling 24 hours, but also about winning with the greatest energy efficiency. For 2016, the Federation Internationale de L’Automobile (FIA) has tightened the fuel allowance for the top-echelon Le Mans Prototype 1 (LMP1) category in an attempt to further link racing and efficiency — and as an attempt to arrest the stunning increases in speed seen during 2015.

This year, the fuel allowed per lap has been reduced by 10%, for an approximate reduction in energy per lap of about 10 megajoules (2.77 kW·h). Peak instantaneous fuel flow also is reduced by 10%.

At the same time they are facing this new energy restriction, the Audi and Toyota works teams are addressing performance deficits that put their hybrids behind the Porsche team that dominated both the World Endurance Championship and the 24 Hours of Le Mans in 2015.

Porsche ran the most electrically intensive of the available formulae in the FIA’s mandatory hybrid-electric specifications, a system that provides 8 MJ (2.22 kW·h) of electric assist per lap to the combustion engine.

Previously, Toyota and Audi opted for more combustion-oriented specifications, with Toyota at a 6 MJ (1.66 kW·h) level with a large-displacement naturally aspirated V8 gasoline engine and Audi with a 4 MJ (1.11 kW·h) electric drive mated to its turbodiesel V6 combustion engine.

Porsche’s speed in 2015, along with the reduced fuel flow for 2016, convinced Audi and Toyota each to move up one step in the electrification hierarchy. Toyota joined Porsche at 8 MJ of assist, while Audi prefers the additional fossil fuel available to its turbodiesel at the 6 MJ level.

According to 2016 rules, gasoline engines with 8 MJ electric assist are allowed 134.9 MJ (37.47 kW·h) per lap of fuel and a maximum fuel flow of 87 kg (192 lb) per hour. Diesels with 6 MJ electric assist have 131.2 MJ (36.44 kW·h) of fuel allowed per lap and 77 kg/h (169.7 lb/h). If Audi had opted for 8 MJ of assist to match its gasoline-powered rivals, its energy per lap would have been limited to 126.3 MJ (35.08 kW·h) and peak fuel flow would have been 74.1 kg/h (163 lb/h).

A review of the LMP1 contenders for 2016 shows all three works team cars measure 4650 mm long, 1900 mm wide and 1050 mm high (183 x 75 x 41 in), with a minimum weight of 875 kg (1929 lb). The three teams rely on Michelin for their tires. A required change to the cars’ design is a 45% enlargement of the openings in the tops of the front fenders. This is intended to reduce lift in the event a car turns sideways at speed.

And LMP1 teams have agreed with the FIA to limit entries to two cars per team, in an effort to contain costs. This means that it is not possible, however difficult and unlikely, for a single team to sweep the top three podium spots. It also might increase the likelihood of each team earning a spot in the final photograph.

Porsche 919 Hybrid

As the incumbent champion, Porsche has unsurprisingly returned with less-dramatic changes than its challengers. The 2016 919 Hybrid retains its 2.0-L 90° V4 gasoline engine boosted by twin Honeywell turbo-chargers that drives the rear wheels through a 7-speed sequential gearbox. The case’s construction is a blend of carbon fiber with titanium inserts and aluminum castings. The combustion engine uses Bosch engine management and is fed from a 62.5-L fuel cell.

Electric power from the 8 MJ-class hybrid drive system is stored in an 800-V A123 Systems lithium-ion battery pack; that power is directed to the front wheels. By doubling the usual system voltage from 400 V to 800, the 919 enjoys quicker recharges and is able to use thinner gauge (and thus lighter) wiring. The 919 rolls on BBS forged magnesium wheels.

However, while the fundamentals of the car are carried over, each of these parts has been massaged and refined, according to Porsche team principal Andreas Seidl. That’s because as fast and dominant as last year’s car was relative to its competitors, the team was nevertheless immediately aware of specific shortcomings that needed to be addressed, he said.

Last year’s car was built after only a year of LMP1 competition for Porsche, so naturally there was the opportunity for many more lessons to be learned. That experience applied to this year’s car, Seidl explained.

