When Honda announced in 2013 they would return to Formula 1 as a power unit supplier to McLaren, many with fond memories of Ayrton Senna and Alain Prost’s dominant title wins of the eighties and nineties hoped for a return to those glory days.
The engine Honda introduced in 2015, RA615H, was designed to meet what McLaren’s then-boss Ron Dennis termed a “size zero” concept. It was not Honda that chose this design direction. McLaren’s strong desire for Honda to keep the power unit as small and as compact as possible was the driving factor.
However the Honda team did not create a compact design at the expense of performance. In hindsight, the team had set specifications for the V6 combustion engine development without a clear idea of the turbo pressures required or the ideal air-fuel mixture. The Honda Power Unit team assumed they would be able to figure it out in due course.
That assumption did not survive its first contact with reality. When the 2015 season rolled around, it was painfully obvious that they were severely lacking performance. So much so that at one race Fernando Alonso famously commented over the team radio to his engineer “It feels like GP2. Embarrassing. Very embarrassing.” His choice of event was no coincidence either: The Japanese Grand Prix at the Honda-owned Suzuka circuit.
Engineers at Honda were already at work trying to improve performance for the 2016 season. The RA616H had a revised plenum and the turbo housed inside the V-bank of the engine was increased in size to keep up with the increasing power demand of the internal combustion engine (ICE).
While the revised plenum increased the overall height of the engine by a few centimetres, it allowed for a more optimised intake system which facilitated smoother airflow into the combustion chamber by reducing the number of bends in the branches of the intake system. The variable intake system housed inside the plenum was also redesigned before that year’s British Grand Prix to further optimise airflow into the engine.
“At the time, we were trying various different technology upgrades, so it is hard to say that the revisions in the intake system were the only effect, but I do recall an improvement of about 7kW in power after the changes with McLaren’s help,” recalls Tetsushi Kakuta, executive chief engineer of F1 power unit development at Honda Racing Development Sakura.
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In a world where 5kW can make a big difference in the overall competitiveness of power units, this was clearly a big step forward. However, the changes in the intake system also brought some major setbacks.
Because the overall height of the engine had increased, the engine’s centre of gravity also moved up by about 8mm. In the pursuit of improved reliability and performance, the power unit also became around 4.2 kg heavier.
For the next season, McLaren’s designers pushed Honda to focus on lowering the centre of gravity and help to ‘shrink wrap’ the top of the engine cover for better aerodynamics. Taking this feedback to heart, the Honda team went back to the drawing board to create an all-new design for the 2017 season.
The resulting RA617H was the first of Honda’s engines to implement the ‘split-turbo’ design which Mercedes adopted from the outset of the regulations with its standard-setting power unit. The compressor sits in the front of the engine and the turbine at the rear, with the MGU-H motor housed inside of the V-bank.
Switching to this design meant Honda’s turbo could now be sized without being limited by the space between the cylinder banks. It also helped lower the centre of gravity by bringing down the position of the shaft connecting the compressor and turbine. With this change, the RA617H was able to drop the CG height by over 25mm compared to the previous year.
However, the most significant technological improvement over the RA616H was the introduction of the pre-combustion chamber. This is a separate chamber inside the cylinder head connected to the main cylinder area that surrounds the tip of the spark plug.
In this chamber, a richer air-fuel mixture is ignited and strong jets of hot gasses escape from tiny holes in the walls of the pre-combustion chamber. These strong jets of hot gasses quickly ignite the rest of the air-fuel mixture in the cylinder. This pre-chamber ignition technology enabled Honda to make large gains in performance that would not have been possible with conventional spark methods.
While the introduction of the pre-combustion chamber provided great potential for performance improvement, development time was required to fully get a handle on pre-chamber ignition. The first iteration of the design looked like a cap with perforations screwed onto the tip of the spark plug.
Optimising the location where the jet of hot gasses emitted from the holes perforated in the pre-combustion chamber landed was a critical development point to gain performance. Additionally, if the surface of the pre-combustion chamber was not cooled properly, the remaining heat from the surface would auto-ignite the main chamber, causing pre-ignition.
