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F1: BURNING LEAN – PRE-CHAMBER COMBUSTION TECHNOLOGY
Unsung at the start of the 2014 season, amongst the talk of lack of noise, reliability and ugly noses, F1 cars cut fuel consumption by a third over the winter. While most of us would grab at the chance to find such a gain with our road cars, while also getting more torque for the same power, this narrative was not seized upon by the media at the time. The FIA brought this reduction with a severe restriction on fuel, by limiting the race fuel load and average fuel flow. This went hand in hand with a reduction in the engine size to just 1.6l, but allowed efficiency improvements by adding a turbo and direct injection. With these changes, the rules made quite an effort to improve F1’s green credentials. But, there was another innovation introduced, initially by Mercedes AMG, that took the new lean burning F1 power units and applied pre-chamber combustion technology. This raised the Power Unit up to an unprecedented level of efficiency.
To make power with so little fuel, you cannot simply burn less fuel, the air-fuel ratio would be too lean and the engine would suffer overheating, knock and a lack of power. Mercedes AMG adapted an existing combustion technology known as ‘pre-chamber combustion’ from the start of 2014, that allowed lean combustion, but still lots of power. Ferrari followed in mid-2015 and Renault in mid-2016. Leaving Honda to come on board with this tech in 2017 The technology pre-chamber combustion, inaccurately termed Turbulent Jet Ignition (TJI) by the media, which is a Mahle branded system (www.mahle-powertrain.com/en/experience/mahle-jet-ignition/) used by Ferrari (like calling a pen a biro or a vacuum cleaner a hoover). This is not a new technology, ironically it was actually used on early Honda Civics in the seventies and in different guises by a range of other manufacturers.
In normal petrol engines, even race car engines, the ‘bang’ happens in the combustion chamber, which is formed inside the cylinder, between the piston crown and the cylinder head. Fuel and air are mixed inside the combustion chamber and ignited by the spark plug. If the fuel and air are mixed correctly there will be full and fast combustion leading to the piston descending on a power stroke. With too little fuel compared to air, you will not see the weak mix fully ignite, creating a less efficiency engine producing less power. In a modern F1 engine if you run the combustion too lean, i.e. too little fuel to air, you’ll lose power and have all the bad associated results mentioned above. Careful combustion and fuel blends will help relieve some of the problems, but only to a point. It’s been evident Renault and Ferrari have both relied on anti-knock agents in the fuel to improve the situation, tell-tale red ‘rust’ deposits on the inside of the exhaust pipes, show the additive ferrocene is being burnt in the fuel, this is well known ferrous based (i.e. iron) anti knock agent. Even in testing for this season, the Ferrari exhaust smell like paraffin, the sharp acrid fumes leading to watery eyes in the pit lane from the unknown fuel additive.
There are many lean burn combustion technologies in use on both diesel and petrol engines, but the F1 rules preclude a lot of these from the demand for spark ignition and limits of fuel single injectors, bans on compression control and other mapping restrictions. Mercedes AMG found the pre-chamber tech met the rules and were able to adapt it to the needs of F1.
In a pre-chamber set up the fuel is split into two separate combustion chambers for each cylinder. Combustion starts in a small separate chamber inside the cylinder head (hence pre-chamber). Above where the spark plug and direct injector would normally be is a chamber, a rich fuel and air mixture is created in the pre-chamber. Below in the main combustion chamber the space is simultaneously filled with a lean fuel mixture, by the portion of fuel injector spanning the two chambers. Ignition commences with the spark plug, as demanded by the rules, inside the pre-chamber. As the burning fuel and air expand, holes in the bottom of the pre-chamber send jets of these burning gasses down into the main combustion chamber, which fully ignites the weak mixture and creates the power stroke. With this set up the spark plug is used effectively with the rich fuel air mix and the leaner mix is fully burnt with the multiple jets exiting the pre-chamber. Simple in principal, but difficult to get right with; injector design, fuel blend and ECU control critical to the process. Thus, allowing a strong power output with less fuel overall.
But, this set up can be developed to be tuned for either efficiency or outright power. Which are needed for the race and qualifying, respectively. By varying the ratio of fuel between pre and main chambers, performance can be altered to give either better economy or more power. But to do this the fuel injector and its control must be very complex, as the single injector spans the two combustion chambers, it needs to be able to delivery different amounts of fuel into each of the two combustion chambers, not something usually achieved with a single injector, so the engine effectively needs two injectors formed into one. Mercedes ability to run well in the race and still get a power boost for qualifying shows they have solved this problem, which is an area that Ferrari are focusing on for this year.