“There’s not a single main evolution,” he said. “We are quite happy with the basic concept of the car. Which means the new car is again more an evolution. They worked on really every single part of the car. Now it is all about getting the maximum performance out of this package as well as making sure the car is reliable.”

The aerodynamic refinements made to the 919 aren’t immediately visible, but they have achieved the goal of reducing drag to help offset the loss of available fuel, according to Seidl. Trilux quad LED headlights are both brighter and more efficient than last year’s lights, saving energy and helping the team’s drivers see better during the long hours of darkness at Le Mans.

Meanwhile, the team toiled to wring additional fractions of percentages of efficiency from each of the car’s parts, seeking to mitigate the speed loss from the reduced fuel allowance.

Audi R18 e-tron

In contrast, Audi was frustrated by last year’s results and built a new car. The clean-sheet design carried over little more than the R18 e-tron name and the 4.0-L diesel combustion engine claimed to produce 514 hp (383 kW). The basic design of the 120° V6, boosted via a single Garrett variable turbine-geometry turbocharger, is now in its sixth racing season after starting life at 3.7 L. The latest iteration drives the R18’s rear wheels through an Audi-designed 6-speed sequential gearbox that uses Xtrac gears. The hybrid’s fuel capacity is 49.9 L.

Audi ditched its previous Flybrid Automotive energy storage system in favor of a lithium-ion battery pack because of the need to recover more energy for its 6-MJ boost system powering the car’s front wheels. The single front electric motor drives the front wheels through a limited-slip differential.

The R18’s chassis features an entirely new aerodynamic concept and completely revised front-suspension design, as the company is pressing for every possible advantage. The biggest innovation may be a change to a central high-pressure hydraulic system for running all the car’s ancillary systems in place of various electric servo motors, which are much heavier.

The new hydraulic system provides boost for the power steering, brakes and clutch. It also operates some engine subsystems. Such a centralized approach may appear to present a single point of critical failure that could knock out multiple systems, but Audi engineers are satisfied with its projected reliability; the central hydraulics did not prove to be an issue during the 2016 Silverstone 6-hour race.

Audi’s diesel engine is heavier than its competitors’ gasoline engines, while the cars’ minimum weights are the same. This handicaps Audi regarding the weight of other components, noted Technical Director Jörg Zander. Considering his car’s heavier combustion engine, the idea of using a battery pack as heavy as competitors’ wasn’t a practical option, which steered the team to the 6 MJ category.

The smaller battery pack also provides a potential packaging advantage compared to rivals’ cars. But the new lithium-ion pack is considerably larger than the previous flywheel storage system, which presented a challenge to designers.

Another concern was uneven tire wear, which handicapped Audi last season because teams expect to go a pit stop or two with just fuel rather than mounting new tires at every stop. Over the course of a 24-hour endurance race, the accumulated time of additional tire changes adds up, so teams typically double- and triple-stint the tires to reduce that time spent stationary in the pits. Audi uses OZ magnesium wheels on its car.

This year’s R18 has a much narrower central section of its monocoque and a raised nose to flow more air between the wheel arches and the nose. According to Zander, this lets the front wing provide more downforce with a smaller angle of attack, for reduced drag.

The team scrapped the linked hydraulic damper system used last year to provide anti-dive characteristics for keeping the R18 aerodynamically stable. This year’s car uses more conventional dampers and also replaces the front unequal-length control arms with individual links to optimize geometry for improved use of the tires.

“We had an issue with tire temperatures last year where we had to accept big differences between the front and the rear,” Zander said. “This looks much better.”

Toyota TS050

Toyota was the furthest adrift last year in terms of performance, so the company moved its TS050 racer much closer to Porsche’s winning 2015 formula. In place of the TS040’s 3.7-L naturally aspirated gasoline V8, the TS050 features a 500-hp (373-kW) 2.4-L twin-turbo gasoline V6.

The team was shocked by Porsche and Audi’s progress last year, admitted Toyota technical director Pascal Vasselon.