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While the Honda team resolved this by making the pre-combustion chamber out of a material with high-heat transfer properties, this caused the spark plug to retain much of the heat, causing them to melt. Honda worked with NGK to design a pre-combustion chamber that was fully integrated into the spark plug to further help with heat dissipation.
The number of holes, the hole diameters, and the volume of the pre-combustion chamber were also optimised for maximum flame propagation. All this enabled Honda to realise a performance gain of around 40kW over their original design introduced in 2015.
Despite their struggles and rocky relationship with McLaren, Honda managed to make performance gains. But so had their rival power unit manufacturers.
McLaren finished the 2015 season, their first year back with Honda, a lowly ninth in the championship, with only the Manor team behind them. They improved to sixth the following year, but chronic unreliability pegged them back to ninth in 2017. McLaren’s patience had run out, and well before the season was over it became clear the two parties would go separate ways.
Earlier in the season Sauber had announced it would become a second Honda user in 2018. But the team was subsequently sold to new owners and within three months new team principal Frederic Vasseur had cancelled the Honda deal.
However within two years Honda became a two-team effort. A successful year supplying engines to Red Bull’s junior team Toro Rosso (now AlphaTauri) paved the way for a fruitful tie-up with the senior squad.
For 2018 season with Toro Rosso, Honda turned their attention to further improving the thermal efficiency of the internal combustion engine. This is typically done by increasing the compression ratio or the air-to-fuel ratio. Pre-chamber ignition technology allowed Honda to make progress on both fronts, but the power unit development team felt that this alone was not enough to catch the leaders. To fill this gap, Honda developed what they called ‘High-Speed Combustion’.
In a conventional combustion process, the flame front fans out from the tip of the spark plug where it is ignited. However, this process takes time for the flame to fully ignite all the fuel in the piston. It can also lead to incomplete combustion and because the flame propagation takes time, meaning full combustion may not be achieved at top-dead centre (maximum compression) where maximum power can be produced.
Pre-chamber ignition speeds up flame propagation by igniting a larger flame front area in the piston using the hot ignited gasses jettisoned out of the pre-combustion chamber. Taking this idea one step further, Honda’s engineers came up with its High-Speed Combustion technique.
During this process, the hot ignited gasses from the pre-combustion chamber are directed towards the cylinder walls and the heat from this auto-ignites air-fuel mixture. Thus, instead of the flame front propagating centre-out, the flame ignites in a ring shape (like a doughnut) and propagates outside-in, in a three-dimensional fashion. This vastly increases the speed of the flame propagation, hence the name High-Speed Combustion. This technology was implemented at the Russian GP in 2018 in the ‘spec three’ version of the RA618H.
This idea was further developed on the RA619H which was used the following year by Toro Rosso and – for the first time – Red Bull.
“By utilising High Speed Combustion, we were able to vastly increase both the compression ratio and the air-fuel ratio” recalled Kakuta. “By optimising the valve geometry and repositioning the injector that used to sit on the exhaust side, the RA619H was able to make significant gains over the previous years.”
Honda’s team was met with a different culture when they switched from McLaren to the two Red Bull teams. “Red Bull offered to provide more cooling if the power output could be increased,” Kakuta explained.
“When we were partnered with McLaren, they claimed that their previous engine supplier (Mercedes) could run their PU hotter and asked us to raise our operating temperature window. With Red Bull, perhaps Renault’s operating temperatures were much lower and they were happy to work with us to optimise the operating temperature window.”
Red Bull started with securing space and airflow for a charge air cooler. By decreasing the inlet temperatures, the compression ratio of the engine could be increased without worrying about knocking. In 2019 Honda also started developing fuel blends and made big leaps on this front. Finally, at the Austrian Grand Prix, Honda achieved its first victory of the V6 hybrid turbo era.
