saviour stivala wrote: ↑
Wed Jul 10, 2019 12:11 pm
“Exhaust and intake pressures”. “Regardless of the aspiration method. The goal of the cam/valve timing design are to optimize cylinder filling and exhaust removal”.
A larger turbo on a small engine will need more RPM to get the most out of the turbo.
As turbochargers have evolved, not only their compressor side got more efficient, but so have the turbine side. These modern turbochargers still cause restrictions in the exhaust tract, but will bring back pressure down to a level equal to the pressurization of the intake tract. Dealing with similar pressures on both sides, with high-density charges on both sides, moving the system to a more similar to a naturally-aspirated model. So piston motion can be used for some wave tuning to get the charge motion at a lower velocity and with less ‘valve timing’. With the more efficient turbine section of modern turbochargers, the intake and exhaust pressures are back to a 1:1 ratio. Although elevated from a naturally-aspirated application, the equalized pressures allow camshaft designers to apply ‘some’ of the theories behind N/A camshafts to turbo cams. With these modern turbochargers there is still an exhaust restriction – there’s no getting around that fact –but they bring the restriction level to that of the amount of boost they are providing the intake tract. That 1:1pressure ratio is what allows wave-tuning to come into play,
A few points on the above post.
1. Turbo efficiency has been high enough to produce positive pressure differential (MAP > EBP) for many years. And yes modern turbochargers are ever more efficient.
2. Wave tuning has the same benefits for all engines - even for a negative pressure differential engine. The benefit of wave tuning on such an engine is to reduce the effective "negative pressure" - in the best case, a negative differential engine can still have positive scavenging during valve overlap thanks to wave tuning.
3. The current formula one engine in self sustaining mode is almost certainly in this category. To maximise MGUH harvesting, the exhaust back pressure will be as high as possible consistent with effective scavenging of the combustion chamber.
Camshaft selection and valve timing must be a nightmare for the engine developers when you think about the range of pressure ratios they have to deal with. In self sustaining mode the pressures are likely 4 bar MAP and 5+ bar EBP. In electric supercharger mode probably 4 bar MAP and 1 bar EBP. I am sure that variable valve timing would be a huge benefit. I have no idea why the rules prohibit this in what is supposed to be an "energy efficient", "road relevant" formula.
Taking this one step further, I believe that full ECU control of valve events is just around the corner (eg Koenigsegg "Freevalve" system). Loosening the rules in this area (perhaps with compulsory sharing of technology) would help develop this tech to "road ready" stage while delivering perhaps 5-10% race fuel reduction and even greater benefits for road cars.