“With stable regulations, when you gain 1-1.5 seconds [in lap time compared to the prior year] you are happy,” he said. “Last year we gained 2.5 seconds. We were quite happy and expecting to be competitive. We found the others have gained 5 seconds.

“Things which are completely unbelievable but are facts,” Vasselon asserted. “At Spa [2015] we realized we were not chasing tenths of seconds, but four seconds.”

It was this stunning realization that drove Toyota to pull ahead adoption of the new engine to this year rather than 2017. The problem with last season’s naturally aspirated V8: while its output was comparable to a turbocharged unit, its powerband was narrower. This left drivers at a disadvantage on the track.

“So the decision was made in May last year to change to turbocharging and not wait for 2017,” revealed Vasselon. “The sweet spot [for naturally aspirated engines] is quite narrow. A turbo will give you a very wide range of rpm with fuel efficiency and at different temperature and pressure.”

The TS050’s gearbox is a redesigned transverse 7-speed sequential unit with a ZF multi-plate clutch. A new gearbox was needed because of the additional torque produced by the boosted engine.

“The gearbox had to be totally different,” Vasselon said. The trouble was that last May, when the team made the decision to switch to a turbocharged engine, they had already passed the deadline for designing a new gearbox for 2016. So the Toyota engineers designed and built a new gearbox using model-based development to represent the expected loads from an engine that did not yet exist. The good news is that this approach worked.

“It is clear that we are able to get a good anticipation of the performance of an engine before building it, as well as the dynamic behavior of the mechanical elements,” said Vasselon.

But the car has yet to race 24 hours. “At the moment so far we have not seen reliability issues in the powertrain,” he said, almost as if hoping it will remain so. If it does, it may be because the team was conservative in its design, producing a gearbox that surely is heavier than is absolutely required. “We could do some weight-saving” on the transmission, Vasselon conceded.

The TS050 uses Akebono monobloc brake calipers and RAYS magnesium wheels. It carries 62.5 L of gasoline.

For its driveline, Toyota dumped the supercapacitor used for last year’s 6 MJ electric drive system, replacing it with a lithium-ion battery pack using a Denso inverter. It drives an Aisin front electric motor and a Denso rear electric motor that assists the combustion engine driving the rear wheels. The electric motors combine for another 500 hp, but 2016 regulations limit electric drive at the Le Mans race to a maximum of 408 hp (304 kW).

Toyota switched from supercapacitors to batteries because batteries’ characteristics better-suited the 8 MJ design, Vasselon said. Also, battery technology development is outpacing that of capacitors. “The batteries initially are lacking power density,” he said. “A battery is good for energy density, good by weight.

“The capacitor is the opposite, it has good power density,” he continued. “These devices require very high power. You have to capture the energy generated in 2 seconds. Two years ago, batteries were not good enough. Now we achieve higher power capacity with the battery than with the capacitor.”

Who will win?

The only reason OEMs compete in a full season of LMP1 racing is for the chance to win the big one: the 24 Hours of Le Mans. The season’s other endurance races are barely more than test sessions when compared to the importance of the iconic French competition. But Le Mans has some of its own rules, such as the cap on electric assistance for 2016 and the cars run a unique aerodynamic configuration not seen at the Silverstone season opener in April.

Nevertheless, as with times from test days, the results hint at the relative performance the might be expected at Le Mans. Audi Sport’s Team Joest won the Silverstone race, closely followed by Porsche. But team Joest was subsequently stripped of the win for a minor technical infraction involving the post-race thickness of a chassis skid plate, a decision team leaders decided not to appeal.

Regardless, it suggests that Audi’s focused effort to regain its competitive position compared to Porsche has worked. When this issue of Automotive Engineering went to press in late May, both teams showed they had an excellent chance to win at Le Mans. Toyota did not look as good at Silverstone, with much slower lap times. But that car’s compressed development schedule likely means there could be more to be extracted before the June 18-19 race. It surely will be exciting to see each team’s technical strategies play out on the track.

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