Engine tokens: love them or loathe them, they're here to stay - as the means by which F1 can, on the one hand, allow development of the power units introduced in 2014 while, on the other, not let costs spiral out of control.

The system is complicated but there is method in the madness. Because the hybrid technology is relatively new, it was decided not to follow a V8-style engine freeze. Moreover, the fear was that any advantage gained by a manufacturer - Mercedes, as it turned out - would be locked in through to 2020.

A quick refresher: the power unit is divided into groups of functions, with each given a weighting of either one, two or three development tokens. So, for example, if a manufacturer wants to change the power electronics it's one token and if they want to change the turbine inlet it's two tokens; a combustion chamber upgrade is three.

The complete power unit is made up of the sum of these ranked items - 66 tokens in all - and each year manufacturers are allowed to choose what parts of the engine they wish to develop.

In 2016, they will have 32 tokens (almost 50 per cent of possible development) to spend. That allowance will then be steadily reduced with 25 tokens permitted in 2017, 20 in 2018 and 15 in 2019. Previously, manufacturers could spend only two tokens that year.

But on the face of it, with less than half of their hybrids now able to be developed, it looks like time is already running out for Renault and Honda in their bid to close in on Ferrari and Mercedes.

Yet according to Sky F1 pundit Mark Hughes, there's still plenty of opportunity. "When you look at the weights, quite significant parts of the engine, like the compressor or the combustion chamber, they're quite small proportions of what's allowed, certainly within the next couple of years," he says.

"32 tokens are allowed next year. A compressor change requires three and a combustion chamber change three, so there's still scope to effectively completely re-design the engine. If you've badly messed up at this stage, you can still easily put it right.

"At the moment, you're not being strangled by the regulations - they aren't preventing you from putting the design right. They will do eventually, if you leave it long enough before putting it right, but that's 2019.

"If you haven't sorted it out by then, you don't really deserve to be in F1."

Renault are to return as a fully-fledged team in 2016 while Red Bull will use Tag Heuer-branded Renault units

What's also helped the stragglers is the loophole Ferrari spotted which circumvents the cut-off date for engine homologation - the end of February - and allowed for in-season development this year.

"For me, it is a much better system because if a modification doesn't work, you can find out right away and use more tokens to put it right again, rather than being stuck with it all year. We saw, for example, in 2014 that Ferrari completely messed up on the sizing and layout of their MGU-H and compressor; they knew exactly what the problem was but they were handcuffed," says Hughes.

"So had this year's interpretation been enforced then, they could have fixed that well before the end of the season and had a much more respectful season."

So what areas will they likely target for the most performance benefit?

"The most gain is always in the internal combustion engine itself because everything else is just derived from that. It's not like you're craving additional power from the other [ERS] systems; it's just that you're capturing a percentage of the power that would normally get wasted. But it still originates from the internal combustion engine, so the more power you can get from that the better everything works.

"The whole system then works better: it compounds the advantages you get by improving the internal combustion engine. The more power you produce, the more heat you're creating so the better the MGU-H can work. And the more power you're producing, the faster you're going so the harder you're braking and the more MGU-K power you get. The whole thing works better."

Honda's focus for 2016 is expected to be on changing their engine's turbo compressor - part of a 'size zero' concept to fit a particularly svelte McLaren MP4-30 chassis - which has been found wanting in 2015.

"Honda have decided they wanted their compressor within the vee of the engine," Hughes explains, "which for a conventionally-sized compressor you couldn't do, you'd have to have it forward of the vee. But to get equivalent power from the compressor, a smaller one has to be turning faster."

Which is something they haven't managed to achieve this year. But at least Honda have the excuse of arriving late in F1's new era; Renault have struggled for two seasons already.

"The job that Mercedes did first time out, resolving the conflicting demands of this kind of engine, has made it look bad for the others," adds Hughes. "Ferrari pretty much got there in one leap from '14 to '15 so it's competitive with the Merc on power.

"Renault is difficult to assess because it's had a reliability problem - which didn't show itself in simulation, it only showed itself at the start of the season - which has taken half a season to put right. And until that was put right, there was no point in putting any performance development on the engine.

"The one they've had available since Austin [and which ran for the first time in Brazil] is supposedly worth a couple of tenths, but the big gains will be in the 2016 engine - or so they believe."