ptuomov wrote: ↑Sun Sep 15, 2019 9:32 am
ptuomov wrote: ↑Fri Sep 13, 2019 9:03 am
Ok, so if the problem is low speed preignition, I can see how more tumble is better. If the problem is spark knock, then more tumble is not necessarily always better. If so, for downsized passenger car engines, or engines running very high octane fuel, more tumble the better. For a "tuner engine" running on pump gas and making power at higher rpms, there may be a point where more tumble is no longer better.
LoganD wrote: ↑Sun Sep 15, 2019 9:08 am
This used to be the old thinking, and it was based on the fact that as RPM rises charge motion naturally gets more chaotic so you don't need to design a port specifically for tumble at high RPM. Now we know you want to control that charge motion and design it to do things deliberately, this involves designing the entire intake system correctly. This is how you've got Ferrari and McLaren turning 8500+ RPM with turbocharged engines making 200hp/L on 91 octane.
The new quote display for this forum sucks hairy scrotum.
With port-injected engines, the "old thinking" certainly looks it has been the dominant thinking. Maybe the solutions have changed. My impression was that in the port-injected engine era, the idea was to get a lot of tumble at low rpms or low loads such that the burn would be fast enough in those conditions even with a lot of EGR. In the port-injected engine era, the high rpms and high loads would take care of themselves. At high boosts and high rpms, sometimes charge motion was considered excessive and an intent was to slow it down.
That McLaren is a low compression, port injection engine "relic" so it's of particular interest to me. What's inside there? It has long intake runners and I am guessing very much a tumble port feeding a conventional four-valve head. But that's just a guess. Another guess is that the variable valve timing and using them right with turbos is largely responsible for the wide power band. With those log exhaust manifolds, the narrow bore spacing is going to allow it to rev higher, especially if the VVT takes out the valve overlap, I am guessing.
Well, I guess it depends on what you consider "old". You have to be careful because an engine released in 2005 had the combustion system designed at least 5 years earlier, so there's a time lag. Basically any engine with the combustion system designed in the last 15 years is going to have very high tumble due to shallow valve angles. So that would basically be any production engine after 2010. In the late 90's and very early 2000's high end CFD was still so expensive that it was being used sparingly and time-dependent CFD was virtually impossible with automotive development budgets. It was also pretty impractical from a time perspective, what used to take 6 months to run now takes 2 weeks and can be done on computers that are 1/100th the cost you would have paid 20 years ago.
I guess what I'm saying is that the drastic increase in speed and drastic reduction in cost for high end CFD has caused us to change a lot of our previous perceptions about engine design. The new Mercedes M139 is a perfect example, 20 years ago if someone were going to design a 2.0 turbo 4-cyl to make over 400 hp on pump fuel they wouldn't make it heavily undersquare (83x92) with almost no valve angle. That engine revs to 7400 RPM and makes 370 lb-ft from 121 ci.
It makes a lot of sense if you think about it. If you're knock limited and not airflow limited, which most is the case for most heavily turbocharged engines, it makes sense to design the entire engine around reducing knock instead of just trying to make it flow more. This is where the aftermarket is behind, a Coyote or LS head doesn't need more flow to make 1000 hp on pump fuel, it needs more knock resistance.
This thinking spilled over into naturally aspirated engine design. They now give the engine just enough airflow to make the RPM/power target, and then they design the rest of the engine around maximizing efficiency and cylinder pressure. That's why the new GT3 engine has 13.5:1 compression and pretty shallow valve angles for an engine that revs to 9000 RPM. They spent a great deal of time making the large bore engine very knock resistant, and that's a hard thing to do.