It was the Japanese manufacturer’s first F1 win since 2006. But while that proved a one-off for its short-lived manufacturer team, there was more to come for its hybrid engine project.
For 2020 their goal was to “maximise the potential” of the power unit design based on the RA619H. One of the major disadvantages of utilising High-Speed Combustion was the decline in performance over the lifetime of the engine. Because combustion pressure was increased due to the High-Speed Combustion process, the pressure applied onto the cylinder walls by the piston ring also increased and caused the cylinder walls to deteriorate quicker. As a result, surface wear accelerated and the seal between the piston ring and the cylinder wall leaked, leading to pressure loss and power output loss.
To combat this, Honda Racing Development (HRD) Sakura collaborated with Honda’s Kumamoto Manufacturing centre to develop a special plating process to coat the inside of the cylinder walls, dubbed “Kuma-Mekki” (“Kuma” from “Kumamoto” and “Mekki” meaning ‘plating’). This plating was a breakthrough for Honda as it allowed the power units to maintain their performance until the end of life. This increase in reliability also allowed the team to run the engines in higher modes for longer durations over the race weekends. This was precisely what “maximising the potential” was all about.
Upon deepening their understanding of the High-Speed Combustion process and optimising compression ratio and air-fuel mixture to maximise power output and reliability, it became clear to the power unit development team that the ICE design Honda had been using since 2017 had reached its limits and a whole new concept was required to further improve performance.
Originally, the HRD Sakura team planned to introduce this new design in line with the Formula 1 regulation change in 2022 (originally planned for 2021 but pushed back a year due to the Covid-19 pandemic). However in 2020 the Honda board made an unexpected decision to pull out of Formula 1 entirely at the end of the following season, prompting the team to bring this design forward.
The RA621H was a full bottom-up redesign for the ICE. One of the goals of this redesign was to integrate better into the Red Bull chassis and help contribute towards better packaging for aerodynamic performance. On the RA621H, the valve angle was reduced to optimise for a higher compression ratio, and the length of the valves was also reduced to help with packaging.
The team also brought the exhaust-side camshaft further inboard by reducing the size of the gear train. Overall, this brought the outer-facing exhaust valves over 30mm further inboard and allowed for tighter packaging on Red Bull’s end. To further contribute to a smaller engine cover, Honda also removed a rib that was on the exhaust side originally intended to increase the rigidity of the cylinder head cover.
However increasing the strength of the cylinder head and block was also necessary as the pressures they were subjected to had increased over the years of development of the ICE. In 2018 and 2019, the aluminium alloy used for the cylinder heads was changed to a stronger alloy. The cylinder block also followed suit in 2021 when the design of the block was completely changed for the RA621H. The new, stronger material also allowed Honda to cut down some of the material thicknesses, allowing for a lighter but stronger package.
Having powered Red Bull to three wins in 2019, three further victories followed in the shortened 2020 season, one of which came courtesy of AlphaTauri. This fell short of the desired progress on both sides. Honda publicly claimed its withdrawal from F1 was for other reasons, but the limited success it enjoyed up to that point surely had a bearing on its fourth withdrawal from the sport.
But from the start of 2021 it was clear the Red Bull-Honda package was capable of fighting Mercedes for the championship. This was the toughest opposition the Silver Arrows had faced since the V6 hybrid turbos were introduced. The fight went to the final round, and while Mercedes prevailed in the constructors championship, the drivers trophy went the way of Red Bull’s Max Verstappen, albeit in contentious circumstances.
It would not have happened without an incredible effort from Honda which, from their lower point in 2015, propelled them past Ferrari and Renault to the same level as Mercedes. Over seven seasons, Honda increased their overall power output by some 100kW (around 134 bhp) – a monumental feat.
Honda is unquestionably the “most improved” power unit manufacturer in this turbo-hybrid era. Its engines are now Red Bull branded, and while they have had some problems at the beginning of the new season, they have already taken their first victory of the year in Jeddah, and could yet propel Red Bull to even greater successes than they enjoyed last year.
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