In the second of three features on Ferrari's 2015 season, ESPN looks at how the team unlocked a big improvement in performance from its engine.

In order to fully appreciate the progress Ferrari made with its power unit this year, it helps to understand the importance of efficiency under the current regulations. Formula One's V6 turbo hybrids have been criticised since they first started testing in 2014, yet they are remarkable pieces of engineering. After years of running high-revving, naturally-aspirated V8s and V10s, F1 took on the challenge of making something more frugal. What it ended up with was some of the most efficient engines ever made in terms of power per fuel consumption.

"Generally speaking, if you want to find efficiency in a combustion engine you tend to look at diesel engines on huge ships that go at 100rpm or something," Ferrari technical director James Allison explained to ESPN. "They are so slow that they have very little friction and they are so steady in the way that they operate that they can be set up to work on one cycle for optimum efficiency. They work like that for days on end without anyone touching them and they have great big heat recovery plants the size of a house.

"That's the sort benchmark for a thermally efficient internal combustion engine and we've got these little screaming V6s that don't just sit there at 100rpm, they zap up and down, go round laps and fight with one another on the track. Yet they are as efficient, in fact you could argue more, than this one-purpose, solely-designed-for-fuel-efficiency engine that has got, on the face of it, a lot more in its favour than what we do. It's very, very impressive that these little racing engines can achieve that level of fuel efficiency."

Of course, beating a ship engine for efficiency is not what Ferrari set out to do over the winter, its target was to close the 50 bhp gap to rivals Mercedes. Yet the strict fuel flow limit in F1's regulations means the only way to unlock more power is to get more energy from the fuel at your disposal.

"It's a really strange formula to get your head round," Allison adds. "I've often seen non-F1 engineers look at me like they don't believe me when I tell them this, but everyone thinks more power means more fuel consumption, but this set of rules is absolutely not set up that way. More power means less fuel consumption in this sport. The way you get more power is by making a power unit that has less fuel consumption.

"Everybody's fuel flow is limited to the same value and every single car on the grid is going down the straight with their fuel pump pumping at exactly the same number of kg per second, so with that fixed fuel flow, the more efficient the engine is the more horsepower you deliver to the wheels.

"The more horsepower you deliver to the wheels, the less time you spend on the straight because you get to the end of the straight quicker and that means the less time you spend at the full fuel flow on the straight and the less fuel you consume.

"So fuel efficiency delivers you lap time in two ways, it both gives you the normal thing of extra horsepower at the wheels but also you the added benefit that you use less fuel. That means at some of the races that are very tough on fuel consumption, you are not having to lift off at all.

"A less powerful engine, a less efficient engine, will have to lift off at the end of the straight to save fuel, whereas a car with an efficient engine can keep the foot planted until the end of the straight. This formula is all about efficiency and that is what it was designed to achieve."

Early in 2014 Ferrari already knew where its power unit's inefficiencies were. A decision to prioritise aerodynamic packaging at the rear of the car meant the team compromised some aspects of the engine, including the turbocharger and the MGU-H part of the energy recovery system. It had been a conscious decision in an attempt to maximise downforce, but it did not pay off and left the team a long way off the pace at most circuits.

A ban on in-season engine development in 2014 meant Ferrari was stuck with its decision all year, but it could start making sure it rectified the issues for 2015. Alongside improvements to the internal combustion engine, a larger turbocharger was fitted, which in turn produced more energy for the MGU-H to recover. More energy for the MGU-H meant more power to deploy on the straights, thus shortening the time spent on full throttle and saving more precious fuel. In short: Win, win, win.

Peter J Fox/Getty Images

But in order to maximise the advantage of its new, more efficient engine and energy recovery system, Ferrari needed improvements from fuel supplier Shell. Although fuel is at the very core of the upward spiral of efficiency explained above, Allison admits the team overlooked the importance of its partnership with Shell in 2014.

"The new leadership on the Ferrari side was very keen to bring the power of all their technical partners and the engineers inside Ferrari to try to make sure that we were competitive in 2015," Allison says. "I think Shell were always waiting for us to be involved in that way and the change was on our side to open our doors."

In total Ferrari and Shell claim the performance gain from fuel upgrades alone to be worth 0.5s a lap and an average of 30 seconds per race. In the fight against Mercedes those are big numbers and worth roughly 25% of the overall performance gain Ferrari has made with its power unit this year. Pinpointing exactly when Ferrari levelled up with Mercedes is harder. The opposition points to Maranello's big combustion upgrade at the Italian Grand Prix.

"Since they introduced the updated engine at Monza, I would say that they matched us in terms of power and efficiency," Mercedes' team chairman Niki Lauda said towards the end of the season. "The 20bhp advantage we had is now gone. Now we are at the same level of power. We know for certain because of the reference we have."

But Allison insists it was a gradual improvement from all sides of the engine programme rather than one single step.

"Monza wasn't the magic bullet in the season, it was several steps, all of which were very useful. We would have made a step forward without this, but Shell's input was very useful."

And that is the best way to look at Ferrari's progress this year. The realisation that it had got its priorities dramatically wrong at the start of the new engine formula in 2014 triggered the need for change that snowballed through the winter to deliver a big step change in performance for 2015. From there constant progress has been made with the efficiency of the power unit to level up with Mercedes by the end of the year. The next step, and undoubtedly the hardest yet, will be to move ahead in 2016.

The final part of ESPN's analysis of Ferrari will assess the team's chances of closing the gap to Mercedes next year.