Lean burn
‘The dream of an engine engineer working in this area of lean burn today, is to be able to burn as a diesel, and do that more or less automatically – a sort of auto combustion,’says Engelric. ‘To do that right, firstly you needto have direct injection, then you need to use spark plugs with pre-chamber and you need knowledge of the right set-up. But there are no fixed rules that guarantee you the best result with one attempt. Why do you think F1 teams spend so much money on engine testing? They know what they want, they know the rough direction to go in, but they have to experiment with a trial and error approach. ‘The talent of the engineer is to extract the knowledge from the results of each test, and build understanding,’Engelric adds. ‘We know that swirl, tumble and pre-chamber all
help with the combustion, but it is still an area that requires a lot of experimentation to gain that last five per cent of efficiency, and that is where you can easily explode your budget. For us, it makes no sense to spend all our money
in this area, it is a constant fight between the technology we want to develop, the budget of a privateer team, the racing we want to create and the profitable business we want to be.’ Another factor that required careful
consideration when redesigning the combustion chamber was controlling the area where knock is most likely to occur. This can be done through ensuring smoother geometries, Mecachrome used to manufacture 300 Formula 1 engines per
season. Today it supplies both Formula 2 and GP3 with its V6, as well as making the new LMP1 version available to privateers higher squish (the sudden turbulence of the air and fuel mixture as the piston approaches TDC), and instigating turbulence in the knock-prone regions to create conditions that minimise the likelihood of such an effect. The electronic control systems have to be capable of not only detecting the beginning of knock, but also have the correct mapping to react and prevent any development of knock.
‘The key is to push this limitation as far as possible, whilst using electronics to control the moment that you start to see this phenomenon because you know that it can destroy your engine,’ says Engelric. ‘We have eliminated 90 per cent of the risks of the first order of knock in our current LMP1 engine, whilst achieving the level of power possible
from the fuel limitations by managing the introduction of direct injection.’
Structural role
A further consequence of switching to direct injection was that the cam cover required modification as the high-pressure pump was now driven by the camshaft. Furthermore, the engine plays a much more structural role in a
prototype racecar as opposed to a single seater. ‘This means we have to achieve much higher stiffness with our LMP1 engine compared to F2.
In F2 we have four points to fix the engine to the chassis, however for LMP1 we have a total
of six attachment points, two of which are by the cam cover,’ explains Engelric. ‘We have used FEA to analyse the stiffness as well as the fatigue stress of the parts to guarantee that our new components never fail under the stresses we see during the life cycle of the engine. With this new design we have improved the stiffness of the engine by 40 per cent, which is essential for such a car and why we redesigned the cam cover.’ Aside from the direct injection-related redesigns, the LMP1 and F2 engines are very similar; sharing approximately 90 per cent of the same components including the block, head castings, crank and conrod. ‘The total distance raced in a full season of F2 is around 8000kms, Le Mans is around 7000kms. Our engine has already done a full season of F2 on the dynamometer, so we are not far away from completing Le Mans,’ Engelric says. ‘Of course it is somewhat different because at Le Mans, you start and never stop, but the majority of the larger parts will all work the same way for Le Mans as they do in GP3 and F2. The more critical parts, such as the pistons, valves and other valve-train components need to be
re-evaluated to guarantee they can survive Le Mans and the WEC season. But if we had started designing this LMP1 engine from scratch, I am pretty sure we would have arrived very close to the design we have today. The V6 is a good
compromise and the single turbocharger in the centre fits well with the installation of the car.’
From Race Car Engineering mag.